Process for catalytic co2 absorption and catalyst for said process

Abstract

The present invention generally relates to the field of carbon dioxide (CO2) capture and storage. More particularly, the present invention pertains to a process for CO2 absorption in an overall CO2 absorption / desorption procedure, the process comprising absorbing CO2 in an absorbent composition comprising amines and a metal organic framework (MOF). The present invention further encompasses methods for preparing the MOF and the absorbent composition.

Description

[0001] PROCESS FOR CATALYTIC CO2ABSORPTION AND CATALYST FOR SAID PROCESS

[0002] FIELD OF THE INVENTION

[0003] The present invention generally relates to the field of carbon dioxide (CO2) capture and storage. More particularly, the present invention pertains to a process for CO2 absorption in an overall CO2 absorption / desorption procedure comprising absorbing CO2 in an absorbent composition comprising amines and a metal organic framework (MOF). The present invention further encompasses methods for preparing the MOF and the absorbent composition.

[0004] BACKGROUND

[0005] The increasing concentration of carbon dioxide (CO2) in the Earth's atmosphere is a major contributor to global climate change. As a greenhouse gas, CO2 traps heat, leading to the warming of the planet, which has significant environmental, economic, and societal impacts. To mitigate these effects, there has been a growing focus on reducing CO2 emissions from industrial sources and developing technologies for capturing and storing CO2. Under this premise, CO2 capture and storage (CCS) and CO2 capture and utilization (CCU) have become promising routes to limit increasing CO2 emission. Importantly, for both routes, CO2 capture is always the first step. In order to meet mid-to-long-term CO2 reduction targets, it is necessary to develop a cost-effective CO2 capture technology.

[0006] Existing CO2 capture technologies can be broadly classified into three categories: precombustion capture, post-combustion capture, and oxy-fuel combustion. Each of these technologies aims to isolate CO2 from other gases in flue streams or process gases before it is released into the atmosphere. Post-combustion capture, using amine solvent as a CO2 absorber, is one of the most widely used methods due to its applicability to existing power plants. A typical amine-based CO2 capture and regeneration process involves the entire absorption-desorption cycle of CO2 using amine solvents, as shown in FIG. 1. The CO2- containing flue gas stream is introduced into an absorber column where it flows counter currently as it contacts lean amine, allowing for efficient absorption. After absorption, the rich amine solvent flows through a rich-lean heat exchanger before being introduced into a stripper column for thermal regeneration. In the stripper column, heated steam is supplied to strip out the captured CO2 and increase the temperature. This free (lean) amine solvent then returns to the absorber column for a new cycle of absorption.

[0007] Importantly, the slow absorption rate of existing amine solvents as a CO2 absorber is one of the important limitations for efficient CO2 capture. Reaching the ideal loading capacity of the amine solvent is a particularly difficult challenge within the typically limited gas-liquid contact time during operation, leading to an increase in operational expenses (OPEX).

[0008] From a catalytic perspective, using a proper catalyst can decrease the activation energy of CO2 absorption and boost the reaction rate, thereby decreasing the absorption time and increasing the CO2 capture efficiency. Nevertheless, while showing some improvement, various solid catalysts are unable to meet application requirements due to limitations such as poor catalytic performance or catalyst dissolution problems. Finding proper catalysts for CO2 absorption therefore requires balancing multiple performance metrics, including efficiency, stability, selectivity, and cost.

[0009] Therefore, there remains a need for improved catalysts and improved processes for CO2 absorption.

[0010] SUMMARY OF THE INVENTION

[0011] It has now been found that the above objectives can be attained either individually or in any combination by using the specific and well-defined processes and catalysts as disclosed herein.

[0012] More specifically, the present inventors have developed an improved process for CO2 absorption comprising the use of a metal organic framework (MOF) that can act as a catalyst and increase the rate of absorption of CC>2-absorbing compositions, such as amine-containing compositions. It has been found that MOFs comprising metal ions, in particular zinc ions, and nitrogen-containing ligands in a defined nitrogen-to-metal ratio shows advantageous properties when applied in a CO2 absorption process.

[0013] The MOFs of the present invention are distinguished by structural defects that cause the metal ions to be bound to fewer ligands than theoretically possible based on their coordination number or ligancy. This advantageously improves the catalytic action of the MOF as demonstrated herein in the example section. Hence, this allows to increase the CO2 absorption rate of CO2-absorbing compositions. In some cases this even allows high throughput operation in a conventional production plant.

[0014] Another advantage of the present process for CO2 absorption is that the MOFs as described herein can be efficiently separated from the CO2-containing composition, to avoid contamination in further downstream processing steps (e.g., a CO2 desorption process). Moreover, it has been found herein that the (separated) MOFs can be re-used in another CO2 absorption process. In particular, the MOFs can be repeatedly re-used with retention of catalytic activity as demonstrated herein in the example section.

[0015] In a first aspect, the present invention provides a process for CO2 absorption. The process preferably comprises the step of absorbing CO2 in an absorbent composition comprising a metal-organic framework (MOF); wherein the MOF comprises metal ions M, preferably zinc ions, and nitrogen-containing ligands L; wherein the (theoretical) coordination number of the metal ions M is 4; and wherein the MOF has an atomic ratio of nitrogen to metal of at most 3.5, preferably at most 3.0, more preferably at most 2.7. This means that the MOFs of the present invention are distinguished by structural defects that cause the metal ions to be bound to fewer ligands than theoretically possible based on their coordination number or ligancy.

[0016] It has been found herein that the process according to the present invention, or embodiments thereof, provides for a more efficient CO2 absorption with improved reaction rates when compared to existing processes.

[0017] Another advantage of the present invention is that the present process may operate at lower temperatures, resulting in reduced OPEX.

[0018] An advantage of the absorbent composition as described herein is that it may be easily fixed on an absorber column. In addition, the present process may reduce the need for larger equipment, which advantageously reduces capital expenditures (CAPEX).

[0019] In some preferred embodiments, the absorbent composition comprises one or more amine compounds. This has the advantage that the amine compounds can selectively react with CO2 to form carbamates (for amine compounds comprising primary amines and secondary amines) or bicarbonates (for amine compounds comprising tertiary amines), leading to efficient capture of CO2 even at low concentrations. Another advantage is that CO2 has a good solubility in various amine compounds, which can enhance the overall absorption capacity. This is especially beneficial in processes that require high throughput.

[0020] In some preferred embodiments, the metal ion M comprises zinc (Zn) ions, i.e. , the metal M is zinc. The present inventors have found that MOFs comprising zinc ions are particularly efficient catalysts for CO2 absorption. Moreover, it has been found herein that the resulting zinc complex can be readily modified to introduce framework defects and further improve catalytic activity. In some preferred embodiments, the present process is a batch process, wherein preferably the absorbent composition comprises at least 0.01 wt.% of the MOF, preferably at least 0.1 wt.% of the MOF; with wt.% relative to the total weight of the absorbent composition. Advantageously, the present process provides for fast CO2 absorption even with low amounts of MOF, thereby providing a cost-effective technology

[0021] In some preferred embodiments, the present process is a continuous process, wherein preferably the Weight Hourly Space Velocity (WHSV) of the absorbent composition is between 0.1 and 1000 h’1, preferably between 0.5 and 100 h’1, more preferably between 1 and 10 h’1.

[0022] In some preferred embodiments, the step of absorbing CO2 in the absorbent composition is performed by contacting a CO2 containing stream with the absorbent composition, preferably counter currently at a temperature of from 0°C to 60°C and / or at a total pressure of from 0.1 bar to 5.0 bar.

[0023] In some preferred embodiments, the nitrogen-containing ligand L is an aromatic heterocyclic amine, preferably comprising two or more nitrogen atoms. In some preferred embodiments, the nitrogen-containing ligand is an imidazolate of Formula (I): wherein R1, R2, R3are each independently selected from the group comprising hydrogen, alkyl, alkenyl, alkynyl, halo, amino, nitro and cyano; or R2and R3together with the carbon atoms to which they are attached can form a cycloalkyl, a cycloalkenyl, an aryl, or a heteroaryl wherein each of said cycloalkyl, cycloalkenyl, aryl, and heteroaryl can be unsubstituted or substituted with one or more substituents R4, wherein each R4is independently selected from the group comprising alkyl, alkenyl, hydroxyl, amino, cyano, nitro and halo.

[0024] In some preferred embodiments, the metal ions M comprise zinc (Zn) ions. Preferably, the MOF is a zeolitic imidazole framework (ZIF) comprising zinc ions. It has been found herein that the porous structure and controllable functionality of ZIFs may be particularly advantageous in the present process for CO2 absorption.

[0025] In some preferred embodiments, the absorbent composition further comprises a solvent; preferably comprising water or an alcohol.

[0026] In some preferred embodiments, the MOF has a BET surface area of at least 200 m2 / g, preferably at least 300 m2 / g; preferably determined by means of N2 physisorption according to ASTM D3663-20 or ISO 9277:2022.

[0027] The MOF may be obtained by various methods. In some preferred embodiments, the MOF is obtained by means of a method comprising the steps of: providing a MOF comprising metal ions M with (theoretical) coordination number 4 and nitrogen-containing ligands L; and, applying a mechanical force to the MOF, thereby forming framework defects such that the atomic ratio of coordinating nitrogen to metal in the MOF is at most 3.8.

[0028] Preferably, the mechanical force is applied to the MOF comprises by means of ball milling, jet milling, hammer milling, ultrasonic milling, cryogenic grinding, or vibratory milling.

[0029] In some preferred embodiments, the one or more framework defects are missing atoms and / or missing ligands L that provide openings in the framework structure. Optionally the openings are stabilised with an oxide selected from the group comprising of silicon, aluminium, boron, gallium, titanium, zirconium, or hafnium, and mixtures thereof; preferably a silicon oxide.

[0030] In some preferred embodiments, the MOF is obtained by means of a method comprising the steps of: contacting a solution comprising a metal source with (theoretical) coordination number 4 and a Lewis acid comprising B, Al, Ga, In, or combinations thereof, with nitrogen-containing ligands, thereby obtaining a MOF precursor; and heating the MOF precursor to a temperature of from 25 to 250°C, thereby obtaining the MOF comprising metal ions M and nitrogen-containing ligands L with an atomic ratio of coordinating nitrogen to metal of at most 3.8.

[0031] In some preferred embodiments, the metal source comprises zinc (Zn), i.e., preferably the metal M is Zn. In some preferred embodiments, the Lewis acid is a boron-containing acid; preferably boric acid or boronic acid. The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, which illustrate, by way of example, the principles of the invention.

[0032] BRIEF DESCRIPTION OF THE FIGURES

[0033] FIG. 1 illustrates a carbon capture process schematic.

[0034] FIG. 2 illustrates a reactor for CO2 absorption in an amine solution.

[0035] FIG. 3 illustrates (a) operando ATR-IR spectra of CO2 absorption in MDEA solution; and (b) the relationship between IR signal (1353 cm-1) and CO2 loading.

[0036] FIG. 4 illustrates a comparison between CO2 loading results collect from ATR-IR and titration. FIG. 5 illustrates CO2 absorption performance in 30 wt% MDEA water solution with different catalysts under 30°C.

[0037] FIG. 6 illustrates a kinetic study for CO2 absorption in 30 wt% MDEA water solution with different catalysts under 30°C.

[0038] FIG. 7 illustrates the relationship between kcAr and catalyst crystallinity in 30 wt% MDEA water solution with ZIF-8-B through different ball milling times under 30°C.

[0039] FIG. 8 illustrates the relationship between kcAT and catalyst surface area in 30 wt% MDEA water solution with ZIF-8-B through different ball milling times under 30°C.

[0040] FIG. 9 illustrates the relationship between kcAT and catalyst missing linker ratio in 30 wt% MDEA water solution with catalysts that went through different treatment methods under 30°C. FIG. 10 illustrates stability tests for different catalysts.

[0041] FIG. 11 illustrates CO2 absorption performance with different catalysts amounts and stirring conditions.

[0042] DETAILED DESCRIPTION OF THE INVENTION

[0043] Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

[0044] The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known, or part of the common general knowledge in any country as of the priority date of any of the claims. As used herein, the singular forms “a”, “an”, and “the” include both singular and plural referents unless the context clearly dictates otherwise. By way of example, "a step" means one step or more than one step.

[0045] The terms “comprising”, “comprises” and “comprised of’ as used herein are synonymous with “including”, “includes” or “containing”, “contains”, and are inclusive or open-ended and do not exclude additional, non-recited members, elements, or method steps. The terms also encompass “consisting of” and “consisting essentially of”, which enjoy well-established meanings in patent terminology.

[0046] Whereas the terms “one or more” or “at least one”, such as one or more members or at least one member of a group of members, is clear per se, by means of further exemplification, the term encompasses inter alia a reference to any one of said members, or to any two or more of said members, such as, e.g., any >3, >4, >5, >6 or >7 etc. of said members, and up to all said members. In another example, “one or more” or “at least one” may refer to 1 , 2, 3, 4, 5, 6, 7 or more.

[0047] The terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order, unless specified. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other sequences than described or illustrated herein.

[0048] As used herein, the term “and / or” when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a list is described as comprising group A, B, and / or C, the list can comprise A alone, B alone, C alone, A and B in combination, A and C in combination, B and C in combination, or A, B, and C in combination.

[0049] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” or “in a particular embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while certain embodiments described herein include some, but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art.

[0050] The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1 , 2, 3, 4 when referring to, for example, a number of elements, and can also include 1 .5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of endpoints also includes the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein. This applies to numerical ranges irrespective of whether they are introduced by the expression “from... to...” or the expression “between... and...” or another expression.

[0051] As used herein, the terms “about” or “approximately” are used to provide flexibility to a numerical value or range endpoint by providing that a given value may be “a little above” or “a little below” said value or endpoint, depending on the specific context. Hence, the terms “about” or “approximately” as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, are meant to encompass variations of and from the specified value or endpoint, such as variations of + / -10% or less, preferably + / -5% or less, more preferably + / -1 % or less, and still more preferably + / -0.1 % or less of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention.

[0052] Unless otherwise stated, use of the terms “about” or “approximately” in accordance with a specific number or numerical range should also be understood to provide support for such numerical terms or range without the term “about”. For example, the recitation of “about 30” should be construed as not only providing support for values a little above and a little below 30, but also for the actual numerical value of 30 as well.

[0053] As used herein, the term “substantially” refers to the complete or nearly complete extent or degree of an action, characteristic, property, state, structure, item, or result. For example, an object that is “substantially” enclosed would mean that the object is either completely enclosed or nearly completely enclosed. The exact allowable degree of deviation from absolute completeness may in some cases depend on the specific context. However, generally speaking the nearness of completion will be so as to have the same overall result as if absolute and total completion were obtained. The use of “substantially” is equally applicable when used in a negative connotation to refer to the complete or near complete lack of an action, characteristic, property, state, structure, item, or result.

[0054] The terms “wt.%,” “vol%”, or “mol%” refers to a weight percentage of a component, a volume percentage of a component, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, which includes the component.

[0055] Whenever the term “substituted” is used in the present invention, it is meant to indicate that one or more hydrogens on the atom indicated in the expression using “substituted” is replaced with a selection from the indicated group, provided that the indicated atom’s normal valency is not exceeded, and that the substitution results in a chemically stable compound. Where groups can be substituted, such groups may be substituted with one or more, and preferably one, two or three substituents.

[0056] The term “halo” or “halogen” as a group or part of a group is generic for fluoro, chloro, bromo, or iodo. The term “hydroxyl” or “hydroxy” as used herein refers to the group -OH. The term “cyano” as used herein refers to the group -C=N. The term “amino” as used herein refers to the group -NR1R2, wherein R1, R2are each independently selected from the group comprising hydrogen, alkyl, alkenyl, alkynyl or aryl.

[0057] The term "alkyl" as a group or part of a group, refers to a hydrocarbyl group of formula CnH2n+i wherein n is a number greater than or equal to 1 , with no site of unsaturation. Alkyl groups may be linear or branched and may be substituted as indicated herein. Generally, alkyl groups can comprise from 1 to 12 carbon atoms, preferably from 1 to 10 carbon atoms, more preferably from 1 to 6 carbon atoms, more preferably from 1 to 4 carbon atoms. When a subscript is used herein following a carbon atom, the subscript refers to the number of carbon atoms that the named group may contain. For example, the term "Ci-ealkyl", as a group or part of a group, refers to a hydrocarbyl group of formula CnH2n+i wherein n is a number ranging from 1 to 6. Thus, for example, “Ci-ealkyl” includes all linear or branched alkyl groups with between 1 and 6 carbon atoms, and thus includes methyl, ethyl, n-propyl, / -propyl, butyl, and its isomers (e.g., n-butyl, / -butyl, and f-butyl); pentyl and its isomers, hexyl, and its isomers, etc. For example, Ci-4alkyl includes all linear or branched alkyl groups having 1 to 4 carbon atoms, and thus includes for example methyl, ethyl, n-propyl, / -propyl, 2-methyl-ethyl, butyl, and its isomers (e.g., n-butyl, / -butyl, and f-butyl), and the like. In particular embodiments, the term alkyl refers to Ci-ealkyl (C1-12 hydrocarbons), yet more in particular to Ci-walkyl (C1-10 hydrocarbons), yet more in particular to C-i-galkyl (C1-9 hydrocarbons), yet more in particular to Ci-ealkyl (C1-6 hydrocarbons) as further defined herein above. Non-limiting examples of alkyl include methyl, ethyl, 1 -propyl (n-propyl), 2-propyl ( / Pr), 1 -butyl, 2-methyl-1-propyl(i-Bu), 2- butyl (s-Bu), 2-dimethyl-2-propyl (t-Bu), 1 -pentyl (n-pentyl), 2-pentyl, 3-pentyl, 2-methyl-2- butyl, 3-methyl-2-butyl, 3-methyl-1 -butyl, 2-methyl-1 -butyl, 1 -hexyl, 2-hexyl, 3-hexyl, 2-methyl- 2-pentyl, 3-methyl-2-pentyl, 4-methyl-2-pentyl, 3-methyl-3-pentyl, 2-methyl-3-pentyl, 2,3- dimethyl-2-butyl, 3,3-dimethyl-2-butyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n- dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, n-octadecyl, n- nonadecyl, and n-icosyl.

[0058] The term “alkenyl” as a group or part of a group, refers to an unsaturated hydrocarbyl group which may be linear, or branched, comprising one or more with at least one site (usually 1 to 3, preferably 1 ) of unsaturation, namely at least one sp2carbon-sp2carbon double bond. Generally, alkenyl groups can comprise from 2 to 12 carbon atoms, preferably from 2 to 10 carbon atoms, preferably from 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms. When a subscript is used herein following a carbon atom, the subscript refers to the number of carbon atoms that the named group may contain. Examples of C2-ealkenyl groups are ethenyl, 2-propenyl, 2-butenyl, 3-butenyl, 2-pentenyl and its isomers, 2-hexenyl and its isomers, 2,4-pentadienyl, and the like. The double bond may be in the cis or trans configuration.

[0059] The term “alkynyl” as a group or part of a group, refers to a branched or straight chain hydrocarbon comprising at least one site (usually 1 to 3, preferably 1 ) of unsaturation, namely a sp1carbon-sp1carbon triple bond. In particular embodiments, the term alkynyl refers to C2-12 alkynyl (C2-12 hydrocarbons), preferably to C2-9 alkynyl (C2-9 hydrocarbons) yet more preferably to C2-6 alkynyl (C2-6 hydrocarbons) as further defined herein above with at least one site (usually 1 to 3, preferably 1 ) of unsaturation, namely at least one sp1carbon-sp1carbon triple bond. Examples of alkynyl include but are not limited to: ethynyl (-C=CH), 3-ethyl-cyclohept- 1-ynylene, and 1-propynyl (propargyl, -CH2C=CH).

[0060] The term “cycloalkyl”, as a group or part of a group, refers to a cyclic alkyl group, that is a monovalent, saturated, hydrocarbyl group having 1 or more cyclic structure, and comprising from 3 to 20 carbon atoms, more preferably from 3 to 10 carbon atoms, more preferably from 3 to 8 carbon atoms; more preferably from 3 to 6 carbon atoms. Cycloalkyl includes all saturated hydrocarbon groups containing one or more rings, including monocyclic, bicyclic groups or tricyclic. For example, cycloalkyl comprises a C3-10 monocyclic or C7-18 polycyclic saturated hydrocarbon, such as for instance cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclooctylmethylene, norbornyl, fenchyl, trimethyltricycloheptyl, decalinyl, adamantyl and the like. The further rings of multi-ring cycloalkyls may be either fused, bridged and / or joined through one or more spiro atoms. When a subscript is used herein following a carbon atom, the subscript refers to the number of carbon atoms that the named group may contain. For example, the term “Cs-iocycloalkyl”, refers to a cyclic alkyl group comprising from 3 to 10 carbon atoms. For example, the term “Cs-scycloalkyl”, refers to a cyclic alkyl group comprising from 3 to 8 carbon atoms. For example, the term “Cs-ecycloalkyl”, refers to a cyclic alkyl group comprising from 3 to 6 carbon atoms. For the avoidance of doubt, fused systems of a cycloalkyl ring with a heterocyclic ring are considered as heterocycle irrespective of the ring that is bound to the core structure. Fused systems of a cycloalkyl ring with an aryl ring are considered as aryl irrespective of the ring that is bound to the core structure. Fused systems of a cycloalkyl ring with a heteroaryl ring are considered as heteroaryl irrespective of the ring that is bound to the core structure.

[0061] The term “cycloalkenyl” as a group or part of a group, refers to a non-aromatic cyclic alkenyl group, with at least one site (usually 1 to 3, preferably 1 ) of unsaturation, namely a sp2carbon- sp2carbon double bond; preferably from 5 to 18 carbon atoms, more preferably from 5 to 10 carbon atoms, more preferably from 5 to 6 carbon atoms. Cycloalkenyl includes all unsaturated hydrocarbon groups containing one or more rings, including monocyclic, bicyclic, or tricyclic groups. For example, cycloalkenyl can comprise C5-10 monocyclic or C7-18 polycyclic hydrocarbon. The further rings may be either fused, bridged and / or joined through one or more spiro atoms. When a subscript is used herein following a carbon atom, the subscript refers to the number of carbon atoms that the named group may contain. For example, the term “C5- iocycloalkenyl”, refers to a cyclic alkenyl group comprising from 5 to 10 carbon atoms. For example, the term “Cs-scycloalkenyl”, refers to a cyclic alkenyl group comprising from 5 to 8 carbon atoms. For example, the term “Cs-ecycloalkyl”, refers to a cyclic alkenyl group comprising from 5 to 6 carbon atoms. Examples include but are not limited to: cyclobutenyl, cyclopentenyl (-C5H7), cyclopentenylpropylene, methylcyclohexenylene, and cyclohexenyl (- CeHg). The double bond may be in the cis or trans configuration. For the avoidance of doubt, fused systems of a cycloalkenyl ring with a heterocyclic ring are considered as heterocycle irrespective of the ring that is bound to the core structure. Fused systems of a cycloalkenyl ring with an aryl ring are considered as aryl irrespective of the ring that is bound to the core structure. Fused systems of a cycloalkenyl ring with a heteroaryl ring are considered as heteroaryl irrespective of the ring that is bound to the core structure.

[0062] The term “aryl”, as a group or part of a group, refers to a polyunsaturated, aromatic hydrocarbyl group having a single ring (i.e. phenyl) or multiple aromatic rings fused together (e.g. naphthyl), or linked covalently, typically comprising 6 to 12 carbon atoms; wherein at least one ring is aromatic, preferably comprising 6 to 10 carbon atoms, wherein at least one ring is aromatic. The aromatic ring may optionally include one to two additional rings (either cycloalkyl, heterocyclyl or heteroaryl) fused thereto. Examples of suitable aryl include C6-i2aryl, preferably Ce- aryl, more preferably Ce-saryL Non-limiting examples of aryl comprise phenyl, biphenylyl, biphenylenyl, or 1-or 2-naphthanelyl; 5- or 6-tetralinyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- or 8- azulenyl, 4-, 5-, 6 or 7-indenyl, 4- or 5-indanyl, 5-, 6-, 7- or 8-tetrahydronaphthyl, 1 , 2,3,4- tetrahydronaphthyl, and 1 ,4-dihydronaphthyl; 1-, 2-, 3-, 4- or 5-pyrenyL A “substituted aryl” refers to an aryl group having one or more substituent(s) (for example 1 , 2 or 3 substituent(s), or 1 to 2 substituent(s)), at any available point of attachment.

[0063] The term “heteroaryl” as a group or part of a group, refers but is not limited to 5 to 12 carbon- atom aromatic rings or ring systems containing 1 or 2 rings which can be fused together or linked covalently, typically containing 5 to 6 atoms; at least one of which is aromatic in which one or more carbon atoms in one or more of these rings can be replaced by N, O and / or S atoms where the N and S heteroatoms may optionally be oxidized and the N heteroatoms may optionally be quaternized, and wherein at least one carbon atom of said heteroaryl can be oxidized to form at least one C=O. Such rings may be fused to an aryl, cycloalkyl, heteroaryl or heterocyclyl ring. Non-limiting examples of such heteroaryl, include: pyrrolyl, furanyl, thiophenyl, pyrazolyl, imidazolyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, triazolyl, oxadiazolyl, thiadiazolyl, tetrazolyl, oxatriazolyl, thiatriazolyl, pyridinyl, pyrimidyl, pyrazinyl, pyridazinyl, oxazinyl, dioxinyl, thiazinyl, triazinyl, imidazo[2,1-b][1 ,3]thiazolyl, thieno[3,2- b]furanyl, thieno[3,2-b]thiophenyl, thieno[2,3-d][1 ,3]thiazolyl, thieno[2,3-d]imidazolyl, tetrazolo[1 ,5-a]pyridinyl, indolyl, indolizinyl, isoindolyl, benzofuranyl, isobenzofuranyl, benzothiophenyl, isobenzothiophenyl, indazolyl, benzimidazolyl, 1 ,3-benzoxazolyl, 1 ,2- benzisoxazolyl, 2,1-benzisoxazolyl, 1 ,3-benzothiazolyl, 1 ,2-benzoisothiazolyl, 2,1- benzoisothiazolyl, benzotriazolyl, 1 ,2,3-benzoxadiazolyl, 2,1 ,3-benzoxadiazolyl, 1 ,2,3- benzothiadiazolyl, 2,1 ,3-benzothiadiazolyl, benzo[d]oxazol-2(3H)-one, 2,3-dihydro- benzofuranyl, thienopyridinyl, purinyl, imidazo[1 ,2-a]pyridinyl, 6-oxo-pyridazin-1 (6H)-yl, 2- oxopyridin-1 (2H)-yl, 6-oxo-pyridazin-1 (6H)-yl, 2-oxopyridin-1 (2H)-yl, 1 ,3-benzodioxolyl, quinolinyl, isoquinolinyl, cinnolinyl, quinazolinyl, quinoxalinyl; preferably said heteroaryl group is selected from the group consisting of pyridyl, 1 ,3-benzodioxolyl, benzo[d]oxazol-2(3H)-one, 2,3-dihydro-benzofuranyl, pyrazinyl, pyrazolyl, pyrrolyl, isoxazolyl, thiophenyl, imidazolyl, benzimidazolyl, pyrimidinyl, triazolyl and thiazolyl.

[0064] Preferred features, embodiments, and uses of this invention are set herein below. Each embodiment of the invention so defined may be combined with any other embodiment unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. Hereto, the present invention is in particular captured by any one or any combination of one or more of the below embodiments, with any other aspect and / or embodiment.

[0065] In a first aspect, the present invention provides a process for CO2 absorption, applicable in an overall CO2 absorption / desorption process. The present invention also encompasses an overall CO2 absorption / desorption procedure comprising the process for CO2 absorption as described herein. The process for CO2 absorption preferably comprises the step of absorbing CO2 in an absorbent composition comprising a metal-organic framework (MOF); wherein the MOF comprises metal ions M, preferably zinc ions, and nitrogen-containing ligands L; wherein the (theoretical) coordination number of the metal ions M is 4; and wherein the MOF has an atomic ratio of coordinating nitrogen to metal of at most 3.8. In other words, the present application describes technology for accelerated CO2 absorption by using a specifically designed catalyst. The term “catalyst” as used herein refers to a substance or compound which causes a change in the rate of a reaction (e.g., CO2 absorption).

[0066] In the following, (preferred) embodiments, particularities, and features of the present process and materials applied therein are explained in further detail.

[0067] In accordance with the present invention, a specific metal-organic framework (MOF) is used herein as a catalyst for CO2 absorption. The term “metal organic framework (MOF)” as used herein has a well-established meaning within the prior art. More specifically, a MOF is a material, typically a crystalline material, comprising organic ligands coordinated to central metal ions, thereby forming a porous, three-dimensional structure.

[0068] In the context of the present invention, the MOF as described herein comprises metal ions M, preferably zinc ions, and nitrogen-containing ligands L and is characterized by the following features: a (theoretical) coordination number of the metal ions M is 4; and, the MOF has an atomic ratio of coordinating nitrogen-to-metal of at most 3.8.

[0069] Put differently, the present MOFs may comprise unsaturated metal complexes wherein the central metal ion is bound to fewer ligands L than its typical coordination number, in the case of the present invention being typically 4 ligands per metal ion. The metal ion therefore has an incomplete coordination sphere, resulting in a MOF with unsaturated or “open” (vacant) coordination sites, which are defined herein as “defects”.

[0070] Extensive experimentation by the present inventors has revealed that the presence of such defects in the MOF structure makes it more reactive, when compared to a MOF structure having less or no defects, thus providing a MOF with improved catalytic activity as demonstrated herein in the example section. In particular, it has been found that a larger amount of defects can improve catalytic performance of the respective MOF.

[0071] Metal complexes with fewer ligands than the metal ion's preferred coordination number are generally considered less stable within the art. In particular, they may readily undergo reactions to fill their coordination sites, or they might decompose or undergo structural rearrangement. In contrast, the present inventors have found that the MOF as described herein may advantageously remain stable, even after multiple recycling steps.

[0072] Depending on the desired application, the number of defects (i.e., reactive sites) of the present MOF may be tuned. Hence, some embodiments may provide a MOF comprising metal complexes with a smaller or larger deviation from its typical coordination number. In some preferred embodiments, the present MOF has an atomic ratio of nitrogen, coordinated to the metal, to metal of less of at most 3.8, or at most 3.7, or at most 3.6, or at most 3.5, or at most 3.4, or at most 3.3, or at most 3.2, at most 3.1 , preferably at most 3.0, or at most 2.9, or at most 2.8, more preferably at most 2.7, or at most 2.6, or at most 2.5, or at most 2.4.

[0073] It should be understood that the “atomic ratio of coordinating nitrogen to metal” generally refers herein to nitrogen atoms as part of the ligand L that are coordinated with the metal ion M. For instance, an imidazole or imidazolate compound comprises two nitrogen atoms of which one nitrogen atom may coordinate with the metal ion M. Hence, the nitrogen-containing ligand L as described herein may comprise nitrogen atoms in its molecular structure that can coordinate with the metal ion M, and nitrogen atoms that do not coordinate with the metal ion M.

[0074] In some preferred embodiments, the present MOF has an atomic ratio of coordinating nitrogen to metal of at least 2.1 , or at least 2.2. In some preferred embodiments, the present MOF has an atomic ratio of coordinating nitrogen to metal of from at least 2.1 to at most 3.8, or at least 2.1 to at most 3.7, or at least 2.1 to at most 3.6, or at least 2.1 to at most 3.5, or at least 2.1 to at most 3.4, or at least 2.1 to at most 3.3, or at least 2.1 to at most 3.2, or at least 2.1 to at most 3.1 , preferably at least 2.1 to at most 3.0, or at least 2.1 to at most 2.9, or at least 2.1 to at most 2.8, or at least 2.1 to at most 2.8, more preferably at least 2.1 to at most 2.7, or at least 2.1 to at most 2.6, or at least 2.1 to at most 2.5, or at least 2.1 to at most 2.4.

[0075] The metal ions comprised in the present MOF are not particularly limited in terms of size, electronic configuration, or oxidation state, as long as they have a preferred coordination number of 4. This has the advantage that the central metal ion may be varied to further tune the catalytic reactivity of the MOF towards specific applications.

[0076] In some embodiments, the metal ions M comprise an ion selected from the group comprising zinc (Zn), cupper (Cu), cobalt (Co), manganese (Mn), cadmium (Cd), iron (Fe), or mercury (Hg) ions, and mixtures thereof. In some preferred embodiments, the metal ion M comprises zinc (Zn) ions, preferably the metal ion M is a zinc ion, for instance Zn2+. The present inventors have found that MOFs comprising zinc ions are particularly efficient catalysts for CO2 absorption.

[0077] Alternatively, and in some embodiments, the metal ion M comprises zinc (Zn) ions and one or more other metal ions, such as cupper (Cu), cobalt (Co), manganese (Mn), cadmium (Cd), iron (Fe), or mercury (Hg) ions.

[0078] The nitrogen-containing ligands L comprised in the present MOF are not particularly limited, as long as they comprise one or more nitrogen atoms that can coordinate with the metal ion M. Suitable nitrogen-containing ligands are negatively charged compounds or neutral compounds, preferably negatively charged compounds. They may be monodentate (i.e., binding to the central metal ion through a single donor atom) or polydentate (binding to the central metal ion through multiple donor atoms). The nitrogen-containing ligands can be bridging ligands, having multiple donor atoms that can simultaneously coordinate with two or more metal ions, forming a bridge between them, resulting in the formation of polynuclear complexes. In some embodiments, the nitrogen-containing ligands are at least bridging ligands.

[0079] In some preferred embodiments, the nitrogen-containing ligand L is an aromatic heterocyclic amine, preferably comprising two or more nitrogen atoms. In some preferred embodiments, the nitrogen-containing ligand is an imidazolate of Formula (I) wherein R1, R2, R3are each independently selected from the group comprising hydrogen, alkyl, alkenyl, alkynyl, halo, amino, nitro, and cyano; or R2and R3together with the carbon atoms to which they are attached can form a cycloalkyl, a cycloalkenyl, an aryl, or a heteroaryl wherein each of said cycloalkyl, cycloalkenyl, aryl, and heteroaryl can be unsubstituted or substituted with one or more substituents R4, wherein each R4is independently selected from the group comprising alkyl, alkenyl, hydroxyl, amino, cyano, nitro, and halo.

[0080] In some embodiments, the nitrogen-containing ligand is an imidazolate of Formula (I) wherein R1, R2, R3are each independently selected from the group comprising hydrogen, alkyl, halo, nitro, and cyano; or R2and R3together with the carbon atoms to which they are attached can form a cycloalkyl, a cycloalkenyl, an aryl, or a heteroaryl wherein each of said cycloalkyl, cycloalkenyl, aryl, and heteroaryl can be unsubstituted or substituted with one or more substituents R4, wherein each R4is independently selected from the group comprising alkyl, alkenyl, hydroxyl, amino, cyano, nitro and halo.

[0081] The term “imidazolate” as used herein has a well-established meaning within the prior art. More specifically, it refers to a negatively charged compound, which is the deprotonated form of the corresponding imidazole compound. Non-limiting examples of suitable imidazolate ligands are deprotonated forms of imidazole (Im), 2-methylimidazole (2-Melm), 2- ethylimidazole (2-Etlm), 2-nitro-imidazole (2-Nlm), 2-propylimidazole, imidazolate-2- carboxyaldehyde (ICA), 4-methyl-5-imidazolecarboxaldehyde (4-MelCA), 2-cyano-imidazole (cnlm), benzimidazole (blm), 3-methylbenzimidazole (3-Meblm), 3-nitro-benzimidazole (3- Nblm), 3-bromo-benzimidazole (3-Brblm), Purine, 4,5,6,7-tetrahydrobenzimidazole, 4- azabenzimidazole (4-ablm), 5-azabenzimidazole (5-ablm), 4,5-dichloroimidazole (dclm), 2- mercaptobenzimidazole (SHBzIm), 2-aminobenzimidazole (NH2Bzlm), 2-phenylimidazole (Phlm), 4-methyl-5-imidazolecarboxaldehyde, 2-bromoimidazole (2-blm), 5- chlorobenzimidazole (cblm) or 2-chloroimidazole (2-clm). It should be understood that the same or different nitrogen-containing ligands may be coordinated to the central metal ion, as is apparent to the person skilled in the art. For instance, the nitrogen-containing ligands may comprise 2-methylimidazolate and carboxaldehyde-2- imidazolate, or 2-methylimidazolate and benzimidazolate, or benzimidazolate and 2- aminobenzimidazolate, or 2-methylimidazolate and imidazolate.

[0082] In some exemplary embodiments, the present MOF comprises zinc ions and imidazolate anions of Formula (I) as defined herein above.

[0083] In some embodiments, for example when the MOF comprises zinc ions and imidazolate anions of Formula (I), the MOF is also known as a zeolitic imidazolate framework (ZIF). The term “zeolitic imidazolate framework (ZIF)” as used herein has a well-established meaning within the prior art. Zeolitic imidazolate frameworks (ZIFs) are a class of metal-organic frameworks (MOFs) that are topologically isomorphic with zeolites. Zeolitic imidazolate frameworks or ZIFs have properties similar to inorganic zeolitic materials. ZIFs are based on [T(lm)2] tetrahedral bonds in which Im is an imidazolate type linking moiety and T is one or more transition metal(s). Each T is tetrahedrally coordinated to four Im, and each Im is coordinated to two T cations. These materials are generally referred to as zeolitic imidazolate frameworks or ZIFs since the angle formed by imidazolates (Im’s) when bridging transition metals is similar to the 145° angle of the Si — O — Si bond in zeolites. ZIFs are composed of tetrahedrally-coordinated transition metal ions (e.g. Zn, Fe, Co, preferably Zn) connected by imidazolate linkers. Since the metal-imidazole-metal angle is similar to the 145° Si-O-Si angle in zeolites, ZIFs have zeolite-like topologies. Preferably the zeolitic imidazolate framework comprises zinc. Advantageously, ZIFs can be thermally stable and base tolerant, which provides for a robust CO2 absorption process.

[0084] The T metal ion may be selected from metal ions of groups 9, 10, 11 , 12 and 14 of the periodic table. Thus, the T metal ion may be selected from a cobalt (Co) ion, a rhodium (Rh) ion, an iridium (Ir) ion, a nickel (Ni) ion, a palladium (Pd) ion, a platinum (Pt) ion, a copper (Cu) ion, a silver (Ag) ion, a gold (Au) ion, a zinc (Zn) ion, a cadmium (Cd) ion, a mercury (Hg) ion and a lead (Pb) ion. Typically, the metal ion is an Ag, Au, Cu, Zn, Co, Cd, Ir, Pt, Pd or Pb ion. Often, the metal ion is selected from Ag, Au, Cu, Zn, Co, Cd, Ir, Pt and Pd ions. In some preferred embodiments, the metal ion is a Zn ion, for instance Zn2+. One or more T metal ions can be used during synthesis, preferably each time combined with Zn cations. The metal precursors can be metal halides, metal nitrates, metal oxides, metal hydroxides, metal alkynoates, like acetates and metal formates or mixtures thereof. In some embodiments, the molar ratio of the metal cations in the metal compounds used to the organic linker during synthesis is from 1 :1 to 1 :100, or from 1 :1 to 1 :80, or from 1 :1 to 1 :50, or from 1 :1 to 1 :16, preferably may be 1 :1.5, 1 :2, 1 :3, 1 :4, 1 :5, 1 :6, 1 :7, 1 :8, 1 :9.

[0085] Exemplary ZIF structures include (between brackets is given the typical linker reference), but are not limited to, ZIF-1 (2-Melm), ZIF-2 (2-Melm), ZIF-3 (2-Melm), ZIF-4 (2-Melm), ZIF-5(2- Melm), ZIF-6 (2-Melm), ZIF-7 (2-Melm, bIM or Im / blm), ZIF-8 (2-Melm), ZIF-9 (2-Melm or blm), ZIF-10 (2-Melm), ZIF-11 (2-Melm or blm), ZIF-12 (2-Melm), ZIF-14 (2-Etlm), ZIF-20 (Purine), ZIF-21 (Purine), ZIF-22 (5-ablm), ZIF-23 (4-ablm), ZIF-25, ZIF-60 (lm / 2-Melm), ZIF- 61 (lm / 2-Melm), ZIF-62 (2-Nlm) or Im / blm), ZIF-63, ZIF-64 (2-Melm), ZIF-65 (2-Nlm), ZIF-66, ZIF-67 (2-Melm), ZIF-68 (Im / blm or 2-Nlm / blm), ZIF-69 (Im / blm or 2-Nlm / cblm), ZIF-70 (Im / blm or lm / 2-Nlm), ZIF-71 (dclm), ZIF-72 (dclm), ZIF-73 (2-Nlm / 3-Meblm), ZIF-74 (2- Nlm / 3-Meblm), ZIF-75 (2-Nlm / 3-Meblm), ZIF-76 (Im / cblm), ZIF-77 (2-Nlm), ZIF-78 (3-Nblm / 2- Nlm), ZIF-79 (3-Meblm / 2-Nlm), ZIF-80 (dclm / 2-Nlm), ZIF-81 (3-Brblm / 2-Nlm), ZIF-82 (cnlm / 2-Nlm), ZIF-90 (ICA), ZIF-91 (NaBH4 reduced ZIF-90), ZIF-92 (ZIF-90 functionalized with ethanolamine), ZIF-93 (4-MelCA), ZIF-95 (cblm), ZIF-96, ZIF-97, ZIF-100 (cblm). According to the number of imidazolate, ZIFs can have single imidazolate ligands such as ZIF- 2 to -23, 64-67, 71 , 72, 77 or they can have two types of imidazolate ligands such as ZIF-68 to ZIF-70 and ZIF-73 to ZIF-82 where each metal ion (preferably zinc ion) is connected by two different ligands.

[0086] Advantageously, the present process provides for fast CO2 absorption even with low amounts of MOF, thereby providing a cost-effective technology. The present process may be (part of) a batch process or a continuous process. The term “continuous process” as used herein may refer to continuously cycling the absorbent composition over the MOF that itself remains confined in a given space of the absorption equipment, particularly since it can act as a nonsolubilized heterogeneous catalyst as demonstrated herein in the example section. In a “batch process” the absorbent composition is contacted with the MOF in a single vessel.

[0087] In the event that the present process is (part of) a batch process, the absorbent composition may comprise at least 0.01 wt.% of the MOF, or at least 0.05 wt.%, preferably at least 0.1 wt.% of the MOF; with wt.% relative to the total weight of the absorbent composition. In some preferred embodiments, the absorbent composition comprises at most 30.0 wt.% of the MOF, preferably at most 25.0 wt.% of the MOF; with wt.% relative to the total weight of the absorbent composition. In some preferred embodiments, the absorbent composition comprises of from at least 0.01 wt.% to at most 30.0 wt.% of the MOF, or at least 0.05 wt.% to at most 30.0 wt.%, or at least 0.1 wt.% to at most 30.0 wt.%, or at least 0.1 wt.% to at most 25.0 wt.% of the MOF; with wt.% relative to the total weight of the absorbent composition. These amounts were found to provide optimal results, as illustrated in the example section.

[0088] In the event that the present process is (part of) a continuous process, the Weight Hourly Space Velocity (WHSV, being the weight amount of the absorbent composition that treated per weight amount of catalyst) of the absorbent composition may be at least 0.1 h’1, or at least 0.5 h’1, preferably at least 1 h’1. In some preferred embodiments, in continuous operation mode, the WHSV is at most 1000 h’1, or at most 500 h’1, preferably at most 50 h’1. In some preferred embodiments, in continuous operation mode, the WHSV is at least 0.1 IT1to at most 1000 h’1, or at least 0.5 IT1to at most 1000 h’1, or at least 1 IT1to at most 1000 IT1or at least 0.5 IT1to at most 50 h’1, or at least 1 IT1to at most 10 IT1.

[0089] In such embodiments, a continuous stream of absorbent composition may cycle between two process units where absorption and desorption take place.

[0090] The present MOFs have the advantage that they may have desirable properties for catalysis such as a high BET surface area, large pore volumes, and the ability to selectively adsorb, store, and separate gases, liquids, and other molecules. In some preferred embodiments, the MOF has a BET surface area of at least 200 m2 / g, preferably at least 300 m2 / g. Preferably, the BET surface area of the MOF is determined by means of Nitrogen (N2) physisorption according to ASTM D3663-20 or ISO 9277:2022.

[0091] An important finding of the present invention is that the specific MOF as described herein above can increase the CO2 absorption rates of an absorbent composition. The term “absorbent composition” as used herein refers to a formulation or mixture that is able to capture, preferably selectively capture, carbon dioxide (CO2). The absorbent composition may for instance absorb at least a portion of CO2from a gas stream comprising CO2.

[0092] In some preferred embodiments, the present process for CO2 absorption provides that the step of absorbing CO2 in an absorbent composition comprising a metal-organic framework (MOF) as described herein, comprises contacting the absorbent composition with a gas stream comprising CO2. The gas stream may be provided to the absorbent composition in various ways. For instance, suitable gas streams include those produced in a chemical plant (including refineries), a power plant, a cement plant, a steel plant as the result of combustion of a carbon-containing energy carrier or a fermentation plant as the result of biochemical conversion of biological material during which CO2 is released. In some preferred embodiments, the step of absorbing CO2 in the absorbent composition is performed by contacting a CO2 containing stream with the absorbent composition, preferably counter currently at a temperature of from 0°C to 60°C and / or at a total pressure of from 0.1 bar to 5.0 bar.

[0093] In particular embodiments, the gas stream comprising CO2 may be any gas stream that comprises CO2 in amounts greater than that in air. In various embodiments, the gas stream may comprise at least 0.1 mol.% CO2, at least 1 mol.% CO2, at least 5 mol.% CO2, at least 10 mol.% CO2, at least mol.% CO2, at least 30 mol.% CO2, at least 40 mol.% CO2, at least 50 mol.% CO2, at least 60 mol.% CO2, at least 70 mol.% CO2, at least 80 mol.% CO2, or at least 90 mol.% CO2. In one or more embodiments, the gas stream may comprise from 0.1 mol.% CO2 to 100 mol.% CO2, such as from 0.1 mol.% CO2 to 90 mol.% CO2, 0.1 mol.% CO2 to 80 mol.% CO2, 0.1 mol.% CO2 to 70 mol.% CO2, 0.1 mol.% CO2 to 60 mol.% CO2, 0.1 mol.% CO2 to 50 mol.% CO2, 0.1 mol.% CO2 to 40 mol.% CO2, 0.1 mol.% CO2 to 30 mol.% CO2, 0.1 mol.% CO2 to 20 mol.% CO2, or 0.1 mol.% CO2 to 10 mol.% CO2. In one or more embodiments, the gas stream may comprise from mol.% CO2 to 100 mol.% CO2, such as from 20 mol.% CO2 to 100 mol.% CO2, 30 mol.% CO2 to 100 mol.% CO2, 40 mol.% CO2 to 100 mol.% CO2, 50 mol.% CO2 to 100 mol.% CO2, 60 mol.% CO2 to 100 mol.% CO2, 70 mol.% CO2 to 100 mol.% CO2, 80 mol.% CO2 to 100 mol.% CO2, or mol.% CO2 to 100 mol.% CO2.

[0094] The present absorbent composition may be in liquid, solid, or gel form, and is optimized to maximize CO2uptake, minimize energy requirements for regeneration (i.e., CO2 release or desorption), and maintain stability under operational conditions. An advantage of the present absorbent composition is that it can be used in various industrial processes including processes directed to carbon capture or gas purification, and in air treatment systems.

[0095] In some preferred embodiments, the absorbent composition comprises one or more active chemical agents, such as amine compounds, carbonate compounds, or ionic liquids, that are configured to react and / or physically absorb CO2.

[0096] In some preferred embodiments, the absorbent composition comprises one or more amine compounds. Suitable amine compounds include alkanolamines and compounds comprising one or more primary amine functional groups, secondary amine functional groups, or tertiary amine functional groups. In some embodiments, the absorbent composition comprises one or more amine compounds selected from the group comprising N-methyl-diethanolamine (MDEA), triethylamine (TEA), triethanolamine (TEA), N-ethyl-diethanolamine (EDEA), N- isopropyl-diethanolamine (IPDEA), N-tert-butyl-diethanolamine (BDEA), dimethylmonoethanolamine (DMMEA), diethyl-monoethanolamine (DEMEA), triisopropanolamine (TIPA), 1-dimethylamino-2-propanol (1 DMA2P), 1-diethylamino-2-propanol (1 DEA2P), 3- dimethylamino-1 -propanol (3DMA1 P), 3-diethylamino-1 -propanol (3DEA1 P), 2-

[0097] (diisopropylamino)ethanol (DI PAE), 2-(dimethylamino)-2-methyl-1 -propanol (DMA-2M-1 P), 3- dimethylamino-2,2-dimethyl-1 -propanol (DMA-2, 2-DM-1 P), 4-(dimethylamino)-2-butanol (DMAB), 4-(dipropylamino)-2-butanol (DPAB), 4-diethyl-amino-2-butanol (DEAB), 4-((2- hydroxyethyl)(methyl)amino)-2-butanol (HEMAB), 4-ethyl-methyl-amino-2-butanol (4EMA- 2B), 4-((2-hydroxyethyl)(ethyl)amino)-2-butanol (HEEAB), 1-(2-Hydroxyethyl)-piperidine (1- (2-HE)PP), 1-methyl-2-piperidineethanol (1 M-2PPE), 1-(2-Hydroxyethyl)pyrrolidine (1-(2- HE)PRLD), 3-(dimethylamino)-1 ,2-propanediol (DMA-1 ,2-PD), 3-diethylamino-1 ,2- propanediol (DEA-1 ,2-PD), 3-pyrrolidino-1 ,2-propanediol (PRLD-1 ,2-PD), 3-piperidino-1 ,2- propanediol (3PP-1 ,2-PD), 3-hydroxy-1 -methylpiperidine (3H-1 MPP), 1-ethyl-3- hydroxypiperidine (1 E-3HPP), 2-(1-piperazinyl)-ethylamine (PZEA), 3-diethylamino-1 ,2- propanediol (DEA-1 ,2-PD), N-methyl-4-piperidinol (MPDL), 2-amino-2-methyl-1 -propanol (AMP), 2-amino-2-hydroxymethyl-1 ,3-propenediol (AHPD), N-methyl-N-secondary-butyl- glycine, diethylglycine, dimethylglycine, and mixtures thereof.

[0098] In some embodiments, suitable amine compounds may comprise one or more tertiary amine functional group.

[0099] In some embodiments, the absorbent composition comprises one or more carbonate compounds and / or one or more bicarbonate compounds, such as potassium carbonate, potassium bicarbonate, sodium bicarbonate, or sodium carbonate.

[0100] The present absorbent composition may further comprise a solvent. In some preferred embodiments, the absorbent composition further comprises a solvent; preferably comprising water or an alcohol. Suitable alcohols are compounds comprising one or more hydroxyl groups, including methanol, ethanol, isopropanol, n-propanol, ethylene glycol, glycerol or mixtures thereof. For instance, an alcohol in the present context may comprise one, two, three, four, five, six, or more hydroxyl groups.

[0101] The present absorbent composition may further comprise an absorption promoter. The term “absorption promoter” as used herein refers to a substance or additive that may enhance the efficiency, rate, or capacity of CO2 capture within an absorbent composition. In some preferred embodiments, the absorbent composition further comprises an absorption promoter, preferably selected from the group comprising piperazine, monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), methyl-aminopropylamine (MAPA), 3- aminopropanol (AP), 2,2-dimethyl-1 ,3-propanediamine (DMPDA), diglycolamine (DGA), 2- amino-2-methylpropanol (AMP), 1 -amino- 2-propanol (Ml PA), 2-methyl-methanolamine (MMEA), piperidine (PE), and mixtures thereof.

[0102] The present invention further encompasses a CC>2-containing absorbent composition comprising a metal-organic framework (MOF); wherein the MOF comprises metal ions M, preferably zinc ions, and nitrogen-containing ligands L; wherein the (theoretical) coordination number of the metal ions M is 4; wherein the MOF has an atomic ratio of nitrogen to metal of at most 3.8; obtained or obtainable by means of the process according to the first aspect of the present invention, or embodiments thereof.

[0103] The present invention further encompasses methods for preparing the specific MOFs used in the process according to the first aspect of the present invention, or embodiments thereof.

[0104] In some preferred embodiments, the MOF is obtained by means of a method comprising the steps of: providing a MOF comprising metal ions M with (theoretical) coordination number 4 and nitrogen-containing ligands L; and, applying a mechanical force to the MOF, thereby forming framework defects such that the atomic ratio of coordinating nitrogen to metal in the MOF is at most 3.8; preferably wherein the mechanical force is applied to the MOF comprises by means of ball milling, jet milling, hammer milling, ultrasonic milling, cryogenic grinding, or vibratory milling.

[0105] In some embodiments, the framework defects can be introduced by exerting mechanical forces on the MOF (crystals), preferably by ball milling or pressurizing during a given time. These mechanical forces result in defects accompanied eventually with linker vacancies. The metal-nitrogen bonds may be broken upon ball milling, leading to unsaturated metal-sites (electron poor) and nitrogen-sites (electron-rich). It has been found herein that this may advantageously improve the reactivity of the resulting defect-rich MOF. For instance, the defect-rich MOF may coordinate, react and / or activate nucleophilic species such as water or alcohols, which may be further comprised in the absorbent composition as a solvent.

[0106] The term “ball milling” refers to a method used for grinding and mixing materials in a rotating cylinder or conical mill. The mill is partially filled with grinding media, such as balls or pebbles. As the mill rotates, the impact and attrition between the balls and the material result in the creation of defects and eventually in size reduction. The term “jet milling” refers to the use of high-speed jets of gas (usually air or nitrogen) to impact particles, causing them to break apart.

[0107] The term “hammer milling” refers to the use of rotating hammers to crush and disintegrate materials.

[0108] The term “ultrasonic milling” refers to the use of ultrasonic waves that create intense vibrations that break down particles. This method is useful for delicate materials and nano-sized particles.

[0109] The term “cryogenic grinding” refers to the method wherein materials are cooled using liquid nitrogen or other cryogenic gases, to make the material more brittle hence more easily fractured.

[0110] The term “vibratory milling” refers to the use of vibrations to impact and grind particles, and is suitable for both wet and dry grinding.

[0111] In some preferred embodiments, the one or more framework defects are missing atoms providing openings in the framework structure. Optionally the openings are stabilised with an oxide of metals or metalloids, preferably selected from the group comprising silicon, aluminium, boron, gallium, titanium, zirconium, or hafnium, and mixtures thereof. In some embodiments, soluble compounds of the metals or metalloids are added to the MOF, containing the defects, in the presence of a suitable solvent. Subsequently, the resulting mixture may be treated with a suitable acid or base to precipitate the compounds of the metal or metalloid into its corresponding oxide, oxyhydroxide or hydroxide. Without willing to be bound by any theory, said precipitation step is typically a hydrolysis reaction of the metal or metalloid precursor. The addition of soluble compounds of the metals or metalloids to the MOF, containing the defects, can be done in presence of an excess of suitable solvent to which the MOF and soluble compounds of the metals or metalloids is added in any sequence or the soluble compounds of the metals or metalloids are dissolved in a minimum amount of solvent are added to the MOF, containing the defects, either in a dry or wet impregnation procedure. After the addition, the resulting slurry may be left maturing at a temperature between 0 and 150°C during 0.1 to 24 hours. After the precipitation step, the solvent may be removed by evaporation, filtration and / or centrifugation; and the obtained solid, containing the MOF and the precipitated oxide, oxyhydroxide or hydroxide, may be subsequently thermally treated between 80 to 300°C. In one embodiment, after the exertion of mechanical forces and creating defects, the obtained material can be stabilized by introducing silica (SiO2) or silicic acids [H2xSiOx+2]n. For example, the framework may be stabilised by dispersing the MOF and hydrolysing tetraethyl orthosilicate (TEOS). The provided MOF (prior to applying mechanical forces), preferably a ZIF, may be prepared using any of the methods known in the art.

[0112] A hydrothermal method may be used to prepare said (defect-free / defect-depleted) MOFs. Said preparation method involves heating chemical reagents, typically in the presence of a solvent such as water, in a (closed) reactor vessel, such as an autoclave. More particularly, and in exemplary embodiments, hydrothermal synthesis of MOFs may involve heating a mixture of organic linkers and metal salt(s) in an aqueous solution inside a sealed vessel at high temperature, typically above the normal boiling point of at least one solvent comprised in the solution, and pressure. A solvothermal method is similar to the hydrothermal method, except that instead of an aqueous solution, organic solvents such as ethanol, methanol, isopropyl alcohol, N-dimethyl formaldehyde (DMF), or mixtures thereof are used below their normal boiling point.

[0113] In some embodiments, hybrid ZIFs can be synthesized, comprising the use of at least two different imidazole structures in the synthesis. In some embodiments, the first imidazole is different from the second imidazole. In embodiments, the first imidazole can comprise 2- methylimidazole and the second imidazole can comprise carboxaldehyde-2-imidazole. In an alternative embodiment, the first imidazole can comprise 2-methylimidazole and the second imidazole can comprise benzimidazole. In another embodiment, the first imidazole can comprise benzimidazole and the second imidazole can comprise 2-aminobenzimidazole. In yet another embodiment, the first imidazole can comprise 2-methylimidazole and the second imidazole can comprise imidazole. In some embodiments, the first imidazole can comprise 2- methylimidazole or benzimidazole. In other embodiments, the second imidazole can comprise benzimidazole, 2-aminoimidazole, imidazole, or carboxaldehyde-2-imidazole.

[0114] Synthesis of the provided MOF (prior to applying mechanical forces), preferably a ZIF, may include the use of a suitable solvent.

[0115] The solvent can advantageously be polar and aprotic, for instance, the solvent may comprise an amide, like N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), N,N- dimethylacetamide (DMAc), 1 ,3-dimethylpropyleneurea (DMPLI) or mixtures thereof, sulfoxide (for example, dimethyl sulfoxide), phosphoramide (e.g., hexamethylphosphoramide), acetonitrile (MeCN), an ether (e.g., dimethyl ether, diethyl ether, methylethyl ether, or the like, or combinations thereof), or a combination thereof.

[0116] Alternatively, aqueous solvents such as aqueous ammonia and alcohols, like methanol, ethanol, propanol and mixtures thereof can be used as a suitable solvent. In particular embodiments, a base may be added to the reaction mixture used to prepare the provided MOF (prior to applying mechanical forces). Suitable inorganic bases include NaOH, KOH. Suitable organic bases include mono-alkylamines, like butylamine, dialkylamines, trialkylamines, like trimethylamine, triethylamine, tripropylamine, monoethanolamine (MEA), 2-amino-2-methyl-1 -propanol (AMP), monoethanolamine (MEA), diethanolamine (DEA), diglycolamine (DGA), methyldiethanolamine (MDEA), piperazine (PZ), tetramethylammoniumhydroxide (TMAOH), tetraethylammoniumhydroxide (TEAOH), tetrapropylammoniumhydroxide (TPAOH), or tetrabutylammoniumhydroxide (TBAOH) in water or mixtures thereof, piperazine and / or 1 ,4-dimethylpiperazine. While not wishing to be bound by any theory of operation, it is believed that the organic base may activate the relatively weaker imidazole base, into an imidazolate anion, to react with electrophilic metal cations, such as Zn2+cations. The use of such bases may advantageously reduce the time of reaction.

[0117] If desired, the synthesis can be conducted in the presence of a template, which can typically comprise or be a neutral organic compound, such as an ether, ketone, ester, amine, nitrile, nitro compound, phosphine, hydrocarbon, halide, or the like, or combination thereof.

[0118] In most cases, the reagents, metal precursors, imidazole(s), solvent and template are combined into a solution, either aqueous or nonaqueous, with synthesis reaction temperatures ranging from 0°C to 300°C, preferably between 20°C and 200°C, most preferably between 40°C and 175°C and with synthesis times ranging from 5 minutes to 5 days, preferably from 20 minutes to 100 hours, most preferably from 30 minutes to 48 hours.

[0119] In some embodiments, the process can further comprise activating a (hybrid) ZIF to remove impurities by washing the obtained solid product with water, alcohols, ethers, acetonitrile, acetone, hydrocarbons, or aromatics or mixtures thereof.

[0120] In some embodiments, after synthesis, ligands can be removed by treating the MOF with copper salts, preferably copper halides, copper nitrate or copper acetate.

[0121] In some preferred embodiments, framework defects can be introduced by using Lewis acids during the synthesis of the MOFs, preferably ZIFs. Preferably, the Lewis acids comprise the following elements of the boron group or group 13 of the periodic table, consisting of boron (B), aluminium (Al), gallium (Ga) or indium (In). Preferably, hydroxides or any precursor of these hydroxides are used, like halides, nitrates, acetates, sulphates, oxides (AI2O3, B2O3, Ga2C>3 and ln2O3), oxyhydroxides (BO(OH), AIO(OH), GaO(OH), InO(OH)), tetraboric acid (H2B4O7), sodium tetraborate (NaB(OH)4), sodium aluminate (NaAI(OH)4, NaAIO2), and sodium gallate (NaGa(OH)4).

[0122] The listed Lewis acids may bind hydroxide ions by means of the chemical reaction equations described below (as exemplified for oxides or hydroxides of boron, aluminium, gallium, and indium):

[0123] This forms four coordinated hydroxide complexe anions (tetrahydroxy anions) that can coordinate with other cations like Zn2+.

[0124] Preferably, the ratio of elements of group 13 to metal ions (e.g., Zn2+) in the synthesis solutions is from 0.05 to 0.50, preferably from 0.10 to 0.40 more preferably from 0.15 to 0.30. Without willing to be bound by any theory, it is believed that these tetrahydroxy anions bind to the metal cations described herein and hence occupy a coordination that would otherwise be occupied by an imidazolate anion. After synthesis, these tetrahydroxy anions may be removed from the MOF structure by washing the modified MOF with a suitable aqueous solution, thereby replacing the aforementioned tetrahydroxy anions with other anions and / or water molecules.

[0125] In some preferred embodiments, the MOF is obtained by means of a method comprising the steps of: contacting a solution comprising a metal source with (theoretical) coordination number 4 and a Lewis acid comprising B, Al, Ga, In, or combinations thereof with nitrogen-containing ligands, thereby obtaining a MOF precursor; and, heating the MOF precursor to a temperature of from 25 to 250°C, thereby obtaining the MOF comprising metal ions M and nitrogen-containing ligands L with an atomic ratio of nitrogen to metal of at most 3.8.

[0126] In some preferred embodiments, the metal source comprises zinc (Zn). In some preferred embodiments, the Lewis acid is a boron-containing acid; preferably boric acid or boronic acid.

[0127] The present invention also encompasses a process for optimizing CO2 absorption, preferably comprising the steps of contacting a CO2-containing stream with an absorbent composition comprising a MOF having the features as defined herein. The present invention further encompasses the use of a MOF comprising metal ions M, preferably zinc (Zn) ions, and nitrogen-containing ligands L; wherein the (theoretical) coordination number of the metal ions M is 4; and wherein the MOF has an atomic ratio of coordinated nitrogen to metal of at most 3.8; for increasing the CO2 absorption rate of an absorbent composition.

[0128] A particular advantage of using the present MOF in a process for CO2 absorption is the reduced cost in the overall absorption / desorption process. Moreover, use of a MOF as described herein allows to reduce the time needed for CO2 absorption, which in turn leads to lower energy requirements (OPEX), and / or allows for high throughput operation or smaller equipment and hence lower capital investment (CAPEX).

[0129] The present invention further encompasses a process for CO2 absorption and desorption. Preferably, the process comprises the steps of: absorbing CO2 in an absorbent composition using the process as described herein above, or (preferred) embodiments thereof, thereby obtaining a CC>2-containing absorbent composition; and, desorbing CO2 from the CC>2-containing absorbent composition by heating said composition to a desorption temperature Td, optionally in the presence of a catalyst.

[0130] In some embodiments, the desorption temperature Td is at most 500 K, preferably at most 450 K, preferably at most 400K, preferably at most 380 K, preferably about 361 K.

[0131] In some embodiments, the process is a batch process. In some more preferred embodiments, the process is a continuous process.

[0132] In some embodiments, the process further comprises the step of: recycling the MOF into the process.

[0133] The MOF of the present invention has the advantage that it can be easily recycled, particularly since it can act as a non-solubilized heterogeneous catalyst as demonstrated herein in the example section.

[0134] In some embodiments, the step of recycling the MOF comprises the step of: directly recycling the MOF and absorbent composition into the process, by using the MOF and absorbent composition in a re-absorption step without separation.

[0135] In some embodiments, the step of recycling the MOF comprises the steps of: removing the MOF from the absorbent composition, preferably through filtration; washing the MOF with a suitable solvent; optionally, centrifuging the MOF; drying the MOF; and, adding the dried MOF to an absorbent composition.

[0136] Alternatively, in some embodiments, the process comprises the use of the MOF in a continuous mode.

[0137] The MOF of the present invention has the advantage that it can be used easily in a continuous mode, where the absorption composition is sent continuously over the MOF that itself remains confined in a given space of the absorption equipment, particularly since it can act as a nonsolubilized heterogeneous catalyst as demonstrated herein in the example section. The MOF in such continuous mode is preferably used in particulate shapes, like balls, pellets, extrudates or powder. The particulate shapes of the MOF can be confined as a fixed bed, fluidized bed, entrained bed or can be fixed on the trays or packing of an absorption equipment.

[0138] EXAMPLES

[0139] The following examples serve to merely illustrate the invention and should not be construed as limiting its scope in any way. While the invention has been shown in only some of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes and modifications without departing from the scope of the invention.

[0140] Experimental Methods

[0141] Chemicals and materials

[0142] MDEA (Methyl diethanolamine, 99%), 2-methylimidazole (99%), methanol (99.9%), Boric acid (99.99%) were ordered from Sigma Aldrich. Zinc (II) acetate (99.99%) was ordered from ACROS organics. Tetraethyl orthosilicate (99%) was purchased from Fisher Scientific.

[0143] Metal organic framework (MOF) synthesis

[0144] 35.1 g of Zn(CH3COO)2-2H2O was dissolved in 240 mL H2O and 26.2 g of 2-methylimidazole and 300.0 g NH3-H2O (25%) solution was added to the mixture under violent stirring. After stirring at 20°C for 10 minutes, a white precipitate was collected through a centrifuge and washed with ethanol and water to remove any excess 2-methylimidazole. The collected precipitate was dried in a convection oven at 60°C for 24 hours. The resulting MOF was obtained as a white powder and had a zeolitic imidazolate framework-8 (ZIF-8) structure.

[0145] Metal organic framework (MOF) modification The obtained MOF, termed herein ZIF-8, was further subjected to a ball milling procedure at 650 RPM for 30 minutes to introduce various framework defects. The obtained MOF, termed herein ZIF-8-B was used for further characterization and testing of catalytic behaviour for absorption of CO2 in an absorbent composition.

[0146] To stabilize the formed framework defects, the 0.5 g of the modified MOF ZIF-8-B was dispersed in 400 mL ethanol. Next, 5 mL 28 wt% ammonia and 100 mL H2O were added to the mixture under continuous stirring. After successful mixing, 0.25 g of tetraethyl orthosilicate (TEOS) was added to the mixture and stirring was continued at 20°C for 6 h. After completion, the reaction mixture was centrifuged and the obtained solid washed with ethanol. The collected powder was dried in a convection oven at 60°C for 18 hours. The resulting material is termed ZIF-8-Si herein.

[0147] Modified metal organic framework (MOF) synthesis

[0148] In another experiment, instead of mechanically modifying a synthesized MOF, an intrinsically modified MOF was synthesized.

[0149] First, Zn(CH3COO)2-2H2O (5 mmol) was mixed with 0.3 g H3BO3 in 30 mL deionized water. Next, 2-methylimidazole (80 mmol) dissolved in 30 mL deionized water was added to the mixture under continuous stirring. After stirring at 20°C for 1 hour, the mixture was transferred to a Teflon-lined stainless steel, autoclave (100 mL), which was heated to 175°C and maintained at said temperature for 3 hours. After cooling to 20°C, the modified MOF (termed herein ZIF-8-H) was collected, washed with deionized water, and dried in a convection oven at 60°C for 18 hours.

[0150] The same procedure was repeated to synthesize ZIF-8-H (0.1 ), except by using 0.1 g of H3BO3 instead.

[0151] Characterization methods

[0152] X-ray powder diffraction (XRD) was conducted on a high-throughput STOE STADI P Combi diffractometer operated in transmission mode with focusing Ge(111 ) monochromatic X-ray inlet beams (A=1.5406 A, Cu Ko source).

[0153] N2 physisorption measurements were performed using Micromeritics Instruments Tristar 3000 at 77 K. The samples were degassed under N2 flow at 200°C for 6 h prior to measurement. The relative nitrogen pressure varied between 0.01 and 0.99 (p / pO). Pore volumes were calculated by the t-plot method. The BET surface area of the MOF is determined by means of Nitrogen (N2) physisorption according to ASTM D3663-20 or ISO 9277:2022.

[0154] 1H-MAS-NMR spectra were registered using a Bruker Avance I HD 500 MHz (equipped with a 11 .74 T Oxford magnet) and a 2.5mm HXY Chemagnetics probe. The samples were spun at 20 kHz and a single pulse acquisition experiment was performed with a pulse duration of 2.5ps. The free induction decay (FID) was measured with 2048 complex points, spectral window of 20 kHz for ~50ms. Before performing the Fourier transform (FFT), the FID was zero filled to 16384 complex points. 64 scans were accumulated for each sample with a repetition delay of 5 s.

[0155] Evaluation of catalytic behaviour

[0156] CO2 absorption was carried out in a Parr reactor linked to an Operando attenuated total reflection Fourier transform infrared (ATR-IR), which is ReactIR iC10, equipped with an MCT detector. The scheme for the reactor is shown in FIG. 2. The spectra were obtained by collecting 128 scans at 4 cm-1resolution every 30 s. The change of the signal intensity at 1353 cm-1was used to track the concentration change of HCOs', which is indicative for CO2 absorption in an amine solution (absorbent composition). Liquid samples were taken after 60 minutes of absorption, which was titrated by Chittick equipment with an average absolute relative deviation of less than 5%. Typically, 500 pL CC>2-loaded amine solution (absorbent composition) was added to the flask by a calibrated pipette, and then 1 mL of 1.5 M H2SO4 was injected into the solution under a stirring. The volume of released CO2 was recorded to calculate the CO2 loading of the sample. Every sample was tested three times, and the average value was adopted.

[0157] Calculation of kcat

[0158] Assuming the CO2 absorption is a fist order reaction,

[0159] Where Co and Ct represent the CO2 loading in the amine solvent (molco2 molamine'1) at the beginning of the absorption (0 min) and t min.

[0160] For CO2 absorption with MOFs as a catalyst: cat koveraii I^blan

[0161] Quantification of the ATR-IR signal

[0162] The amine solution as used herein comprises methyl diethanolamine (MDEA). MDEA can be readily followed by the operando ATR-IR system, built into a batch autoclave. FIG. 3a below shows the spectra of a typical CO2 absorption experiment in an aqueous MDEA solution. The signal intensity of bicarbonate (at 1353 cm-1) increases with absorption time, while the signal intensity at 1353 cm-1follows a nice linear relationship with the CO2 absorbed as measured using the titration method (FIG. 3b). Due to the linearity, CO2 absorption rates can be directly related to the intensity change of the 1353 cm-1vibration. Figure b below shows a validation of the method. It collects the results of two separate CO2 absorption experiments, one monitored with the Operando ATR-IR detector and the other with titration. The curve achieved by Operando ATR-IR is consistent with the titration results. (FIG. 4).

[0163] The CO2 loading can be then calculated as follows: ct(HC03“) = [At(1353 cm-1) - Ao(1353 cm-1)] / ft

[0164] At(1353 cm-1): IR Signal intensity on 1353 cm-1at t min.

[0165] Ao(1353 cm-1): IR Signal intensity on 1353 cm-1at 0 min. ftthe slope of IR signal intensity versus CO2 loading in MDEA solution determined by titration.

[0166] Example 1 : CO2 absorption performance of MOFs according to the invention

[0167] In a first example, the metal organic frameworks (MOFs) according to the present invention (ZIF-8-B and ZIF-8-H) characterized by more defects were compared to an as synthesized MOF (ZIF-8) comprising less defects.

[0168] Absorbent compositions comprising 30 wt% MDEA water solution were prepared to test the CO2 absorption efficiency of the different MOFs. In addition, an absorbent composition was prepared without the addition of any MOF, which served as a “blank sample” to identify the background CO2 absorption rate of the composition. The CO2 absorption performance of the different MOFs is shown in FIG. 5 and FIG. 6 as well as Table 1.

[0169] From the presented data it can be concluded that the comparative ZIF-8 sample shows almost identical CO2 absorption performance compared to the blank sample. In contrast, the ZIF-8-B and ZIF-8-H MOFs according to the present invention show an improved catalytic activity. Assuming a first-order reaction, ZIF-8-B and ZIF-8-H show a 5-fold increment to ZIF-8.

[0170] Table 1. CO2 absorption performance with different MOFs.

[0171] Average kcAT

[0172] Sample absorption rate

[0173] (*10'2min-1)

[0174] (mmol min'1) Blank 1.5 0.0

[0175] ZIF-8 1.5 0.6

[0176] ZIF-8-B 3.0 3.0

[0177] ZIF-8-H 2.8 2.9

[0178] Reaction conditions: 30°C, 30 wt% aqueous MDEA, MOF amount 0.33 wt% (based on solution), 15% CO2 (50 mL min-1), / 85% N2 (288 mL min-1) at 1 bar total pressure, and stirring under 1000 rpm.

[0179] Example 2: Separability of ZIF-8-B from the absorbent composition comprising an amine solution

[0180] In a second example, the separability of the modified MOF ZIF-8-B from the absorbent composition comprising CO2-rich MDEA amine solution was tested.

[0181] In particular, the CO2-rich MDEA amine solution after absorption with ZIF-8-B is collected through centrifuging to remove the solid catalyst, and went through a regeneration process to get a “recycled solution”. A new CO2 absorption reaction was then carried out with the “recycled solution” without the addition of any MOFs. As shown in Table 2, the recycled solution shows merely no catalytic effect, indicating that the MOF can be efficiently removed from the amine solution and cannot influence further downstream processing steps such as CO2 desorption.

[0182] Table 2. CO2 absorption performance of recycled solution. cAT

[0183] Sample

[0184] (*10'2min-1)

[0185] Blank 0.0

[0186] ZIF-8-B 3.0

[0187] Recycled solution 0.3

[0188] Reaction conditions: 30°C, 30 wt% aqueous MDEA, MOF amount 0.33 wt% (based on solution), 15% CO2 (50 mL min'1), / 85% N2 (288 mL min'1) at 1 bar total pressure, and stirring under 1000 rpm.

[0189] Example 3: Influence of defects of the MOF on catalytic performance In a third example, the influence of defects on the catalytic behaviour of the MOF was further tested. In particular, different ZIF-8-B MOFs were prepared by ball milling as described above, but using different Ball milling times (15, 30, and 60 minutes). In addition, these MOF were compared to the MOFs prepared with 0.3 g of H3BO3 (ZIF-8-H) and 0.1 of H3BO3 (ZIF-8-H (0.1 )) as described above.

[0190] It has been found that by increasing the ball milling time, the MOF became more disordered, associated with more structural defects. In addition, the material rich in structural defects has lower crystallinity compared with a material having less or no defects. FIG. 7 shows the relationship between crystallinity and the kcAT value for ZIF-8-B after different Ball milling time (15, 30, 60 min), considering the crystallinity of not-treated ZIF-8 as 100%. The kcAT starts to increase from 0.06 min-1to 0.03 min-1with the decrease of the crystallinity. Worth noting, that with a too-intense treatment, e.g. too long ball milling time, the crystallinity drops to 8% which is almost amorphous, but still shows a kcAT of 0.02 min-1. A similar trend was found between BET surface area and kcAT, which can be found in FIG. 8, indicating a proper treatment intensity (e.g. ball milling time), can create more active sites.

[0191] By measuring1H-MAS-NMR, as described in the experimental method section above, the different MOFs according to the present invention (ZIF-8-B and ZIF-8-H) could be compared to the comparative ZIF-8 having less defects by calculating a defect ratio as follows:

[0192] A(Zn — H2O Peak area of Zn-OF (0.25 ppm) from the deconvolution results1H-MAS-NMR A(C - Hf. Peak area of C-H (6.9 ppm) from the deconvolution results1H-MAS-NMR.

[0193] FIG. 9 shows the relationship between the calculated defect ratio and the kcAT. It has been found that when 15.6% of the nitrogen-containing ligands were disconnected from the central zinc ion, the highest kcAT can be reached. Moreover, it has been found that this value is independent of the preparation method of the respective MOF. Table 3 summarizes the catalytic effect for CO2 absorption.

[0194] Table 3. Summary of CO2 absorption performance with MOFs having different amounts of defects.

[0195] Missing-linker

[0196] ,r, kcAT

[0197] Sample defect ratio

[0198] (*10'2min-1)

[0199] (%) ZIF-8 5.2 0.6

[0200] ZIF-8-B (15 min) 9.1 1.4

[0201] ZIF-8-B 15.6 3.0

[0202] ZIF-8-B (60 min) 23.9 2.0

[0203] ZIF-8-H (0.1 ) 10.0 1.4

[0204] ZIF-8-H 13.5 2.9

[0205] Reaction conditions: 30°C, 30 wt% aqueous MDEA, MOF amount 0.33 wt% (based on solution), 15% CO2 (50 mL min-1), / 85% N2 (288 mL min-1) at 1 bar total pressure, and stirring under 1000 rpm.

[0206] Example 4: MOF stability

[0207] In a fourth example, the relative stability of the MOFs according to the present invention (ZIF- 8-B, ZIF-8-Si, and ZIF-8-H) was compared across different CO2 absorption cycles.

[0208] The relative performance of the MOFs after 5 cycles of CO2 absorption is shown in FIG. 10. From the presented data it can be concluded that the different preparation methods of the respective MOFs can influence their long-term stability. For instance, it can be observed that ZIF-8-B loses its activity after 2 cycles. However, it has been found that coating the MOF structure with SiO2 (as shown for ZIF-8-B-Si) can improve the long-term stability of the ball- milled MOF. In particular, the KCAT keeps around 66% activity after 5 cycles. Notably, ZIF-8-H exhibited good durability, showing no noticeable reduction in performance after five recycling cycles. The differences in catalytic activity after 5 cycles is summarized in Table 4.

[0209] Table 4. Summary of CO2 absorption performance with different catalysts through 5 absorption cycles. cAT

[0210] (*10'2min-1)

[0211] Sample

[0212] Fresh Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5

[0213] ZIF-8-B 3.0 1.6 0.7 0.1 0 0

[0214] ZIF-8-B-Si 3.0 1.8 1.6 1.6 1.9 1.9

[0215] ZIF-8-H 2.8 2.9 2.7 2.7 2.7 2.8 Reaction conditions: 30°C, 30 wt% aqueous MDEA, MOF amount 0.33 wt% (based on solution), 15% CO2 (50 mL min-1), / 85% N2 (288 mL min-1) at 1 bar total pressure, and stirring under 1000 rpm.

[0216] Example 5: CO2 absorption performance with different MOF amounts

[0217] In a fifth example, different absorbent compositions comprising 30 wt% MDEA water solution were prepared with different amounts of ZIF-8-B MOF.

[0218] The CO2 absorption performance is shown in FIG. 11 and summarized in Table 5. The linear increase of the kinetic absorption rate constant below 0.3 wt% ZIF-8-B, corresponding to a 10.5 ± 1.2 g'1min-1normalized catalytic activity, reflects a first order in ZIF-8-B loading. Usage of higher ZIF-8-B loadings deviates from that linearity, barely showing faster absorption at the highest loadings.

[0219] Table 5. CO2 absorption performance with different catalysts amount.

[0220] ZIF-8-B kcAT

[0221] Amount

[0222] (*10'2min'1)

[0223] (wt.%)

[0224] 0.06 0.7

[0225] 0.13 1.4

[0226] 0.2 1.9

[0227] 0.3 3.0

[0228] 0.8 3.0

[0229] 1.3 3.0

[0230] Reaction conditions: 30°C, 30 wt% aqueous MDEA, MOF amount 0-1.3 wt% (based on solution), 15% CO2 (50 mL min'1), / 85% N2 (288 mL min'1) at 1 bar total pressure.

[0231] Without wishing to be bound by theory, the present inventors found that the limit in absorption rate increase at the highest ZIF-8-B loadings was most likely the consequence of rate limitation due to gas-to-liquid mass transfer resistance. For instance when changing the stirring conditions at the same ZIF-8-B loading (e.g., 0.3 wt.%) from 500 rpm to 1000 rpm, it was found that the kinetic absorption rate increased from 0.5 10'2min'1to 3.0 10'2min'1.

Claims

CLAIMS1. A process for CO2 absorption, the process comprising the step of absorbing CO2 in an absorbent composition comprising a metal-organic framework (MOF); wherein the MOF comprises metal ions M and nitrogen-containing ligands L; wherein the coordination number of the metal ions M is 4; and wherein the MOF has an atomic ratio of coordinating nitrogen to metal of at most 3.8, preferably at most 3.5, preferably at most 3.0, more preferably at most 2.7.

2. The process according to claim 1 , wherein the absorbent composition comprises one or more amine compounds.

3. The process according to claim 1 or 2, wherein the metal ions M comprise zinc (Zn) ions, and preferably wherein the MOF is a zeolitic imidazole framework (ZIF).

4. The process according to any one of claims 1 to 3, wherein said process is a batch process, and wherein the absorbent composition comprises at least 0.01 wt.% of the MOF, preferably at least 0.1 wt.% of the MOF; with wt.% relative to the total weight of the absorbent composition.

5. The process according to any one of claims 1 to 4, wherein said process is a continuous process, and wherein the Weight Hourly Space Velocity (WHSV) of the absorbent composition is between 0.1 and 1000 h’1, preferably between 0.5 and 100 h’1, more preferably between 1 and 10 IT1.

6. The process according to any one of claims 1 to 5, wherein the step of absorbing CO2 in the absorbent composition is performed by contacting a CO2 containing stream with the absorbent composition, preferably counter currently, at a temperature of from 0°C to 60°C and / or at a total pressure of from 0.1 bar to 5.0 bar.

7. The process according to any one of claims 1 to 6, wherein the nitrogen-containing ligand L is an aromatic heterocyclic amine, preferably comprising two or more nitrogen atoms.

8. The process according to any one of claims 1 to 7, wherein the nitrogen-containing ligand is an imidazolate of Formula (I)wherein R1, R2, R3are each independently selected from the group comprising hydrogen, alkyl, alkenyl, alkynyl, halo, amino, nitro and cyano; or R2and R3together with the carbon atoms to which they are attached can form a cycloalkyl, a cycloalkenyl, an aryl, or a heteroaryl wherein each of said cycloalkyl, cycloalkenyl, aryl, and heteroaryl can be unsubstituted or substituted with one or more substituents R4, wherein each R4is independently selected from the group comprising alkyl, alkenyl, hydroxyl, amino, nitro, cyano and halo.

9. The process according to any one of claims 1 to 8, wherein the absorbent composition further comprises a solvent; preferably comprising water or an alcohol.

10. The process according to any one of claims 1 to 9, wherein the MOF has a BET surface area of at least 200 m2 / g, preferably at least 300 m2 / g.

11. The process according to any one of claims 1 to 10, wherein the MOF is obtained by means of a method comprising the steps of: providing a MOF comprising metal ions M with coordination number 4 and nitrogencontaining ligands L; and applying a mechanical force to the MOF, thereby forming framework defects such that the atomic ratio of coordinating nitrogen to metal in the MOF is at most 3.8; preferably wherein the mechanical force is applied to the MOF comprises by means of ball milling, jet milling, hammer milling, ultrasonic milling, cryogenic grinding, or vibratory milling.

12. The process according to any one of claims 1 to 11 , wherein the one or more framework defects are missing atoms and / or nitrogen-containing ligands L, providing openings in the framework structure, and optionally wherein the openings are stabilised with an oxide selected from the group comprising of silicon, aluminium, boron, gallium, titanium, zirconium, hafnium and mixtures thereof.

13. The process according to any one of claims 1 to 10, wherein the MOF is obtained by means of a method comprising the steps of: contacting a solution comprising a metal source with coordination number 4 and a Lewis acid comprising B, Al, Ga, In, or combinations thereof with nitrogencontaining ligands, thereby obtaining a MOF precursor; and heating the MOF precursor to a temperature of from 25 to 250°C, thereby obtaining the MOF comprising metal ions M and nitrogen-containing ligands L with an atomic ratio of coordinating nitrogen to metal of at most 3.8.

14. The process according to claim 13, wherein the Lewis acid is a boron-containing acid; preferably boric acid or boronic acid.

15. Use of an absorbent composition comprising a metal-organic framework (MOF); wherein the MOF comprises metal ions M and nitrogen-containing ligands L; wherein the coordination number of the metal ions M is 4; and wherein the MOF has an atomic ratio of coordinating nitrogen to metal of at most 3.8, preferably at most 3.5, preferably at most 3.0, more preferably at most 2.7, for a CO2 absorption process.

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