Sorbent comprising a silica structure supporting a nitrogen containing polymer, process of preparation, and use in gas adsorption

Abstract

The invention defines a sorbent suitable for adsorbing gases having pKa greater than 5, preferably carbon dioxide, comprising a silica structure supporting a nitrogen-containing polymer (NP), the nitrogen-containing polymer (NP) having a backbone comprising nitrogen atoms, the solid adsorbent being obtainable by a process comprising the steps of: (a) providing an aqueous alkaline medium comprising, (i) the nitrogen-containing polymer (NP), (ii) a silica source, wherein the silica source is selected from the group consisting of an aqueous solution of a silicate salt and a colloidal silica sol; (b) contacting the aqueous alkaline medium of step (a) with carbon dioxide to form a mixture, the mixture optionally comprising gel, wherein the aqueous alkaline medium is contacted with carbon dioxide by feeding a stream of carbon dioxide into the aqueous alkaline medium, or the source of carbon dioxide is in the form of a material that will release carbon dioxide under the reaction conditions of the process; (c) ageing the mixture of step (b) to produce an aged mixture comprising gel; (d) subjecting the aged mixture of step (c) to a drying step in order to remove at least some of the water from the aged gel to produce the sorbent. The invention is also directed to a process for producing the sorbent and the use of the sorbent for capturing gases having a pKa greater than 5, preferably carbon dioxide, typically from a mixture of gases, for instance by direct air capture or by other means.

Description

[0001] SORBENT COMPRISING A SILICA STRUCTURE SUPPORTING A NITROGEN CONTAINING POLYMER, PROCESS OF PREPARATION, AND USE IN GAS ADSORPTION

[0002] Field of the Invention

[0003] The invention relates to a sorbent useful for absorbing carbon dioxide, the sorbent comprising a silica structure which supports a nitrogen-containing polymer (NP). This nitrogen-containing polymer (NP) has a backbone containing nitrogen atoms and may for instance be a polyalkyleneimine or a polymer containing di- or oligomeric amines bridged together by a suitable bridging compound (BC). The invention involves forming the silica structure in the presence of the nitrogen-containing polymer (NP). The sorbent is effective for capturing of acidic gases, typically having a pKa greater than 5, preferably carbon dioxide, typically from a mixture of gases, for instance by direct air capture or by other means.

[0004] Background of the Invention

[0005] The increasing levels of greenhouse gases in the atmosphere is of growing global concern in view of the predicted impact on climate change. This is particularly so in view of the rising levels of carbon dioxide. It is widely accepted that even the present concentration of carbon dioxide in atmospheric air is responsible for increasing dramatic environmental changes, including droughts, flooding and disruption of ecosystems around the world. It is predicted that as carbon dioxide levels continue to rise that we are predicted see significant increases in the average temperatures of the atmosphere and oceans leading to increasing melting of polar and glacier ice which in turn would bring about rising sea levels with the inevitable flooding of low- lying lands. Increased atmospheric temperatures are also expected to increase the likelihood of powerful cyclonic storms globally.

[0006] The governments of many countries are aiming to act by legislating with the aim of reducing emissions of greenhouse gases, particularly carbon dioxide, and ultimately limit global warming. Many nations have adopted The Paris Agreement which is a legally binding international treaty on climate change. Its goal is to limit global warming to well below 2, preferably to 1 ,5°C, compared to preindustrial levels. In recent years there has been a great deal of effort placed in developing technologies that can achieve the goal of reducing carbon dioxide levels from atmospheric air and / or gaseous emissions. Capturing carbon dioxide at source is generally regarded as the most cost-effective. Typically, this could be large carbonbased energy facilities, natural gas processing, synthetic fuel plants, industries with major carbon dioxide emissions, for instance steelmaking and cement production, and hydrogen production plants which employ fossil fuels.

[0007] One dominant carbon capture technology involves absorption or sequestering of the carbon dioxide. By far the most common active compounds used for absorbing carbon dioxide has relied on amine chemistry. Typical amines used for this purpose include alkanolamines, including monoethanolamine, diethanolamine, diisopropanolamine, pentaethylenehexamine, tetraethylenepentamine, triethylenetetramine, tetraethylenetetramine, bis (2-hydroxypropyl) amine, N,N’-bis (2-hydroxy ethyl) ethylene diamine, alkyl amines, methyl amine, linear polyethyleneimine, branched polyethyleneimine, dimethyl amine, diethyl amine, methyl diethanolamine, methyl ethanol amine, polyethylene polyamine, diethylene tri-amine, N,N’-bis-(3-aminopropyl) ethylene diamine.

[0008] US Patent No 9,084,960 B2 discloses a method for reducing the CO2 content of a gas and employs CO2 capture agents that may include mono amines (in particular secondary amines, such as diethanolamine), polyamines, monoguanidines and polyguanidines and mixtures of these compounds.

[0009] US Patent No 9,533,250 B2 concerns CO2 reduction from indoor air from an enclosed space. The reference describes an amine-based compound, and it is suggested that the amine-based compound may comprise any suitable amine, such as a primary or secondary coming, or a combination thereof. The disclosure reveals that the amine-based compound may range from simple single molecules, such as ethanolamine, to large molecule amine polymers such as polyethyleneimine. Suggested in the document are monoethanolamine, ethanol amine, methylamine, branched polyethyleneimine, linear polyethyleneimine, diethanolamine, dimethylamine, diethylamine, diisopropanolamine, tetraethylenepentamine, methyldiethanolamine, methylethanolamine, and any of several polyamines such as polyethyleneimine, or a combination thereof.

[0010] US Patent No 11 ,229,897 B2 discloses a gas absorbing material that includes a polyamine produced using a process that is free of formaldehyde as a reaction product and / or a reactant. The disclosure describes producing reaction solution of a first amine compound and a reactant. The reactant is said to comprise a carbonate ester compound or a ketone compound. The first amine compound would react with the reactant to produce a second amine compound.

[0011] US Patent No 10,010,861 B2 describes a polymeric amine in the context of absorbing carbon dioxide. The polymeric amine is said to consist of a polymer skeleton containing nitrogen atoms and branched chains bonded to the nitrogen atoms of the polymer skeleton. Each of the branched chains contains at least one nitrogen and the polymeric amine is modified by substitution of at least one of the nitrogen atoms of the polymer skeleton or the branched chains with a hydroxyl group containing carbon chain. Example 1 describes the synthesis of polyethyleneimines modified by a partial substitution with butylene oxide. This synthesis involves dissolving a polyethyleneimine (MN = 1200, 19 mmol N / g) in methanol. The disclosure reveals adding the butylene oxide to the polyethyleneimine / methanol solution in different amounts such that the mole ratio of the butylene oxide to nitrogen atoms present in the polyethyleneimine were 0.15:1 , 0.37:1 , and 0.54:1. The disclosure reveals removing the solvent by subjecting the solutions of the modified polyethyleneimine to heating in a vacuum oven.

[0012] US Patent No 10,751 ,689 B2 discloses a modified polyamine in the context of absorbing carbon dioxide. The modified polyamine is the reaction product of an amine and an epoxide. The amines described are relatively low molecular weight amines such as pentaethylenehexamine (PEHA) and tetraethylenepentamine (TEPA) and such amines are known as oligoamines. Example 1 reveals preparing a modified polyamine species based on pentaethylenehexamine (PEHA) and propylene oxide (PO). The preparation describes dissolving 10 g of the PEHA in 40 mL of water and adding 5 g of PO to the PEHA solution followed by stirring for 20 hours at room temperature. The temperature of the reaction mixture was said to be raised progressively to 60°C which was maintained for two hours. The water was said to be removed by rotary evaporator followed by overnight vacuum at below 1 mmHg.

[0013] Polyethyleneimines (PEI) and other polyalkyleneimines, such as polypropyleneimines (PPI), are known for their superior performance with respect to stability as CO2 adsorbents in DAC and CO2 capture in post-combustion processes and from CO2 point sources. These sources are typically present in three areas, namely fuel combustion activities, industrial processes and natural gas processing.

[0014] It is known that linear PEI and linear PPI are better than branched PEI and branched PPI and this is believed to be as a result of the relatively high content of secondary amino groups in the linear products compared to the corresponding branched products. It is known that a low fraction of primary NH functionality is more beneficial for absorbing CO2. This can be improved by alkoxylation of the PEI or PPI by alkoxylation, but such improvement tends to be limited.

[0015] An article entitled, "Stable ordered mesoporous silica materials templated by high- temperature stable surfactant micelle in alkaline media", by Xiaoyu Yang et al., J. Phys. Chem. B, Vol. 108, No. 15, 2004, pp 4696 - 4700, describes synthesis of ordered hexagonal mesoporous silica material (JLll-30) in alkaline media at high temperature (>160°C), using cationic (1 ,3-dimethyl-2-imidazolidin-2-ylidene- hexadecylmethyl ammonium bromide (DIHAB) as a template. The article does not mention CO2 capturing.

[0016] An article entitled, "High-performance polyethyleneimine / sodium silicate material: one-step strategy at ambient temperature and application in removing heavy metal ion", by Airong Xu et al., J Mater Sci (2022) 57:4221-4238, describes polyethyleneimine (PEI) composites containing silicon which showed potential for scavenging heavy metal ions. The PEI composite is said to exhibit enhanced elasticities and flexibility by the inclusion of Na2SiO3. A description of preparing the composite is given involving forming a mixture of PEI (Mw 10,000, 99%) and Na2SiOs to which was added poly (ethylene glycol) diglycidyl ether (PEGDE) (Mn 500). The mixture was stirred for 20 minutes and then transferred to a glass mould and then it was allowed to self-polymerise over about 30 minutes at ambient temperatures to obtain a wet sample. The wet sample was rinsed 3 to 4 times with deionised water to wash off residual unreacted starting materials and the resulting sample was freeze-dried to obtain a dry sample. The article described experiments where the PEI silicon composites were used to absorb Cu2+ions. The article does not mention CO2 capturing.

[0017] The article entitled, "One step process using CO2 for preparation of amino functionalised mesoporous silica for CO2 capture application", by Dang Viet Quang et al., ACS Sustainable Chem. Eng. 2017, 5, pp 3170 - 3178, describes a one-step method for preparing amino functionalised mesoporous silica for CO2 capture by bubbling CO2 with 3-aminopropyl triethoxy silane (APTES, 30 wt % in water) and then mixing with sodium silicate. Three absorbents, namely what precipitated silica absorbent (PSA), silica gel absorbent (SGA), and condense silica gel absorbent (C- SGA) was synthesised by tuning CO2 pressure when preparing CO2 absorbed AP TES solution and ageing temperatures of the obtained hydrogel.

[0018] Sushilkumar A Jadhav et al. describes in an article entitled, "Porous silica particles: Synthesis, physio chemical characterisation and evaluation of suspension stability", Physical Chemistry: An Indian Journal - May 2017; S1 : S102 (ISSN: 0974-7524) describes the synthesis and / or physicochemical characterisation of five important types of porous silica particles with different characteristic properties.

[0019] The article by Qianxi Fang et al., entitled, "Role of additives in silica supported polyethyleneimine absorbents for CO2 absorption, Mater. Res. Express 7 (2020) 303526, describes SiO2 aerogels prepared by sol-gel method and methyl alkylation reaction using TEOS as silicon sources. Silica supported polyethyleneimine (PEI / SiO2) adsorbents for CO2 adsorption were prepared by grafting techniques, in which mesoporous silica aerogel was loaded with PEI organic compounds functionalised by hydroxyl or amino groups as dopants. CO2 adsorption isotherm is of PEI amine modified silica aerogels were measured.

[0020] Traditional solid CO2 adsorbents containing amine polymers, based on silica, involve providing a silica with a specific pore volume and pore diameter and then subsequently loading the silica with liquid amine polymer adsorbents, for instance as solutions with organic solvents or water. However, such technology is limited to about 55% loading, but the efficiency of such sorbents tends to diminish once the loading exceeds 45%, based on mmol CO2 per mol of NH. The inventors believe that this disadvantage is probably as a result of the pores becoming overloaded. In addition, this can be exacerbated during CO2 adsorption by the CO2 reacting with the amino groups which can have an effect on the size of the system. It would be desirable to provide silica-based CO2 sorbents which overcome this problem.

[0021] A further objective of the present invention is to provide effective silica-based adsorbents containing amine-based polymers for carbon dioxide capturing and employs a convenient route to said silica-based sorbents which avoids sophisticated and / or expensive silica compounds for building such silica products.

[0022] Summary of the Invention

[0023] A first aspect of the present invention defines a sorbent suitable for adsorbing a gas having a pKa greater than 5, preferably carbon dioxide, comprising a silica structure supporting a nitrogen-containing polymer (NP), the nitrogen-containing polymer (NP) having a backbone comprising nitrogen atoms, the solid adsorbent being obtainable by a process comprising the steps of:

[0024] (a) providing an aqueous alkaline medium comprising,

[0025] (i) the nitrogen-containing polymer (NP),

[0026] (ii) a silica source, wherein the silica source is selected from the group consisting of an aqueous solution of a silicate salt or a colloidal silica sol;

[0027] (b) contacting the aqueous alkaline medium of step (a) with carbon dioxide to form a mixture, the mixture optionally comprising gel, wherein the aqueous alkaline medium is contacted with carbon dioxide by feeding a stream of carbon dioxide into the aqueous alkaline medium, or the source of carbon dioxide is in the form of a material that will release carbon dioxide under the reaction conditions of the process;

[0028] (c) ageing the mixture of step (b) to produce an aged mixture comprising gel; (d) subjecting the aged mixture of step (c) to a drying step in order to remove at least some of the water from the aged mixture to produce the sorbent.

[0029] A further aspect of the present invention relates to a process for preparing the sorbent, the process comprising the steps of:

[0030] (a) providing an aqueous alkaline medium comprising,

[0031] (i) a nitrogen-containing polymer (NP), the nitrogen-containing polymer (NP) having a backbone comprising nitrogen atoms,

[0032] (ii) a silica source, wherein the silica source is selected from the group consisting of an aqueous solution of a silicate salt or a colloidal silica sol;

[0033] (b) contacting the aqueous alkaline medium of step (a) with carbon dioxide to form a mixture, the mixture optionally comprising gel, wherein the aqueous alkaline medium is contacted with carbon dioxide by feeding a stream of carbon dioxide into the aqueous alkaline medium, or the source of carbon dioxide is in the form of a material that will release carbon dioxide under the reaction conditions of the process;

[0034] (c) ageing the mixture of step (b) to produce an aged mixture comprising gel;

[0035] (d) subjecting the aged mixture of step (c) to a drying step in order to remove at least some of the water from the aged mixture to produce the sorbent.

[0036] Another aspect of the present invention relates to the use of the sorbent for absorbing a gas having a pKa greater than 5, preferably carbon dioxide.

[0037] A still further aspect of the present invention relates to a process for reducing the concentration of gas with pKa greater than 5, preferably carbon dioxide, present in a gas mixture produced as a gas stream from a combustion process, comprising the steps of

[0038] (a) passing the gas stream containing the gas with pKa greater than 5 at an initial concentration (C1) through a gas scrubbing stage, comprising contacting the gas stream with an amine, preferably comprising an alkanolamine, to reduce the concentration of the gas with pKa greater than 5 to an intermediate concentration (C2) which is below the initial concentration (C1 ), (b) passing the gas stream resulting from step (a) through a purifying stage, comprising contacting the gas stream with the sorbent defined according to first aspect of the present invention (including its preferred embodiments) or prepared by the process defined by the further aspect of the present invention (including its preferred embodiments) thereby reducing the concentration of the gas with pKa greater than 5 in the gas stream to a final concentration (C3) which is below the intermediate concentration (C2).

[0039] Detailed Description of the Invention

[0040] The sorbent according to the present invention comprises A) silica and B) the nitrogen-containing polymer (NP). Suitably the silica should be present in the sorbent in an amount from 20 to 60% by weight, based on the dry weight of the sorbent. Preferably the sorbent should comprise silica in an amount from 30 to 60% by weight, based on the dry weight of the sorbent. The amount of nitrogen-containing polymer (NP) comprised in the sorbent may be from 40 to 80% by weight, based on the dry weight of the sorbent. In a preferred form, the sorbent comprises from 40 to 70% by weight, based on the dry weight of the sorbent.

[0041] Thus, in a preferred embodiment the sorbent comprises:

[0042] A) silica in an amount from 20 to 60% by weight based on the dry weight of the sorbent, preferably from 30 to 60%, more preferably from 30 to 50%; and

[0043] B) the nitrogen-containing polymer (NP) in an amount from 40 to 80% by weight based on the dry weight of the sorbent, preferably from 40 to 70%, more preferably from 50 to 70%.

[0044] The nitrogen-containing polymer (NP) may be any suitable nitrogen-containing polymer (NP) having a backbone comprising nitrogen atoms. Generally, the backbone of the nitrogen-containing polymer (NP) would comprise organic repeating units containing at least one nitrogen atom. The nitrogen-containing polymer (NP) may be linear, or it may be structured, for instance branched. A preferred class of suitable nitrogen-containing polymers (NP) includes polyalkyleneimines. The polyalkyleneimines may be linear or branched. In particular, for the branched polyalkyleneimine, branching may occur at its nitrogen fractions. Linear polyalkyleneimines are composed exclusively of repeat units of formula A; branched polyalkyleneimines have, besides the linear repeat units, tertiary nitrogen atoms according to the formula B

[0045] A B in which Q may be C2-C8 alkylene, suitably ethylene, propylene or butylene and * represents the connection point to an adjacent repeating unit or a terminal group, for instance a hydrogen atom when directly bonded to a nitrogen atom or -NH2 when bonded to Q. Preferably the polyalkyleneimine is a polyethyleneimine.

[0046] Polyalkyleneimines, such as polyethyleneimine, are commercially available or can be made by various methods understood in the art. For example, polyethyleneimine can be made by a ring opening reaction of aziridine by acid catalysed polymerisation.

[0047] Preference is given to polyalkyleneimines, especially polyethyleneimines, having a degree of branching (DB) of more than 50 and preferably more than 60. Polyalkyleneimines, including polyethyleneimines, can be characterised by their degree of branching (DB). To define the degree of branching, reference is made to H. Frey et al., Acata Polym. 1997, 48, 30 it the degree of branching DB is defined as

[0048] DB (%) = (T+Z) / (T+Z+L) x 100, where

[0049] T is the average number of terminally bound monomeric units (primary amino groups),

[0050] Z is the average number of branching monomeric units (tertiary amino groups), L is the average number of linearly bound monomeric units (secondary amino groups). T, Z, and L can be determined via13C-NMR in D2O. The degree of branching DB of the polyalkyleneimines, especially polyethyleneimines, according to the present invention is preferably in the range from 55 to 95%, preferably in the range from 57 to 90% and more preferably in the range from 60 to 80%.

[0051] Preferably, the polyalkyleneimine is branched and more preferably the polyalkyleneimine is a branched polyethyleneimine. Branched polyethyleneimines may be represented by the following exemplary structure: wherein * represents the remainder of the polyethyleneimine molecule.

[0052] The polyalkyleneimine and polyethyleneimine mentioned in the foregoing text desirably may be an alkoxylated polyalkyleneimine and alkoxylated polyethyleneimine respectively.

[0053] Another suitable type of nitrogen-containing polymer (NP) includes the reaction product of

[0054] (i) (A) a di- or oligoamine or a polyamine with

[0055] (ii) (ii) a bridging compound (BC), the bridging compound (BC) comprising at least one amine reactive group (ARG), wherein the bridging compound (BC) is capable of bonding to amine groups of at least two di- or oligoamine (A) molecules. This type of nitrogen-containing polymer (NP) may be linear or structured, for instance having branching or even a degree of cross-linking.

[0056] One suitable bridging compound (BC) includes phosgene. Phosgene will react with a primary amine group of the di- or oligoamine or polyamine (A) to form an isocyanate group which will further react with amine group of another molecule of the di- or oligoamine or polyamine (A), thereby forming the breach between molecules of the di- or oligoamine or polyamine (A). Typically, such amine reactive groups (ARG) include functional groups, such as epoxide, isocyanate, blocked isocyanate, ester and acid anhydride. It is preferred that the bridging compound (BC) comprises at least two such amine reactive groups (ARG).

[0057] The bridging compound (BC) may be the reaction product of a di- or polyol with epichlorohydrin. Such reaction product will contain glycidyl groups bonded to the di- or polyol radical by ether linkage, typically in place of the hydroxyl groups.

[0058] Preferably the bridging compound (BC) comprises at least two epoxide groups and at least two epoxide groups as amine reactive groups (ARG) will readily react with the amine groups, preferably the primary amine groups, of the di- or oligoamine or polyamine (A) to form amino alcohol linkages between the di- or oligoamine or polyamine (A). More preferably each epoxide is part of a glycidyl group and more preferably still the at least two glycidyl ether groups include diglycidyl ether, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, 1 ,4-butane diol diglycidyl ether, neopentyl diglycidyl ether, 1 ,6-hexane diol diglycidyl ether, trimethylol propane triglycidyl ether, 5,5-dimethyl-3,7-dioxa-1 ,9(2)-bis (oxirana)-4, 6(2,4)- dibenzenanonaphane.

[0059] Other suitable bridging compound includes diisocyanates which can readily react with amine groups of two di- or oligoamine or polyamine (A) molecules and link them together by a urea linkage. Examples of suitable diisocyanates include methylene diphenyl diisocyanate (MDI), toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), methylene dicyclohexyl diisocyanate or hydrogenated MDI (HMDI) and isophorone (IPDI). Suitable di- or oligoamines (A) desirably have at least 2 amino groups, for instance 2-8 amino groups; and have at least one carbon atom per nitrogen atom, preferably 1-5 carbon atoms per nitrogen atom; and comprise at least two primary and / or secondary amino groups, preferably at least two primary amino groups; and has a molecular weight (Mw) that lies in the range from 50 to 500 g / mol, preferably from 60 to 300 g / mol.

[0060] Preferred di- or oligoamines (A) are selected from the group consisting of ethylene diamine, hexamethylene diamine, methylcyclohexane diamine (MCDA), propylene diamine (PDA), tetra methylene diamine (TMDA), pentamethylenediamine (PMDA), dipropylenetriamine (DPTA), tripropylenetetramine (TPTA), tris (2-amino ethyl) amine (TAEA), tetra propylene pentamine (TPPA), N,N'-Bis-(3-aminopropyl)- ethylenediamine (N4-amine), 3-(2-aminoethylamino)propylamine (N3-amine), spermine, spermidine, triamino nonane, diethylentriamin (DETA), triethylentetramine (TETA), tetraethylenpentamine (TEPA), pentaethylenhexamine (PEHA).

[0061] Suitable polyamines (A) desirably have at least 7 amino groups, for instance from 2 to 23 amino groups; and at least one carbon atom per nitrogen atom, preferably from 1 to 5 carbon atoms per nitrogen atom; and comprise at least two primary and / or secondary amino, preferably at least two primary groups. Each amino group would be separated by an alkylene group, typically having between 2 and 8 carbon atoms. The alkylene groups may each be of the same carbon chain length or alternatively may have different carbon chain lengths. Polyamines (A) may be linear or branched. Suitable polyamines have a weight average molecular weight (Mw) of up to 1000 g / mol. Preferred polyamines (A) have a weight average molecular weight (Mw) in the range from 300 to 1000 g / mol, desirably from 500 to 800 g / mol.

[0062] Suitable polyamines (A) include compounds containing molecules with repeating alkylene amine units, for instance having between 6 and 22 repeating alkylene amine units. Such polyamines (A) may be represented by molecules having one or both repeating units C or D:

[0063] C D where

[0064] X is an alkylene unit having from 2 to 8 carbon atoms, n and m are each independently integers between 0 and 22 provided that the sum of n and m are in the range from 6 to 22.

[0065] Each * represents the connecting point to an adjacent repeating unit or a terminal group, typically a hydrogen atom directly bonded to a nitrogen atom or -NH2 when directly bonded to X.

[0066] Examples of polyamines (A) include hexaethyleneheptamine, heptaethyleneoctamine, octaethylenenonaamine.

[0067] The nitrogen-containing polymer (NP) is preferably alkoxylated. Such alkoxylated polymer may be an alkoxylated reaction product of di- or oligoamine or polyamine (A) with a bridging compound (BC). Preferably, however, the alkoxylated nitrogencontaining polymer (NP) is an alkoxylated polyalkyleneimine, and more preferably is an alkoxylated polyethyleneimine.

[0068] Alkoxylation of the nitrogen-containing polymer (NP) is normally achieved by reacting an alkylene oxide (AO). The alkylene oxide (AO) reacts with an NH group of the nitrogen-containing polymer (NP) to provide an alkoxylated substituent bonded to the nitrogen atom. Preferably the alkoxylated containing polymer (NP), suitably alkoxylated polyalkyleneimine, preferably alkoxylated polyethyleneimine, has been alkoxylated using an alkylene oxide selected from at least one C3-Ci2-alkylene oxide. The least one of C3-Ci2-alkylene oxide may be a mixture of different C3-Ci2-alkylene oxides, for instance including C3 and / or C4 alkylene oxides with higher alkylene oxides, for instance at least one of C8-C12 alkylene oxides, such as 1 ,2-octylene oxide, 1 ,2-decylene oxide or 1 ,2-dodecylene oxide. When the mixture of alkylene oxides is a mixture of C3-C4 alkylene oxides and C8-C12 alkylene oxides, the ratio of C3-C4 alkylene oxides to C8-C12 alkylene oxides may lie in the range from 2:1 to 20:1 , more desirably from 5:1 to 10:1 . Preferably mixtures of alkylene oxides should comprise propylene oxide and / or butylene oxide. Throughout this specification the reference to butylene oxide means 1 ,2-butylene oxide. More preferably at least one alkylene oxide should be propylene oxide, butylene oxide or a mixture of propylene oxide and butylene oxide.

[0069] The alkoxylated nitrogen-containing polymer (NP), preferably alkoxylated polyalkyleneimine, more preferably alkoxylated polyethyleneimine, desirably has a molar ratio of alkylene oxide (AO) to NH functionality of the nitrogen containing polymer (NP), preferably alkoxylated polyalkyleneimine, more preferably alkoxylated polyethyleneimine, of up to 0.35, for instance from 0.1 to 0.35. Preferably the molar ratio of alkylene oxide (AO) to NH functionality is in the range from 0.15 to 0.32, more preferably from 0.18 to 0.3.

[0070] NH represents the amine number and is calculated by determination of the secondary amino groups and primary amino groups, where NH = (number of secondary amino groups) + (2 x (number of primary amino groups)). NH is determined by titration of the respective nitrogen-containing polymer (NP) with trifluoromethansulphonic acid.

[0071] In a typical representation, the alkoxylated nitrogen-containing polymer (NP), as illustrated below as a branched alkoxylated polyethyleneimine:

[0072] where AB represents one alkylene oxide side chain and * represents the remainder of the polyethyleneimine molecule.

[0073] The weight average molecular weight (MW) of the nitrogen-containing polymer (NP), suitably polyalkyleneimine, preferably polyethyleneimine, suitably lies in the range from 300 to 10,000 g / mol, suitably from 500 to 10,000 g / mol, desirably from 500 to 5000 g / mol, more desirably from 500 to 2500 g / mol, preferably from 500 to 1500 g / mol. The weight average molecular weight (Mw) is defined as the weight average molecular weight (Mw) of the nitrogen-containing polymer (NP) which is not alkoxylated or the nitrogen-containing polymer (NP) prior to alkoxylation.

[0074] The weight average molecular weight (Mw) of the nitrogen-containing polymer (NP) may be determined by high-performance liquid chromatography (HPLC) using HPLC equipment that is commercially available and using known methods described in the literature.

[0075] Polyalkyleneimines, particularly as polyethyleneimines, are well known in the literature and available commercially, for instance the Lupasol® range of products available from BASF. The silica structure component of sorbent may be described as mesoporous silica, porous silica, silica gel, silica microgel. The silica structure component may comprise of individual particles or macrostructures, for instance comprising polyparticulate structures.

[0076] Sorbent according to the present invention is produced by a new process of forming the silicate structure in the presence of the nitrogen-containing polymer (NP). The fact that the inventors were able to form the silica structure around the nitrogencontaining polymer (NP) was unexpected. It was also surprising that this process led to silica-based sorbents containing a higher loading of the nitrogen-containing polymer (NP). Furthermore, the inventors found that the sorbent produced by this route was particularly suitable for capturing acidic gases, typically having pKa greater than 5, preferably carbon dioxide.

[0077] The process involves the steps of:

[0078] (a) providing an aqueous alkaline medium comprising,

[0079] (i) the nitrogen-containing polymer (NP),

[0080] (ii) a silica source, wherein the silica source is selected from the group consisting of an aqueous solution of a silicate salt and a colloidal silica sol;

[0081] (b) contacting the aqueous alkaline medium step (a) with carbon dioxide to form a mixture, the mixture optionally comprising gel, wherein the aqueous alkaline medium is contacted with carbon dioxide by feeding a stream of carbon dioxide into the aqueous alkaline medium, or the source of carbon dioxide is in the form of a material that will release carbon dioxide under the reaction conditions of the process;

[0082] (c) ageing the mixture of step (b) to produce an aged mixture comprising gel;

[0083] (d) subjecting the aged mixture of step (c) to a drying step in order to remove at least some of the water from the aged mixture to produce the sorbent.

[0084] The aqueous alkaline medium in step (a) may be provided by combining the nitrogen-containing polymer (NP), suitably polyalkyleneimine, more suitably an alkoxylated polyalkyleneimine, preferably polyethyleneimine, more preferably an alkoxylated polyethyleneimine, with an aqueous alkali silicate solution and optionally diluting the combined aqueous solutions. The aqueous alkali silicate solution is typically a solution of an alkali metal silicate or ammonium silicate, desirably selected from the group consisting of potassium silicate, sodium silicate and ammonium silicate, preferably sodium silicate. Silicates such as sodium silicate and potassium silicate are available commercially, for instance from the CIECH Group, for instance under the Vitrosi I® name.

[0085] The aqueous alkaline medium may instead be formed by combining a silica sol and aqueous alkali, for instance ammonium hydroxide or an alkali metal hydroxide, such as sodium hydroxide or potassium hydroxide and combining with the nitrogencontaining polymer (NP). Silica sols are commercially available, for instance from WR Grace & Co under the name Ludox®.

[0086] In general, it may be desirable to adjust the pH when forming the aqueous alkaline medium by including alkali such as sodium hydroxide, potassium hydroxide or ammonium hydroxide. This may have the advantage of ensuring the pH of the aqueous alkaline medium it is sufficiently high to prevent any premature gelation of the silicate.

[0087] The aqueous alkaline medium of step (a) would typically be at a pH of greater than 10, for instance at least 10.5, for instance at least 11.

[0088] The nitrogen-containing polymer (NP), suitably polyalkyleneimine, more suitably an alkoxylated polyalkyleneimine, preferably polyethyleneimine, more preferably an alkoxylated polyethyleneimine, may be present in the aqueous alkaline medium of step (a) in any suitable amount depending upon the level of loading required in the sorbent. Preferably the aqueous alkaline medium of step (a) comprises the nitrogencontaining polymer (NP), suitably polyalkyleneimine, more suitably an alkoxylated polyalkyleneimine, preferably polyethyleneimine, more preferably an alkoxylated polyethyleneimine, in an amount from 5 wt.% to 60 wt.%, based on the dry weight of the nitrogen-containing polymer (NP) on the weight of the aqueous solution, such as from 10 wt.% to 50 wt.%, for instance from 10 wt.% to 40 wt.%, for example from 10 wt. % to 30 wt. %. A loading of greater than 50 wt.% is more easily achievable when the nitrogen-containing polymer (NP) is an alkoxylated polyalkyleneimine, especially an alkoxylated polyethyleneimine.

[0089] The aqueous alkaline medium of step (a) may contain the silicate in any suitable amount. Preferably the silicate may be present in the aqueous alkaline medium in an amount from 1 wt.% to 20 wt.%, based on the dry weight of the silicate on the dry weight of the aqueous solution, preferably from 5 wt.% to 15 wt.%, for instance from 5 wt. % to 10 wt. %.

[0090] The level of loading of nitrogen-containing polymer (NP), suitably polyalkyleneimine, more suitably an alkoxylated polyalkyleneimine, preferably polyethyleneimine, more preferably an alkoxylated polyethyleneimine, on the sorbent will to some extent depend on the relative amounts of the nitrogen-containing polymer (NP) to the silicate employed in forming the sorbent. Typical dry weight ratios of nitrogencontaining polymer (NP) to silicate employed in the aqueous alkaline medium may range from 0.8: 1 to 3: 1 , for instance from 1 :1 to 2: 1.

[0091] In step (b) carbon dioxide is brought into contact with the aqueous alkaline medium by feeding a stream of carbon dioxide into the aqueous alkaline medium, or the source of carbon dioxide is in the form of a material that will release carbon dioxide under the reaction conditions of the process. Preferably this is achieved by feeding a stream of carbon dioxide into the aqueous alkaline medium. As the carbon dioxide passes through the medium it serves to reduce the pH of the aqueous medium at a controllable rate. The flow rates of the carbon dioxide feed stream may be controlled volumetrically (typically within the pH range of 6 to 10) due to the buffering effect of the resulting carbonates that would form in situ. Volumetric control offers the advantage of avoiding pH sensors, which may require frequent cleaning, calibration and replacement.

[0092] The carbon dioxide, typically as a feed stream, is desirably passed through the aqueous alkaline medium at a rate from 0.005 to 0.15, preferably from 0.01 to 0.06, litres of carbon dioxide per kg of dry silica source, preferably the silicate salt, contained in the aqueous alkaline medium over a period of 1 to 5 hours, the volume of carbon dioxide based on a temperature of 25°C. As the stream of carbon dioxide is fed into the aqueous alkaline medium, the carbon dioxide gas would pass through the medium, typically forming bubbles or a stream of bubbles. It may also be desirable to feed the carbon dioxide gas in a number of different places throughout the aqueous alkaline medium. In this case, the stream of carbon dioxide would comprise a multiplicity of streams, for instance two or more streams that are fed into and for streams of gas, typically as bubbles, through the aqueous medium.

[0093] Although it is preferred to feed carbon dioxide as free carbon dioxide in the form of a gas or liquid in a feed stream, the source of carbon dioxide may be in the form of a material that will release carbon dioxide under reaction conditions, for example, sodium bicarbonate. Other components, liquids or gases may also be present in the carbon dioxide feed stream.

[0094] The carbon dioxide may be regarded as a silica gelation initiator. Additional gel initiators may be added, for example, aluminium compounds, especially if it is desirable for the silica structure of the sorbent to be aluminosilicate or polyaluminosilicate and the like, which may serve to lower the pH of the aqueous alkaline medium. Other gel initiators may also be fed into the aqueous alkaline medium, either with the carbon dioxide, or as a separate addition. These include for example, inorganic and organic acids, such as sulphuric and acetic acids, acid salts, such as borax, sodium bisulphite, ammonium sulphate, alkali metal salts of amphoteric metal acids, such as aluminate and certain organic compounds, such as organic anhydrides, amides and esters. Preferably, however, the process should be carried out in the absence of strong mineral acids which depending on the amounts added could lower the pH to quickly and risk a chaotic gel formation. Normally, it would not be necessary to add any other gel initiators.

[0095] As the carbon dioxide contacts the aqueous alkaline medium, typically by feeding a stream of carbon dioxide into the medium, and the pH of the medium reduces to be below pH 10 some gel formation can be expected. During this step (b) the pH of the medium may be in the range from 6 to 10, preferably from 6.5 to 7.5. This is based on the use of carbon dioxide in the absence of additional gel initiators. The operable pH may be different where additional gel initiators are used. On completion of step (b) it is preferred that the resulting mixture comprises gel. Normally a significant amount of gelation occurs during this step.

[0096] The gel may be regarded as an amorphous colloid comprising a three-dimensional network formed from silica molecules or colloidal silica fine particles with a continuous aqueous medium throughout. The gel may appear and behave more as a solid than a liquid. The gel formation during the process of the present invention may comprise numerous gel entities each individually comprising the aforementioned three-dimensional network and typically behaving as individual solids.

[0097] It may be desirable to carry out step (b) under pressure, typically by carrying out the step in a pressurised vessel. Pressures of up to 1380 kPa may be employed, for instance from 110 to 1380 kPa, such as 130 to 800 kPa, for example from 170 to 500 kPa.

[0098] On completion of step (b) it is important that the so formed mixture is aged for a time sufficient to achieve gelation or further gelation. The aged mixture will comprise gel, generally comprising three-dimensional networks and chains of high surface area silica particles.

[0099] The length of time required for the ageing step (c) generally depends on factors such as the solids content of the aqueous alkaline medium of step (a), particularly the solids content of the silica source, the pH of the aqueous mixture at the end of step (b), the presence or absence of other gelation initiators, and the amount, if any, of gelation that has already occurred in step (b). Preferably the ageing step (c) is over a period from 1 to 48 hours. Suitably the ageing step (c) may be from 2 to 36 hours, for instance from 3 to 30 hours, such as from 6 to 24 hours, for example from 10 to 18 hours.

[0100] Ageing may be conducted at temperatures in the range of 0°C to 50°C but preferably at temperatures of from 10°C to 30°C, suitably from 15°C to 25°C. Ageing would usually be carried out at normal atmospheric pressures. However, in some cases it may be desirable to employ reduced pressures or increased pressures. When high pressures are desired, it may be suitable to use pressures as high as 200 kPa, for instance from above 100 to 200 kPa, such as above 100 to 150 kPa. Alternatively, if low pressures are desired, it may be suitable to employ pressures as low as 20 kPa, such as from 20 to below 100 kPa, such as 15 to below 100 kPa. Generally, the pressure employed will be from 80 to 120 kPa, suitably from 90 to 110 kPa and usually at atmospheric pressure.

[0101] Subsequent to the ageing step (c), the aged mixture would then be subjected to a drying step (d) to remove at least some of the water from the aged mixture. Generally, the drying step can typically be carried out at elevated temperatures, for instance at temperatures of at least 80°C. Preferably the drying step may be carried out at temperatures from 90°C to 140°C. It is also preferable to use reduced pressure in combination with the elevated temperatures, for instance reduced pressure of from 50 to 5 mbar, preferably from 300 to 10 mbar. A more preferably the drying conditions comprise temperatures of at least 80°C, preferably from 90°C to 140°C and reduced pressure of from 300 to 10 mbar. The drying step may reduce the moisture content to produce essentially dry sorbent particles. The moisture content of the sorbent may be below 1 .7% by weight of the sorbent, for instance from 0.1 wt.% to 1.5 wt.%. Alternatively, higher moisture contents may be desired, for instance up to 5 wt.% or higher, for instance from 3 wt.% to 5 wt.%. It may be desirable to subject the sorbent to a treatment of an inert gas such as nitrogen and / or elevated temperature to remove any residual moisture, for instance not removed during the drying step and / or taken on during storage of the sorbent, prior to using the sorbent.

[0102] The drying step results in the sorbent according to the present invention. The sorbent may take on various solid forms from small particulate structures to microstructures. Typically, the sorbent is solid and particulate i.e in the form of particles, for instance in the form of a powder. Examples of suitable drying equipment include, for instance a Taumeltrockner, which is an industrial -sized tumble dryer suitable for this application, for instance as indicated in the webpage: https : / / www.schuettgut-portal.com / fachzentrum / taumeltrockner-(3778).html. Other suitable drying equipment includes mixing and drying equipment, for instance as described in the webpage: http : H pdf.directindustry.de / pdf / j-engelsmann- ag / broschuere-doppeldonustrockner-taumeltrockner / 161911-675657. html

[0103] Another suitable type of drying equipment includes horizontal vacuum contact dryers which employ shafts mounted on the terminal ends of the equipment. Further suitable drying equipment may include vertical dryers, for instance having a conical bottom and one or more baffles that prevent co-rotating of the product to ensure homogenous drying. Both of these types of drying equipment are described in the webpage: https : / / prozesstechnik.industrie.de / food / verfahren-mechanisch-food / von- pastoes-bis-rieselfaehig /

[0104] Any of these pieces of equipment would be suitable for such drying operations for making the sorbent according to the present invention. However, the above- mentioned vertical dryer may be particularly suitable for drying the product of step (c) containing the silica material formed around the nitrogen-containing polymer (NP) which can be of a viscous liquid consistency.

[0105] In addition, it may be desirable to include an aqueous solution of a polyamine into the product of step (c) during the drying process to improve mixing.

[0106] The sorbent according to the present invention may desirably have an average particle size (D50) which lies in the range from 5 to 1000 pm, preferably from 10 to 250 pm. Desirably fine particles (fines) which represent particle size is of less than 5 pm should be kept to a minimum, typically below 10% by weight of the sorbent, normally below 5% by weight of the sorbent, generally below 1 % by weight of the sorbent.

[0107] The sorbent of the present invention is particularly suitable for adsorbing a gas having pKa greater the 5, preferably carbon dioxide. Preferably the sorbent is used for capturing carbon dioxide from a mixture of gases. More preferably the mixture of gases is either atmospheric air or any sort of exhaust fumes. Typically, the exhaust fumes may for instance be gases emitted from industrial process, including power generating plants. Additionally, the exhaust fumes may be produced from various other devices such as heat generating devices, including commercial and domestic boilers, or other devices such as motion generating devices, for instance combustion engines for vehicles.

[0108] In one preferred application the sorbent may be used for capturing carbon dioxide in direct air capture (DAC). Direct air capture (DAC) relates to technologies that extract carbon dioxide directly from the atmosphere. Such technology typically relies on employing carbon dioxide sorbent which can initially absorb carbon dioxide directly from air and then later in a controlled environment desorb the carbon dioxide where it can be further processed to produce a more permanent storage for the carbon dioxide.

[0109] The sorbent according to the present invention can be used in direct air capture (DAC) as the sorbent for initially capturing the carbon dioxide from air. The sorbent may be incorporated into articles and devices typically used for direct air capture (DAC).

[0110] The present invention also provides a process for reducing the concentration of gas with pKa greater the 5, preferably carbon dioxide, present in a gas mixture. Suitably the gas mixture is produced as a gas stream from a combustion process. Such combustion process can be any operation in which fossil fuels are combusted, typically in energy generation, especially industrial power plants and industrial processes, but could also relate to any of the aforementioned operations described above in regard to exhaust fumes. The process of treating the gas mixture may comprise the steps of

[0111] (a) passing the gas stream containing the gas with pKa greater than 5 at an initial concentration (C1) through a gas scrubbing stage, comprising contacting the gas stream with an amine, preferably comprising an alkanolamine, to reduce the concentration of the gas with pKa greater than 5 to an intermediate concentration (C2) which is below the initial concentration (C1 ),

[0112] (b) passing the gas stream resulting from step (a) through a purifying stage, comprising contacting the gas stream with the sorbent defined according to the first aspect of the present invention (including its preferred embodiments described herein) or prepared by the process defined in the further aspect of the present invention (including its preferred embodiments described herein) thereby reducing the concentration of the gas with pKa greater than 5 in the gas stream to a final concentration (C3) which is below the intermediate concentration (C2).

[0113] The amines employed in the gas scrubbing stage may be any amine customarily employed for absorbing gases with pKa greater than 5, typically carbon dioxide. Suitable amines include any of alkanolamines (including monoethanolamine, diethanolamine, diisopropanolamine, methyl diethanolamine, methyl ethanol amine), pentaethylenehexamine, tetraethylenepentamine, triethylenetetramine, tetraethylenetetramine, bis (2-hydroxypropyl) amine, N,N’-bis (2-hydroxy ethyl) ethylene diamine, alkyl amines, methyl amine, linear polyethyleneimine, branched polyethyleneimine, dimethyl amine, diethyl amine, polyethylene polyamine, diethylene tri-amine, N,N’-bis-(3-aminopropyl) ethylene diamine, monoguanidines and polyguanidines and mixtures thereof. Preferably amines employed in the gas scrubbing stage comprise alkanolamines, suitably one or more of monoethanolamine, diethanolamine, diisopropanolamine, methyl diethanolamine, methyl ethanol amine.

[0114] The gas scrubbing stage may be any conventional system employed for treating gas streams containing high levels of gases having pKa greater than 5, for instance hydrogen sulphide, carbon dioxide, or sulphur dioxide, commonly referred to as sour gases or acid gases. A suitable gas scrubbing stage may comprise a typical amine gas treating process (e.g. the Girbotol process) which would include an absorber unit comprising an amine in which gas of pKa greater than 5 would be absorbed from the gas stream and a regenerator unit in which the amine would be regenerated.

[0115] Typically, the initial concentration (C1) of gas of pKa greater than 5 in the gas stream should be greater than 2 wt.%, preferably greater than 5 wt. %. Generally, the concentration of gas of pKa greater than 5 would be much higher than this, for instance greater than 8 wt.%, suitably greater than 9 wt.%, often greater than 10 wt.%. The gas scrubbing stage in step (a) will reduce the concentration of gas having pKa greater than 5 to a lower concentration, termed intermediate concentration (C2). This gas scrubbing stage will normally remove a significant amount of the gas of pKa greater than 5. Thus, the intermediate concentration (C2) will normally be significantly below the initial concentration (C1 ). Typically, the intermediate concentration (C2) will be below 2 wt.% and normally below 1 .5 wt.%. Preferably the intermediate concentration (C2) may be in the range from 0.2 to below 1 .5 wt.%, for example from 0.5 to 1 wt.%.

[0116] The purification stage in step (b) will remove the gas having pKa greater than 5 from the low levels of the intermediate concentration (C2) to the final concentration (C3) having significantly lower levels in the ppm range of concentrations. The sorbent of the present invention is particularly suitable for providing the gas stream with a final concentration (C3) of gas having pKa greater than 5. Generally, the final concentration (C3) will be below 1000 ppm, suitably below 800 ppm, normally below 600 ppm, and preferably below 400 ppm.

[0117] Following examples are intended to illustrate the present invention and are not to be viewed in any way as limiting the scope of the present invention.

[0118] Examples

[0119] Preparation of Nitrogen containing polymers (NP)

[0120] NP 1

[0121] A 5 L stainless steel reactor with stirrer was charged with 2700 g PEI (polyethyleneimine) (Mw 800 g / mol, amine number 18.2 mmol / g). The reactor was evacuated (60 mbar) and purged with nitrogen 3 times while increasing the temperature to 100°C. The reactor was pressurized to 2 bar and dosage of propylene oxide was initiated with 112 g PO within 5 minutes. While stirring at 150 rpm a further 700 g PO was dosed over a period of 3 hours. The temperature was raised to 115°C and stirred for another 3 hours. Then the reactor was cooled to 60°C and depressurized. Finally, the reactor was treated for 20 minutes at 100 mbar and purged with nitrogen. 3409 g of a slightly yellowish viscous liquid was obtained.

[0122] NP 2

[0123] A 5 L stainless steel reactor with stirrer was charged with 2700 g PEI (Mw 1200 g / mol, amine number 17.9 mmol / g). The reactor was evacuated (60 mbar) and purged with nitrogen 3 times while increasing the temperature to 110°C. The reactor was pressurized to 2 bar and dosage of propylene oxide (PO) was initiated with 105 g PO within 5 minutes. While stirring at 150 rpm a further 400 g PO was dosed over a period of 2.5 hours. The temperature was raised to 115°C and stirred for another 3 hours. Then the reactor was cooled to 60°C and depressurized. Finally, the reactor was treated for 20 minutes at 100 mbar and purged with nitrogen. 3202 g of a slightly yellowish viscous liquid was obtained.

[0124] NP 3

[0125] A 5 L stainless steel reactor with stirrer was charged with 2500 g PEI (Mw 800g / mol, amine number 18.2 mmol / g) followed by 590 g water. The reactor was evacuated (60 mbar) and purged with nitrogen 3 times while increasing the temperature to 100°C. The reactor was pressurized to 2 bar and dosage of butylene oxide (BuO) was initiated with 152 g BuO within 5 minutes. While stirring at 150rpm a further 700 g BuO was dosed over a period of 3 hours. The temperature was raised to 120°C and stirred for another 3.5 hours. Then the reactor was cooled to 60°C and depressurized. Finally, the reactor was treated for 10 minutes at 100 mbar and purged with nitrogen. 3933 g of a clear liquid was obtained.

[0126] NP4

[0127] A 2 L glass flask with stirrer and a refluxing funnel was charged with 500 g polyethylenimine (PEI, Mw 5000 g / mol, amine number 17.7 mmol / g) and 342 g water was added (300rpm) and nitrogen was purged 20 minutes while the mixture heated to 50°C. While the temperature was kept within 50-65°C, 135 g butylene oxide (BuO) was dosed over a period of 3 h. The mixture was stirred at 60°C overnight. Then water was partially removed while the temperature was raised to 100-105°C (30 minutes) and kept for 30 Minutes at 100°C and then 40 mbar of vacuum was applied for 10 minutes. The obtained yellowish mixture was quenched with nitrogen and cooled to room temperature (approximately 20°C). 825 g of a yellowish viscous liquid was obtained that was diluted with 445 g water to achieve a 50% aqueous solution.

[0128] NP5

[0129] 185.6 g (0.8 mol) of pentaethylenehexamine (PEHA) was placed in a 500mL 3-neck flask equipped with an anchor stirrer and a dropping funnel. The aqueous solution was warmed to 50°C and stirred at 150 rpm while 97 g (0.48 mol) butanediol diglycidyl ether was dosed through a funnel within 90 minutes, always keeping the temperature below 85°C. The reaction mixture was maintained at 80°C for another

[0130] 1 h and is then allowed to cool to room temperature (RT) to yield 280 g of a yellow viscous liquid.

[0131] NP6

[0132] 264.6 g (0.7 mol) Tetraethylenepentamine (TEPA) was placed in a 500 mL 4-neck flask equipped with an anchor stirrer, a reflux condenser and dropping funnel and combined with methanol to give a 50% by weight solution. The solution was warmed to 35°C and stirred at 150 rpm. 92 g (0.46 mol) Butanediol diglycidylether was dosed through a funnel within 90 minutes. After a further 30 minutes the viscous liquid was transferred to a rotation evaporator (Buechi) and the solvent was removed at 90°C resulting in 222 g of a yellowish viscous liquid.

[0133] The nitrogen-containing polymers (NP) are characterised in Table 1 and Table 2. Table 1

[0134] AO - alkylene oxide PO - propylene oxide

[0135] BuO - butylene oxide

[0136] Table 2 Example 1 - Preparation of the Sorbent

[0137] The preparation of 14 samples of Sorbent employed the following procedure with the respective dosages components where applicable shown in Table 3 for each experiment.

[0138] The required amount of Nitrogen-containing polymer (NP) was placed in a 500 mL 4- neck flask equipped with an anchor stirrer, a reflux condenser and dropping funnel. 100 g of water was then added followed by the required dose of 40% by weight aqueous solution of sodium silicate sol. Further water was added to the mixture to provide a reaction mixture of 450 g total weight.

[0139] CO2 gas was bubbled through the reaction mixture as it is being stirred at a CO2 flow rate of 0.8 ml / s for a period of time specified in Table 3. After the period of time specified in Table 3, a gel was formed which was aged for a period of time specified in Table 3. Subsequently water is removed at 90-140°C under vacuum (i.e. reduced pressure of 300-10 mbar). A free-flowing slightly yellowish powder was then obtained.

[0140] BASF SE 230143

[0141] Table 3

[0142] Example 2 - Determination of Carbon Dioxide (CO2) Adsorption Capacity and Rates

[0143] The respective CO2 adsorption capacities of the Sorbent samples were determined using a TGA (TGA Q500, TA Instruments Inc, New Castle, DE, USA). About 20-25 mg of the Sorbent samples underwent preheating to 110°C for 30 mins to remove any adsorbed moisture in N2 (1 bar, 100 ml / min). Then Sorbent samples were cooled to the temperature to be employed for the CO2 adsorption determination and equilibrated. One set of tests measured the total CO2 uptake at 50°C after 30 minutes in wt.% and a second set of tests measured the total CO2 uptake at 25°C after 10 minutes in wt.%. To start the CO2 determination, the composition of the gas was changed to 15% CO2 balanced by N2 (1 bar, 100 ml / min). The adsorption capacity on a dry basis was calculated by determining the weight change prior to introducing the 15% CO2 in the gas stream. The determination of both the total CO2 adsorption capacity the two temperatures and for the required period of time and the time to achieve 90% of the total uptake with the first set of tests at 50°C were established.

[0144] The results of this test work are presented in Table 4.

[0145] Table 4

Claims

Claims1 . A sorbent suitable for adsorbing gases and having pKa greater than 5, preferably carbon dioxide, comprising a silica structure supporting a nitrogencontaining polymer (NP), the nitrogen-containing polymer (NP) having a backbone comprising nitrogen atoms, the solid adsorbent being obtainable by a process comprising the steps of:(a) providing an aqueous alkaline medium comprising,(i) the nitrogen-containing polymer (NP),(ii) a silica source, wherein the silica source is selected from the group consisting of an aqueous solution of a silicate salt and a colloidal silica sol;(b) contacting the aqueous alkaline medium of step (a) with carbon dioxide to form a mixture, the mixture optionally comprising gel, wherein the aqueous alkaline medium is contacted with carbon dioxide by feeding a stream of carbon dioxide into the aqueous alkaline medium, or the source of carbon dioxide is in the form of a material that will release carbon dioxide under the reaction conditions of the process;(c) ageing the mixture of step (b) to produce an aged mixture comprising gel;(d) subjecting the aged mixture of step (c) to a drying step in order to remove at least some of the water from the aged gel to produce the sorbent.

2. The sorbent according to claim 1 , wherein the sorbent comprises:A) silica in an amount from 20 to 60% by weight based on the dry weight of the sorbent, preferably from 30 to 60%; andB) the nitrogen-containing polymer (NP) in an amount from 40 to 80% by weight based on the dry weight of the sorbent, preferably from 40 to 70%.

3. The sorbent according to claim 1 or claim 2, wherein the nitrogen containing polymer (NP) is a polyalkyleneimine and preferably is a polyethyleneimine.

4. The sorbent according to claim 1 or claim 2, wherein the nitrogen-containing polymer (NP) is the reaction product of (i) a di- or oligoamine or polyamine (A) with (ii) a bridging compound (BC), the bridging compound (BC) comprising at least one amine reactive group (ARG), wherein the bridging compound (BC) is capable of bonding to amine groups of at least two di- or oligoamine or polyamine (A) molecules.

5. The sorbent according to any of claims 1 to 4, wherein the nitrogen-containing polymer (NP), suitably polyalkyleneimine, preferably polyethyleneimine, is an alkoxylated nitrogen-containing polymer (NP), suitably an alkoxylated polyalkyleneimine, preferably an alkoxylated polyethyleneimine.

6. The sorbent according to claim 5, wherein the alkoxylated nitrogen-containing polymer (NP), suitably alkoxylated polyalkyleneimine, preferably alkoxylated polyethyleneimine, has been alkoxylated using an alkylene oxide selected from at least one C3-Ci2-alkylene oxide, preferably propylene oxide or butylene oxide.

7. The sorbent according to any of claims 1 to 6, wherein the nitrogen-containing polymer (NP), suitably polyalkyleneimine, preferably polyethyleneimine, has a weight average molecular weight (Mw) from 300 to 10,000 g / mol, preferably from 500 to 1500 g / mol.

8. The sorbent according to any of claims 1 to 7, wherein the nitrogen-containing polymer (NP), suitably polyalkyleneimine, preferably polyethyleneimine, is branched.

9. The sorbent according to any of claims 1 to 8, wherein the aqueous alkaline medium of step (a) is at a pH of greater than 10, for instance at least 10.5 for instance at least 11 .

10. The sorbent according to any of claims 1 to 9, wherein the silica source is an aqueous solution of a silicate salt and preferably the silicate salt is selected from the group consisting of potassium silicate, sodium silicate and ammonium silicate, more preferably sodium silicate.11 . The sorbent according to any of claims 1 to 10, wherein the aqueous alkaline medium of step (a) is provided by combining an aqueous solution of the alkali metal silicate salt with an aqueous solution of the nitrogen-containing polymer (NP), suitably polyalkyleneimine, preferably polyethyleneimine, and optionally diluting the combined aqueous solutions.

12. The sorbent according to any of claims 1 to 11 , wherein the aqueous alkaline medium of step (a) comprises the nitrogen-containing polymer (NP), suitably polyalkyleneimine, preferably polyethyleneimine, in an amount from 5 wt.% to 40 wt.%, by weight of the aqueous alkaline medium, preferably from 10 wt. % to 30 wt. %.

13. The sorbent according to any of claims 1 to 12, wherein the aqueous alkaline medium of step (a) comprises the alkali metal silicate salt in an amount from 1 wt.% to 15 wt.%, by weight of the aqueous alkaline medium, preferably from 5 wt. % to 10 wt. %.

14. The sorbent according to any of claims 1 to 13, wherein the carbon dioxide of step (b) is passed through the aqueous alkaline medium of step (a) at a rate from 0.005 to 0.15, preferably from 0.01 to 0.06, litres of carbon dioxide per kilogram of alkali metal silicate salt contained in the aqueous alkaline medium over a period of 1 to 5 hours, the volume of carbon dioxide based on a temperature of 25°C.

15. The sorbent according to any of claims 1 to 14, wherein the ageing step (c) is over a period from 1 to 48 hours.

16. The sorbent according to any of claims 1 to 15, wherein the drying step (d) is carried out at a temperature of at least 80°C, preferably from 90°C to 140°C, and a reduced pressure of from 300 to 10 mbar.

17. The sorbent according to any of claims 1 to 16, wherein the sorbent is a solid which is in the form of particles.

18. A process for preparing the sorbent defined according to claim 1 , the process comprising the steps of:(a) providing an aqueous alkaline medium comprising,(i) a nitrogen-containing polymer (NP), the nitrogen-containing polymer (NP) having a backbone comprising nitrogen atoms,(ii) a silica source, wherein the silica source is selected from the group consisting of an aqueous solution of a silicate salt and a colloidal silica sol;(b) contacting the aqueous alkaline medium of step (a) with carbon dioxide to form a mixture, the mixture optionally comprising gel, wherein the aqueous alkaline medium is contacted with carbon dioxide by feeding a stream of carbon dioxide into the aqueous alkaline medium, or the source of carbon dioxide is in the form of a material that will release carbon dioxide under the reaction conditions of the process;(c) ageing the mixture of step (b) to produce an aged mixture comprising gel;(d) subjecting the aged mixture of step (c) to a drying step in order to remove at least some of the water from the aged mixture to produce the sorbent.

19. The process according to claim 18, wherein the process comprises any of the features described in any of claims 2 to 16.

20. Use of the sorbent defined according to any of claims 1 to 17 or prepared by the process according to claims 18 to 19 for adsorbing a gas or gases with pKa greater than 5, preferably carbon dioxide.21 . The use according to claim 20, wherein the sorbent is employed for capturing carbon dioxide from a mixture of gases, suitably either atmospheric air or exhaust fumes, preferably employed in direct air capture (DAC).

22. The use according to claim 20 or claim 21 , wherein the sorbent is incorporated into a composition or an article for direct capturing of carbon dioxide.

23. A process for reducing the concentration of gas with pKa greater than 5, preferably carbon dioxide, present in a gas mixture produced as a gas stream from a combustion process, comprising the steps of(a) passing the gas stream containing the gas with pKa greater than 5 at an initial concentration (C1) through a gas scrubbing stage, comprising contacting the gas stream with an amine, preferably comprising an alkanolamine, to reduce the concentration of the gas with pKa greater than 5 to an intermediate concentration (C2) which is below the initial concentration (C1 ),(b) passing the gas stream resulting from step (a) through a purifying stage, comprising contacting the gas stream with the sorbent defined according to any of claims 1 to 17 or prepared by the process according to claims 18 to 19 thereby reducing the concentration of the gas with pKa greater than 5 in the gas stream to a final concentration (C3) which is below the intermediate concentration (C2).

24. A process according to claim 23, wherein the initial concentration (C1 ) is greater than 2 wt.%, preferably greater than 5 wt.%, more preferably greater than 10 wt.%, based on the weight of the gas stream.

25. A process according to claim 23 or claim 24, wherein the intermediate concentration (C2) is below 2 wt.%, preferably from 0.5 to 1 wt.% and the final concentration (C3) is below 1000 ppm, preferably below 400 ppm.

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