REGENERATIVE ADSORBENTS OF MODIFIED AMINES ON SOLID SUPPORTS

Abstract

The invention relates to regenerative, solid sorbents for adsorbing carbon dioxide from a gas mixture, including air, with the sorbent including a modified polyamine and a solid support. The modified polyamine is the reaction product of an amine and an epoxide. The sorbent provides structural integrity, as well as high selectivity and increased capacity for efficiently capturing carbon dioxide from gas mixtures, including the air. The sorbent is regenerative, and can be used through multiple cycles of adsorption-desorption.

Claims

1. A solid carbon dioxide sorbent with structural integrity for adsorbing carbon dioxide from a gas mixture, comprising a modified polyamine which is supported upon and within a solid support, wherein the modified polyamine is a reaction product of an excess of an amine selected from the group consisting of tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine, triethylenetetramine, diethylenetriamine, polyethylenimine and a mixture thereof, and a diepoxide, a triepoxide, a polyepoxide, and a polymeric epoxide to provide a material with amine functionalities, wherein the solid support is (a) a nano-structured support of silica, silica-alumina, alumina, titanium oxide, calcium silicate, carbon nanotubes, carbon, or a mixture thereof and having a primary particle size of less than about 100 nm; or (b) a natural or synthetic clay or a mixture thereof.

2. The carbon dioxide sorbent according to claim 1, in which the modified polyamine is present in an amount of 1% to 90% by weight of the sorbent, or in an approximately equal amount by weight as the support.

3. The carbon dioxide sorbent according to claim 1, which further comprises a polyethylene oxide or a polyol or various mixtures thereof in an amount of 1 up to about 25% by weight of the sorbent, wherein the polyol is glycerol, oligomers of ethylene glycol or polyethylene glycol, and the polyethylene oxides may be present as their corresponding ethers.

4. The carbon dioxide sorbent according to claim 1 in which the modified polyamine is bound to the surface of the support by grafting methods with various species.

5. The carbon dioxide sorbent according to claim 1, which has a high surface area for solid-gas contact, wherein the carbon dioxide sorbent is regenerative for capturing and separating carbon dioxide for at least one cycle at temperatures of 85° C. or less from a gas containing carbon dioxide.

6. The carbon dioxide sorbent according to claim 1, which has a high surface area for solid-gas contact, wherein the carbon dioxide sorbent is regenerative for capturing and separating carbon dioxide for at least one cycle at temperatures of 55° C. or less from a gas containing carbon dioxide.

7. The carbon dioxide sorbent according to claim 1, wherein the solid support has a primary particle size of between 3 and 50 nm.

8. The carbon dioxide sorbent according to claim 1, wherein the solid support has a primary particle size of between 3 and 30 nm.

9. The carbon dioxide sorbent according to claim 1, wherein the solid support has a primary particle size of between 3 and 15 nm.

10. The carbon dioxide sorbent according to claim 1, wherein the modified polyamine is present in an amount of 1% to 70% by weight of the sorbent, or in an approximately equal amount by weight as the support.

11. The carbon dioxide sorbent according to claim 1, wherein the modified polyamine is present in an amount of 1% to 40% by weight of the sorbent, or in an approximately equal amount by weight as the support.

12. The carbon dioxide sorbent according to claim 1, wherein the natural or synthetic clay or a mixture thereof comprises montmorillonite.

13. The carbon dioxide sorbent according to claim 1, which has a high surface area for solid-gas contact, wherein the carbon dioxide sorbent is regenerative for capturing and separating carbon dioxide for at least one cycle at temperatures of 55° C. or less from air or a gas mixture containing carbon dioxide in a concentration below 1 percent by volume.

14. The carbon dioxide sorbent according to claim 1, which has a high surface area for solid-gas contact, wherein the carbon dioxide sorbent is regenerative for capturing and separating carbon dioxide for at least one cycle at temperatures of 55° C or less from ambient air with carbon dioxide concentrations between 200 ppm and 5000 ppm by volume.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 illustrates the reaction of amines with epoxy resins. FIG. 2 illustrates: CO.sub.2 desorption on unmodified and epoxy resin modified amine based solid adsorbents. TGA measurements under dry conditions. Desorption conditions: 85° C. under pure nitrogen. PO: propylene oxide; BO: 1,2-epoxybutane.

[0040] FIG. 3 illustrates CO.sub.2 adsorption/desorption cycles on unmodified and propylene oxide (PO ) modified pentaethylenehaxamine (PEHA) based solid adsorbents. TGA measurements under thy conditions. Adsorption and desorption under isotherm conditions at 85° C. Adsorption under pure CO.sub.2. Desorption under pure nitrogen.

[0041] FIG. 4 illustrates CO.sub.2 adsorption capacity over 10 adsorption/desorption cycles on PEHA-PO-1-2/precipitated silica (61/39 wt % prepared in “one pot”). Adsorption at 1000 ppm CO.sub.2 in air at 2.5° C. Desorption at 400 ppm CO.sub.2 in air at 50° C. Under humid conditions.

[0042] FIG. 5 illustrates CO.sub.2 concentration at the outlet of the adsorbent bed over the first 5 adsorption/desorption cycles on PEHA-PO-1-2/precipitated silica (61/39 wt % prepared in “one pot”), Adsorption at 1000 ppm CO.sub.2 in air at 25° C. Desorption at 400 ppm CO.sub.2 in air at 50° C. Under humid condition.

[0043] FIG. 6 illustrates CO.sub.2 adsorption capacity over 35 adsorption/desorption cycles on TEPA-PO-1-2/precipitated silica (61/39 wt % prepared in “one pot”). Adsorption at 1000 ppm CO.sub.2 in air at 25° C. Desorption at 400 ppm CO.sub.2 in air at 50° C. Under humid conditions.

DETAILED DESCRIPTION OF THE INVENTION

[0044] The invention relates to regenerative supported modified polyamine sorbents for absorbing CO.sub.2. The sorbent comprises a modified polyamine on a nano-structured support, e.g., a nanosilica support, for adsorbing and desorbing CO.sub.2. Carbon dioxide can be adsorbed from any desired source, including industrial exhausts, flue gases of fossil fuel-burning power plants, as well as natural sources such as ambient air The nano-structured support according to the invention provides structural integrity to the polyamine as well as a high surface area for solid-gas contact. The support can also include natural and synthetic clays.

[0045] The modified polyamine sorbent with nano-scale support according to the invention provides significant advantages over the absorbents of the prior art, e.g., adsorbents having a polymeric support, including a high CO.sub.2-selectivity and removal capacity at ambient and elevated temperatures. Thus, the present sorbent allows selective capture and separation of CO.sub.2 from various gas mixtures under various conditions and temperatures.

[0046] The present sorbent is also easy to regenerate and recycle at ambient to moderate temperatures, enabling multiple adsorption-desorption cycles with no or minimal loss of activity. The sorbent also addresses the corrosion and evaporation problems of the prior art adsorbents.

[0047] Thus, the present sorbent system is practical for separating CO.sub.2 from industrial effluent gases such as those from fossil fuel-burning power plants and other industrial factories, as well as other gas streams, particularly natural gas containing significant CO.sub.2 concentrations. Significantly, the sorbent can also be used to separate CO.sub.2 from atmospheric air.

[0048] The sorbent according to the invention is suggested to adsorb CO.sub.2 by the following mechanism. Upon contact with a gaseous stream containing CO.sub.2, the supported modified amine chemically adsorbs CO.sub.2 by forming a carbamate complex.

##STR00001##

[0049] In the presence of water, the carbamate further reacts to form a bicarbonate and releases the amine, which can further react with CO.sub.2, thereby increasing the overall CO.sub.2 adsorption capacity.

##STR00002##

[0050] According to an embodiment of the invention, the adsorbed CO.sub.2 can be readily desorbed and the supported modified polyamine can be regenerated. The desorption of CO.sub.2 and regeneration of the sorbent can be achieved by modest heating of the sorbent, applying reduced pressure or vacuum, gas purge, and/or a carbon dioxide lean sweep gas, which releases CO.sub.2 from the sorbent. The ready regeneration enables the sorbent to undergo repeated absorption-desorption cycles with ease.

[0051] A large variety of amines can be used in the present invention. Suitable amines include primary, secondary and tertiary alkyl- and alkanolamines, aromatics, mixed amines, and combinations thereof. Polyamines are preferred. Primary and secondary amines are the most active for CO.sub.2 absorption. The polyamine should, therefore, preferably contain a sufficient amount of primary and secondary amine sites. Specific examples of amines include, but are not limited to, tetraethylenepentaamine, pentaethylenehexamine, triethylenetetramine, diethylenetriamine, ethylenediamine, hexaethyleneheptamine, polyethylenimines, polyallylamines, polyvinylamines and the like, including various polymeric amine compounds and mixtures thereof.

[0052] Preferred polyamines include various higher ethyleneamines which are sometimes referred to as polyethyleneamines. A general formula for such polyamines is: H(NH(CH.sub.2).sub.n).sub.pNH.sub.2 where n is 1 to 4 and p is 2 to about 10,000. The polyamine preferably contains a sufficient amount of repeating NH(CH.sub.2CH.sub.2) or NH(CH.sub.2) units so that they possess relatively low volatility to avoid or minimize loss of amine, which would contaminate the gas stream and decrease the effectiveness of the adsorption system over time. Specifically preferred linear polyamines include triethylenetetramine, (TETA), tetraethylenepentamine pentaethylenehexamine (PEHA) and hexaethyleneheptamine (HETA).

[0053] Epoxides that can be used in this invention to modify the amine include single epoxides, as well as diepoxides, triepoxides and higher homologues. Examples of epoxide components include, but are not limited to, ethylene oxide, propylene oxide, 1,2-epoxybutane, 2,3-epoxybutane, glycidol, butyl glycidyl ether, teri-butyl glycidyl ether, dodecyl and tetradecyl glycidyl ethers, octl/decyl glycidyl ether, 1,2-epoxycyclohexane, epichlorohydrin, glycerol diglycidyl ether, 1,4-cyclohexanedimethanol diglycidyl ether, neopentyl glycol diglycidyl ether, poly(ethylene glycol) diglycidyl ether, resorcinol diglycidyl ether, pol(propylene: glycol) diglycidyl ether, 4,4′-isopropylidenediphenol diglycidyl ether, 1,2,5,6-diepoxycyclooctane, trimethylolpropane triglycidyl ether, N,N-diglycidyl-4-glycidyloxyaniline, 4,4′-methylenebis(N,N-digtycidylaniline) and the like, including mixtures thereof.

[0054] The support according to the invention is a support having primary particle sizes less than 1,000 nm, preferably less than about 100 nm. Preferred supports are nanosilica, especially so-called fumed silica and precipitated silica. Fumed silica typically has a primary particle size ranging from 5 to 50 nm and a specific surface area between 50 and 500 m.sup.2/g. Fumed silica is generally prepared by vapor phase hydrolysis of a silicon-bearing halide, such as silicon tetrachloride (SiCl.sub.4). Examples of commercially available fumed silica include AEROSIL® from Evonik, CAB-O-SIL® from Cabot, and REOLOSIL® from Tokuyama. Precipitated silica is formed from aqueous solutions by reaction of an alkaline silicate (e.g., sodium silicate) with a mineral acid (e.g., sulfuric acid) under stirring. Primary particles formed by this method are generally between 3 and 50 nm, more specifically between 3 and 30 nm and preferably are between 3 and 15 nm in size. These primary particles can subsequently aggregate to form larger micron size particles. The specific surface area of precipitated silica generally ranges from 50 to 500 m.sup.2/g. Examples of commercially available precipitated silica include HI-SIL® from PPG Industries, SIPERNAT® from Evonik and FINESIL® and TOKUSIL® from Tokuyama.

[0055] Fumed silica and precipitated silica have the appearance of a lightweight, fluffy, white powder. Their small particle size allows them to absorb and retain significant amounts of amines while maintaining free flowing powder characteristics without caking. Another advantage of fumed and precipitated silicas is their non-toxicity. The non-toxicity allows them to be used in food processing, e.g., as anti-caking additives in powdered food products such as milk substitutes, and in cosmetic products, e.g., in abrasive material in a toothpaste. Fumed and precipitated silicas are generally hydrophilic, but their surface can be treated to produce hydrophobic silicas. Both hydrophilic and hydrophobic silicas, as well as other modified silicas, are all suitable for use as the nano-structured polyamine support according to the invention.

[0056] Other nano-structured materials suitable for use in the present polyamine sorbents include fumed or precipitated oxides such as fumed aluminum oxide, fumed zirconium oxide, and fumed titanium oxide, precipitated aluminum oxide, precipitated titanium oxide, precipitated zirconium oxide, calcium silicate, carbon nanotubes, and mixtures thereof. Other supports can also include natural and synthetic clays.

[0057] The supported polyamine sorbent can be prepared by impregnation or by another conventional technique.

[0058] To enhance the CO.sub.2 adsorption and desorption characteristics of the supported amine sorbent, polyols may be incorporated in the sorbent composition, in an amount up to 25% of the total weight of the sorbent. The addition of polyols improves the adsorption and desorption of the sorbent, and decreases the viscosity of the amines, allowing CO.sub.2 to have better access to the active amino sites of the sorbent even at lower temperatures (<50° C.). Polyols used in the invention should have low volatility to avoid or minimize material loss, which could contaminate the gas stream and decreases the effectiveness of the adsorption system over time. Examples of polyols used in the present sorbent include but are not limited to glycerol, oligomers of ethylene glycol, polyethylene glycols, polyethylene oxides, ethers of oligomers of ethylene glycol, ethers of polyethylene glycols, ethers of polyethylene oxides, oligomers or polymers of cyclic ethers such as polytetrahydrofuran, and modifications and mixtures thereof. Preferred polyols have a, molecular weight lower than 10,000. More preferably, polyols have a molecular weight lower than 1,000.

[0059] The modified polyamine is obtained by dissolving the amine in a solvent, preferably water, to form an amine solution; adding the epoxide to the amine solution with agitation or stirring to form a mixture for a period of time and form a liquid reaction product of the amine and epoxide; and then heating the mixture, if needed, to ensure complete reaction, followed by heating, if necessary under vacuum conditions, to remove the solvent. The amine is a primary, secondary or tertiary alkyl- or alkanolamine, an aromatic amine, a mixed amine, or a combination thereof, while the epoxide is a simple epoxide, diepoxide, trtepoxide, a polyepoxide compound, polymeric epoxide or a mixture thereof. A preferred polyamine is tetraethylenepentamine, pentaethylenehexamine, triethylenetetramine, diethylenetriamine, ethylenediamine, hexaethyleneheptamine, a polyethylenimine, or a combination thereof, while a preferred epoxide is ethylene oxide, propylene oxide, 1,2-epoxybutane, 2,3-epoxybutane, glycidol, butyl glycidyl ether, tart-butyl glycidyl ether, dodecyl and tetradecyl glycidyl ethers, octyl/decyl glycidyl ether, epichlorohydrin, glycerol diglycidyl ether, polyethylene glycol) diglycidyl ether. 4,4′-isopropylidenediphenol diglycidyl ether, trimethylolpropane triglycidyl ether or a mixture thereof. In the sorbent the modified polyamine is present in an amount of about 1% to 90% by weight or 40% to 70% by weight or in an approximately equal amount by weight of the support.

[0060] In one embodiment of the method of preparation, wherein the nano-structured support is dispersed in the solvent to form a suspension; the amine is dissolved in the solvent to form an amine solution; the epoxide is dissolved in a solvent to form an epoxide solution; and the suspension and the amine and epoxide solutions are combined. This can be conducted by dissolving the amine in solvent to form an amine solution, adding this solution to the support suspension, and then adding the epoxide solution to the amine/support mixture; mixing the mixture at a temperature of 15 to 30° C. for 0.1 to 50 hours; then heating the mixture to ensure complete reaction, and finally heating to at least 50° C. for 30 seconds to 60 minutes to remove part or all of the solvent, with any remaining solvent removed by heating if necessary under vacuum.

[0061] Alternatively, the amine and epoxide can be reacted separately to obtain a modified amine after removal of the solvent. The sorbent is formed by adding the modified amine to the dispersion of the support in solvent with stirring to disperse the modified polyamine onto the support. Alternatively, the reaction product of amine and epoxide can be added directly to the support suspension, without prior solvent removal.

[0062] If the epoxide is a liquid it can also be added neat without the need of dissolving it in a solvent.

[0063] In another embodiment, the method of preparation further comprises adding a polyol before the removing solvent for the obtention of the sorbent. In particular, the method further comprises adding a polyol to the suspension; drying the suspension after the addition of the polyol to form a supported polyol; dispersing the supported polyol in the solvent; and combining the dispersed supported polyol and the amine solution prior to removing the solvent to obtain the sorbent.

[0064] Because it is environmentally benign and very economic, the preferred solvent is water. Water is able to form solution with most amines described here as well as a number of epoxides including but not limited to propyleneoxide, 1,2-epoxybutane, glycidol, glycerol diglycidyl ether, polyethylene glycol) diglycidyl ether. However, in some cases the use of other solvents including but not limited to methanol, ethanol and isopropanol might be necessary.

[0065] To enhance the stability of the modified amines further, chemical bonding of these modified amines to surface of the supports by any known method including grafting with various species is also possible.

[0066] The amine-epoxide based CO.sub.2 adsorbents described here are efficient, regenerable under mild conditions, easy to prepare from readily available starting materials, economical and have high CO.sub.2 adsorption capacity. As such they fill most if not all of the desirable characteristics for a CO.sub.2 adsorbent for post combustion CO.sub.2 capture and CO.sub.2 capture from various dilute sources such as ambient air which include:

[0067] Fast adsorption of CO.sub.2 at mild temperatures or room temperature

[0068] Able to work under humid conditions

[0069] Fast desorption under mild conditions

[0070] No leaching of the active part

[0071] Long term stability under working conditions

[0072] Low cost

[0073] Easy to produce on a large scale

[0074] The invention also relates to a method of capturing and separating carbon dioxide from a gas source by adsorbing the carbon dioxide on the sorbent. The sorbent is regenerative in that it can be desorbed and regenerated by applying heat, reduced pressure, vacuum, gas purge, lean sweep gas, or a combination thereof. In this regard, the invention also relates to the use of a modified polyamine to provide a solid sorbent for adsorbing carbon dioxide from a gas mixture, characterized in that the modified polyamine is the reaction product of an amine and an epoxide and is provided upon a nano-structured solid support.

[0075] The released carbon dioxide can be used in a method to produce a renewable fuel such as methanol. In one embodiment, this method comprises reduction of carbon dioxide and water, or reduction of carbon dioxide under conditions sufficient to produce methyl formate as an intermediate compound followed by catalytic hydrogenation of the intermediate compound with hydrogen to form methanol.

[0076] In another embodiment, methanol is produced by catalytic hydrogenation of the intermediate compound wherein the hydrogen used in the hydrogenation is obtained by electrolysis of water obtained from the air. In another embodiment, methanol is produced by reducing the carbon dioxide under conditions sufficient to carbon monoxide, reacting the carbon monoxide with methanol under conditions sufficient to obtain methyl formate, and catalytically hydrogenating the methyl formate under conditions sufficient to produce methanol.

[0077] Methanol produced according to the invention can be further processed to any desired derivative or modified compounds. For example, methanol can be dehydrated to produce dimethyl ether, which can also be further treated under conditions sufficient to form compounds such as ethylene and propylene. Ethylene and propylene can be converted to higher olefins, a synthetic hydrocarbons, aromatics, or related products, and therefore are useful as a feedstock for chemicals or as transportation fuel. In a further embodiment, methanol can be further used for microbiological production of single cell proteins.

[0078] The methods for preparing polyamine supported sorbents according to the invention are inexpensive and easy to carry out, yet produce sorbents that are superior to the sorbents prepared by previously known methods.

[0079] For example, the modified polyamine can be prepared by first dissolving the amine in water to form an amine solution. Next, an aqueous solution of epoxide is added to the amine solution to form a mixture. The mixture is initially stirred at room temperature (i.e., 15 to 30° C.) for 0.01 to 50 hours and preferably 0.01 to 10 hours and then is heated for 30 seconds to 1000 minutes and preferably from 10 to 300 minutes to allow the reaction to run to completion and then heated to a higher temperature to remove part or all of the water. Any remaining water can be removed by heating under vacuum. The obtained modified amine is generally a viscous liquid.

[0080] To form the sorbent, the obtained modified amine can be dissolved in water and added to the support (e.g., silica) in suspension in water with stirring to disperse the modified polyamine onto the support. The amount of modified amine would be between 1 and 90% or between 40 and 70% of the combined weight of the modified amine and support or in approximately equal amount by weights with the support. Thereafter, the water can be removed as described above. The supported modified polyamine sorbent can be obtained as a solid, which could be crushed and sieved to produce a solid with a uniform size particle distribution for use in the adsorption of carbon dioxide.

[0081] Alternatively, the modified amine and sorbent can be prepared together by mixing the amine and silica into water as noted above, followed by the addition of a solution of the epoxide. Stirring or agitation of the mixture is maintained to form the sorbent and then the water is removed to obtain the sorbent as a solid.

[0082] Alternatively, the reaction product of amine and epoxide can be added directly to the support suspension, without prior water removal.

[0083] If the epoxide is a liquid it can also be added neat to the reaction mixture without the need of dissolving it first in a solvent.

[0084] As noted above, polyols can be added to enhance the adsorption/desorption characteristics of the supported amine sorbent. When a polyol is used, the polyol can be mixed together with the modified amine solution and added to the suspension of the support. The polyol can also be separately dissolved in the solvent and combined with the suspension of the support. In that case, the polyol solution is preferably added first to the suspension of the support, and the solvent is then removed to obtain the supported polyol material. The obtained solid is then dispersed in the solvent and a solution of the modified amine in the solvent is added under stirring. Finally, solvent is removed to form the supported modified amine/'polyol sorbent. The sorbent can be used as is or can be crushed and sieved to obtain a solid with a uniform particle size distribution. It can also be crushed to a powder. The formation of the modified amine by reaction of an amine and an epoxide can also be conducted in-situ in the presence of the polyol.

[0085] Any solvent which is capable of dissolving, but which does not react, at least rapidly, with the amine, the epoxide and the polyol can be utilized. The solvent should preferably be easily separated from the sorbent by mild heating and/or vacuum. Preferred solvents include but are not limited to water and alcohols, which can dissolve amines, epoxides and polyols and can be easily removed from the sorbent. For example, water, methanol, ethanol, and isopropyl alcohol, and various mixtures thereof can be used.

[0086] Advantageously, the invention enables a wide range of CO.sub.2 adsorbing capabilities for use with various natural and industrial gas sources. The adsorption can be performed under various conditions, e.g., over a temperature range of 0 to 100° C., and in any suitable manner, e.g., in a regular flow system or in a fixed, moving, or fluidized adsorption bed. The ability of the sorbent to capture CO.sub.2 can be demonstrated by measuring adsorption by thermogravimetry using a thermogravimetric analyzer (TGA), in a flow system over a sorbent cartridge or by measuring CO.sub.2 absorption under static conditions.

[0087] Once the amine containing sorbent is saturated with CO.sub.2, the sorbent can be regenerated. As used herein, the term “regeneration” or “regenerative” is understood to mean that the sorbent can be re-used by releasing or desorbing the adsorbed gas from the sorbent. The adsorbed gas is released by treating the sorbent with any process that effects the release, e.g., heating, reduced pressure, vacuum, gas purge, and combinations thereof. Thus, the regenerated sorbent according to the invention can be used repeatedly, through multiple adsorption-desorption cycles. In an example, the sorbent maintains its adsorption efficiency even after repeated absorption-desorption cycles. Preferably, the sorbent maintains its absorption efficiency for many adsorption-desorption cycles. It is convenient to use parallel adsorption units, which allow adsorption and desorption/regeneration to be carried out continuously.

[0088] For example, for a CO.sub.2 sorbent, the regeneration is endothermic, so the absorbed CO.sub.2 is released by subjecting the absorbent to elevated temperature (e.g., by heating the sorbent at temperatures from about 25° C. to about 120° C.), reduced pressure (e.g., by pressure swing absorption (PSA)), gas purge, vacuum, lean gas sweep, or any combinations thereof. The regeneration treatment allows essentially most of the CO.sub.2 that is complexed with the modified amine of the sorbent to be released. The CO.sub.2 can then be stored or used in any desired manner, and the sorbent freed (regenerated) from CO.sub.2 is reused in further CO.sub.2 adsorption-desorption cycles.

[0089] FIG. 2 and Table 1 illustrate the advantage, during the CO.sub.2 desorption step, of modifying amines with epoxy resins compared to an unmodified amine. The adsorbent composed of PEHA/Sipemat 50S required more than 8 minutes to desorb 90% of the CO.sub.2 at 85° C. On the other hand, the adsorbent were PEHA was modified with propylene oxide (PO) required only 1.73 min to achieve a similar desorption level and desorption was essentially over in about 3 min. The adsorbent with PEHA modified with 1,2-epoxybutane needed only 1.13 min to desorb 90% of the CO.sub.2.

TABLE-US-00001 TABLE 1 CO.sub.2 desorption characteristics of unmodified and epoxy resin modified amine based solid adsorbents. TGA measurements under dry conditions. Desorption conditions: 85° C. under pure nitrogen. time to achieve time to achieve 50% desorption 90% desorption CO.sub.2 adsorption at CO.sub.2 adsorption at (min) (min) 25° C. (mg CO.sub.2/g) 55° C. (mg CO.sub.2/g) PEHA-PO-1-2/Sipernat 50S (61/39 wt %) 0.73 1.73 117 144 TEPA-PO-1-2/Sipernat 50S (61/39 wt %) 0.67 1.43  94 135 PEHA-BO-1-2/Sipernat 50S (61/39 wt %) 0.57 1.13 109 122 PEHA/Sipernat 50S (50/50 wt %) 2.26 8.17 167 194 PO: Propylene oxide; BO: 1,2-epoxybutane; TGA measurements. Adsorption under pure CO.sub.2. Desorption under pure nitrogen at 85° C.

[0090] FIG. 3 shows an example of higher stability obtained by the reaction of propylene oxide with pentaethylenehexamine. The adsorbent containing only PEHA on Sipernat 50S exhibited a steady decrease in weight and CO.sub.2 adsorption capacity over 10 adsorption/desorption cycles in a TGA experiment under isotherm conditions. On the other hand, the adsorbent containing PEHA modified with PO on Sipernat 50S did not show a decrease in either weight or CO.sub.2 adsorption capacity over the 10 adsorption/desorption cycles.

[0091] Uses and reactions of CO.sub.2 include those mentioned above and as further disclosed in co-pending U.S. Pat. Nos. 7,605,293, 8,212,088 and 8,440,729, among others. The entire content of these three patents is expressly incorporated herein by reference thereto.

[0092] The sorbent according to the invention is thermally stable and does not release the supported polyamine in the temperature and/or pressure range of the adsorption operation. Further, because it is capable of regeneration and effective operation at a temperature range that can be easily maintained throughout the process, the sorbent is cost-effective for providing a high efficacy and a long life span, in addition to a high selectivity and capacity for CO.sub.2 capture and separation. Because of its flexibility and versatility, the sorbent can also advantageously be used to treat large volumes of CO.sub.2-containing gases from various sources.

EXAMPLES

[0093] The following examples are illustrative only and should not be interpreted as limiting the scope of the invention.

Example 1

Preparation of a Regenerable CO.SUB.2 .Adsorbent

[0094] An adsorbent according to the invention is conveniently prepared in two steps.

Step 1: Preparation of a Modified Polyamine Species

[0095] A modified polyamine species based on pentaethylenehexamine (H(NHCH.sub.2CH.sub.2).sub.5NH.sub.2,PEHA) and propylene oxide (PO) was prepared as follows. 10 g of PEHA (0.043 mol) was dissolved in 40 mL water. 5 g of PO (0,086 mol) was drawn with a syringe and then added drop-wise to the PEHA solution. The mixture was stirred for 20 hours at room temperature. After that, the temperature was progressively raised to 60° C. and kept at that temperature for 2 hours. The water was removed by rotary evaporator and followed by overnight vacuum (<1 mm Hg). The obtained product was a viscous yellow liquid. The modified polyamine is named PEHA-PO-1-2.

Step 2: Preparation of a Supported Polyamine Sorbent

[0096] A supported modified polyamine sorbent composed of 61 wt. % PEHA-PO-1-2 and 39 wt. % Sipernat 50S was prepared as follows. 3 g of PEHA-PO-1-2 was dissolved in 10 mL of water. 2 g of Sipernat 50s was suspended in 40 mL of water. PEHA-PO-1-2 solution was then slowly added to the Sipernat 50S suspension under stirring to ensure good dispersion of the modified polyamine on the support. The mixture was stirred for an additional 20 hours at room temperature. The water was then removed from the mixture by rotary evaporator and followed by overnight vacuum (<1 mm Hg). The supported polyamine adsorbent obtained was a white solid, which could be crushed and sieved to produce a solid with a uniform particle size distribution.

Example 2

Preparation of Adsorbent Based on Modified Polyamines and Precipitated Silica Sipernat 50S in “One Pot”

[0097] This example illustrates the preparation in “one pot” of a supported modified polyamine sorbent composed of 61 wt. % PEHA-PO-1-2 and 39 wt. % Sipernat. 50s. 3.33 g of PEHA (0.0143 mol) was dissolved in 30 mL of water. 3.33 g of Sipernat 50S was suspended in 70 mL of water. The PENIA solution was then slowly added to the Sipernat 50S suspension under stirring to ensure good dispersion of PEHA on the support. The mixture was stirred (magnetic stirring 400 rpm) at room temperature for 2 hours. 0.0287 mol of PO (2 mL) was drawn with a syringe and then added drop-wise to the PEHA-Sipernat 50S mixture. The mixture was stirred for an additional 20 hours. After that, the temperature was progressively raised to 60° C. and kept at that temperature for 2 hours. The water was removed from the mixture by rotary evaporator and followed by overnight vacuum (<1 mm Hg). The supported polyamine adsorbent obtained was a white solid, which could be crushed and sieved to produce a solid with a uniform particle size distribution.

Example 3

[0098] Measurement of CO.sub.2 adsorption capacity using a PEHA-PO-1-2/precipitated silica (Sipernat 50S) adsorbent placed in a cartridge in a flow system. CO.sub.2 Adsorption from a mixture containing 1000 ppm CO.sub.2 and 50% humidity for air quality purposes.

[0099] This example illustrates the removal of CO.sub.2 from a gas mixture containing 1000 ppm CO.sub.2 in air for indoor air quality purposes. The adsorbent used was PEHA-PO-1-2/precipitated silica (61/39 wt % prepared in “one pot”) prepared according to example 2.

[0100] CO.sub.2 adsorption data were obtained using an all-glass grease free flow system. The adsorbent was first placed in round bottom flask and evacuated (˜30 mTorr) at 85° C. for 3 hours to desorb CO.sub.2 and water present on the adsorbent. After this pretreatment, 1 g of the adsorbent was placed in a straight glass tube between two glass wool plugs thermostated at 25° C. The adsorbent weight (1 g) after pretreatment was used for the later calculation of the CO.sub.2 adsorption capacities. For the adsorption measurements a Horiba VIA-510 CO.sub.2 analyzer equipped with an IR detector specifically intended for CO.sub.2 measurements was placed in-line with the adsorption setup. Before the experiment, the analyzer was calibrated with reference gases; CO.sub.2 in air and ultra zero grade air for the zero. An air mixture containing 1000 ppm CO.sub.2 and 50% moisture (dew point of 14° C.) was used for the adsorption measurements. The air flow (˜335 mL,/min) was then opened on the adsorbent bed. Almost immediately the CO.sub.2 concentration in the gas outlet fell to a value lower than 10 ppm, signaling essentially complete CO.sub.2 adsorption from the air. The CO.sub.2 concentration was recorded as a function of time via Lab View 8.6. After an initial period close to 0 ppm CO.sub.2, the concentration in the outlet gas started to increase. After saturation of the adsorbent, when the CO.sub.2 concentration reached a value close to the inlet value (1000 ppm), the gas flow was stopped. The total adsorption capacity was determined to be 106 mg CO.sub.2/g adsorbent (2.4 mmol CO.sub.2/g adsorbent) after 5 hours of adsorption.

[0101] The desorption of the CO.sub.2 on the adsorbent was performed by heating the adsorbent containing glass tube to 50° C. with a heating tape and then passing a flow of air containing 400 ppm CO.sub.2 and 13% humidity (dew point of 14° C.) (335 ml/min) through it for 1 hour. The CO.sub.2 concentration was recorded as a function of time via LabView 8.6. Heating resulted in an increase of the CO.sub.2 concentration to values above 5000 ppm followed by a decrease until a CO.sub.2 concentration close to the inlet concentration (400 ppm CO.sub.2) was reached.

[0102] This initial adsorption/desorption cycles was followed by 10 additional adsorption/desorption cycles under the same conditions except for the adsorption time which was reduced to 3 h (adsorption at 25° C. for 3 h, 1000 ppm CO.sub.2 in air, 50% humidity (dew point of 14° C.), 335 mL/min and desorption at 50° C. for 1 h, 400 ppm CO.sub.2 in air, 13% humidity (dew point of 14° C.), 335 mL/min). The adsorption capacity remained stable at around 96-98 mg CO.sub.2/g adsorbent as can be seen in FIG. 4. The CO.sub.2 concentration profile during the adsorption/desorption cycles was very similar from cycle to cycle as observed in FIG. 5 showing the CO.sub.2 concentration as measured at the outlet of the adsorbent bed.

Example 4

[0103] Measurement of CO.sub.2 adsorption capacity using a TEPA-PO-1-2/precipitated silica adsorbent placed in a cartridge in a flow system. CO.sub.2 Adsorption from a mixture containing 1000 ppm CO.sub.2 and 50 humidity for air quality purposes.

[0104] This example illustrates the removal of CO.sub.2 from a gas mixture containing 1000 ppm CO.sub.2 in air for indoor air quality purposes. The adsorbent used was TEPA-PO-1-2/precipitated silica (61/39 wt % prepared in “one pot”).

[0105] The same procedure as described in example 3 was used. Over 35 cycles of adsorption/desorption the adsorption capacity remained stable at around 84-87 mg CO.sub.2/g adsorbent as can be seen in FIG. 6.