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CO2 Absorption and Desorption in an Aqueous Solution of Heavily Hindered Alkanolamine: Structural Elucidation of CO2‑Containing Species Young-Seop Choi,‡,§ Jinkyu Im,†,§ Jun Kyo Jeong,† Sung Yun Hong,† Ho Gyeom Jang,‡ Minserk Cheong,† Je Seung Lee,*,† and Hoon Sik Kim*,† †

Department of Chemistry and Research Institute of Basic Sciences, Kyung Hee University, 26 Kyungheedae-ro, Dongdaemun-gu, Seoul 130-701, Republic of Korea ‡ Department of Chemistry, Korea University, 145 Anam-ro, Seougbuk-gu, Seoul 136-701, Republic of Korea S Supporting Information *

ABSTRACT: The pathways for the CO2 absorption and desorption in an aqueous solution of a heavily hindered alkanolamine, 2-(tbutylamino)ethanol (TBAE) were elucidated by X-ray crystallographic and 13C NMR spectroscopic analysis. In the early stage of the CO 2 absorption, the formation of carbonate species ([TBAEH]2CO3) was predominant, along with the generation of small amounts of zwitterionic species. With the progress of the absorption, the carbonate species was rapidly transformed into bicarbonate species ([TBAEH]HCO3), and the amounts of the zwitterionic species increased gradually. During desorption at elevated temperature in the absence of CO2, [TBAEH]HCO3 was found to transform into [TBAEH]2CO3, where CO32− strongly interacts with two [TBAEH]+ via hydrogen bondings.

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INTRODUCTION The chemical absorption of CO2 has received particular interest as the most practical technology to reduce the emission of CO2 from flue gases of fossil fuel-fired power plants.1−3 On industrial scales, aqueous solutions of alkanolamines, such as monoethanolamine (MEA), diethanolamine (DEA), and 2amino-2-methyl-1-propanol (AMP) are typically used as absorption media, due to their relatively rapid CO2 absorption at low partial pressures.4−8 However, the chemical absorption processes based on aqueous alkanolamine solutions have one or more drawbacks, including high energy costs during cooling or regeneration, poor cyclic capacity due to the formation of thermally stable carbamate species, or slow kinetics.9,10 In order to circumvent the problems associated with the use of aqueous alkanolamines, a variety of CO2 capture solvent systems have been developed, which are based on superbases,11−14 phase changing silicon compounds,15,16 diamines,10,17,18 cyclic amines,12−14,19 lipophilic amines,17,20 and amino acid.21,22 The modification of MEA by introducing a functional group or groups has also been attempted to improve the absorption and desorption performance of MEA.10,23−25 While the advances with these alternative CO2 capture systems are significant, there still remain many obstacles to overcome for their practical application, especially in terms of absorption rate, cost of capture media, and processability. Accordingly, tremendous efforts have been devoted to develop high performance alkanolamine-based CO2 capture solvents with low regeneration energy and high absorption capacities and rates, and as a result, considerable amounts of kinetic, © 2014 American Chemical Society

thermodynamic, and spectroscopic data have been accumulated on the CO2 absorption in alkanolamines.4,7,26−29 Nonetheless, the understanding of the nature of CO2-containing species involved is still primitive due to the lack of unambiguous evidence on the types of amine-CO2 salts formed during absorption and regeneration, especially for hindered amines.6 Recently, we have reported that the alcoholic solutions of the heavily hindered alkanolamines, 2-(t-butylamino)ethanol (TBAE) and 1-(t-butylamino)-2-propanol (TBAP), reversibly interact with CO2 in a 1:1 molar ratio through their hydroxyl groups, forming zwitterionic carbonate species, which completely desorb CO2 at 80 °C (see Scheme 1).30 In a continuation of our study on the CO2 absorption behavior in hindered amines, we have also investigated CO2 Scheme 1. Formation of Zwitterionic Species from the Interaction of a Heavily Hindered Amine with CO2 in An Alcoholic Solvent30

Received: Revised: Accepted: Published: 4163

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measured, 4312 independent reflections (Rint = 0.0443). The final R1 values were 0.0468 (I > 2σ(I)). The final wR(F2) values were 0.1206 (I > 2σ(I)). The final R1 values were 0.0699 (all data). The final wR(F2) values were 0.1369 (all data). The goodness of fit on F2 was 0.986. CCDC number CCDC 948841. Crystal Data for [TBAEH]2CO3 (2). C13H32N2O5, M = 296.41, monoclinic, a = 14.850(19) Å, b = 6.060(8) Å, c = 18.66(2) Å, α = 90.00°, β = 90.40(2)°, γ = 90.00°, V = 1679(4) Å3, T = 100(2) K, space group C2/c, Z = 4, 5171 reflections measured, 1121 independent reflections (Rint = 0.0321). The final R1 values were 0.0540 (I > 2σ(I)). The final wR(F2) values were 0.1414 (I > 2σ(I)). The final R1 values were 0.0679 (all data). The final wR(F2) values were 0.1487 (all data). The goodness of fit on F2 was 0.970. CCDC number CCDC 948842. Theoretical Investigation. The interaction of carbonate species with CO2 was theoretically investigated using the Gaussian 09 program.36 The geometry optimizations and thermodynamic corrections were performed with hybrid Becke 3-Lee−Yang−Parr (B3LYP) exchange−correlation functional with the 6-31+G* basis sets for C, H, N, and O. All stationary points were verified as minima by full calculation of the Hessian and a harmonic frequency analysis. The effect of bulk solvent was taken into account in the single point calculations through the self-consistent reaction field theory (SCRF) based on the Polarisable Continuum Model (IEFPCM-UFF) implemented in the Gaussian program.37,38

absorption and desorption in TBAE and TBAP in water in place of alcohols. We now report the X-ray structural and 13C NMR spectroscopic evidence on the speciation involved during the CO2 absorption and desorption in an aqueous solution of a heavily hindered amine, TBAE.

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EXPERIMENTAL SECTION General. TBAP was prepared according to the literature procedure.30,31 All other chemicals were purchased from Aldrich Chemical Co. and used as received. CO2 and 13CO2 with a purity of 99.9% were obtained from Shin Yang Gas Co. (Korea) and Cambridge Isotope Laboratories Inc., respectively. 13 C NMR spectra were recorded on a 400 MHz Bruker NMR spectrometer for room temperature experiments, and on a 600 MHz Varian NMR spectrometer for variable temperature experiments. Absorption and Desorption of CO2. Absorption and desorption measurements were conducted using an apparatus similar to that described elsewhere.32,33 In a typical experiment, 10 g of aqueous amine (30 wt %) solution was loaded into a glass tube (OD = 1.5 cm, L = 30 cm) equipped with a thermocouple, and inlet and outlet Teflon valves. The absorption tube was placed into a glass container tube (OD = 2.5 cm, L = 38 cm) attached on the lid of a balance (OHAUS, EP613C) with an accuracy of 0.001 g. The container tube was then carefully positioned in the middle of a vertical electric heater without contact with the heater wall. The temperature was controlled by means of a temperature controller. CO2 (99.9%) gas was introduced into the absorption tube containing an alkanolamine solution at 40 °C at a rate of 30 mL min−1 for 60 min. The weight change upon absorption of CO2 was monitored using the balance and recorded on a computer. Once the absorption was completed, the absorbed CO2 was desorbed at elevated temperature of 80 or 100 °C for 30 min by flowing N2 into the absorption tube at a rate of 20 mL min−1 using a mass flow controller (see Figure S1 in the Supporting Information, SI). For the absorption and desorption experiments using 13CO2, a 60 mL high pressure glass vessel containing 30 wt % TBAE in D2O was pressured with 0.1 MPa of 13CO2. Samples were withdrawn at certain intervals from the vessel through a sampling valve and subjected to 13C NMR analysis (see Figure S2 in the SI). Crystallographic Data and Packing Diagrams. Single crystal X-ray diffraction data for [TBAEH]HCO3 (1) and [TBAEH]2CO2 (2) were collected on a Bruker SMART APEXII diffractometer equipped with a CCD area detector using graphite monochromated MoKα radiation (λ = 0.71073 Å). Preliminary orientation matrix and cell parameters were determined from three sets of ω scans at different starting angles. Data frames were obtained at scan intervals of 0.5° with an exposure time of 10 s per frame at 100 K. The reflection data were corrected for Lorentz and polarization factors. Absorption corrections were carried out using SADABS.34 The structures were solved by direct methods and refined by full-matrix leastsquares analysis using anisotropic thermal parameters for nonhydrogen atoms with the SHELXTL program.35 All hydrogen atoms were calculated at idealized positions and refined with the riding models. Crystal Data for [TBAEH]HCO3 (1). C7H17NO4, M = 179.22, monoclinic, a = 8.424(3) Å, b = 9.711(3) Å, c = 23.089(6) Å, α = 90.00°, β = 93.937(19)°, γ = 90.00°, V = 1884.3(10) Å3, T = 100(2) K, space group P2(1)/c, Z = 8, 11 215 reflections

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RESULTS AND DISCUSSION CO2 Absorption and Desorption in TBAE. Recently, we have demonstrated that the alcoholic solution of the heavily hindered alkanolamines, 2-(t-butylamino)ethanol (TBAE) and 1-(t-butylamino)-2-propanol (TBAP), can be used as easily regenerable nonaqueous CO2 absorbents.30 We have also shown that, in an alcohol solvent such as methanol, ethylene glycol (EG), or propylene glycol (PG), TBAE and TBAP react with CO2 in a 1:1 molar ratio through their hydroxyl groups, forming zwitterionic carbonate species with a covalently bound carbonate group, which completely desorb CO2 at 80 °C (see Scheme 1). From these results, it has been anticipated that the aqueous solutions of TBAE and TBAP behave similarly to their alcoholic solutions toward the interaction with CO2, producing the same zwitterionic species. It has also been expected that the zwitterionic species formed in water solvent could desorb CO2 at around 80 °C. However, contrary to our expectation, the regenerations of the CO2-loaded aqueous solutions of TBAE and TBAP were far from completion even at the elevated temperature of 100 °C, implying that the zwitterionic carbonate species is not a major CO2-containing species in water solvent. As can be seen in Table 1, 30 wt % aqueous solutions of TBAE and TBAP showed the absorption capacities close to 1 mol CO2 per mole of amine, but their cyclic capacities at 80 °C were less than 50% of those achieved in alcohols. X-ray Crystallography. We were curious about the reason for such low cyclic capacities of TBAE and TBAP in water and thus attempted to structurally characterize the CO2-containing species of TBAE formed after absorption and during regeneration by X-ray crystallographic analysis. To this end, CO2 was absorbed into a 30 wt % aqueous solution of TBAE at 40 °C for 1 h and desorbed at 80 °C for 30 min by flowing N2. 4164

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173.2°, O7···O3 2.5845(18) Å, H7···O3 1.75 Å, ∠O7−H7··· O3 173.0°). Single crystals were also obtained from a CO2-loaded aqueous solution of TBAE desorbed at 80 °C under a N2 flow. Diffusion of acetone into the CO2-loaded aqueous solution of TBAE after a partial desorption of CO2 produced needle-type crystals suitable for X-ray structure determination. The X-ray structural analysis of the crystals clearly shows that the major species formed during the desorption is [TBAEH]2CO3 (2) composed of two [TBAEH]+ cations and a CO32− anion with C2 symmetry as shown in Figure 2 (see also

Table 1. CO2 Capture Performance of Hindered Amine Solutionsa absorbent TBAE/EG TBAP/EG TBAE/ water TBAP/ water TBAP/ watere

rich CO2 loading (mol/mol)b

cyclic capacityc regenerability (%)d

0.99 1.05 0.94

0.98 1.05 0.42

99 100 45

0.95

0.46

48

0.94

0.71

76

a

Absorption measurements were conducted with amine (30 wt %) solutions in EG or water at 40 °C for 1 h. Desorption was carried out at 80 °C for 30 min by N2 stripping.30 bmol CO2 absorbed/mol amine. c Rich CO2 loading−lean CO2 loading. d(Cyclic capacity/Rich CO2 loading) × 100. eDesorption at 100 °C.

Single crystals suitable for the X-ray structural analysis were grown by diffusing acetone into a CO2-loaded 30 wt % aqueous solution of TBAE at room temperature. The ORTEP drawing of the crystal structure shown in Figure 1 reveals that the major

Figure 2. ORTEP drawing of the crystal structure of 2 displayed with thermal ellipsoids at 50% probability levels. Hydrogen bonds are represented as dotted lines. O3* and O3′* have been generated by applying symmetry operation (x, y, z) and (x, y+1, z), respectively.

Figure S4 and Tables S7−S12 in the SI). The carbonyl oxygen atom of CO32− interacts with each N−H hydrogen atom of two [TBAEH]+ cations (N1···O2 2.715(3) Å, H1C···O2 1.82 Å, ∠N1−H1C···O2 163.9°). The other two oxygen atoms of CO32− are found to interact with each O−H hydrogen atom of two [TBAEH]+ cations through hydrogen bonds in the lattice (O1···O3 2.672(3) Å, H1···O3 1.85 Å, ∠O1−H1···O3 165.8°). Intermolecular hydrogen bonding interactions are also observed between the CO32− and two adjacent [TBAEH]+ cations (N1···O3* 2.707(3) Å, H1D···O3* 1.82 Å, ∠N1− H1D···O3* 162.5°). Such strong intra- and intermolecular hydrogen bonding interactions present in the crystal unit of 2 could be responsible for the lower cyclic capacity of TBAE in water. In consideration of the absorption capacity close to 1, the cyclic capacity around 0.5, and the isolation of 2 from the partially regenerated solution of 1, it is most likely that at least some portion of the bicarbonate species, 1 is transformed into the carbonate species, 2 during regeneration at elevated temperature in the absence of CO2. The driving force for the transformation of 1 into 2 at elevated temperature is believed to be the additional stabilization of 2 exerted by the presence of two hydroxyl groups of [TBAEH]+, forming a stable hydrogen bonding network among N−H, O−H, and CO32− (Figure 2). The interconversion between NaHCO3 and Na2CO3 is well established in the Solvay process.40 Nonetheless, it is worth mentioning here that the structural characterization of bicarbonate and carbonate species derived from hindered amines has not yet been reported. Moreover, the transformation of bicarbonate species into carbonate species has never been proven by X-ray crystallography.

Figure 1. ORTEP drawing of the crystal structure of 1 displayed with thermal ellipsoids at 50% probability levels. Hydrogen bonds are represented as dotted lines. N1*, H1C*, N2*, H2C*, and O5* have been generated by applying symmetry operation (x, y+1, z).

species formed by the interaction of CO2 and TBAE in water is a bicarbonate species, [TBAEH]HCO3 (1) (see Figure S3 and Tables S1−S6 in the SI). Each H atom attached to the N atom of amino group and O atom of hydroxyl group is found to interact strongly with an oxygen atom of bicarbonate anion (N1···O2 2.8155(16) Å, H1D···O2 1.96 Å, ∠N1−H1D···O2 154.0°, N1···O5* 2.7375(16) Å, H1C···O5* 1.86 Å, ∠N1− H1C···O5* 159.3°, N2···O2 2.8038(16) Å, H2D···O2 1.93 Å, ∠N2−H2D···O2 157.2°, N2···O5* 2.7110(16) Å, H2C···O5* 1.83 Å, ∠N2−H2C···O5* 160.6°, O8···O2 2.7638(16) Å, H8··· O2 1.94 Å, ∠O8−H8···O2 168.7°). Similar to the previous report,39 the bicarbonate anion (HCO3−) interacts with an adjacent HCO3− through hydrogen bonds, forming hydrogenbonded dimers around crystallographic center of inversion (O4···O6 2.5896(17) Å, H4···O6 1.75 Å, ∠O4−H4···O6 4165

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13

C NMR Spectroscopic Study. The formation of 1 and 2, and the transformation of 1 into 2 were investigated by 13C NMR spectroscopy in D2O. As can be seen in Figure 3, the

Figure 4. 13C NMR (400 MHz, D2O, 25 °C) spectra of 30 wt % TBAE solution in water under 0.1 MPa 13CO2 for (a) 5 min, (b) 10 min, and (c) 20 min. Figure 3. 13C NMR (400 MHz, D2O, 25 °C) spectra of (a) 30 wt. % TBAE in water, (b) 30 wt % TBAE solution in water under 0.1 MPa CO2 for 30 min, (c) crystals of 1, and (d) desorption of (b) for 30 min at 80 °C under N2 flow, and (e) crystals of 2.

soon as it forms (vide infra).41−46 After 10 min under 13CO2 pressure, the peak at 164.1 ppm shifted to 162.3 ppm, suggesting that 2 started to transform into 1 as the decrease in TBAE concentration (Figure 4(b)). In contrast, the chemical shift of the small peak at 158.2 ppm remained almost unchanged. The carbonyl peak at 162.3 ppm shifted further to 160.5 ppm in the 13C NMR spectrum of the sample taken after 20 min (Figure 4(c)) and remained constant at 160.5 ppm on further contact with 13CO2 (not shown here), indicating that all the carbonate species were converted into bicarbonate species. This is reasonable because free TBAE would not be present at this time, and thus the reaction of 1 with TBAE to afford 2 is no longer possible. It is interesting to notice that the intensity of the unknown peak at 158.2 ppm increased gradually with the progress of CO2 absorption. It is well recognized that the major CO2-containing species generated during the CO2 absorption in aqueous alkanolamines are carbamate, bicarbonate, and carbonate species, and their composition varies with the basicity and structure of alkanolamines.4,41,42,44,46 In this context, the presence of the peak at around 158 ppm is rather unusual. Considering the chemical shift of the carbonyl group and the X-ray structural analysis results, it is unlikely that the unknown carbonyl peak belongs to any of the bicarbonate, carbonate, or carbamate species. One possible assumption is that zwitterionic carbonate species (3) is produced from the interaction of 13CO2 with TBAE in water, similar to the reaction of CO2 with TBAE in an alcohol solvent (Scheme 1). However, in consideration of peak intensity relative to those of carbonate and bicarbonate species, the pathway leading to the formation of zwitterionic carbonate species from the direct interaction of TBAE and CO2 seems to be thermodynamically and kinetically unfavorable. To more precisely identify the species responsible for the peak at 158.2 ppm, several 13C NMR experiments were conducted. For this purpose, TBAE was reacted with CO2 in EG for 30 min, and the supposedly zwitterionic species was isolated as white powders by adding diethyl ether. As can be seen in Figure S5 (see the SI), the zwitterionic carbonate species generated in EG exhibited a peak 158.0 ppm in DMSO-d6. When small amounts of D2O were added, a new peak appeared at 159.8, a characteristic peak of bicarbonate species, along with the concomitant loss of intensity of the peak at 158.0 ppm. This indicates that, in the absence of CO2 pressure, zwitterionic species could be converted into the bicarbonate species by the reaction of D2O. Variable temperature 13C NMR experiments were also conducted in the range of 30−80 °C to see the effect of temperature on the formation of the zwitterionic species, 3

CO2-saturated 30 wt % TBAE solution in water shows a distinct and characteristic carbonyl peak assignable to a bicarbonate species at 160.1 ppm.41−44 The NMR spectrum of the crystals of 1 also showed a carbonyl peak at 161.1 ppm, strongly suggesting that the bicarbonate species, 1 is generated almost exclusively during the absorption of CO2 in an aqueous solution of TBAE. When the CO2-loaded solution of TBAE was heated at 80 °C for 30 min under a nitrogen atmosphere, the bicarbonate carbonyl peak at 160.1 ppm shifted downfield at 165.2 ppm. Such a downfield shift of the carbonyl peak could be attributed in part to the increased basicity of the solution by the presence of basic TBAE formed during the regeneration at elevated temperature.41−44 However, in consideration of the degree of peak shift of 5.1 ppm and the observation of the carbonyl peak at 164.9 ppm in the NMR spectrum of the crystals of 2, it is assumed that the carbonate species, 2 exists in large quantities in the partially regenerated solution of 1.41−46 Although it was not possible to precisely determine the ratio of 2/1 in the above regenerated solution by NMR spectroscopy, the X-ray crystallographic analysis results suggest that 1 could be converted into 2 during regeneration process at an elevated temperature. The pathways for the CO2 absorption and desorption in an aqueous solution of TBAE was further investigated by 13C NMR spectroscopy in D2O. To more clearly observe the species involved during the CO2 absorption and desorption, labeling experiments were conducted using 13CO2 instead of CO2. For 13C NMR analysis, a 60 mL high pressure glass vessel containing 30 wt % TBAE in D2O was pressured with 0.1 MPa of 13CO2 (see Figure S2 in the SI). Samples were withdrawn at certain intervals from the vessel through a sampling valve and subjected to 13C NMR analysis. As can be seen in Figure 4(a), the 13C NMR spectrum of the sample after 5 min shows the two carbonyl peaks at 164.1 and 158.2 ppm. The intense peak at 164.1 ppm can be attributed to the formation of carbonate species. The generation of carbamate species involving C−N bond formation is highly unlikely in consideration of the steric crowding exerted by the bulky t-butyl group bonded to the nitrogen atom of TBAE.30 In the early stage of the CO2 absorption, the solution would be quite basic due to the presence of free TBAE and thus the formation of 2 should be predominant because 1 would react with TBAE to give 2 as 4166

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2, the carbonyl peak at 165.1 ppm shifted to 160.5 ppm, indicating that 2 was transformed into 1. Interestingly, the zwitterionic carbonate species observed during the 13CO2 absorption in TBAE could not be clearly seen in the 12CO2 absorption in TBAE. This again implies that the zwitterionic carbonate species, 3 is generated only in small quantities. The pathway leading to the formation of the zwitterionic species is not clear at the moment, but it is likely that the carbonate species, 2, is responsible for the formation of 3. The transformation of the bicarbonate into the carbonate salt was further studied as a function of temperature in the range from 30 to 80 °C. As the variable temperature NMR experiment using a high pressure tube failed to reflect the phenomenon occurring during regeneration, 13C NMR experiments were conducted with a 30 mL D2O solution of TBAE (30 wt %) in a 100 mL three neck flask equipped with an inlet and an outlet valves. To obtain a CO2 loaded TBAE solution, CO2 flowed into the TBAE solution at 30 °C for 30 min. After the completion of the CO2 absorption, the CO2 flow was stopped, and the temperature of the CO2-loaded solution was raised stepwise to desired temperatures. Samples for NMR experiments were withdrawn from the flask after 20 min of stirring at the specified temperature. Figure S8 in the SI shows that the CO2 saturated solution at 30 °C contains mostly bicarbonate species in consideration of the chemical shift of the carbonyl peak at 160.7 ppm. The bicarbonate carbonyl peak is found to continuously shift downfield with the temperature rise up to 165.2 ppm at 80 °C, again demonstrating that the transformation of 1 into 2 is more facilitated at higher temperature. On the basis of spectral data, equilibrium concentrations of [HCO3−] and [CO32−], and their molar ratios at a given temperature were roughly estimated using literature methods.26 As listed in Table S13 in the SI, the molar ratio of [HCO3−]/[CO32−] was varied from 18.6 to 0.6 when the temperature was increased from 30 to 80 °C, supporting the eqs (4)−(6) in Scheme 2 (vide infra).

using a high pressure NMR tube (see Figures S6 and S7 in the SI). Comparison of the 13C NMR spectra taken at various temperatures for the absorption of 13CO2 (0.2 MPa) in a D2O solution of TBAE shows that the intensity of the peak at 158.2 ppm corresponding to the zwitterionic species decreases with the temperature rise. Reverse phenomenon was observed when the temperature was varied from 80 to 30 °C, suggesting that 3 is thermally less stable than the bicarbonate species, and the formation of 3 is more facilitated in the presence of 2. The carbonyl peak at 160.3 ppm assignable to the mixture of carbonate and bicarbonate species is also found to shift to upfield with the temperature rise, indicating that the formation of 1 is more favored at higher temperature. This is in contrast to the phenomenon observed during desorption in the absence of CO2. However, this can be rationalized in consideration that all the spectra were taken under the CO2 pressure of 0.2 MPa. Under a high pressure of CO2, the transformation of 1 into 2 is greatly restricted even at elevated temperatures because free TBAE regenerated from 1, 2, and/or 3 would react more rapidly with acidic CO2 as soon as it forms than with 1, leading to the predominant formation of 1. Figure 5 shows the 13C NMR spectra for the stripping pathway of 13CO2-containing species. When the aqueous

Figure 5. 13C NMR (400 MHz, D2O, 25 °C) spectra of (a) 30 wt % TBAE solution in water under 0.1 MPa 13CO2 for 30 min, (b) after desorption of (a) at 80 °C for 30 min under N2 flow, (c) after addition of TBAE into (a), and (d) after treatment of (c) with 0.1 MPa 13CO2 for 30 min.

Scheme 2. Pathways to the Formation of 1, 2, and 3 during the Absorption and Desorption of CO2 in an Aqueous Solution of TBAE

solution of 13CO2-loaded TBAE was heated at 80 °C for 30 min, the bicarbonate carbonyl peak at 160.5 shifted to 166.1 ppm (Figure 5(b)). The conversion of 1 into 2 at room temperature in the absence of free TBAE and CO2 pressure was excluded because the CO2-loaded solution exhibited the carbonyl peak assignable to bicarbonate species at around 160 ppm in the 13C NMR spectrum even after storing for 2 d at an ambient temperature. The decomposition of 1 into CO2 and TBAE started to take place at elevated temperature of 80 °C. Once TBAE is generated, the solution becomes basic and thus the reaction of TBAE with 1 to give 2 is facilitated. This again indicates that 1 is converted into 2 under the stripping condition. The absence of the peak at around 158 ppm after stripping at 80 °C strongly suggests that the desorption of 13 CO2 from the zwitterionic species is easier than that from bicarbonate or carbonate species. The transformation of bicarbonate into carbonate species during the stripping process is further supported by the NMR spectrum of the 1:1 mixture of bicarbonate species and free TBAE, which shows a single carbonyl peak at 165.1 ppm assignable to the carbonate species (Figure 5(c)). When 13CO2 was added to the mixture of 1 and

Theoretical Investigation. To examine the possibility of the formation of 3 from 2, computational calculation was conducted at the B3LYP level of the theory (6-31+G* for C, H, N, and O) using the Gaussian 03 program. Figure 6 shows the optimized structures for the interaction of 2 with CO2: reactant (carbonate species, 2 and CO2), transition state, and product. The numbers in parentheses are the energies relative to that of the reactant. From the optimized structures of the reactant and the transition state, it is likely that the interaction between the oxygen atom of the hydroxyl group and CO2 is assisted by the 4167

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Figure 6. Optimized structures showing the interaction of 2 with CO2: (a) reactant (2 + CO2, ΔG = 0.0 kcal mol−1), (b) transition state (ΔG‡ = +18.6 kcal mol−1), and (c) product (3 + 1, ΔG = −4.9 kcal mol−1).

strong hydrogen bond interaction between CO32− and the hydroxyl groups of [TBAEH]+, thereby leading to the simultaneous formation of 1 and 3. The activation energy (ΔG‡) and the Gibbs free energy of formation (ΔG) were calculated as 18.6 and −4.9 kcal mol−1, respectively. Although the activation energy is not sufficiently low enough for the process to take place smoothly, the negative ΔG value implies that the transformation of 2 into 1 and 3 is a feasible process. In combination of NMR and structural analysis results, the absorption and desorption of CO2 in an aqueous solution of TBAE seem to proceed according to eqs (1)−(8) as shown in Scheme 2. We have elucidated CO2 absorption and desorption pathways in an aqueous solution of heavily hindered amine, TBAE, by X-ray crystallography and 13C NMR spectroscopy using 13CO2. In summary, in the very early stage of the CO2 absorption in an aqueous solution of TBAE, carbonate species, 2 is generated as the predominant CO2-containing species. On further contact of 2 with CO2, 2 starts to transform into 1 by the reaction with CO2 and water (see eq (2) of Scheme 2). Zwitterionic species, 3 can also be produced as a minor CO2containing species. The desorption of CO2 from the CO2containing species of TBAE follows the reverse pathway of the absorption of CO2 in TBAE. For a deeper understanding of the absorption and regeneration mechanism, the formation of all kinds of possible CO2-containing species should be considered and characterized. In this context, the X-ray structural and spectroscopic characterization of CO2-containing species of TBAE would provide valuable information to those involved in the development CO2 capture processes using amine-based aqueous solvents.

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NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +82-2-966-3701. E-mail: [email protected] (H.S.K.); [email protected] (J.S.L.). Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea CCS R&D Center (KCRC) grant funded by the Korean government (Ministry of Science, ICT & Future Planning) (NRF-2011-0031972). We wish to thank the Korea Institute of Science and Technology for 600 MHz NMR data acquisition.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic diagram and photograph of experimental equipment used for CO2 absorption/desorption experiments, packing diagrams for 1 and 2, tables for structural information, and 13C 4168

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