Method of capturing co 2

Abstract

The present invention relates to a novel absorption medium for removing CO2 from a gas mixture. In particular, the present invention relates to a method of separating CO2 from a gas mixture comprising CO2 in an absorption-desorption process, said method comprising the steps of: i) providing a gas mixture comprising CO2 to an absorption zone; ii) contacting the gas mixture with at least one absorption medium under conditions suitable for the absorption medium to absorb the CO2, to obtain a CO2-enriched absorption medium; iii) desorbing CO2 from the CO2-enriched absorption medium in a desorption zone under conditions suitable for the CO2-enriched absorption medium to desorb CO2, to regenerate the absorption medium and obtain CO2; wherein said absorption medium is amidine derivatives of formula (I) herein, wherein the absorption and desorption of CO2 is controlled by the relative humidity (RH) in the absorption and desorption zone and / or by the water activity (aw) of the compound of formula (I).

Description

[0001] 83399PC01

[0002] 1

[0003] METHOD OF CAPTURING CO2

[0004] Technical field of the invention

[0005] The present invention relates to the field of carbon dioxide capture, such as direct air capture (DAC). In particular, the present invention relates to a method for separating CO2 from a gas mixture using amidine derivatives, wherein the absorption and desorption of CO2 is controlled by the relative humidity (RH) in the absorption / desorption zone and / or by the water activity (aw) of the amidine derivatives used.

[0006] Background of the invention

[0007] Direct Air Capture (DAC) using chemical sorbents can potentially reduce atmospheric carbon dioxide (CO2) levels, thus fostering and supporting the path towards global net-zero carbon emissions. In an efficient cyclic DAC process, the sorbent must selectively capture a substantial weight percentage of CO2 directly from the air at a sufficient rate. Furthermore, it should be possible to release the captured CO2 with minimal energy input— such as heat or electricity— thus producing pure, concentrated CC gas. This poses significant challenges because conventional carbon capture systems either require high regeneration temperatures (up to 900 °C) or exhibit low working capacity under DAC conditions. Earth's lowest atmospheric layer, the troposphere, has a uniform dry composition mainly consisting of N2 (78%), O2 (21%), Ar (0.9%), and ever-increasing CO2 at 0.04% by volume. However, the water vapor content varies from <0.1 vol% at the South Pole to 5 vol% in warm, humid climates. In addition to these regional differences, the water vapor content fluctuates seasonally and daily. Therefore, DAC technologies must tolerate varying levels of water vapor content, as regulating air humidity before capturing CO2 is both impractical and costly. Additionally, water often deteriorates sorbents and competes with CO2 at adsorption sites, making the realization of an efficient DAC system elusive.

[0008] Another major challenge for achieving efficient DAC is minimizing the operation's energy penalty. The thermodynamic minimum work of separating CO2 from air is ~20 kJ-mol’1(at 25 °C). However, most DAC technologies require an order of magnitude higher energy to accomplish CO2 separation from the air, operating below 10% efficiency. The commercialized calcium-looping DAC technology 83399PC01

[0009] 2 exhibits a very high capture capacity of 18 mol / kg (CCh / CaO), but at the expense of a high energy penalty during the regeneration step; the resulting CaCCh must be exposed to temperatures as high as 900 °C via methane combustion to release pure CO2 (387 kJ-mol’1, overall ca. 5% thermodynamic efficiency). In contrast, aqueous amine scrubbers release CO2 at much lower temperatures but at the expense of a low working capacity under DAC conditions; a solution of 30 wt% monoethanolamine (MEA) can achieve a maximum working capacity of ca. 0.4 mol / kg (CO2 / solution) from air, with CO2 being released at 120 °C under reduced pressure (12.2 kPa).

[0010] Most carbon-capture technologies alternate between the capture and regeneration phases via temperature, pressure, and / or vacuum swings that control the direction of CO2 flux in a cyclic process. In the capture phase, the sorbent is selectively loaded with CO2, which is then released by thermal energy, reduced pressure, or steam stripping in the regeneration phase, ultimately affording pure CO2 (>99%), usually with a high energy penalty. Other stimuli have been studied in recent years, such as electrochemical pH-swings photochemical pH-swings, polarity-swings and humidity-swings. Humidity-swing DAC utilizes polymeric basic resins that release CO2 at a partial pressure of 1-10 kPa without sensible heat input in the regeneration phase. For example, Wang, T. et al. (2011) use quaternary ammonium cations attached to a polymer structure and hydroxide or carbonate groups as mobile counterions to absorb carbon dioxide when dry and release it when wet. Thus, Wang, T. et al. discloses an amine-based humidityswing for CO2 capture.

[0011] Pereira, F. S. et al. (2008) discloses that the guanidine bases tetramethylguanidine (TMG) and 1,3, 4,6,7, 8-hexahydro-2 / 7-pyrimido[l,2-a]- pyrimidine (TBD) can capture CO2. Said study discloses that TMG can capture CO2 in the presence of H2O by forming bicarbonates. The study also discloses that the release of CO2 from the TBD-CO2 products can be controlled thermally, and TBD or TBD related compounds are therefore useful for selective separation of CO2 from complex gas mixtures. Pereira, F. S. et al. does not disclose using guanidine base derivatives, e.g., guanidine base derivatives with an increase hydrophobicity, to capture CO2.

[0012] Han, Y., et al. (2021) discloses that guanidine could be used as a humidity-swing CO2 air-capture sorbent capturing CO2 in humid conditions and releasing it in dry 83399PC01

[0013] 3 conditions. Said results are based on ab initio (simulation) methods and Han, Y., et al. does not disclose using the CO2 absorption mediums of the present invention as a humidity swing for CO2 capture.

[0014] WO16089561 Al relates to methods for separating carbon dioxide from a gas stream. The methods include (a) providing a stream having a gas therein; and (b) contacting the stream with a sorption composition thereby removing at least a portion of the gas from the stream. The sorption compositions tested in WO16089561 Al are 2-amino-2-imidazoline (example 1), 2-amino-l-ethyl- imidazoline (example 2), and 1,1,3,3-tetramethylguanidine (TMG, example 3). US2013164199 Al relates to a solvent system comprising an ionic liquid formed from a relatively acidic component and a nitrogenous base for the removal of acid gases, such as CO2, from mixed gas streams. The guanidines used as a part of the ionic liquids in the examples are tetramethylguanidine (Example 1-3) and N-tert- butyl-l,l,3,3-tetramethylguanidine (Example 4). The conjugate base of an acidic component is never the conjugate base of H2O, i.e., OH', but rather different fluor alcohols (e.g., 2,2,3,3,4,4,5,5-octafluoropentanol). Hence, US2013164199 Al does not disclose using a humidity swing for capturing CO2.

[0015] Hence, an improved method for separating CO2 from a gas mixture would be advantageous. In particular, a method that exhibit high working capacity under DAC conditions and does not require high regeneration temperatures (less than 100 °C) would be advantageous.

[0016] Summary of the invention

[0017] Herein, the inventors report on a series of strong organic bases (amidine derivatives) that use water vapor as a trigger to induce the capture and release of CO2 directly from the air. These organic absorbents are innocent towards CO2 in the absence of moisture but rapidly react with up to one molar equivalent of CO2 (equal to 4.5 mol CC / kg of absorbent) from the air when appropriately hydrated (50-75 RH%). Complete desorption of CO2 occurs from the fully loaded absorbent upon co-evaporation of water at 70 °C when a dew point of 26 °C is reached, without aid from sweep gasses or vacuum (Fig. 1A). The strong temperature dependence of the vapor pressure of water allows a drastic reduction of the energy cost of temperature-swing DAC, thus essentially separating CO2 from air 83399PC01

[0018] 4 using scalable strong lipophilic organic bases. A proposed mechanism of the DAC cycle of the present invention exemplified using alkylated TMG is shown in Fig. IB.

[0019] Thus, an object of the present invention relates to providing an improved method for separating CO2 from a gas mixture. In particular, it is an object of the present invention to provide a CO2 absorption medium that solves the above-mentioned problems of the prior art with low working capacity at DAC conditions and high regeneration temperatures (as high as 900 °C). As mentioned above none of the prior art documents discloses using alkylated guanidine bases as a humidity swing. The technical effect of adding alkyl chains to a guanidine compound, such as a TMG compound, are multiple:

[0020] • The alkyl chain length results in different properties for the alkylated guanidine derivatives, since the short alkyl chains give rise to a glassy gel, whereas the long alkyl chains give rise to solid alkylated compounds. The different guanidine analogs might therefore be useful in different environments (e.g. at different humidities).

[0021] • The alkylation of guanidine compounds increases the hydrophobicity and reduces the volatility of the absorbents to enable humidity-controlled carbon dioxide capture and release.

[0022] • The alkyl chains enhance the chemical stability of the absorbent for a longer process lifetime.

[0023] Thus, one aspect of the invention relates to a method of separating CO2 from a gas mixture comprising CO2 in an absorption-desorption process, said method comprising the steps of: i) providing a gas mixture comprising CO2 to an absorption zone; ii) contacting the gas mixture with at least one absorption medium under conditions suitable for the absorption medium to absorb the CO2, to obtain a CO2-enriched absorption medium; iii) desorbing CO2 from the CC -enriched absorption medium in a desorption zone under conditions suitable for the CC -enriched absorption medium to desorb CO2, to regenerate the absorption medium and obtain CO2; wherein said absorption medium is a compound of formula (I): 83399PC01 wherein

[0024] X is selected from the group consisting of H, -OR1, -NfR1^, and Ci-Ce alkyl, each R1is independently selected from Ci-Ce alkyl, or optionally two R1form a 5- or 6-membered ring in conjunction with X-C-N when X is -OR1, or -NfR1^, or in conjunction with -N- in -NfR1^,

[0025] R2is selected from the group consisting of an optionally substituted C5-C20 alkyl, an optionally substituted C5-C20 alkene, an optionally substituted Cs- C20 alkyne, and -(CH2CH2O)n-R3wherein n is an integer in the range of 2- 10 and R3is Ci-Ce alkyl, wherein the absorption and desorption of CO2 is controlled by the relative humidity (RH) in the absorption and desorption zone and / or by the water activity (aw) of the compound of formula (I).

[0026] Another aspect of the present invention relates to an absorption medium comprising a compound of formula (I): wherein

[0027] X is selected from the group consisting of H, -OR1, -N(R1)2, and Ci-Ce alkyl, each R1is independently selected from Ci-Ce alkyl, or optionally two R1form a 5- or 6-membered ring in conjunction with X-C-N when X is -OR1, or -N(R1)2, or in conjunction with -N- in -N(R1)2,

[0028] R2is selected from the group consisting of an optionally substituted C5-C20 alkyl, an optionally substituted C5-C20 alkene, an optionally substituted Cs- 83399PC01

[0029] 6

[0030] C20 alkyne, and -(CH2CH2O)n-R3wherein n is an integer in the range of 2- 10 and R3is Ci-Ce alkyl, wherein the compound of formula (I) is immobilized on a solid support.

[0031] Yet another aspect relates to the use of the absorption medium according to the present invention for separating CO2 from a gas mixture comprising CO2.

[0032] Brief description of the figures

[0033] Figure 1A shows a schematic representation of DAC cycle using strong organic bases of the present invention.

[0034] Figure IB shows a proposed mechanism of DAC cycle by lipophilic strong organic bases used in the examples.

[0035] Figure 2 shows a graphical representation of the experimental setup for direct air capture experiments.

[0036] Figure 3A shows chemical structures of lipophilic guanidine bases.

[0037] Figure 3B shows CO2 capacity of C10-TMG at different temperatures and water contents under 1 atm CO2.

[0038] Figure 3C shows CO2 capacity of absorbents with 1 equiv. H2O at different temperatures under 1 atm CO2.

[0039] Figure 3D shows water uptake (wt%) of absorbents under N2 atmosphere (circles) and CO2 atmosphere (square).

[0040] Figure 3E shows water uptake (wt%) of absorbents under ambient atmosphere. Figure 4A shows CO2 uptake by absorbents (0.050 g) upon passive exposure to a stream (3 ml_ min-1) of humidified air for 7 days at 25 °C.

[0041] Figure 4B shows initial kinetic capture efficiency (ca. 4 days) of absorbents at various relative humidity (RH%) levels.

[0042] Figure 5A shows absorption and desorption of CO2 by prehydrated C10-TMG. Addition of 1.1 equiv. H2O, absorption under 1 atm CO2, desorption of CO2 and water from [CIO-TMG-H] [HC03] with CaC at ambient temperature. The mass of the system was periodically monitored, and aliquots were withdrawn for NMR analysis.

[0043] Figure 5B shows stacked13C NMR spectra (126 MHz, D2O) of aliquots withdrawn from passive desorption experiment after CO2 absorption (top) and after 10 days 83399PC01

[0044] 7 in closed container with CaCIz (bottom). Complete disappearance of HCOs' / COs2' signal is observed.

[0045] Figure 5C shows CO2 capacity of C10-TMG (mol: mol) under 1 atm CO2 at 11

[0046] RH%.

[0047] Figure 5D shows CO2 capacity of C10-TMG (mol: mol) at various temperatures and RH%.

[0048] Figure 5E shows thermogravimetric Analysis of C10-, C12- and Cie-TMG after undergoing DAC at 75, 84 and 100 RH% respectively. Conditions: CO2 (100%, 90 ml_ min-1), ramp 1 °C min-1.

[0049] Figure 5F shows Cyclic absorption / desorption experiments. Absorption: 2 equiv. H2O added, then placed under 1 atm CO2 at 25 °C for 1 h. Desorption: 11 RH%, 1 atm CO2, 70 °C.

[0050] Figure 6 shows H2O / CO2 uptake ratio, CO2 capture efficiency (%), and H2O capture efficiency (%) comparing 25 wt % C7- and C10-TMG on silica gel (SG) and 10 % hexadecyltrimethoxysilane functionalized silica (10%HDTMS-SG).

[0051] Figure 7 shows a cyclic CO2 breakthrough and desorption profiles for 25 wt % C10-TMG / GW over two capture-release cycles (humid air, 25 °C -> dry air, 70 °C), tb: breakthrough time, C / Co: ratio of outlet concentration (C) of CO2 to inlet concentration (Co) of CO2.

[0052] The present invention will now be described in more detail in the following.

[0053] Detailed description of the invention

[0054] Definitions

[0055] Prior to discussing the present invention in further details, the following terms and conventions will first be defined:

[0056] Carbon dioxide

[0057] Carbon dioxide is a chemical compound with the chemical formula CO2. It is a significant greenhouse gas in Earth's atmosphere, contributing to the greenhouse effect by trapping heat. Carbon dioxide thereby plays a crucial role in regulating Earth's temperature. Carbon dioxide is produced by the respiration of animals, the combustion of organic matter, and various industrial processes, such as combustion of fossil fuels, cement production, steel manufacturing, and oil 83399PC01

[0058] 8 refining. In the present context, 'CO2' and 'carbon dioxide' are used interchangeably.

[0059] Gas mixture comprising CO2

[0060] In the present context, the term 'Gas mixture comprising CO2' refers to a CO2 gas often with one or more different gases that are mixed together without any chemical bonding between them, however, it may also refer to pure CO2 gas. Each gas in the mixture retains its own chemical properties. A gas mixture comprising CO2 includes but is not limited to biogas, post-combustion gas mixture, and flue gas.

[0061] Biogas is produced through the anaerobic digestion of organic matter by microorganisms. It is generated from the decomposition of organic materials like agricultural waste, manure, municipal waste, plant material, sewage, green waste, or food waste. Biogas primarily consists of methane (CH4) and carbon dioxide (CO2), with small amounts of other gases such as hydrogen sulphide (H2S) and ammonia (NH3).

[0062] Post-combustion gas mixture refers to the gases that are emitted after the combustion process in engines, power plants, or industrial facilities. It is produced from the burning of fossil fuels such as coal, oil, natural gas, and biomass in various combustion systems. Post-combustion gas mixture typically includes carbon dioxide (CO2), water vapor (H2O), nitrogen oxides (NOX), sulphur oxides (SOX), carbon monoxide (CO), unburned hydrocarbons, and other trace gases. Flue gas is the exhaust gas that is emitted from the combustion process in industrial facilities, power plants, and other combustion systems. It is produced from the burning of fossil fuels such as coal, oil, natural gas, and biomass in boilers, furnaces, and other combustion equipment. Flue gas typically contains carbon dioxide (CO2), water vapor (H2O), nitrogen oxides (NOX), sulphur oxides (SOX), carbon monoxide (CO), and other trace pollutants.

[0063] In an embodiment, the Gas mixture comprising CO2 is an industrial gas mixture.

[0064] Absorption medium

[0065] 'Absorption medium' refers to a material used to absorb or adsorb CO2 from a gas mixture comprising CO2. In the present context, the term refers to compounds of formula (I) that captures CO2. The absorption medium might be a mixture of 83399PC01

[0066] 9 different compounds of formula (I). In the present context the absorption medium may change physically, for example in terms of weight, volume and density as it absorbs e.g. CO2 and / or water, but the molecular structure is essentially unchanged and reversible, i.e. the absorption medium is still a compound of formula (I), or a mixture thereof. It is also to be understood, that in the present context the absorption medium may be present in conjunction with e.g. additives, such as stabilisers, and it may be pre-hydrated, i.e. water may be pre-absorbed prior to running the first process cycle. The absorption medium may also be positioned e.g. on or in a porous solid, for example if the absorption medium is in liquid form.

[0067] Absorb

[0068] The term 'absorb' refers to the process of taking up or holding a substance. In the present context, the substance is CO2. Absorption involves the uptake of substances into the volume of the sorbent, and in the present context also accumulation of substances on the surface of the sorbent. The sorption medium can sorb and desorb CO2 either in the same place or at different zones. The term 'absorption zone' refers to a specific area or region within a system where the process of absorption occurs. If absorption is conducted in the same place as desorption, absorption / desorption is regulated by regulating relative humidity (RH), water activity, and optionally temperature and / or gas flow.

[0069] Desorb

[0070] The term 'desorb' refers to the process of releasing or removing a substance that has been absorbed within a material (by absorption). In the present context, the substance is CO2. The term 'Desorption zone' refers to a specific area or region within a system where the process of desorption occurs. If absorption is conducted in the same place as desorption, absorption / desorption is regulated by regulating relative humidity (RH), water activity, and optionally temperature and / or gas flow. Preferably the desorption is performed under water vapor (steam), CO2 gas flow, nitrogen gas flow, or argon gas flow, more preferably under water vapor (steam) or CO2 gas flow.

[0071] C02-enriched absorption medium 83399PC01

[0072] 10

[0073] The term 'CC -enriched absorption medium' refers to an absorption medium that has absorbed CO2 from the gas mixture comprising CO2. In the present context, the absorption medium absorbed CO2 using hydroxide, which was generated from humidity or additional water, reacting with the absorption medium. A CO2- enriched absorption medium might be fully saturated with CO2 if every absorbent in an absorption medium mixture has sorbed CO2, but the term also covers absorption mediums that are not fully saturated, i.e., absorption mediums wherein some absorbent has absorbed CO2 whereas others have not.

[0074] Optionally substituted C6-C20 alkyl

[0075] The term 'Optionally substituted C6-C20 alkyl' refers to an alkyl group that contains between 6 and 20 carbon atoms and may have one or more substituents attached to it. The optionally substituted C6-C20 alkyl might be or comprise elements that are linear, branched or cyclic. The substituent includes but is not limited to a halide, Ci-Cs alkyl, C2-C8 alkene, and C2-C8 alkyne.

[0076] Optionally substituted C6-C20 alkene

[0077] The term 'Optionally substituted C6-C20 alkene' refers to an alkene group that contains between 6 and 20 carbon atoms and may have one or more substituents attached to it. Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). For example, a C& alkene is a hexene. The optionally substituted C6-C20 alkene might be or comprise elements that are linear, branched or cyclic. The substituent includes but is not limited to a halide, Ci-Cs alkyl, C2-C8 alkene, and C2-C8 alkyne.

[0078] Optionally substituted C6-C20 alkyne

[0079] The term 'Optionally substituted C6-C20 alkyne' refers to an alkyne group that contains between 6 and 20 carbon atoms and may have one or more substituents attached to it. Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond (C=C). For example, a Ce alkyne is a hexyne. The optionally substituted C6-C20 alkyne might be or comprise elements that are linear, branched or cyclic. The substituent includes but is not limited to a halide, Ci-Cs alkyl, C2-C8 alkene, and C2-C8 alkyne.

[0080] Relative humidity (RH) 83399PC01

[0081] 11

[0082] In the present context relative humidity represents the amount of water vapour in a gas as compared to the maximum amount of water vapour said gas could potentially contain. RH is often expressed as a percentage and is calculated as the partial pressure of water vapour (p) in the gas divided by the saturation vapour pressure (ps) of the gas, i.e. RH = 100% ■ p / ps. Importantly, RH can be controlled by the gas temperature, as e.g. warm air can contain more vapour than cold air (a well-known meteorological phenomenon). Thus, in the present context RH may be controlled by controlling the temperature. There may be other ways of controlling or adjusting RH, e.g. by changing the composition of the gas mixture, pressure changes, volume changes, etc. In the present context RH is thus RH as measured in gas mixture comprising CO2 in the absorption / desorption zone, whether this is the same zone or not. In Direct Air Capture (DAC) applications, the gas mixture will preferably be air.

[0083] Water activity awis closely related to relative humidity (RH) but applies to solutions, semi-solids and solids as well. For a gas, the water activity is the same as relative humidity. Thus, In the present context awrepresents partial vapour pressure of water (p) in a sample divided by the standard state partial vapour pressure of water in the sample (p*), i.e. aw= p / p*. As for RH, awmay be controlled by temperature, and for most products water activity will increase with temperature increases. The relative humidity (RH) of a gas in equilibrium with a sample is also called the Equilibrium Relative Humidity (ERH) and is usually given as a percentage. It is equal to water activity according to ERH = awx 100%.

[0084] Solid support

[0085] In the present context, the term "solid support" refers to an insoluble material used as a physical platform to anchor or immobilize a compound of formula (I). The compound of formula (I) may be immobilized by chemically binding to the solid support or by physisorption. The solid support is preferably porous, i.e., it comprises cavities. Without being bound by theory the porous structure increases the surface area of the solid support, thereby increasing the gas capture (CO2 and H2O) of the compound of formula (I). The solid support comprises but is not limited to silica gel, mesoporous silica, and glass wool. 83399PC01

[0086] 12

[0087] The silica gel or mesoporous silica can be functionalized with at least one hydrophobic compound, i.e., at least one hydrophobic compound can be chemically bonded to the silica or mesoporous silica. The hydrophobic compound is preferable a C4-C20 alkyl trimethoxysilane, such as Ci6 trimethoxysilane (hexadecyltrimethoxysilane).

[0088] Mesoporous silica

[0089] Mesoporous silica is a type of solid support composed of silicon dioxide (SiC ) that contains pores with diameters ranging from 2 to 50 nanometers, according to IUPAC classification.

[0090] The present inventors have surprisingly found that when using certain alkylated amidine derivatives as CO2 capture agents, the absorption and desorption of carbon dioxide may be conveniently controlled by the relative humidity of the gas mixture comprising carbon dioxide in the absorption / desorption zone and / or the water activity of the alkylated amidines. It has further surprisingly been found that alkylation of the amidines brings advantages to the process in terms of e.g. the volatility and stability of the absorption medium comprising the amidines and the amidines per se.

[0091] Thus, a first aspect of the present invention is a method of separating CO2 from a gas mixture comprising CO2 in an absorption-desorption process, said method comprising the steps of: i) providing a gas mixture comprising CO2 to an absorption zone; ii) contacting the gas mixture with at least one absorption medium under conditions suitable for the absorption medium to absorb the CO2, to obtain a CO2-enriched absorption medium; iii) desorbing CO2 from the CC -enriched absorption medium in a desorption zone under conditions suitable for the CC -enriched absorption medium to desorb CO2, to regenerate the absorption medium and obtain CO2; wherein said absorption medium is a compound of formula (I): 83399PC01 wherein

[0092] X is selected from the group consisting of H, -OR1, -NfR1^, and Ci-Ce alkyl, each R1is independently selected from Ci-Ce alkyl, or optionally two R1form a 5- or 6-membered ring in conjunction with X-C-N when X is -OR1, or -NfR1^, or in conjunction with -N- in -NfR1^,

[0093] R2is selected from the group consisting of an optionally substituted C5-C20 alkyl, an optionally substituted C5-C20 alkene, an optionally substituted Cs- C20 alkyne, and -(CH2CH2O)n-R3wherein n is an integer in the range of 2- 10 and R3is Ci-Ce alkyl, wherein the absorption and desorption of CO2 is controlled by the relative humidity (RH) in the absorption and desorption zone and / or by the water activity (aw) of the compound of formula (I).

[0094] An alternative aspect of the present invention relates to a method of separating CO2 from a gas mixture comprising CO2 in an absorption-desorption process, said method comprising the steps of: i) providing a gas mixture comprising CO2 to an absorption zone; ii) contacting the gas mixture with at least one absorption medium under conditions suitable for the absorption medium to absorb the CO2, to obtain a CO2-enriched absorption medium; iii) desorbing CO2 from the CC -enriched absorption medium in a desorption zone under conditions suitable for the CC -enriched absorption medium to desorb CO2, to regenerate the absorption medium and obtain CO2; wherein said absorption medium is a compound of formula (I): 83399PC01 wherein

[0095] X is selected from the group consisting of H, -OR1, -NfR1^, and Ci-Ce alkyl, each R1is independently selected from Ci-Ce alkyl, or optionally two R1form a 5- or 6-membered ring in conjunction with X-C-N when X is -OR1, or -N R1^, or in conjunction with -N- in -NfR1^,

[0096] R2is selected from the group consisting of an optionally substituted C5-C20 alkyl, an optionally substituted C5-C20 alkene, an optionally substituted Cs- C20 alkyne, and -(CH2CH2O)n-R3wherein n is an integer in the range of 2- 10 and R3is Ci-Ce alkyl.

[0097] The compounds of formula (I) may preferably have a heteroatom in the X position, and thus in a preferred embodiment X is selected from the group consisting of -OR1and -N(R1)2, preferably -N(R1)2. Guanidine derivatives have proven to be particularly useful and perform well in the tested systems, and thus said absorption medium is preferably a compound of formula (II): wherein each R1is independently selected from Ci-Ce alkyl, or optionally two R1form a 5- or 6-membered ring in conjunction with X-C-N when X is -OR1, or -N(R1)2, or in conjunction with -N- in -N(R1)2,

[0098] R2is selected from the group consisting of an optionally substituted C5-C20 alkyl, an optionally substituted C5-C20 alkene, an optionally substituted Cs- 83399PC01

[0099] 15

[0100] C20 alkyne, and -(CH2CH2O)n-R3wherein n is an integer in the range of 2- 10 and R3is Ci-Ce alkyl.

[0101] The compounds of the present invention of formular (I) or (II) may preferably have a relatively short chain alkyl in the R1position. Therefore, each R1is preferably independently selected from C1-C5 alkyl, C1-C4 alkyl, C1-C3 alkyl, such as C1-C2 alkyl, such as preferably Ci alkyl. For example, each R1may independently be selected from hexyl, cyclohexyl, pentyl, cyclopentyl, butyl, tertbutyl, iso-butyl, sec-butyl, propyl, iso-propyl, ethyl, and methyl. More preferably, All R1are selected from the group consisting of hexyl, cyclohexyl, pentyl, cyclopentyl, butyl, tert-butyl, iso-butyl, sec-butyl, propyl, iso-propyl, ethyl, and methyl, preferably ethyl and methyl, such as most preferably methyl.

[0102] R2is an alkyl, alkene or alkyne of a certain length, where the length may alter some of the advantageous properties of the compounds for absorption. In preferred embodiments R2is selected from the group consisting of a Ce-Ci9-alkyl, -alkene, and -alkyne, such as a Ce-Cis-alkyl, -alkene, and -alkyne, such as a C7- Ci7-alkyl, -alkene, and -alkyne, such as a C7-Ci6-alkyl, -alkene, and -alkyne, such as preferably Cs-Cio-alkyl, -alkene, and -alkyne. The R2group may preferable be an alkyl, and thus R2may preferably be selected from the group consisting of a C6-C19 alkyl, such as Ce-Cis alkyl, such as a C7-C17 alkyl, such as preferably a C7- Ci6 alkyl, such as preferably Cs-Cio-alkyl. R2may be linear or branched, such as preferably linear. The alkenes described may comprise one or more double bonds, preferably one double bond. The alkynes described may comprise one or more triple bonds, preferably one triple bond. The optional substituents on R2may preferably be selected from the group consisting of halides, and alkylsilyls. More preferably, the optional substituents on R2may be selected from the group consisting of Cl, Br, F, I, methylsilyl, dimethylsilyl and trimethylsilyl.

[0103] The absorption medium is defined herein as a compound of formula (I). The absorption medium may be in different physical states depending e.g. on the substitution pattern of the compound, the temperature, and whether the absorption medium has absorbed carbon dioxide or not, or if it has co-absorbed e.g. water. In one embodiment the absorption medium is a liquid at 20 °C. In such embodiments the absorption medium may be impregnated onto a porous 83399PC01

[0104] 16 solid. Preferably the absorption medium is further capable of absorbing water. The absorption medium may preferably be pre-hydrated with water. The present inventors have shown that co-absorption of water increases absorption of carbon dioxide. Thus, in a preferred embodiment the absorption medium co-absorbs water and CO2. More specifically, the absorption medium may preferably be prehydrated with or co-absorbs 0.5 to 5 equivalents of water as compared to the compound of formula (I), such as 0.7-3 equivalents, such as preferably 1-3 equivalents of water as compared to the compound of formula (I). Preferably, the absorption medium is pre-hydrated with or absorbs water, such as at least 0.5 equivalents of water relative to the compound of formula (I), preferably at least 0.7 equivalents of water relative to the compound of formula (I), such as preferably at least 1.0 equivalents of water relative to the compound of formula (I) , such as preferably at least 2.0 equivalents of water relative to the compound of formula (I) , such as preferably at least 3.0 equivalents of water relative to the compound of formula (I). Further, the absorption medium may preferably have a lower density than the CC -enriched absorption medium. This may be utilised in embodiments where the absorption medium is liquid, since the heavier CO2- enriched absorption medium may fall to the bottom while the absorption medium is exposed to the gas mixture at the surface of the liquid absorption medium. In another preferred embodiment the absorption medium comprises two or more different compounds of formula (I). This may be advantageous if the different compounds have different capabilities at e.g. different concentrations of carbon dioxide or at different RH or different water activities.

[0105] The nature of the gas mixture from which carbon dioxide is extracted in the method of the present invention may depend on whether method is part of e.g. post-combustion gas carbon capture system, a system comprising e.g. biogas or a e.g. a direct air capture (DAC) system. Thus, in a preferred embodiment the gas mixture is selected from the group consisting of air, post-combustion gas mixtures, flue gas, and biogas, or any mixture thereof. Preferably the gas mixture is air, such as ambient air. The concentration of carbon dioxide in the air of the lower atmosphere is relatively low, but nevertheless, if humanity is to lower the carbon dioxide concentration in the atmosphere it will be necessary to remove carbon dioxide from ambient / atmospheric air. Preferably, the CO2 concentration in the gas mixture is at least 200 ppm, such as at least 300 ppm, at least 350 ppm, 83399PC01

[0106] 17 at least 400 ppm, such as preferably at least 420 ppm. Absorption may happen from a gas mixture of 100% carbon dioxide. Desorption may in some cases also be performed into a gas stream of e.g. pure carbon dioxide (100%), i.e. to add the desorbed carbon dioxide to an existing pool of carbon dioxide, which may then be utilised industrially, bound in another solid or otherwise stored. Atmospheric air or ambient air often comprises water in the form of e.g. vapour, and this may also be the case for other gas mixture. Hence, in one embodiment the gas mixture comprises water, such as water vapour.

[0107] The absorption-desorption process of the present invention may preferably be repeated a large number of times using the same absorption medium; hence, the absorption-desorption process is preferably a cyclic process. Preferably the loss of absorption medium per cycle in the cyclic absorption-desorption process is less than 10%, such as less than 5%, such as preferably less than 1%.

[0108] The cyclic process may occur in the same spatial volume, i.e. where the absorption zone and desorption zone are spatially the same zone, and where the conditions are changed in that zone to activate absorption or desorption of carbon dioxide. As described, this may be achieved by controlling the relative humidity (RH) of the gas mixture in the absorption / desorption zone and / or the water activity (aw) of the absorption medium. The RH of the gas mixture and the awof the absorption medium may interact and affect each other. As hot gas holds less vapour than cold gas, the relative humidity (RH) during absorption and desorption may preferably be controlled by controlling the temperature in the absorption and desorption zone. An advantage of the present invention and the absorption medium used is that the temperature difference between absorption conditions and desorption condition is not very high, meaning less energy expenditure. Hence, the difference between the temperature suitable for absorption and the temperature suitable for desorption is preferably less than 100 °C, such as less than 80 °C, such as less than 60°C, such as less than 50 °C, such as less than 40 °C, such as less than 35 °C, such as less than 30 °C, such as less than 25 °C. The RH may also be controlled by adjusting the temperature of the incoming gas mixture. This may preferably be done prior to entering the absorption / desorption zone, or in that zone. Therefore, in one embodiment the relative humidity (RH) during absorption is controlled by the adjusting the temperature of the gas 83399PC01

[0109] 18 mixture. In another embodiment the relative humidity (RH) during desorption is controlled by the adjusting the temperature of the gas mixture. Increasing temperature decreases RH which again increases desorption and vice versa, therefore in a preferred embodiment the relative humidity (RH) during desorption is decreased by increasing the temperature in the desorption zone and the relative humidity (RH) during absorption is increased by decreasing the temperature of in the absorption zone.

[0110] A similar approach may be applied to the water activity of the absorption medium. Thus, preferably the absorption of CO2 in the absorption medium is increased by increasing the water activity (aw) of the absorption medium and wherein the desorption of CO2 in the CC -enriched absorption medium is increased by decreasing the water activity (aw) of the absorption medium. Preferably, the water activity (aw) of the absorption medium is controlled by controlling the temperature of the absorption medium. Alternatively, or simultaneously, the water activity (aw) of the absorption medium may be controlled by controlling the hydration of the absorption medium.

[0111] In a preferred embodiment the gas mixture is contacted with the absorption medium as a gas stream passing through the absorption zone. Likewise, the desorbed carbon dioxide may be delivered from the CO2 enriched absorption medium to gas stream, which may be e.g. pure CO2.

[0112] In some embodiments the absorption zone and the desorption zone are not spatially the same zone, and thus the absorption medium is moved between absorption and desorption. In such embodiments the method comprises a step between step ii) and step iii) of: iia) transferring the CO2-enriched absorption medium to a desorption zone.

[0113] For cyclic processes with spatially different absorption / desorption zones the method then also preferably comprises a step after step iii) of: iiia) transferring the regenerated absorption medium to the absorption zone. 83399PC01

[0114] 19

[0115] Preferably however, the absorption zone may be converted to a desorption zone. Hence, the desorption zone is spatially the same zone as the absorption zone. This is done by e.g. controlling the RH and / or awin the zone, optionally via temperature control, and optionally exchanging the gas mixture while absorbing with another gas stream (optionally a gas mixture), while desorbing.

[0116] In a preferred embodiment, the conditions suitable for the absorption medium to absorb the CO2 provides a relative humidity (RH) in the absorption zone in the range of 5-100%, such as 10-100%, such as 20-100%, such as 30-90%, such as 35-85%, such as preferably 40-80%. The RH may be controlled by the temperature in the absorption zone, and thus, preferably, the conditions suitable for the absorption medium to absorb the CO2 provides a temperature suitable to obtain a relative humidity (RH) as described above. More specifically, the conditions suitable for the absorption medium to absorb the CO2 may preferably provide a temperature in the range of -30-50 °C, such as -10-40 °C, such as 0-35 °C, such as 10-30 °C, such as preferably 15-25 °C. Preferably, the conditions suitable for the absorption medium to absorb the CO2 provides a water activity (aw) of the absorption medium in the range of 0.20-1.00, such as 0.30-0.95, such as 0.40-0.90, such as 0.50-0.80.

[0117] The CO2-enriched absorption medium may be a liquid or a solid at 20 °C, preferably it is a solid at 20 °C. In a preferred embodiment the CC -enriched absorption medium has a higher density than the absorption medium. For liquid absorption mediums, this means that the unsaturated absorption medium stays at the surface for optimum contact with the gas mixture. Preferably, the CO2- enriched absorption medium has co-absorbed water and CO2, and preferably the CO2-enriched absorption medium is further capable of desorbing water. More preferably, the CC -enriched absorption medium does co-desorb water and CO2.

[0118] In a preferred embodiment, the conditions suitable for the CC -enriched absorption medium to desorb the CO2 provides a relative humidity (RH) in the desorption zone in the range of 1-40%, such as 5-35%, such as 10-30%, such as preferably 10-20%. The RH may be controlled by the temperature in the desorption zone and preferably the conditions suitable for the CC -enriched absorption medium to desorb the CO2 provides a temperature suitable to obtain a 83399PC01

[0119] 20 relative humidity (RH) as described just above. Preferably, the conditions suitable for the CO2-enriched absorption medium to desorb the CO2 provides a temperature in the range of 30-85 °C, such as 40-80 °C, such as preferably 55-75 °C.

[0120] Preferably, the conditions suitable for the CCh-enriched absorption medium to desorb the CO2 provides a water activity (aw) of the absorption medium in the range of 0.01-0.40, such as 0.05-0.35, such as 0.10-0.30, such as 0.10-0.20.

[0121] The present method results in a potential up-concentration of CO2 gas as attained during desorption, and preferably the CO2 obtained in step iv) is subsequently used to produce a CC -rich gas-stream or in carbon sequestration. Preferably, the method of the present invention is applied in Carbon capture from postcombustion gas, biogas or flue gas or in Direct Air Capture (DAC), most preferably Direct Air Capture (DAC).

[0122] The inventors demonstrated in Example 5 that immobilizing the compound of formula (I) on a solid support improves the efficiency of repeated absorption / desorption cycles and reduces the energy required for water evaporation during regeneration, compared with using neat Cn-TMGs. Thus, in an embodiment, the compound of formula (I) is immobilized on a solid support. In another embodiment, the solid support is porous. In yet another embodiment, the solid support is selected from the group consisting of silica gel, mesoporous silica, and glass wool. Replacing silica gel (SG) with hydrophobic mesoporous silica (mesoporous hexadecyltrimethoxysilane functionalized silica gel, HDTMS-SG) reduced water uptake and accelerated regeneration (see Figure 6). A lower H2O / CO2 (mol / mol) ratio indicates that less energy is required to regenerate the sorbent, since less water needs to be removed during the desorption step. This enhances the overall energy efficiency of the CO2 capture and release process. Thus, in an embodiment, the silica gel or mesoporous silica is functionalized with at least one hydrophobic compound, preferably C4-C20 alkyl trimethoxysilane, such as Ci6 trimethoxysilane.

[0123] An aspect of the present invention relates to an absorption medium comprising a compound of formula (I): 83399PC01 wherein

[0124] X is selected from the group consisting of H, -OR1, -NfR1^, and Ci-Ce alkyl, each R1is independently selected from Ci-Ce alkyl, or optionally two R1form a 5- or 6-membered ring in conjunction with X-C-N when X is -OR1, or -NfR1^, or in conjunction with -N- in -NfR1^,

[0125] R2is selected from the group consisting of an optionally substituted C5-C20 alkyl, an optionally substituted C5-C20 alkene, an optionally substituted Cs- C20 alkyne, and -(CH2CH2O)n-R3wherein n is an integer in the range of 2- 10 and R3is Ci-Ce alkyl, preferably wherein the compound of formula (I) is immobilized on a solid support.

[0126] Another aspect of the present invention relates to an absorption medium for separating CO2 from a gas mixture comprising CO2, said absorption medium comprising a compound of formula (I): wherein

[0127] X is selected from the group consisting of H, -OR1, -N(R1)2, and Ci-Ce alkyl, each R1is independently selected from Ci-Ce alkyl, or optionally two R1form a 5- or 6-membered ring in conjunction with X-C-N when X is -OR1, or -N(R1)2, or in conjunction with -N- in -N(R1)2,

[0128] R2is selected from the group consisting of an optionally substituted C5-C20 alkyl, an optionally substituted C5-C20 alkene, an optionally substituted Cs- 83399PC01

[0129] 22

[0130] C20 alkyne, and -(CH2CH2O)n-R3wherein n is an integer in the range of 2- 10 and R3is Ci-Ce alkyl, preferably wherein the compound of formula (I) is immobilized on a solid support.

[0131] Yet another aspect relates to the use of the absorption medium according to the present invention for separating CO2 from a gas mixture comprising CO2.

[0132] It should be noted that embodiments and features described in the context of one of the aspects of the present invention also apply to the other aspects of the invention.

[0133] All patent and non-patent references cited in the present application, are hereby incorporated by reference in their entirety.

[0134] The invention will now be described in further details in the following non-limiting examples.

[0135] Examples

[0136] Example 1 - Materials and methods

[0137] Materials and Chemicals

[0138] Solvents, reagents, and chemicals were purchased from commercial vendors and used without further purification. CO2 (99.999%) and compressed air was purchased from Air Liquide and used as received. N2 was supplied in-house.

[0139] Absorbents were stored in tightly capped laboratory bottles at 5 °C. Laboratory bottles containing absorbents were removed from refrigerator approximately 30 minutes prior to use or until they reached ambient temperature.

[0140] General Description

[0141] XH NMR and13C NMR spectra were recorded at 500 MHz and 126 MHz respectively using CDCI3, D2O and MeCN-cfs as solvents on a Bruker Ultrashield Plus 500 spectrometer. All chemical shifts (5) are reported in ppm using the solvent residual peak as a reference (or ethylene glycol 62.50 ppm for13C NMR in D2O) and all coupling constants (J) are expressed in Hertz (Hz). The following abbreviations are used for multiplicity for NMR resonances: s = singlet, d = 83399PC01

[0142] 23 doublet, t = triplet, q = quartet, and m = multiplet. Fourier transform infrared (FT-IR) spectra were collected on a Bruker Alpha-P FT-IR spectrometer equipped with an attenuated total reflectance (ATR) module. The thermogravimetric analysis (TGA) was performed using a Discovery TGA from TA instruments (New Castle, DE, USA). The samples were heated in a platinum TGA pan. LC-MS analyses were carried out by connecting the above mentioned HPLC apparatus to a Bruker MicrOTOF-QII system equipped with an ESI source with nebulizer gas at 1.2 bar, dry gas at 10 L / min, dry temperature at 200 °C, capillary at 4500 V and end plate offset at -500 V. The ion transfer was conducted with funnel 1 and funnel RF's at 200.0 Vpp and hexapole RF at 100.0 Vpp while the quadrupole ion energy was set at 5.0 eV with a low mass cut-off at 100.00 m / z. In the collision cell, collision energy was set at 8.0 eV, collision RF at 100.0 Vpp, and a transfer time of 80.0 ps and pre-pulse storage of 1.0 ps were used. High-resolution mass spectrometry was performed on a Bruker SolariX XR 7 T ESI / MALDIFT-ICR-MS instrument. Conductivity measurements were performed using HI-2003 Edge® Conductivity Meter. Both portable meters were purchased from Hanna instrument, Denmark including the standard solution for calibration of conductivity probe (HI- 7030 to 12880 pS cm’1at 25°C).

[0143] 1,1,3,3-tetramethylguandidine (TMG) was alkylated according to a slightly modified literature procedure (Heldebrant, D. J. et al. (2010)). To a flame-dried 500 mL round bottomed flask equipped with a reflux condenser and an addition funnel, ~60 mL TMG (0.478 mol, 2.3 equiv.) was dissolved in 45 mL xylenes (or toluene) and placed in a preheated 125 °C aluminum block under nitrogen atmosphere. While stirring, alkylbromide (0.211 mol, 1 equiv.) was added dropwise over 2-4 hours and left to stir overnight. The solids were removed by gravity filtration. The solvent (o-xylene or toluene) was removed under reduced pressure and product was isolated via simple distillation. For the heavier boiling analogues, a white solid precipitates from the distillate upon cooling which was removed by gravity filtration.

[0144] No difference in yield was observed when alternating between o-xylene and toluene as solvent.

[0145] 2-heptyl-l,l,3,3-tetramethylguanidine (C7-TMG): 83399PC01

[0146] Compound was synthesized according to general synthesis procedure using TMG (65 mL, 0.518 mol, 2.1 equiv.) and 1-bromoheptane (38 mL, 0.242 mol, 1 equiv.). The product was isolated as a colorless, non-viscous liquid. Yield: 23.7025 g (46%). Boiling point: 74-79 °C (0.83 mm Hg).XH NMR (500 MHz, CDCI3) 5 3.07 (t, J = 7.00 Hz, 2H), 2.71 (s, 6H), 2.62 (s, 6H), 1.49 (p, J = 7.03 Hz, 2H), 1.32-1.19 (m, 8H), 0.86 (t, J = 7.0 Hz, 3H).13C NMR (126 MHz, CDCI3) 5 160.01, 49.78, 39.76, 38.94, 32.96, 32.10, 29.37, 27.60, 22.79, 14.23. FT-IR (neat): 1360, 1621, 2854, 2923 cm4. Elemental analysis. Anal. Calcd C12H27N3: C, 67.55; H, 12.76; N 19.69. Found: C, 64.66; H, 12.61; N, 18.90. HRMS [M + H+]: Calcd 214.22832, found 214.22786.

[0147] 2-decyl-l,l,3,3-tetramethylguanidine (Cio-TMG):

[0148] Compound was synthesized according to general synthesis procedure using TMG (60 mL, 0.478 mol, 2.2 equiv.) and 1-bromodecane (45 mL, 0.218 mol, 1 equiv.). The product was isolated as a colorless, non-viscous liquid. Yield: 27.4207 g (49%). Boiling point: 106-110 °C (0.50 mm Hg).XH NMR (500 MHz, CDCI3) 6 3.08 (t, J = 6.98 Hz, 2H), 2.72 (s, 6H), 2.63 (s, 6H), 1.49 (p, J = 7.00 Hz, 2H), 1.32-1.19 (m, 14H), 0.86 (t, J = 6.92 Hz, 3H).13C NMR (126 MHz, CDCI3), 6 160.03, 49.77, 39.78, 38.96, 32.95, 32.06, 29.87, 29.77, 29.71, 29.49, 27.65, 22.82, 14.24. FT-IR (neat): v = 1360, 1621, 2852, 2922 cm . Elemental analysis. Anal. Calcd C15H33N3: C, 70.53; N 16.45. Found: C, 72.04; N, 16.64. HRMS [M + H+] : Calcd 256.27527, found 256.27476.

[0149] 2-dodecyl-l,l,3,3-tetramethylguanidine (C12-TMG): 83399PC01

[0150] 25

[0151] Compound was synthesized according to general synthesis procedure using TMG (60 mL, 0.478 mol, 2.3 equiv.) and 1-bromododecane (49 mL, 0.204 mol, 1 equiv.). The product was isolated as a colorless, non-viscous liquid. Yield: 29.4928 g (51%). Boiling point: 125-136 °C (0.98 mm Hg).XH NMR (500 MHz, CDCI3) 6 3.08 (t, J = 6.99 Hz, 2H), 2.73 (s, 6H), 2.64 (s, 6H), 1.50 (p, J = 7.1 Hz, 2H), 1.32-1.18 (m, 18H), 0.87 (t, J = 6.91 Hz, 3H).13C NMR (126 MHz, CDCI3) 6 160.05, 49.78, 39.79, 38.98, 32.96, 32.07, 29.89, 29.85, 29.82, 29.81, 29.72, 29.50, 27.66, 22.83, 14.25. FT-IR (neat): 1360, 1622, 2852, 2921 cm . Elemental analysis. Anal. Calcd C17H37N3: C, 72.02; H, 13.16; N 14.82. Found : C, 71.21; H, 13.31; N, 14.34. HRMS [M + H+] : Calcd 256.27527, found 256.27476.

[0152] Compound was synthesized according to general synthesis procedure using TMG (60 mL, 0.478 mol, 2.2 equiv.) and 1-bromohexadecane (66 mL, 0.216 mol, 1 equiv.). The product was isolated as a colorless, non-viscous liquid. Yield: 35.6556 g, 49%. Boiling point: 170-180 °C (1.13 mm Hg).XH NMR (500 MHz, CDCI3) 5 3.08 (t, J = 7.01 Hz, 2H), 2.72 (s, 6H), 2.63 (s, 6H), 1.49 (p, J = 7.15 Hz, 2H), 1.31-1.18 (m, 26H), 0.87 (t, J = 6.99 Hz, 3H).13C NMR (126 MHz, CDCI3) 5 160.01, 49.81, 39.78, 38.96, 32.98, 32.06, 29.89, 29.84 (multiple overlapping peaks), 29.82, 29.80, 29.72, 29.50, 27.66, 22.82, 14.24. FT-IR (neat): 1360, 1623, 2851, 2921 cm4. Elemental analysis. Anal. Calcd C21H45N3: C, 74.27; H, 13.36; N 12.37. Found : C, 72.64; H, 13.16; N, 11.53.

[0153] Procedure for CO2 uptake under CO2 atmosphere with various water amounts added

[0154] To a 20 mL vial, 0.500 g absorbent and water (0-2 equiv.) was added, as well as a magnetic stir bar. The vial was sealed with a screwcap containing a septum and the total mass was recorded. The absorbent and water mixtures were briefly 83399PC01

[0155] 26 stirred to homogenize, where after the system was purged with CO2 supplied by a freshly prepared double-wrapped balloon for 30 seconds at ambient temperature. After purging, the vial was tightly sealed and placed in a pre-heated aluminum block at the desired temperature (25, 45, 55 or 65 °C) and left to stir under CO2 atmosphere. The vials were insulated by cotton and aluminum foil to maintain a constant temperature. After stirring for 20 hours, the vial was removed from the heating block. The balloon was immediately thereafter removed, and the atmosphere was quickly exchanged with air to avoid any further CO2 uptake or water evaporation upon cooling. The mass of the vial was recorded. The experiments were reproduced in triplicates.

[0156] Procedure for Direct Air Capture experiments

[0157] Air was supplied by a pressurized air tank containing ~420 ppm CO2. For DAC experiments, the flowrate was adjusted to 3 mL min-1at 25 °C (2.748 seem) by a flow-controller (N2 as reference gas). Air was passed into a 250 mL gas wash bottle containing approximately 45 mL of a saturated salt solution to regulate the relative humidity. The following saturated salt solutions were used to regulate the relative humidity: MgCI2(33 RH%), ZnCI2(42 RH%), Mg(NO3)2 (53 RH%), NaBr (57 RH%), NH4NO3 (64 RH%), NaCI (75 RH%), KCI (84 RH%) and deionized water (100 RH%) (Greenspan, L. (1977)). Before an experiment, the system was allowed to equilibrate by purging air through the system for up to 24 hours. The entire system was kept at a constant temperature of 25 °C by a temperature- controlled water bath. The humidified air was passed over an 8 mL vial containing 0.050 g of absorbent placed at the tip of another gas wash bottle. The effluent air was passed through a small bubbler containing 2-4 mL water and 1 drop of H2SO4. The mass of the vial was recorded before and after the experiments. For NMR spectroscopic analysis, the absorbent was dissolved in D2O to provide a molal concentration of roughly 0.090 mol kg-1. Graphical representation of the experimental setup is shown in Fig. 2.

[0158] For kinetics experiments, each data point corresponds to one experiment.

[0159] Procedure for quantification of CO2 and water uptake from direct air capture experiments

[0160] The total mass of the vial was recorded before and after each experiment. The entire contents of the vial was then dissolved in D2O to give a molal concentration 83399PC01

[0161] 27 of 0.090 mol(absorbent) kg-1. The CO2 uptake was calculated by13C NMR spectroscopic chemical shift of HCOs' / COs2' using a standard curve. The water uptake was quantified by subtracting the CO2 uptake from the total mass gain. Standard curve: Standard curve was prepared by addition of CC -rich and CC -poor Cn-TMG / ethylene glycol standard solutions in different ratios and dilution by addition of D2O affording a desired molal concentration of 0.090 mol kg-1. The standard curve was prepared in duplicate using C10-TMG as absorbent and replicated once more using C7-TMG as absorbent. Pipetting aqueous solutions of Cn- TMG was found to be inaccurate. For this reason, all quantities are reported as masses.

[0162] CO2-rich standard solution: To a 20 mL vial, C10-TMG (0.5000 g, 1.96 mmol) and ethylene glycol (0.0924 g, 1.49 mmol, 0.76 equiv.) was added, as well as a magnetic stir bar. The vial was sealed with a screwcap containing a septum and the total mass was recorded. The absorbent and ethylene glycol mixture were briefly stirred to homogenize, where after the system was purged for 15 seconds with CO2 supplied by a freshly prepared double-wrapped balloon at ambient temperature. After purging, the vial was tightly sealed and left to stir (100 rpm) at ambient temperature under CO2 atmosphere. After stirring for 2 hours, the balloon was removed, and the atmosphere was exchanged back to air. The mass was immediately recorded which corresponded to a CO2 loading of 0.84 mol(C02): mol(Cio-TMG). The mixture was diluted by addition of D2O (10.5762 g) affording C10-TMG at a molal concentration of 0.182 mol kg-1.

[0163] CO2-poor standard solution: Since the absorbents are water insoluble in the CO2- free state, it was necessary to partially load the CC -poor standard solution to ensure complete water solubility. This was achieved by mixing CC -free C10-TMG with CO2-rich C10-TMG according to the following procedure: To a 20 mL vial, C10- TMG (0.1843 g, 0.721 mmol) was added as well as 3.5307 g of CC -rich standard solution. The mixture was diluted by addition of D2O (4.0087 g) affording C10-TMG at a molal concentration of 0.181 mol kg-1and a CO2 loading of 0.40 mol(CO2): mol(Cn-TMG).

[0164] Procedure for water uptake studies under N2 and CO2 atmosphere at different relative humidity 83399PC01

[0165] 28

[0166] To a 4 mL vial, 0.100 g absorbent was added, as well as a magnetic stir bar. The mass of the system was recorded. The vial was placed in a preheated 40 mL vial at 25 °C containing a wetted salt (or water) to regulate the relative humidity. The following wetted salts were used to regulate the relative humidity: LiCI (11 RH%), MgCI2(33 RH%), Mg(NO3)2 (53 RH%), NaCI (75 RH%), KCI (84 RH%) and deionized water (100 RH%). The system was purged with CO2 or N2 supplied by a freshly prepared double-wrapped balloon. The vials were insulated by cotton and aluminum foil to maintain a constant temperature of 25 °C. No further wateruptake occurred after 24 hours under N2, while the system was allowed to equilibrate for 7 days under CO2 atmosphere. The water uptake was quantified by recording the mass gain. For samples under CO2 atmosphere, the CO2 uptake (quantified by NMR) was subtracted from the mass gain.

[0167] Procedure for determination of CO2 capacity under constant relative humidity and variable temperature

[0168] To a 4 mL vial, 0.200 g C10-TMG was added, as well as a magnetic stir bar. The mass of the system was recorded. The vial was placed in a preheated 20 mL vial at 80 °C containing a wetted salt to regulate the relative humidity. The following wetted salts were used to regulate the relative humidity: LiBr (5.3-5.8 RH%), LiCI (10.5-11.3 RH%), MgC (26.1-32.8 RH%).36The system was insulated by cotton and aluminum foil to maintain a constant temperature. The system was purged with CO2 for 90 seconds supplied by a freshly prepared double-wrapped balloon and allowed to equilibrate for 24 hours. After 24 hours, an aliquot was withdrawn and analyzed by NMR spectroscopy to quantify CO2 capacity. The mass of the system was recorded. Using the same sample, the entire procedure was repeated again at 70 °C, 60 °C, 50 °C and finally 40 °C by lowering the temperature of the aluminum block by 10 °C between each experiment.

[0169] Desorption experiments

[0170] Absorption. To a 4 mL vial, 0.100 g absorbent (C7-, C10- and C12-TMG) and water (2 equiv.) was added, as well as a magnetic stir bar. The vial was sealed with a screwcap containing a septum and the total mass was recorded. The absorbent and water mixtures were briefly stirred to homogenize, where after the system was purged with CO2 supplied by a freshly prepared double-wrapped balloon for 5 seconds at ambient temperature. After purging, the vial was tightly sealed and 83399PC01

[0171] 29 placed in a pre-heated aluminum block at 25 °C and left to stir under CO2 atmosphere. After stirring for 2 hours the vial was removed from the heated aluminum block. The balloon was removed and the atmosphere was quickly exchanged with air. The mass of the vial was recorded corresponding to the CO2 absorption (mol(CO2): mol(Cn-TMG) was 1.15, 1.18 and 1.01 for n = 7, 10, and 12 respectively).

[0172] Desorption. Hereafter the 4 mL vials containing CC -rich Cn-TMG were placed in a 20 mL vial containing wetted LiCI at 70 °C and rapidly supplied with a CO2 atmosphere by purging for 90 seconds. Desorption of CO2 and water was complete after 3 hours (C10- and C12-TMG) or 7 hours (C7-TMG). It is worth noting that the physical appearance of the samples resemble that of the CC -free absorbent (non-viscous liquid) after stirring at 70 °C and 11 RH%. Additionally, the samples of C10 and C12-TMG were poorly soluble in D2O, which suggests complete CO2 desorption. NMR spectra (126 MHz, 1024 scans) in MeCN-ds (C10- and C12-TMG) or D2O (C7-TMG) revealed no detectable CO2 species.

[0173] Simulated cycling experiments

[0174] Absorption. To a 4 mL vial, 0.0747 g C10-TMG and water (0.0107 g, 2 equiv.) was added, as well as a magnetic stir bar. The vial was sealed with a screwcap containing a septum and the total mass was recorded. The absorbent and water mixtures were briefly stirred to homogenize, where after the system was purged with CO2 supplied by a freshly prepared double-wrapped balloon for 5-10 seconds at ambient temperature. After purging, the vial was tightly sealed and placed in a pre-heated aluminum block at 25 °C and left to stir under CO2 atmosphere. After stirring for 1 hour the vial was removed from the heated aluminum block. The balloon was removed and the atmosphere was quickly exchanged with air. The mass of the vial was recorded.

[0175] Desorption. Hereafter the system was placed in a 20 mL vial containing wetted LiCI at 70 °C and rapidly supplied with a CO2 atmosphere by purging for 90 seconds. After 2-3 hours, the vial was removed and the mass was recorded. The procedure was repeated on the same sample.

[0176] Procedure for thermogravimetric analysis under CO2 atmosphere 83399PC01

[0177] 30

[0178] The thermogravimetric analysis (TGA) was performed under a constant flow of 90 ml_ min-1CO2. The samples were heated in a platinum TGA pan from room temperature to 80 °C at a heating rate of 1 °C min-1.

[0179] Example 2 - Conceptualization of DAC using strong organic bases

[0180] Aim of study

[0181] Aprotic nitrogen bases e.g. peralkylated amidines / guanidines, tertiary amines) are innocent towards CO2 in the absence of water. However, the presence of water allows these bases to react with CO2 to form CO32' by effectively consuming two protons released from carbonic acid. Since carbonate anions in aqueous solutions are moderately basic (pKaH 10.3), aqueous carbonate does not further solubilize or capture CO2 from the air efficiently. As a result, aqueous hydroxide solutions capture roughly 0.5 molar equivalents of CO2 from the air. In contrast, non-aqueous low dielectric environments poorly stabilize CO32', strongly impacting its basicity and, therefore, reacting with water and CO2 to form bicarbonate HCOs' even at low CC . Such environments may enable the capture of significantly more than 0.5 molar equivalents of CO2 directly from the air when CO32' is appropriately hydrated. Based on this principle of organic chemistry - that base strength increases in aprotic non-polar organic environments - the inventors postulated that a strong lipophilic base could capture 1 molar equivalent of CO2 directly from the air upon sufficient hydration. Furthermore, removing water from the fully CO2-loaded absorbents at elevated temperatures would hypothetically allow for complete recovery of all the captured CO2 without an induced vacuum and significant heat energy penalty. This system would, therefore, reach a much higher working capacity than amine scrubbers without requiring a vacuum or intense heating above 100 °C by simply evaporating water below the boiling point owing to the low dielectric environment.

[0182] The aim of the present example was therefore to verify this hypothesis and investigate whether strong organic bases could be used as a humidity swing to capture CO2.

[0183] Results

[0184] The inventors began this investigation by testing the CO2 capture capacity of alkylated tetramethylguanidines (Cn-TMG, n = CnH2n+i; Fig. 3A) under a CO2 atmosphere (>99.999%, 1 atm) with different amounts of water added. The CO2 83399PC01

[0185] 31 capture capacity was sharply enhanced upon introducing up to 2 equivalents of water to all the organic bases (data not shown). Further addition of water showed no notable improvement of their CO2 loading, maintaining approximately 0.8 molar capacity in the presence of up to 60 equivalents of water. Since the water content affected the CO2 capacity of the bases mainly within the lower water equivalency, the inventors further investigated the relationship between the water content and the CO2 capacity of C10-TMG in the range of 0-2 equivalents of water at various temperatures (25-65 °C) in more detail (Fig. 3B). The temperature sensitivity was most noticeable at 1 equivalent of water, where a temperature swing of 40 °C resulted in a difference of 50% in molar capacity. The same trend was observed for all Cn-TMG analogues (Fig. 3C). Analysis of13C nuclear magnetic resonance spectroscopy of samples after CO2 absorption suggests that guanidinium (bi)carbonates were formed in D2O when CO2 reacts with Cn-TMGs, which is in agreement with the mass gain observed under CO2 atmosphere.

[0186] Independent infrared spectroscopic analysis further supported this claim (data not shown).

[0187] Since water content strongly affects the CO2 capture capacity of the lipophilic bases, the inventors speculated whether the CO2 concentration in the atmosphere would similarly impact the water uptake. Therefore, the inventors decided to study the impact of relative humidity (RH%) on the water-uptake capacity of each Cn-TMG analogue under atmospheres with different CO2 contents (under N2, air 420 ppm CO2, and 1 atm CO2; Fig. 3D and Fig. 3E). The higher molecular weight bases were less hygroscopic than the lower molecular weight analogues, irrespective of the relative humidity. The water uptake was significantly lower under N2 (Fig. 3D bottom) than under air or CO2 (Fig. 3D top). Interestingly, none of the Cn-TMG analogues absorbed any detectable amounts of water below 40 RH% under ambient air (Fig. 3E); however, above 40% RH, the water uptake increased linearly with the levels of RH%. Conductivity measurements of wetted C10-TMG suggests the formation of guanidinium hydroxides beyond addition of 0.5 equivalents of water (data not shown).

[0188] Conclusion

[0189] This example demonstrates that Cn-TMG, exemplified using n= 7, 10, 12 and 16, can capture CO2. The CO2 absorption effectivity of said absorbents was significantly enhanced by the addition of water. The temperature sensitivity for 83399PC01

[0190] 32 the absorbents was most noticeable at 1 equivalent of water, where a temperature swing of 40 °C for Cio-TMG resulted in a difference of 50% in molar capacity. The higher molecular weight bases were less hygroscopic than the lower molecular weight analogues, irrespective of the relative humidity.

[0191] Example 3 - Direct Air Capture with Cn-TMG

[0192] Aim of study

[0193] After establishing a correlation between the molar CO2 capture capacity, the water content, and the relative humidity of the system, the inventors shifted their focus to conducting direct air capture (DAC) under realistic conditions at various relative humidities. Thus, the aim of this example was to investigate whether Cn-TMG could be used in DAC.

[0194] Results

[0195] By passively allowing Cn-TMG to contact humidified air streams and subsequently analyzing samples by NMR spectroscopy, the highest CO2 capacity of each Cn-TMG absorbent was observed at different RH% (Fig. 4A). Cie-TMG showed its highest DAC performance above 80 RH%, C7- and Cio-TMG below 70 RH%, and C12-TMG peaked between 70-80 RH%. The inventors hypothesized that the hydrophobicity of Cn-TMGs impacts the optimal DAC conditions, with more hydrophobic analogues requiring higher humidity air to reach its full CO2 capture capacity, offering the opportunity to conduct DAC with various levels of relative humidity in the environment. The inventors hypothesized that the lipophilic bases provide higher molar air capture capacity than strong aqueous bases because of the enhanced basicity carbonate anions. It is worth noting that the rate of air capture dramatically decreases below 75 RH% for all tested bases, even while a higher capture capacity for C7- and Cio-TMG was observed below 75 RH% (Fig. 4B).

[0196] Conclusion

[0197] The present example demonstrates that C7-TMG, Cio-TMG, C12-TMG, and Cie-TMG can be applied in DAC, i.e., said absorbents can capture CO2 in ambient air and can therefore be used to remove CO2 from Earth's atmosphere. Cie-TMG showed its highest DAC performance above 80 RH%, C7- and Cio-TMG below 70 RH%, and C12-TMG peaked between 70-80 RH%. The capture efficiency decreased by 83399PC01

[0198] 33 increasing the alkyl chain length, thus C7-TMG has a higher capture efficiency than C16-TMG.

[0199] Example 4 - Low-temperature regeneration and cyclic experiments

[0200] Aim of study

[0201] The aim of this example was to investigate whether the absorbents could desorb CO2 and thereby be used in cyclic absorption-desorption experiments, i.e., can the absorbent be reused.

[0202] Results

[0203] The inventors observed that a pre-CC loaded sample of C10-TMG spontaneously and completely released CO2 at room temperature when it was placed in a dry sealed container containing anhydrous CaC (Figs. 5A-B). Although this experiment demonstrated that water removal facilitates CO2 release, the resulting outflow stream would be diluted CO2 in the air. Therefore, the inventors decided to demonstrate the production of a pure CO2 stream by optimizing the regeneration conditions under a CO2 atmosphere. For C10-TMG, the inventors obtained isohume and isobaric CO2 loadings under a CO2 atmosphere (1 atm) at various temperatures (25-60 °C, 6-33 RH%; Figs. 5C-D). No CO2 absorption was detected under low humidity and elevated temperatures (< 11 RH%, >60 °C). To test for hysteretic behavior, CC -loaded Cn-TMG (n = 7, 10, 12) were exposed to identical desorption conditions. Complete desorption of CO2 was observed at >60 °C for each Cn-TMG analogue, corresponding to a dew point of 26 °C. These results suggest that full desorption can be achieved at 70°C, yielding undiluted CO2 containing <3.3 mol% H2O, provided that the process had a heat sink below 26°C for water condensation. The water content of the desorbed CO2 would change depending on the temperature of the heat sink. The thermogravimetric analysis (TGA) of CO2 / H2O adducts— hydrated guanidinium bicarbonates, independently prepared from DAC experiments— showed water / CC co-desorption with a theoretical mass loss of CO2 and water under a >99.9% CO2 stream (1 atm) at a decomposition temperature of 50-65 °C (Fig. 5E). Therefore, the inventors have shown that a mere 45 °C temperature swing is sufficient to achieve a >90% working capacity of Cn-TMG analogues (up to 4.5 mol CC / kg absorbent or 21 wt% gravimetric capacity). The inventors have further demonstrated that 83399PC01

[0204] 34 the Cio-TMG could absorb and desorb CO2 for at least 6 cycles without a significant loss in weight% of the absorption medium (Fig. 5F).

[0205] Conclusion

[0206] The present example demonstrates that Cn-TMG absorbents can absorb and desorb CO2 by varying the temperature or relative humidity.

[0207] Example 5 - Immobilized Cn-TMG for humidity-swing CO2 capture

[0208] Aim of study

[0209] While effective, neat Cn-TMGs are viscous liquids with high intrinsic hygroscopicity, leading to slow cycling and energy penalty from water evaporation. To overcome these limitations, the aim of this study was to investigate whether amphiphilic Cn-TMGs could be immobilized on solid supports, preferable on high-surface-area solid supports.

[0210] Materials and methods

[0211] Preparation of HDTMS-SG

[0212] Mesoporous hexadecyltrimethoxysilane functionalized silica gel (HDTMS-SG) was prepared by pre-treating silica gel under N2 flow at 150 °C for 2 hours. Next, 5 g of the pre-treated silica gel was mixed with 60 mL EtOH / H2O (H2O: 5 vol%) and stirred for 30 minutes at 50 °C. Subsequently, 0.5 mL of NH4OH (NH3 : 25 wt%) was added to the mixture and stirred for 1 hour before dropwise adding HDTMS. The mixture was stirred for 24 hours at 50 °C to generate HDTMS-SG. In the present example, X in X% HDTMS-SG refers to the weight percentage of HDTMS relative to silica gel (SG) used in the synthesis.

[0213] Preparation of immobilized Cn-TMG

[0214] Cn-TMG + Silica gel or mesoporous silica - - - - ► Immobilized Cn-TMGs

[0215] Organic solvent Temperature, Time

[0216] In a round bottom flask, Cn-TMG (25-50 wt%) and porous solid materials (silica gel or mesoporous silica, base material, such as glass wool (GW), 1-2 grams) were added together with organic solvent to dissolve Cn-TMG. The reaction mixture was stirred at room temperature for overnight and the organic solvent, 83399PC01

[0217] 35 which may include e.g. dichloromethane, ethers, ethyl acetate, and DMF, was evaporated under reduced pressure to afford immobilized Cn-TMG on the solid support with quantitative yield. The ratio between Cn-TMG was adjusted in terms of weight percentage based on the weight of the solid support. The immobilized Cn-TMG are denoted X % Cn-TMG / solid support in the present example, wherein n is 7, 10, or 12, solid support is silica gel (SG), mesoporous hexadecyltrimethoxysilane functionalized silica gel (HDTMS-SG), or glass wool (GW), and X is the weight percentage of Cn-TMG relative to the solid support.

[0218] Results

[0219] Support polarity critically affected humidity swing behavior (Fig. 6). Replacing silica gel (SG) with hydrophobic mesoporous silica (mesoporous hexadecyltrimethoxysilane functionalized silica gel, HDTMS-SG) reduced water uptake and accelerated regeneration. A lower H2O / CO2 (mol / mol) ratio indicates that less energy is required to regenerate the sorbent, since less water needs to be removed during the desorption step. This enhances the overall energy efficiency of the CO2 capture and release process. The moderate hydrophobic surface functionalization (10 % HDTMS-SG) balanced water management and maintained CO2 capacity.

[0220] The 25 wt % Cio-TMG / 10 % HDTMS-SG composite reached a steady-state CO2 capacity of up to 2.5 mmol g-1with a desorption at 70 °C, with reduced water uptake compared to bare silica gel and Cn-TMGs. Cyclic capture-release experiments were conducted using the optimized 25 wt % Cio-TMG / glass wool (GW) sorbent under humid air (capture) and 70 °C dry air (release). Figure 7 shows two consecutive cycles maintaining approximately 40% capture efficiency and facile regeneration by flowing hot air (70 °C).

[0221] Conclusion

[0222] The present example demonstrates that Cn-TMG can be immobilized on a solid support, such as silica gel, mesoporous hexadecyltrimethoxysilane functionalized silica gel (HDTMS-SG), or glass wool (GW). Using hydrophobic mesoporous silica (HDTMS-SG) instead of regular silica gel reduces water uptake and speeds up regeneration. A moderate level of hydrophobic surface functionalization (10% HDTMS-SG) optimizes water management while maintaining CO2 capacity. Cyclic experiments with Cio-TMG / glass wool (GW) show stable capture efficiency and 83399PC01

[0223] 36 easy regeneration using hot air. Varying alkyl chain length (C?, Cio, C12), loading fraction (25-50 wt %), and surface hydrophobicity enabled systematic control of hydration, CO2 uptake, and regeneration behaviour.

[0224] Overall conclusion on the examples

[0225] The inventors envisage that the humidity swing of the present invention can be used in large-scale applications through a passive absorption chamber where liquid Cn-TMG can be exposed to air. Once loaded with CO2, regeneration will occur with mild heat (70 °C) and water condensation by a passive heat sink. Based on stability tests, C10-TMG has high thermal stability (boiling point: 106- 110 °C at 0.50 mm Hg), negligible vapor pressure, and hydrolytic stability at low pHs. The accelerated cyclic experiments showed that C10-TMG efficiently captures CO2 when moistened with 2 equivalents H2O, and releases water and CO2 - without diluting the CO2 - at 70 °C and 11 RH% (Fig. 5F).

[0226] Therefore, full CO2 desorption and up-concentration of CO2 directly from the air can be achieved with minimum energy costs by using waste heat or renewable energy sources. This process is tolerant to moisture and is even more effective with moderately humid air, while offering the opportunity for water harvesting. Considering the average humidity of the world (ca. 50 RH%) and the growing potential of renewable power sources, the process of the present invention can stand alone for DAC by taking advantage of the natural humidity in the air and its record-low thermal energy requirements.

[0227] The inventors have further demonstrated that immobilizing Cn-TMG on a solid support improves both the efficiency of cyclic CO2 capture and release, and lowers the energy required for regeneration, compared to neat Cn-TMG.

[0228] References

[0229] • Wang, T., et al. : Moisture swing sorbent for carbon dioxide capture from ambient air. Environ. Sci. Technol. 45, 6670-6675 (2011)

[0230] • Pereira, F. S. et ai. \ Study of the carbon dioxide chemical fixationactivation by guanidines, Tetrahedron, Volume 64, Issue 43, 2008, Pages 10097-10106 83399PC01

[0231] 37

[0232] • Han, Y., et al. : Strong bases behave as weak bases in nanoscale chemical environments: implication in humidity-swing CO2 air capture. Phys Chem Chem Phys. 2021 Jul 14;23(27): 14811-14817

[0233] • WO16089561 Al • US2013164199 Al

[0234] • Heldebrant, D. J. et al. Reversible zwitterionic liquids, the reaction of alkanol guanidines, alkanol amidines, and diamines with CO2. Green Chem. 12, 713-772 (2010)

[0235] • Greenspan, L. Humidity fixed points of binary saturated aqueous solutions. J. Res. Natl. Bur. Stand. A Phys. Chem. Vois. 81A, 89-96 (1977).

Claims

83399PC0138Claims1. A method of separating CO2 from a gas mixture comprising CO2 in an absorption-desorption process, said method comprising the steps of: i) providing a gas mixture comprising CO2 to an absorption zone; ii) contacting the gas mixture with at least one absorption medium under conditions suitable for the absorption medium to absorb the CO2, to obtain a CO2-enriched absorption medium; iii) desorbing CO2 from the CC -enriched absorption medium in a desorption zone under conditions suitable for the CC -enriched absorption medium to desorb CO2, to regenerate the absorption medium and obtain CO2; wherein said absorption medium is a compound of formula (I):whereinX is selected from the group consisting of H, -OR1, -N(R1)2, and Ci-Ce alkyl, each R1is independently selected from Ci-Ce alkyl, or optionally two R1form a 5- or 6-membered ring in conjunction with X-C-N when X is -OR1, or -N(R1)2, or in conjunction with -N- in -N(R1)2,R2is selected from the group consisting of an optionally substituted C5-C20 alkyl, an optionally substituted C5-C20 alkene, an optionally substituted Cs- C20 alkyne, and -(CH2CH2O)n-R3wherein n is an integer in the range of 2- 10 and R3is Ci-Ce alkyl, wherein the absorption and desorption of CO2 is controlled by the relative humidity (RH) in the absorption and desorption zone and / or by the water activity (aw) of the compound of formula (I).

2. The method according to claim 1, wherein X is selected from the group consisting of -OR1and -N(R1)2, preferably -N(R1)2.83399PC013. The method according to any one of the preceding claims, wherein said absorption medium is a compound of formula (II):wherein each R1is independently selected from Ci-Ce alkyl, or optionally two R1form a 5- or 6-membered ring in conjunction with X-C-N when X is -OR1, or -NfR1^, or in conjunction with -N- in -NfR1^,R2is selected from the group consisting of an optionally substituted C5-C20 alkyl, an optionally substituted C5-C20 alkene, an optionally substituted Cs- C20 alkyne, and -(CH2CH2O)n-R3wherein n is an integer in the range of 2- 10 and R3is Ci-Ce alkyl.

4. The method according to any one of the preceding claims, wherein each R1is independently selected from C1-C5 alkyl, C1-C4 alkyl, C1-C3 alkyl, such as C1-C2 alkyl, such as preferably Ci alkyl.

5. The method according to any one of the preceding claims, wherein R2is selected from the group consisting of a Ce-Ci9-alkyl, -alkene, and -alkyne, such as a Ce-Cis-alkyl, -alkene, and -alkyne, such as a C?-Ci7-alkyl, -alkene, and -alkyne, such as a C?-Ci6-alkyl, -alkene, and -alkyne, such as preferably Cs-Cio-alkyl, - alkene, and -alkyne.

6. The method according to any one of the preceding claims, wherein the absorption medium co-absorbs water and CO2.

7. The method according to any one of the preceding claims, wherein the gas mixture is selected from the group consisting of air, post-combustion gas mixtures, flue gas, and biogas, or any mixture thereof.83399PC01408. The method according to any one of the preceding claims, wherein the CO2 concentration in the gas mixture is at least 200 ppm, such as at least 300 ppm, at least 350 ppm, at least 400 ppm, such as preferably at least 420 ppm.

9. The method according to any one of the preceding claims, wherein the conditions suitable for the absorption medium to absorb the CO2 provides a relative humidity (RH) in the absorption zone in the range of 5-100%, such as 10- 100%, such as 20-100%, such as 30-90%, such as 35-85%, such as preferably 40-80%.

10. The method according to any one of the preceding claims, wherein the conditions suitable for the absorption medium to absorb the CO2 provides a temperature in the range of -30-50 °C, such as -10-40 °C, such as 0-35 °C, such as 10-30 °C, such as preferably 15-25 °C.

11. The method according to any one of the preceding claims, wherein the conditions suitable for the absorption medium to absorb the CO2 provides a water activity (aw) of the absorption medium in the range of 0.20-1.00, such as 0.30- 0.95, such as 0.40-0.90, such as 0.50-0.80.

12. The method according to any one of the preceding claims, wherein the conditions suitable for the CC -enriched absorption medium to desorb the CO2 provides a relative humidity (RH) in the desorption zone in the range of 1-40%, such as 5-35%, such as 10-30%, such as preferably 10-20%.

13. The method according to any one of the preceding claims, wherein the conditions suitable for the CC -enriched absorption medium to desorb the CO2 provides a temperature in the range of 30-85 °C, such as 40-80 °C, such as preferably 55-75 °C.

14. The method according to any one of the preceding claims, wherein the conditions suitable for the CC -enriched absorption medium to desorb the CO2 provides a water activity (aw) of the absorption medium in the range of 0.01-0.40, such as 0.05-0.35, such as 0.10-0.30, such as 0.10-0.20.83399PC014115. The method according to any one of the preceding claims, wherein the method is applied in Carbon capture from post-combustion gas, biogas or flue gas or in Direct Air Capture (DAC), preferably Direct Air Capture (DAC).

16. The method according to any one of the preceding claims, wherein the compound of formula (I) is immobilized on a solid support.

17. The method according to claim 16, wherein the solid support is porous.

18. The method according to any one of claims 16 or 17, wherein the solid support is selected from the group consisting of silica gel, mesoporous silica, and glass wool.

19. The method according to claim 18, wherein the silica gel or mesoporous silica is functionalized with at least one hydrophobic compound, preferably C4-C20 alkyl trimethoxysilane, such as Ci6 trimethoxysilane.

20. An absorption medium comprising a compound of formula (I):whereinX is selected from the group consisting of H, -OR1, -NfR1^, and Ci-Ce alkyl, each R1is independently selected from Ci-Ce alkyl, or optionally two R1form a 5- or 6-membered ring in conjunction with X-C-N when X is -OR1, or -NfR1^, or in conjunction with -N- in -NfR1^,R2is selected from the group consisting of an optionally substituted C5-C20 alkyl, an optionally substituted C5-C20 alkene, an optionally substituted Cs- C20 alkyne, and -(CH2CH2O)n-R3wherein n is an integer in the range of 2- 10 and R3is Ci-Ce alkyl, wherein the compound of formula (I) is immobilized on a solid support.83399PC014221. Use of the absorption medium according to claim 20 for separating CO2 from a gas mixture comprising CO2.

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