Ethylenediamine-Modified SBA-15 as Regenerable CO2 Sorbent

Mar 25, 2005 - The CO2 adsorption properties of an ethylenediamine-modified mesoporous silica, EDA-SBA-15, have been examined. Adsorption isotherms we...
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Ind. Eng. Chem. Res. 2005, 44, 3099-3105

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Ethylenediamine-Modified SBA-15 as Regenerable CO2 Sorbent Feng Zheng,* Diana N. Tran, Brad J. Busche, Glen E. Fryxell, R. Shane Addleman, Thomas S. Zemanian, and Christopher L. Aardahl Pacific Northwest National Laboratory, P.O. Box 999, MSIN: K6-28, Richland, Washington 99352

The CO2 adsorption properties of an ethylenediamine-modified mesoporous silica, EDA-SBA15, have been examined. Adsorption isotherms were collected by TGA measurements, and the breakthrough time and adsorption capacity were measured using a fixed-bed flow system. The EDA-SBA-15 sorbent adsorbs around 20 mg/g of CO2 from 15% CO2 in N2 at 25 °C and 1 atm total pressure. In pure CO2 at 1 atm, its adsorption capacity is 86 mg/g at 22 °C. The EDASBA-15 sorbent is fully regenerable by thermal swings during cyclic adsorption/desorption. Desorption of CO2 occurs at 110 °C on EDA-SBA-15 and the sorbent is stable in air up to 200 °C. The CO2 uptake by EDA-SBA-15 is not influenced by humidity. The adsorption capacity data are compared with those of previously reported amine-modified silica sorbents. Introduction The linkage between anthropogenic CO2 and global climate change has been proposed. Significant reduction of CO2 emission from the current level is necessary to stabilize atmospheric concentration of CO2. It is vital to the success of carbon management to capture CO2 from large point sources of emission such as fossil fuelbased power plants.1 The current technologies deployed at commercial scale for CO2 scrubbing from power plant flue gas are based on absorption using liquid amines. The major drawbacks of these liquid amine absorbents are the large amount of energy required for regeneration, equipment corrosion, and solvent degradation in the presence of oxygen. Regenerable solid sorbents as an alternative can potentially offer several advantages over liquid amine systems for postcombustion CO2 removal such as reduced toxicity and corrosiveness. Regenerable solid CO2 sorbents are also favorable for applications in enclosed environments, such as submarines and spacecraft. While physical sorbents such as zeolites and carbon molecular sieves can reversibly adsorb a large quantity of CO2 at room temperature, their capacity diminishes quickly at elevated temperature and the selectivity over water is poor. The incorporation of organic amines into a porous support is a promising approach for CO2 sorbents combining good capacity and selectivity at moderate temperature. Amine functional groups are useful for CO2 removal because of their ability to form ammonium carbamates and carbonates reversibly. Some of such solid amine sorbents reported in the literature are reviewed below. The regeneration temperature of these sorbents is typically from 50 to 100 °C. Various porous supports impregnated with liquid amines have been reported and such hybrid sorbents have been used successfully onboard space vehicles for crew air scrubbing.2-5 However, loss of amine components due to evaporation is a problem at moderate temperature. By grafting of amine functional groups directly to the surface of a physical sorbent, the evaporation problem is eliminated and the overall thermal stability can be improved. The ability of amine-modified silica gels and polymers to reversibly adsorb CO2 has been previously demonstrated.6-9

Alkylamine groups can be grafted on to silica by the condensation of appropriate aminoalkoxysilanes. In the literature, simple amine functional groups such as 3-aminopropyl group have been examined for their CO2 uptake properties.6,7,10,11 For example, Leal et al. reported IR and NMR characterization of silica gels grafted with aminopropyl groups and measurements of CO2 adsorption isotherms.6 The adsorption capacity in pure CO2 at 1 atm and 23 °C is about 27 mg/g. CO2 uptake measurements and IR spectroscopy data suggest that in the absence of water it takes two surface amino groups to capture one CO2 molecule in the form of an ammonium carbamate salt. However, when water is available, each surface amino group can capture one CO2 molecule as an ammonium bicarbonate adduct. Huang and co-workers showed that aminopropylMCM-48 adsorbs significantly more CO2 (∼90 mg/g) at 25 °C from pure CO2 at 1 atm than previously reported for modified silica gel.10 IR spectroscopy on sorbent wafers confirmed the identity of the surface amine groups. Their data also revealed that the aminopropyl groups are stable up to 200 °C. Adsorption/desorption isotherms and cyclic adsorption/desorption measurements showed that CO2 adsorption is reversible. Their sorbent could be regenerated by heating to 75 °C in helium. Adsorption characteristics of other gases relevant to the purification of natural gas were also reported. In the presence of water, the amount of CO2 adsorbed doubled as the adsorption mechanism changes from carbamate formation to bicarbonate formation. Temperature-programmed desorption data show that CO2 desorption peak is 10 °C higher when water is present during adsorption. The authors suggested that the CO2-amine bonding was enhanced in the presence of water. Chang et al. reported that aminopropyl-SBA-15 can adsorb up to 17.6 mg of CO2 per gram of sorbent at 25 °C from a gas mixture of 4% CO2 and helium.11 The sorbent could be regenerated by heating to 100 °C and is stable during repeated adsorption and regeneration cycles. Regeneration in He/H2O flow enhanced the CO2 adsorption capacity of the sorbent. The identity of the surface species and their evolution during CO2 adsorption/desorption was probed by in situ diffuse reflectance FTIR. As CO2 adsorbed, the IR absorbance peaks of

10.1021/ie049488t CCC: $30.25 © 2005 American Chemical Society Published on Web 03/25/2005

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monodentate carbonate, bidentate carbonate, monodentate bicarbonate, and bidentate bicarbonate increased. During temperature-programmed desorption, the reverse happens and the N-H and C-H peaks increased. When desorption was done in He flow, bidentate bicarbonate desorbed strongly at 97 °C while other adsorbed species desorbed at 57 °C. When water was present during desorption, however, there was only one CO2 desorption peak and all of the adsorbed species eluted simultaneously. The SBA-15 support used in this work had relatively low BET surface area (∼200 m2/g) and the authors proposed that by using higher surface area samples the CO2 adsorption capacity could be higher than that of polyethyleneimine-based commercial sorbents.12-15 Aresta and Dibenedetto studied the reversible absorption of CO2 by several amino-silane liquids.16 The absorption of CO2 by N-[3-(trimethoxysilyl)propyl] ethylenediamine approaches a 1:1 molar ratio at 22 °C and is reversible upon heating to 60 °C. They have shown that the reaction product was an intramolecular carbamate salt. The authors also obtained a modified solgel sorbent using the above ethylenediamine silane. The resulting sorbent is active for cyclic CO2 adsorption/ desorption but no details on the adsorption capacity and kinetics were reported. Diaf et al. reported thermally reversible adsorption of CO2 by polystyrene-based copolymers with covalently attached diamine functional groups such as ethylenediamine.8 Compared to the linear copolymers, the crosslinked copolymers showed higher CO2 adsorption capacity due to their porous structures. It was shown that the CO2 adsorption capacity increased with the amine content but the CO2 diffusion coefficient exhibited a maximum at moderate amine content. At 0.4 mole fraction of ethylenediamine comonomers, the crosslinked copolymer showed good CO2 adsorption capacity around 92 mg/g of polymer in pure CO2 at 25 °C. No adsorption kinetics data were reported but as the BET surface area of the cross-linked copolymer was relatively low at 50 m2/g, the adsorption kinetics was likely limited by diffusion through the polymer phase. It is conceivable, based upon the results discussed above, that better CO2 sorbents may be produced by covalently tethering choice amine terminal groups as a monolayer on a higher surface area support. Mesoporous silica SBA-15 as a support material can provide the desired high surface area in a highly porous structure that favors facile mass transport. The siloxane framework also provides excellent thermal stability. The formation of the intramolecular carbamate salt between ethylenediamine and CO2 proceeds at a 1:1 mole ratio and therefore ethylenediamine terminal groups can potentially provide higher CO2 capacity than monoamine groups, which react with CO2 at a 2:1 mole ratio in the absence of water. The inherently fast kinetics of intramolecular carbamate formation is also an advantage for ethylenediamine terminal groups. The goal of the present work is to synthesize and characterize the CO2 adsorption properties of a new solid sorbent material, EDA-SBA-15. The EDA-SBA-15 sorbent was prepared by the deposition of a monolayer of N-[3(trimethoxysilyl)propyl] ethylenediamine onto the surface of SBA-15 mesoporous silica. In this manuscript we report the CO2 adsorption capacity and thermal stability of the EDA-SBA-15 sorbent.

Materials and Methods Material Synthesis. The SBA-15 mesoporous silica substrates were prepared following established procedures.17 Briefly, 80.2 g of BASF Pluronic P123 block copolymer was dissolved with stirring at 40 °C in a solution of 2000 mL of water and 400 mL of concentrated HCl. Next, 170 g of tetraethyl orthosilicate (TEOS) was added to the solution with stirring at 40 °C for 16 h, followed by aging of the mixture at 92 °C for 18 h without stirring. The solid product was separated, washed with deionized water, and air-dried at room temperature. After calcination in air at 500 °C for 6 h, 60.6 g of SBA-15 product was obtained. The BET surface area of the SBA-15 substrate was 700 m2/g and the pore diameters ranged from 5.5 to 7.8 nm. The synthesis of a typical EDA-SBA-15 batch began by dispersing 10 g of SBA-15 silica in 150 mL of toluene and 3.2 mL of water and stirring the mixture for 1 h for surface hydration. Next, 10 mL of N-[3-(trimethoxysilyl)propyl] ethylenediamine (EDA-silane) was added and the reaction mixture was heated to reflux. The amount of EDA-silane was approximately one monolayer equivalent (approximately 2.3 silane groups per square nanometer, as measured by solid state 13C NMR). After 4 h at reflux, the reflux condenser was removed and replaced with a short-path still-head and the methanol and water azeotrope were distilled off. After the distillation was complete, the reaction mixture was allowed to cool, and product was collected by filtration, washed twice with 100 mL isopropyl alcohol, and air-dried. Sorbent Characterization. The unmodified and the EDA-modified SBA-15 mesoporous silica samples were characterized by nitrogen adsorption measurements for BET surface area using a Quantachrome Autosorb system. Fourier transform infrared (FTIR) spectra were collected on a Nicolet Magna 860 spectrometer with 4 cm-1 resolution using KBr pellets at room temperature in nitrogen flow. Additional FTIR data were collected using thin wafers of the SBA-15 samples pressed on to tungsten grids. The sample wafer was mounted in a reactor vessel (In-Situ Research Instruments) that allows temperature control, gas flow, and infrared transmission through the wafer. Solid-state 29Si nuclear magnetic resonance (NMR) experiments were performed at frequencies of 75.0 and 59.3 MHz on a Chemagnetics spectrometer. The samples were loaded in 7-mm zirconia rotors and spun at 3.0 kHz in a variable temperature double-resonance probe. Single-pulse Bloch-decay methods were used with 1H decoupling. Typically 1000-3000 scans were collected using 4.5-µs (90°) 29Si pulses and a repetition delay of 60 s. A weighed amount of tetrakis(trimethylsilyl)silane (TTMS) was added to the samples as the internal reference and the loading of EDA-silane was calculated based on TTMS peak areas. 29Si NMR chemical shifts were referenced to tetramethylsilane. Thermal gravimetric analysis (TGA) measurements were made using a Netzsch STD 409 system coupled with a mass spectrometer. The thermal stability of the modified SBA-15 in helium and air was studied with a temperature ramp of 10 °C /min to 650 °C. CO2 adsorption isotherms were also measured using the TGA system from 0 to 750 Torr CO2 partial pressure. Sorption Studies. Breakthrough measurements and temperature-programmed desorption (TPD) were carried out using a fixed-bed flow system shown in Figure 1. The sorbent beds were 5.8 mm in diameter and 30

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Figure 1. The flow system for CO2 sorbent testing.

mm in length. The sorbent materials were supported in the pores of reticular vitreous carbon foam pellets (EG&G Duocell, 80 pores per inch), which improved the thermal response of the sorbent bed. The CO2 concentration in the feed gas was controlled at the desired level from 1 to 100 vol % with balance N2. The gas flow to the sorbent bed was switched between the CO2/N2 mixture and pure argon using a four-way valve. The flow rates of all gas streams were controlled by mass flow controllers. Up to 2% water vapor by volume was added to the feed gas in some experiments. Deionized water was injected by a syringe pump to an evaporator, in which the water was vaporized at 140 °C and the steam was blended with the dry feed gas. The concentration of water vapor was limited such that the sorbent bed temperature was above the dew point. The gas composition downstream of the sorbent bed was continuously monitored by a quadrupole mass spectrometer (Kurt J. Lesker Quad 200) through a glass capillary sampling line. In a typical experiment, pure argon was initially flowed through the sorbent bed at 20-40 sccm. Then the gas flow was switched to the CO2 mixture at the same flow rate. Upon complete breakthrough of CO2, as indicated by the downstream gas composition reaching that of the feed gas, the sorbent bed was purged with pure argon and the sorbent bed was heated at the rate of 0.2-1 °C/s to remove the CO2 adsorbed. Desorption temperature was typically 135-150 °C. Results and Discussions Sorbent Characterization. The FTIR spectra of the unmodified and the EDA-modified SBA-15 sorbents are shown in Figure 2. When EDA groups are grafted to the surface of SBA-15, new IR absorption bands characteristic of alkylamines appeared while those due to Si-O-Si backbone vibrations remain essentially unchanged. In both spectra the strong absorption bands near 1200-1000 cm-1 are due to Si-O-Si asymmetric stretching vibrations. There are also the same absorption bands in both spectra at lower frequencies, not shown in Figure 1, that are assigned to the bending and symmetric stretching vibrations of the Si-O-Si bonds. For the unmodified SBA-15 silica, the broad band at 3432 cm-1 can be assigned to the O-H stretching vibrations of the hydrogen-bonded silanol groups and adsorbed water molecules. The unassociated silanol

Figure 2. FTIR spectra of (a) the SBA-15 substrate and (b) the EDA-modified SBA-15 (previously activated by heating to 135 °C in helium).

groups have a small but distinct absorption at 3746 cm-1. The 1629 cm-1 band is attributed to adsorbed water. For the EDA-modified SBA-15, the doublets at 3362 and 3299 cm-1 can be assigned to the amine N-H stretching vibrations, while the doublets at 2931 and 2881 cm-1 are assigned to C-H stretching vibrations of the methylene groups.18 The broad shape of the N-H stretching bands is due to hydrogen bonding. The free silanol O-H stretching band found in SBA-15 at 3746 cm-1 has disappeared and the absorption due to the stretching vibrations of associated O-H bonds has also been weakened considerably. Both of the above are in agreement with the expected changes on silica surface where the silanol groups have been consumed by the alkoxysilane condensation reaction. The peak at 1601 cm-1 can be assigned to N-H deformations of the primary amine groups. The absorption bands at 1457 and 1346 cm-1 are assigned to the CH2 scissoring and wagging vibrations, respectively. The identity of the peak at 1410 cm-1 is unclear but this peak is also present in the FTIR spectrum of the precursor EDAsilane (not shown) and therefore it is associated with EDA-silane structure. A typical 29Si NMR spectrum of the EDA-modified SBA-15 silica is shown in Figure 3. The resonances at -111, -101, and -94 ppm are assigned to the silicon atoms associated with the Q4 (internal siloxane), Q3 (isolated silanol), and Q2 (geminal silanol) groups, respectively.19-22 The resonance peaks of the above silicon species are broad and overlapping due to a large variation of bond angles typical of mesoporous silica materials. The resonance peaks at -69 and -64 ppm, assigned to the T3 and T2 silicon environments,19 respectively, correspond to the silicon atoms of the EDAsilane origins: (tSiO)3SiR and (dSiO)2Si(OH)R. The presence of the T3 and T2 species confirmed the covalent attachment of the EDA anchors to the silica surface via Si-O-Si linkages. Based on the peak area of the group T3 + T2 and that of the external standard TMMS, the loading density of the EDA ligands in the modified SBA15 was estimated to be 1.32 mmol/g. The surface density of the EDA terminal groups was estimated to be 2.3

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Figure 3. 29Si MAS NMR spectra of the as-synthesized EDAmodified SBA-15.

Figure 5. Thermal stability data of the EDA-modified SBA-15 sorbent in air: (a) TGA weight loss profiles; (b) simultaneous mass spectra of the eluted gas.

Figure 4. Thermal stability data of the EDA-modified SBA-15 sorbent in helium: (a) TGA weight loss profiles; (b) simultaneous mass spectra of the eluted gas.

ligands per square nanometer based on the above EDA loading and the BET surface area data. TGA Measurements. In Figure 4, the TGA weight loss profile of the EDA-SBA-15 sorbent in helium and simultaneous mass spectra of the gas phase are shown. Significant weight loss occurred when samples were heated to 99.3 and 435.9 °C. Carbon dioxide was released during the first weight loss event, which is indicated by peaks in the mass spectrum at a mass-tocharge ratio m/q ) 44 and 12. The CO2 released is believed to have been previously adsorbed by the sample from ambient air. The second weight loss peak, having an onset around 300 °C, was accompanied by significant increase in mass spectrometer signal intensity for species with a mass-to-charge ratio of 30 and 17. This

peak is identified as the detachment and decomposition of the EDA ligands because organic amines such as ethylenediamine, N-propyl-ethylenediamine, and propylamine all have major mass spec fragments at m/q ) 30. The increase on channel m/q ) 17 is likely due to ammonia as a decomposition product. The absence of change in m/q ) 30 signal during the first weight loss stage suggests that the EDA ligands remained tethered to the silica framework during CO2 desorption. The small weight loss between 100 and 400 °C was due to the condensation of the remaining silanol groups on the mesoporous silica surface. The thermal stability of the EDA-SBA-15 sorbent in air was also determined. The data plots are shown in Figure 5. Similar to the experiment in helium, when CO2 was released by desorption in air, simultaneous TGA weight loss peak and mass spectrometer peaks characteristic of CO2 gas were observed. However, the CO2 desorption occurred at a slightly higher temperature of 114.1 °C. The onset of the oxidization of the EDA anchors was around 200 °C and the destruction of the amine groups peaked at 224.6 °C, as indicated by a sharp increase in CO2 and ammonia (m/q ) 17) concentrations. At higher temperature around 320 °C some additional CO2 was released while ammonia concentration gradually declined, indicating a two-step oxidization process. In summary, the EDA-SBA-15 sorbent is thermally stable in helium up to 300 °C and the CO2 desorption temperature is about 100 °C. In air, the EDA-SBA-15 sorbent is resistant to oxidation up to 200 °C and the CO2 desorption temperature is slightly higher at 114 °C. The EDA ligands are stable upon heating in air or helium at temperatures required for CO2 desorption so it is possible to use the sorbent in cyclic adsorption/ desorption operations.

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Figure 6. CO2 concentration in the gas downstream of the sorbent bed during cyclic adsorption/desorption tests with a feed gas of 10 vol % CO2 in N2 at 20 sccm: (a) the first adsorption and breakthrough, (b) the first desorption, (c) the tenth adsorption and breakthrough, and (d) the tenth desorption.

Cyclic Adsorption/Desorption. The variation of the CO2 concentration in the gas downstream of the sorbent bed during the first and the last of 10 cyclic adsorption/ desorption operations, as measured by the mass spectrometer, was plotted in Figure 6. When the previously regenerated sorbent was exposed to a 10% CO2/N2 gas flow, initially no CO2 was detected downstream of the sorbent bed. When the sorbent bed approached saturation, CO2 breakthrough was observed and the CO2 concentration increased rapidly to feed-gas level, indicating fast adsorption kinetics. A CO2 peak was observed when the sorbent was subsequently subjected to a thermal swing to 135 °C in argon flow. The sorbent was fully regenerable, as evidenced by the same height of the two desorption peaks shown in Figure 6. It was found that the CO2 adsorption capacity was stable during the cyclic operations. The adsorption and desorption of CO2 on the EDASBA-15 sorbent were also studied using in situ FTIR to provide a more detailed picture of the surface reactions. The FTIR spectra collected during temperatureprogrammed desorption of CO2 in helium flow are shown in Figure 7. The sample was exposed to CO2 prior to the TPD. The FTIR spectra collected during subsequent readsorption of CO2 are shown in Figure 8. Reversible changes in infrared absorption bands were observed during these cyclic adsorption-desorption tests. With CO2 adsorbed on the EDA-SBA-15 sorbent, the 1576 cm-1 band can be assigned to the deformation of NH2+ or the combination of N-H deformation and C-N stretch vibrations in carbamates.18 It is known that EDA-silane readily reacts with CO2 to form an intramolecular carbamate ammonium salt.16 It is expected that our EDA-SBA-15 sorbent interacts with CO2 similarly as shown in Scheme 1. Upon heating, the 1576 cm-1 band decreased in intensity and shifted to higher wavenumbers as the amine N-H deformation band at 1601 cm-1 emerged. Additionally, the doublet at 3285 and 3360 cm-1 due to the N-H stretching vibrations also became more defined. The emergence of amine signature absorptions indicates that any previously adsorbed CO2 is readily desorbed upon heating of the EDA-SBA-15 sorbent. During CO2 adsorption the above changes were reversed, as can be seen in Figure 8. The carbamate N-H deformation band re-emerged at 1576 cm-1 while the

Figure 7. FTIR spectra of the EDA-SBA-15 sorbent during CO2 TPD in helium flow at various sorbent temperatures: from top to bottom, 30, 40, 50, 60, 70, 80, 90, and 100 °C.

Figure 8. FTIR spectra of the EDA-SBA-15 sorbent during CO2 uptake at various times of exposure to the CO2 stream: from bottom to top, 0, 6, 11, 16, 31, 51, and 73 min.

amine N-H stretch peaks broadened. The reversibility of the above infrared absorption signatures obviously agrees with the regenerability of the EDA-SBA-15 sorbent during cyclic operations. CO2 Adsorption Isotherms. The CO2 adsorption capacity of the EDA-SBA-15 sorbent was determined from the cyclic adsorption/desorption data using the flow system, and also independently from direct gravimetric measurements using TGA. The results are summarized in Figure 9. For comparison, the CO2 adsorption isotherm of the unmodified SBA-15 is also shown in the same figure, where the improvement in adsorption capacity due to the incorporation of amine functionality on the surface can be clearly seen. For the EDA-SBA15 sorbent, the adsorption isotherm based on the TGA

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each amine group can capture one CO2 molecule as bicarbonate ammonium salt:

Figure 9. CO2 adsorption isotherms of the EDA-SBA-15 sorbent (circles: with 2% water vapor; triangles: without water vapor, both based on breakthrough test data at 25 °C; diamonds: isotherm at 22 °C by TGA measurements), as compared to that of the unmodified SBA-15 at 20 °C without water vapor (squares; the fitted line is the Langmuir model).

Scheme 1. Formation of Intramolecular Carbamate Salt through the Reaction of CO2 with the Surface-Tethered EDA

data agrees with the capacity data from breakthrough tests within experimental error. One gram of the EDASBA-15 silica adsorbs around 20 mg of CO2 when exposed to 15% CO2 (by volume) in N2 at 25 °C and 1 atm in the flow system. At the same partial pressure of pure CO2, the sorbent uptakes about 25 mg/g of CO2 at 22 °C based on the TGA data. At 1 atm CO2 partial pressure, the adsorption capacity is 86 mg/g. The capacity of our current batch of the EDA-SBA-15 sorbent is comparable to or better than those of the aminopropyl-modified silica sorbents reported in the literature. For example, Leal et al. reported 27 mg/g capacity for aminopropyl silica gel in pure CO2 at 1 atm and 23 °C.6 Aminopropyl MCM-48 sorbent was reported to adsorb about 90 mg/g of CO2 at 1 atm and 25 °C,10 while aminopropyl SBA-15 was reported to have a capacity of 17.6 mg/g in a 4% CO2 mixture at 25 °C.11 The silane density is around 2.3 EDA-ligands/nm2 in our current EDA-SBA-15 sorbent. With improvements in EDA-silane deposition techniques, it should be possible to achieve higher site density and hence higher CO2 adsorption capacity. The presence of 2% water vapor did not influence the CO2 uptake. This behavior is different from those previously reported for aminopropyl-modified silica. Each aminopropyl group has one amine site, so two aminopropyl groups are necessary to bond one CO2 molecule if no water is present. When water is added,

CO2 + 2RNH2 f RNHCOO- + RNH3+

(1)

CO2 + H2O + RNH2 f RNH3+ + HCO3-

(2)

On the other hand, the reaction of CO2 and the EDASBA-15 is expected to be dominated by the formation of an intramolecular carbamate salt where each EDA groups can capture one CO2 molecule according to Scheme 1.16 The observed rapid uptake of CO2 on EDASBA-15 probably reflects the fast kinetics nature of the intramolecular type carbamate formation. Based on the experiments on the effect of water, it appears that such intramolecular carbamate salts are favored over bicarbonates on the EDA-SBA-15 sorbent even when water is present. Conclusions The present study has shown that EDA-modified SBA-15 mesoporous silica can adsorb and desorb CO2 reversibly. The stability of this solid sorbent over cyclic adsorption/desorption was established. The reaction of CO2 with the EDA-SBA-15 sorbent is believed to proceed via the formation of an intramolecular carbamate salt that provides favorable kinetics. The large pores and cross-channel connectivity of the SBA-15 silica also promote fast mass transport within the pore structures. The CO2 adsorption capacity of the EDA-SBA-15 sorbent is around 20 mg/g at 25 °C and 1 atm with 15% CO2 (by volume) in N2. This compares favorably to the adsorption capacity of aminopropyl-modified mesoporous silica previously reported. However, while the adsorption capacity of the latter decreases in the absence of water, the CO2 adsorption capacity of the EDA-modified mesoporous silica is not influenced by humidity. The adsorption capacity of the current EDA-SBA-15 sorbent is not adequate for the reduction of CO2 emission at large scale economically. Further increase in capacity is necessary. Based on the EDA-ligand loading density of the current EDA-SBA-15 sorbent, the CO2 adsorption capacity can be increased by using more efficient silane deposition methods. It is also conceivable that the affinity of the sorbent to CO2 can be tailored for improved adsorption capacity and desorption requirements by chemically modifying the terminal diamine groups. Acknowledgment This work was funded by the Carbon Management Initiative at the Pacific Northwest National Laboratory. PNNL is operated by Battelle for the U.S. Department of Energy under Contract DE-AC06-76RLO 1830. TGA and FTIR measurements were performed at the William R. Wiley Environmental Molecular Sciences Laboratory. Literature Cited (1) White, C. M.; Strazisar, B. R.; Granite, E. J.; Hoffman, J. S.; Pennline, H. W. J. Air Waste Manage. Assoc. 2003, 53, 645. (2) Xu, X. C.; Song, C. S.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Microporous Mesoporous Mater. 2003, 62, 29. (3) Xu, X. C.; Song, C. S.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Energy Fuels 2002, 16, 1463. (4) Chen, Z. W.; Chanda, M. J. Polym. Mater. 2002, 19, 381.

Ind. Eng. Chem. Res., Vol. 44, No. 9, 2005 3105 (5) Satyapal, S.; Filburn, T.; Trela, J.; Strange, J. Energy Fuels 2001, 15, 250. (6) Leal, O.; Bolivar, C.; Ovalles, C.; Garcia, J. J.; Espidel, Y. Inorg. Chim. Acta 1995, 240, 183. (7) Leal, O.; Bolivar, C.; Sepulveda, G.; Molleja, G.; Martinez, G.; Esparragoz, L. U.S. Patent 5087597, 1992. (8) Diaf, A.; Garcia, J. L.; Beckman, E. J. J. Appl. Polym. Sci. 1994, 53, 857. (9) Diaf, A.; Beckman, E. J. React. Funct. Polym. 1995, 27, 45. (10) Huang, H. Y.; Yang, R. T.; Chinn, D.; Munson, C. L. Ind. Eng. Chem. Res. 2003, 42, 2427. (11) Chang, A. C. C.; Chuang, S. S. C.; Gray, M.; Soong, Y. Energy Fuels 2003, 17, 468. (12) Birbara, P. J.; Filburn, T. P.; Nalette, T. A. U.S. Patent 5876488, 1999. (13) Birbara, P. J.; Nalette, T. A. U.S. Patent 5376614, 1994. (14) Birbara, P. J.; Nalette, T. A. U.S. Patent 5492683, 1996. (15) Birbara, P. J.; Nalette, T. A. U.S. Patent 5620940, 1997. (16) Dibenedetto, A.; Aresta, M.; Fragale, C.; Narracci, M. Green Chem. 2002, 4, 439.

(17) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (18) Socrates, G. Infrared and Raman Characteristic Group Frequencies, 3rd ed.; John Wiley & Sons: Chichester, 2001. (19) Huh, S.; Wiench, J. W.; Yoo, J. C.; Pruski, M.; Lin, V. S. Y. Chem. Mater. 2003, 15, 4247. (20) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 3767. (21) Lindner, E.; Schneller, T.; Auer, F.; Mayer, H. A. Angew. Chem., Int. Ed. 1999, 38, 2155. (22) Maciel, G. E.; Sindorf, D. W. J. Am. Chem. Soc. 1980, 102, 7606.

Received for review June 11, 2004 Revised manuscript received January 20, 2005 Accepted February 28, 2005 IE049488T