REGENERATIVE ADSORBENTS OF MODIFIED AMINES ON NANO-STRUCTURED SUPPORTS

Abstract

The invention relates to regenerative, solid sorbent for adsorbing carbon dioxide from a gas mixture, with the sorbent including a modified polyamine and a nano-structured solid support. The modified polyamine is the reaction product of an amine and an aldehyde. 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 operations of absorption-desorption cycles.

Claims

1. A solid sorbent for adsorbing carbon dioxide from a gas mixture, comprising a modified polyamine and a nano-structured solid support, wherein the modified polyamine is the reaction product of an amine and an aldehyde.

2. The sorbent according to claim 1, wherein the nano-structured support has a primary particle size less than about 100 nm and preferably between 3 and 50, 3 and 30 or 3 and 15 nm.

3. The sorbent according to claim 1 or 2, wherein the nano-structured support is a silica, silica-alumina, calcium silicate, carbon nanotube, carbon or a mixture thereof.

4. The sorbent according to claim 1, 2 or 3, wherein the modified polyamine is obtained as a viscous liquid by dissolving the amine in water to form an amine solution; adding the aldehyde to the amine solution with agitation or stirring to form a mixture for a period of time to form a liquid reaction product of the amine and aldehyde; and then heating the mixture, if necessary under vacuum conditions, to remove water,

5. The sorbent according to any preceding claim, wherein the amine is a primary, secondary or tertiary alkyl- or alkanolamine, an aromatic amine, a mixed amine, or a combination thereof.

6. The sorbent according to any preceding claim, wherein the amine is tetraethylenepentamine, pentaethylenehexamine, triethylenetetramine, diethylenetriamine, ethylenediamine, hexaethyleneheptamine, a polyethylenimine, or a combination thereof.

7. The sorbent according to any preceding claim, wherein the aldehyde is a simple aldehyde, dialdehyde, trialdehyde, a polymeric aldehyde compound or a mixture thereof.

8. The sorbent according to any preceding claim, wherein the aldehyde is formaldehyde, glyoxal (ethanedial), glutaraldehyde (pentane-1,5-dial), succindialdehyde (butanedial), o-phthalaldehyde, m-phthalaldehyde, p-phthalaldehyde or a mixture thereof.

9. The sorbent according to claim 1, in which the modified polyamine is present in an amount of about 25% to 75% or 40% to 60% or in an approximately equal amount by weight of the support.

10. The sorbent according to claim 1, which further comprises a polyol in an amount up to about 25% by weight of the sorbent.

11. The sorbent according to claim 10, wherein the polyol is selected from the group consisting of glycerol, oligomers of ethylene glycol, polyethylene glycol, polyethylene oxides, and ethers, modifications and mixtures thereof.

12. The sorbent according to claim 1, wherein the nano-structured support is nanosilica, the modified amine is present in an amount of about 25% to 75% by weight of the sorbent, and the sorbent further comprises polyethylene glycol in an amount up to 25% by weight of the sorbent.

13. A method for preparing the sorbent of claim 1, which comprises combining the amine, aldehyde and support in a solvent with mixing and heating for a sufficient time to allow the amine and aldehyde to combine and be provided upon the support, followed by removal of the water to obtain the sorbent as a solid.

14. The method of claim 13, 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 aldehyde is dissolved in a solvent to form an aldehyde solution; and the suspension and the amine and aldehyde solutions are combined.

15. The method of claim 13, which further comprises dissolving the amine in water to form an amine solution, adding an aqueous solution of the aldehyde to the amine solution to form a mixture; mixing the mixture at a temperature of 15 to 30° C. for 1 to 50 hours; then heating the mixture to at least 100° C. for 30 seconds to 60 minutes to remove part or all of the water, with any remaining water removed by heating under vacuum, to obtain the modified amine as a viscous liquid.

16. The method of claim 15, wherein the sorbent is formed by adding the viscous liquid to the dispersion the support with stirring to disperse the modified polyamine onto the support.

17. The method according to one of claim 13, 14 or 15 which further comprises adding a polyol before the removing water for the obtention of the sorbent.

18. The method according to claim 17, which 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 form the sorbent.

19. A method for continuously capturing and separating carbon dioxide from a gas mixture with a sorbent, which comprises exposing the sorbent according to claim 1 to the gas mixture to effect absorption of carbon dioxide by the sorbent and treating the sorbent that contains absorbed or entrapped carbon dioxide to release it as purified carbon dioxide.

20. The method according to claim 19, wherein the sorbent is provided in a fixed, moving, or fluidized bed and the gas and bed are in contact for a sufficient time to trap the carbon dioxide in the sorbent.

21. The method according to claim 19 or 20, wherein the sorbent is treated with sufficient heat, reduced pressure, vacuum, gas purge, or a combination thereof to release a substantial amount or all of the absorbed carbon dioxide.

22. The method according to one of claims 19 to 21, which further comprises reacting the released carbon dioxide to form useful products.

23. The method according to claim 22, wherein carbon dioxide is used to produce methanol by (a) electrochemical reduction of carbon dioxide in water or (b) reducing carbon dioxide under conditions sufficient to produce methyl formate as an intermediate compound and catalytically hydrogenating the intermediate compound with hydrogen under conditions sufficient to form methanol.

24. The method according to claim 22, which further comprises 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.

25. The method according to claim 23 or 24, which further comprises dehydrating the methanol under conditions sufficient to produce dimethyl ether.

26. The method according to claim 25, which further comprises heating the dimethyl ether in the presence of an acidic-basic or zeolitic catalyst under conditions sufficient to form ethylene and/or propylene.

27. The method according to claim 26, which further comprises converting the ethylene and/or propylene under conditions sufficient to higher olefins, a synthetic hydrocarbons, aromatics, or a product produced therefrom, for use as a feedstock for chemicals or as transportation fuel.

28. The method according to claim 27, which further comprises hydrating the ethylene or propylene under conditions sufficient to form ethanol or propanol.

29. Use of a modified polyamine to provide a solid sorbent for absorbing carbon dioxide from a gas mixture, characterized in that the modified polyamine is the reaction product of an amine and an aldehyde and is provided upon a nano-structured solid support.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a graph of the adsorption and desorption of CO.sub.2 on a sorbent based on pentaethylenehexamine formaldehyde named PEHA-HCHO-1-1 (see Example 1) which is supported by precipitated silica, 53/47 wt %, and with the adsorption/desorption measured by a thermogravimetric analyzer.

[0026] FIG. 2 is a graph of the adsorption and desorption of CO.sub.2 under humid conditions on a sorbent based on pentaethylenehexamine/formaldehyde sorbent named PEHA-HCHO-1-1 (see Example 1) supported on precipitated silica, 53/47 wt %, and with adsorption/desorption measured in flow system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] 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 absorbing and desorbing CO.sub.2. CO.sub.2 can be absorbed from any desired source, including industrial exhausts, flue gases of fossil fuel-burning power plants, as well as natural sources. 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.

[0028] The modified polyamine sorbent with nano-scale support according to the invention provides significant advantages over the absorbents of the prior art, e.g., absorbents 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.

[0029] The present sorbent is also easy to regenerate and recycle at ambient to moderate temperatures, enabling multiple absorption-desorption cycles with no or minimal loss of activity. The sorbent also addresses the corrosion and evaporation problems of the prior art absorbents.

[0030] 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 the atmospheric air.

[0031] The sorbent according to the invention is suggested to absorb CO.sub.2 by the following mechanism. Upon contact with a gaseous stream containing CO.sub.2, the supported modified amine chemically absorbs CO.sub.2 by forming a carbamate complex.

##STR00001##

[0032] 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 absorption capacity.

##STR00002##

[0033] According to an embodiment of the invention, the absorbed 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.

[0034] 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, polyamines, mixture of polyamine and combinations and modifications thereof. 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 amino components. Specific examples of amino components include, but are not limited to, tetraethylenepentaamine, pentaethylenehexamine, triethylenetetramine, diethylenetriamine, ethylenediamine, hexaethyleneheptamine, polyethylenimines, and the likes, including various polymeric amine compounds and mixtures thereof.

[0035] Preferred polyamines include various higher ethyleneamines which are sometimes referred to as polyethyleneamines. A general formula for such polyamines is: H.sub.2N(NH(CH.sub.2).sub.n).sub.pNH.sub.2 where n is 1 to 4 and p is 2 to 10000. The polyamine preferably contains a sufficient amount of repeating NH(CH.sub.2CH.sub.2) units so that they possess low volatility to avoid or minimize loss of amine, which would contaminate the gas stream and decrease the effectiveness of the absorption system over time. Specifically preferred linear polyamines include tetraethylenepentaamine (TEPA) and pentaethylenehexamine (PEHA) and hexaethyleneheptamine (HEHA).

[0036] Aldehydes that can be used in this invention to modify the amine include single aldehydes, as well as dialdehydes, trialdehydes and higher homologues. Examples of aldehyde components include, but are not limited to, formaldehyde, glyoxal (ethanedial), glutaraldehyde (pentane-1,5-dial), succindialdehyde (butanedial), o-phthalaldehyde, m-phthalaldehyde, p-phthalaldehyde, and the likes, including various polymeric aldehyde compounds and mixtures thereof.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] The supported polyamine sorbent can be prepared by impregnation or by another conventional technique.

[0041] To enhance the CO.sub.2 absorption 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 additions of polyols improves the absorption 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 be unreactive toward amines, and should have low volatility to avoid or minimize loss, which contaminates the gas stream and decreases the effectiveness of the absorption 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.

[0042] 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.

[0043] For example, the modified polyamine can be prepared by first dissolving the amine in water to form an amine solution. Next an aqueous solution an aldehyde is added to the amine solution to form a mixture. The mixture is initially stirred at room temperature (i.e., 15 to 100° C.) for 1 min to 50 hrs and preferably 1 to 30 hrs and then is heated to at least 100° C. for 30 seconds to 300 minutes and preferably from 10 to 60 minutes to remove part or all of the water. Any remaining water can be removed by heating under vacuum. The obtained modified amine is a viscous liquid.

[0044] To form the sorbent, the viscous liquid 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 viscous liquid would be between 25 and 75% or between 40 and 60% of the combined weight of the viscous liquid and support: preferably approximately equal weights of these are used. 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 uniform powder for use the absorption of carbon dioxide.

[0045] 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 aldehyde to prepare a mixture. Stirring or agitation of the mixture is maintained to form the sorbent and then the water is removed to obtain the sorbent as a powder like solid.

[0046] As noted above, polyols can be added to enhance the absorption/desorption characteristics of the supported amine sorbent. When a polyol is used, the polyol can be mixed together with the 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 amine in the solvent is added under stirring. Finally, solvent is removed to form the supported amine/polyol sorbent. The sorbent can be used as is or can be crushed and sieved to obtain a uniform powder.

[0047] Any solvent which is capable of dissolving, but which does not react with, the amine 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 alcohols, which can dissolve amines and polyols and can be easily removed from the sorbent. For example, methanol, ethanol, and isopropyl alcohol, and various mixtures thereof can be used. The preferred solvent is water.

[0048] Advantageously, the invention enables a wide range of CO.sub.2 absorbing capabilities for use with various natural and industrial gas sources. The absorption 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 absorption bed. The ability of the sorbent to capture CO.sub.2 can be demonstrated by measuring absorption 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.

[0049] Once the bulk of the amines is complexed 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 absorbed gas from the sorbent. The absorbed 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 absorption-desorption cycles. In an example, the sorbent maintains its absorption efficiency even after repeated absorption-desorption cycles. Preferably, the sorbent maintains its absorption efficiency for many absorption-desorption cycles. It is convenient to use parallel absorption beds, which allow absorption and desorption/regeneration to be carried out continuously.

[0050] 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 85° 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 polyamine of the sorbent to be released. The CO.sub.2 can then be released, stored or used in any desired manner, and the sorbent freed (regenerated) from CO.sub.2 is reused in further CO.sub.2 absorption-desorption cycles.

[0051] 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.

[0052] 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 absorption 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.

[0053] It is generally observed that primary amines are more reactive than secondary amines towards CO.sub.2 and bind CO.sub.2 more strongly than secondary amine. While this could be an advantage during the CO.sub.2 adsorption step it also means that more energy is needed during the regeneration (desorption) step to liberate the adsorbed CO.sub.2 which could possibly lead to slower desorption kinetics. To lower the energy requirements during the regeneration step, the utilization of polyamines containing only or mostly secondary amines could therefore be advantageous.

[0054] Aldehydes react preferentially with primary amines to form imines. In the presence of formaldehyde containing no alpha-hydrogen the reaction with a primary amine results in the introduction of a —CH.sub.2—OH group and the formation of a secondary amine. Further reaction with another primary amine from another polyamine molecule can lead to oligomerization by a condensation reaction and elimination of water.

[0055] The elimination of part or all of the primary amines, or transformation of the primary amines into secondary amines, in polyamines results in the formation of a modified polyamine with improved CO.sub.2 desorption characteristics requiring a lower energy input during the regeneration step. Thus the desorption step could for example be performed at a significantly lower temperature and/or in a shorter time period, decreasing the overall cost and time of the process. The adsorbents described in this patent offer a substantial improvement compared to the present state of the art.

EXAMPLES

[0056] 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

[0057] An absorbent according to the invention is conveniently prepared in two steps.

Step 1: Preparation of a Modified Polyamine Species

[0058] A modified polyamine species based on pentaethylenehexamine (PEHA), H(NHCH.sub.2CH.sub.2).sub.5NH.sub.2, and formaldehyde is prepared as follows. 30 g of PEHA was dissolved in water. 10.45 g of an aqueous solution of formaldehyde (37% formaldehyde in water) diluted in 20 mL of water was 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 100° C. and kept at that temperature for 90 minutes to remove parts of the water. The remaining water was removed by heating under vacuum on a rotavap followed by overnight vacuum (<1 mm Hg). The obtained product was a viscous yellow to orange liquid. The modified polyamine was named PEHA-HCHO-1-1.

Step 2: Preparation of a Supported Polyamine Sorbent

[0059] A supported modified polyamine sorbent composed of 50 wt. % PEHA-HCHO-1-1 and 50 wt. % fumed silica having an average primary particle size of 7 nm and a specific surface area of 390 m.sup.2/g+/−40 m.sup.2/g.

[0060] PEHA-HCHO-1-1 was dissolved in 100 mL of water. This solution was then added slowly under stirring to an equal amount of fumed silica in suspension in 300 mL water to ensure good dispersion of the modified polyamine on the support. The mixture was stirred for an additional 20 hours at room temperature, and the water was then removed from the mixture by heating under vacuum on a rotovap followed by overnight vacuum (<1 mm Hg). The supported polyamine sorbent obtained was a white solid, which could be crushed and sieved to produce a uniform powder.

Example 2: Preparation of Adsorbent Based on Modified Polyamines and Fumed Silica

[0061] This example illustrates the preparation in “one pot” of a supported modified polyamine sorbent composed of 53 wt. % PEHA-HCHO-1-1 and 47 wt. % fumed silica having an average primary particle size of 7 nm and a specific surface area of 390 m.sup.2/g+/−40 m.sup.2/g 30.1 g (0.1295 mol) of PEHA was dissolved in 120 mL of water. This solution was then added under stirring to 30.1 g of fumed silica in suspension in 300 mL of water to ensure good dispersion of PEHA on the support. Stirring was maintained for 1 hour before 10.5 g (0.1295 mol) of an aqueous solution of formaldehyde (37% formaldehyde in water) was added drop-wise to the PEHA/fumed silica solution (rate of addition of 0.3 mL/min) at room temperature. The mixture was stirred for an additional 20 hours. The water was removed by heating under vacuum on a rotavap followed by overnight vacuum (<1 mm Hg). The obtained product was a white powder like solid.

Example 3

[0062] Measurement of CO.sub.2 Absorption Capacity Using an 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

[0063] 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-HCHO-1-1/precipitated silica (53/47 wt % prepared in “one pot”).

[0064] CO.sub.2 adsorption data were obtained using an all-glass grease free flow system. The adsorbent, generally 1 g was placed in a glass tube between two glass wool plugs. The U-tube was then evacuated (˜30 mTorr) at 85° C. for 3 hours. The weight of the adsorbent after this treatment was measured. The adsorbent weight after pretreatment was used for the later calculation of the CO.sub.2 adsorption capacities. After pretreatment the adsorbent containing U-tube was placed in a thermostated oil bath at 25° C. 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. The concentration range used was 0-2000 ppm. Before each run, 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 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 0 ppm, signaling complete CO.sub.2 adsorption from the air. The CO.sub.2 concentration was recorded as a function of time via LabView 8.6. After an initial period at 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 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).

[0065] The desorption of the CO on the adsorbent was performed by heating the adsorbent containing U-tube to 60° C. and then passing a flow of air containing 400 ppm CO.sub.2 and 50% humidity (335 ml/min) through it. The CO.sub.2 concentration was recorded as a function of time via LabView 8.6. Immediately after opening of the air flow onto the saturated adsorbent the concentration in CO.sub.2 spiked to 3-5% CO.sub.2 and then slowly decreased until reaching the inlet CO.sub.2 concentration (400 ppm CO.sub.2).

[0066] The adsorption/desorption cycling was repeated 4 times under these conditions (adsorption at 25° C., 1000 ppm CO.sub.2 in air, 50% humidity, 335 mL/min and desorption at 60° C., 400 ppm CO.sub.2 in air, 50% humidity, 335 mL/min). The adsorption capacity remained stable at around 95-105 mg CO.sub.2/g adsorbent.

Example 4

[0067] Measurement of CO.sub.2 Absorption Capacity by Thermogravimetric Analysis. CO.sub.2 Adsorption from a Mixture Containing 1000 ppm CO.sub.2 for Air Quality Purposes

[0068] CO.sub.2 absorption data was obtained using a thermogravimetric analyzer (Shimadzu TGA-50). The absorbent (5-20 mg) was loaded into a platinum crucible and placed on the instrument balance. The solid absorbent was then pretreated at the desired temperature, generally 90 to 110° C. for 1 hr. under a flow of air. Subsequently, the sample was cooled to the desired adsorption temperature and the gas flow switched to a gas mixture containing 1000 ppm CO.sub.2 in air. The change in mass in the sample was recorded over time to determine the CO.sub.2 adsorption capacity. Desorption was performed by heating the adsorbent to a higher temperature (generally 50 to 80° C.) in a gas mixture containing 400 ppm CO.sub.2 air. The change in mass in the sample was recorded over time to determine the CO.sub.2 desorption capacity. These adsorption/desorption cycles were repeated a number of times to determine the stability of the adsorbent.

[0069] An example of adsorption/desorption measurements obtained with this method for the adsorbent prepared according to the “one pot” synthesis (PEHA-HCHO-1-1/precipitated silica, 53/47 wt %) is presented in FIG. 1. The adsorption was performed at 25° C. for 90 minutes in a gas mixture containing 1000 ppm CO.sub.2 in air. Desorption was performed at 60° C. for 45 minutes in a gas mixture containing 400 ppm CO.sub.2. The adsorption capacity did not decrease over more than 35 adsorption/desorption cycles and remained around 79 mg CO.sub.2/g adsorbent.