Structure of Monoethanolamine and Diethanolamine Carbamates in

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Ind. Eng. Chem. Res. 2010, 49, 6–13

Structure of Monoethanolamine and Diethanolamine Carbamates in Aqueous Solutions Determined by High-Energy X-ray Scattering Hiroshi Deguchi,*,† Yoshiyuki Kubota,† Yasuyuki Yagi,† Ikuko Mitani,† Yoshihiro Imai,† Masahiko Tatsumi,† Noriko Watari,‡ Takuya Hirata,§ and Yasuo Kameda| Power Engineering R&D Center, The Kansai Electric Power Co., Inc., 3-11-20, Nakoji, Amagasaki, Hyogo 661-0974, Japan, AdVanced Technology Research Center, Mitsubishi HeaVy Industries, LTD., 1-8-1, Sachiura, Kanazawa-ku, Yokohama 236-8515, Japan, Hiroshima Research & DeVelopment Center, Mitsubishi HeaVy Industries, LTD., 6-22, 4-chome, Kan-on-shin-machi, Nishi-ku, Hiroshima 733-8553, Japan, and Department of Material and Biological Chemistry, Faculty of Science, Yamagata UniVersity, Yamagata 990-8560, Japan

A more detailed understanding of interactions between amine solution and carbon dioxide (CO2) is required for economical capture and recovery of CO2 emitted from thermal power plants. In order to investigate the molecular structure of the carbamate molecules which originate from CO2 gas, a high-energy X-ray scattering method was applied to monoethanolamine and diethanolamine aqueous solutions with several concentrations of CO2. Distinct peaks attributable to C-O (0.126 nm) and O · · · O (0.223 nm) interactions within the carbamate molecule were found in the difference distribution function derived from observed interference functions. To our knowledge, this is the first observation of the CO2-captured molecule. The experimental difference distribution functions were well reproduced by theoretical difference distribution functions evaluated from information obtained from NMR and computer conformation analysis. The present results indicate that the X-ray scattering method can contribute to the structural analysis of amine-H2O-CO2 systems. Introduction Problems associated with increasing greenhouse gases is an urgent subject to be solved in this century. In a rough estimation, one-third of the total emissions of carbon dioxide (CO2) gas is now caused by the energy supply sector.1-3 Reducing emissions of CO2 from thermal power plants is therefore indispensable. In order to reduce the emission of CO2 from the thermal power plants, it is necessary to capture CO2 in the flue gas. A chemical absorption method4-8 is suitable for capturing lowly concentrated, low pressure, and a large amount of CO2 like the flue gas. Typically, CO2 in the flue gas is chemically captured by aqueous amine solutions. The captured CO2 can be recovered by heating the solution. However, the current technology of the chemical absorption system costs substantial energy to recover CO2 from the amine solution. When an aqueous solution of monoethanolamine is used as the absorber, the thermal energy of about 900 kcal/kg-CO2 is required for recovering CO2.5 Since the thermal energy is provided by the steam generated in the boiler, the efficiency of generating the electric power should be considerably lowered. One way to solve this problem is to develop a more efficient solution system. In order to design a new solution, it is necessary to understand how the recovering energy is determined at the molecular level. The binding strength between amine and CO2 influences the recovering energy. Since the structure of CO2bound amine can be thought to reflect the binding strength, clarification of the structure of CO2-bound amine will help to reveal the binding strength. It is reported that water molecules * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +81-6-6494-9700. Fax: +81-66494-9703. † The Kansai Electric Power Co., Inc. ‡ Advanced Technology Research Center, Mitsubishi Heavy Industries, LTD. § Hiroshima Research & Development Center, Mitsubishi Heavy Industries, LTD. | Yamagata University.

stabilize the amine-CO2 complex by forming hydrogen bonds in the 2-amino-2-methyl-1-propanol(AMP)-H2O-CO2 system using density functional theory (DFT) calculations.9 The existence of the hydrogen bonds also affects the recovering energy. Thus, it is important to analyze the local structure around CO2 to determine the dominating factor which contributes to the recovering energy. However, the structure of the hydrated carbamate molecule has not been satisfactory elucidated. The NMR method is frequently employed to distinguish chemical species in the amine-H2O-CO2 solutions. Composition of the chemical species has been investigated for solutions involving monoethanolamine (MEA), butyl-ethanolamine (BEA), methyl-diethanolamine (MDEA), and 2-[(2-aminoethyl)amino]ethanol.10-12 These studies showed that the captured CO2 exists mainly as amine carbamate and HCO3-. However, detailed information on the structure of the carbamate molecule cannot be provided by NMR. IR and Raman spectroscopic methods have been applied to analyze the structure of the amineH2O-CO2 system.13-15 These spectroscopic methods can provide information on chemical bonds within the constituent molecules. Efforts to combine results from the IR/Raman and ab initio calculation have been made to assign observed vibrational bands; however, definite information on the molecular geometry and interatomic bond lengths in the carbamate molecule has not been obtained from these spectroscopic methods. On the other hand, neutron and X-ray scattering methods are well-known tools for the structural analysis of disordered materials such as liquids and amorphous solids.16-19 Compared to the spectroscopic method, scattering methods have an advantage that the molecular structure can be derived directly through the observed distribution function. However, there are few studies that apply the scattering method to the aqueous amine solutions. In the present work, we applied the high-energy X-ray scattering method to the amine-H2O-CO2 systems to investigate the molecular structures of the carbamate molecules. We

10.1021/ie9009556  2010 American Chemical Society Published on Web 11/23/2009

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010

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derived distribution functions around the CO2-captured molecule in MEA and diethanolamine (DEA) aqueous solutions with several concentrations of the CO2. As a result, distinct peaks were observed in the difference distribution function. NMR measurements and computer conformation analysis were implemented to obtain theoretical difference distribution functions of the CO2-captured molecules. We discuss the local structures of amine carabamates by comparison of the theoretical and experimental difference distribution functions. To our knowledge, this is the first observation of the CO2scaptured molecule in the amine-H2O-CO2 system.

scattering factor for the constituent atoms was evaluated from analytical expression proposed in the International Tables for Crystallography.20 Incoherent scattering intensities were calculated by interpolating the tabulated values from Cromer.21 Data normalization procedure was carried out by applying the highangle region method,22 where the corrected observed intensities were adjusted to the theoretical values, sum of the coherent and the incoherent intensity, in the high-Q region (180-220 nm-1). The observed total interference term, i(Q), is given by

Method and Materials

where

Materials. MEA (>99%, Tokyo Kasei Kogyo Co., Ltd.) and DEA (>99%, Tokyo Kasei Kogyo Co., Ltd.) were respectively dissolved in distilled water to obtain 30 wt % aqueous solutions, which were adjusted to the typical concentration that is used in the chemical absorption method. The molar fractions of amine and water were (MEA)0.112(H2O)0.888 for MEA solution and (DEA)0.068(H2O)0.932 for DEA solution. Absorption of CO2 into the aqueous amine solution was performed by bubbling the CO2 gas in the solution for a predetermined time. The CO2 content was controlled by changing the bubbling time. The CO2 concentration was determined using a total organic carbon analyzer (TOC-VCSH, Shimadzu Corporation). The CO2 concentration, RCO2, examined in this study were 0, 0.25, and 0.52 for MEA solutions, and 0, 0.24, 0.47 for DEA solutions in the units of moles CO2 per mole amine. Hereafter, the solutions of RCO2 ) 0, 0.25, and 0.52 for MEA are referred to as MEA0, MEA1, and MEA2, respectively. Similarly, the DEA solutions of RCO2 ) 0, 0.24, and 0.47 are referred to as DEA0, DEA1, and DEA2, respectively. X-ray Scattering Measurements and Data Reduction. In order to extract information on the molecular structures of the carbamate molecules existing in the sample solutions, X-ray scattering data in the wider Q-range with higher statistical accuracies are required. The high-energy X-ray scattering measurements were carried out with the diffractometer installed at the BL16XU undulator beamline on the SPring-8 storage ring (operated at 8 GeV and 100 mA) at the Japan Synchrotron Radiation Research Institute (JASRI), Hyogo, Japan. The incident X-ray wavelength, λ ) 0.03356 nm, was determined by 33 Bragg reflections from the standard Si powder. Scattered X-rays were collected by a YAP (YAlO3:Ce) detector over an angular range of 0.5 e 2θ e 80°, corresponding to the scattering vector magnitude range of 1.6 e Q e 240.2 nm-1 (Q ) 4π sin θ/λ). Measurements were carried out in two parts. The angular step interval was chosen to be ∆(2θ) ) 0.1° in the range of 0.5 e 2θ e 30°, and ∆(2θ) ) 0.2° in the range of 20 e 2θ e 80°, respectively. The total exposure time was 5.0 h for each sample solution. All the measurements were done at 26 ( 2 °C. The sample solution was sealed in a flat plate acrylate resin cell with a thickness of 2.0 mm, which had X-ray transmission windows made of a Kapton film with a thickness of 25 µm. A measurement on an empty cell was made in advance. The observed scattering intensities were corrected for background and absorption. Mass attenuation coefficients used for the absorption correction were calculated by interpolating the tabulated values from Physical Reference Data of NIST (http:// physics.nist.gov/ PhysRefData/XrayMassCoef/cover.html). Since the polarizing direction of X-ray from the BL16XU beamline is horizontal and the 2θ scan is done vertically in the present experiment, the polarization correction is not needed. Atomic

i(Q) ) [Ieu(Q) - 〈 f 2〉]/〈 f 〉2

∑c f

〈 f 2〉 )

i

(1)

2 i (Q)

and 〈 f 〉2 ) [

∑ c f (Q)]

2

i i

Here, Ieu(Q) is the normalized coherent scattering intensity in electron units. ci denotes the number of the ith atom in the stoichiometric unit, for example, (H2NCH2CH2OH)0.112(H2O)0.888(CO2)x (x ) 0, 0.028, and 0.058) for the MEA solution. fi(Q) corresponds to the atomic scattering factor of the ith atom. The distribution function, g(r), is evaluated by the Fourier transform of the corrected i(Q) g(r) ) 1 +

1 2π2Fr

∫

Qmax

0

Qi(Q) sin(Qr) dQ

(2)

where F is the number density of the stoichiometric unit. In order to enhance the information from the captured CO2, the difference interference term, ∆i(Q), was evaluated by the following equation ∆iR-β(Q) )

iR(Q)〈 f 〉R2 - iβ(Q)〈 f 〉β2 〈 f 〉R2 - 〈 f 〉β2

(3)

where R and β stand for solutions with and without CO2, respectively. The difference distribution function, ∆g(r), can be calculated by ∆gR-β(r) ) 1 +

1 2π2Fr

∫

Qmax

0

Q∆iR-β(Q) sin(Qr) dQ (4)

The upper limit of the integral in eqs 2 and 4, Qmax, was set to 196 nm-1 for the MEA solution and 190 nm-1 for the DEA solution considering the statistical uncertainties involved in the observed interference terms. NMR Measurements. In order to confirm the chemical species and its mole fraction, NMR measurements were performed. These results were used for calculation of theoretical difference distribution functions. 1H and 13C NMR spectra of the MEA2 and the DEA2 solutions were recorded on a Bruker DRX-500 spectrometer at 30 °C. We used the inverse gated decoupling mode in 13C NMR measurements. The resonance frequencies were 500.1 MHz for 1H NMR measurements and 125.8 MHz for 13C NMR measurements. C6D6 with tetramethylsilane (TMS) was used as an external reference using a doublewalled sample tube. The sample tubes were sealed during the measurements. The repetition time and the number of accumulations were 5.6 s and 128 for 1H NMR and 4.0 s and 2704-8000 for 13C NMR, respectively.

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Computer Conformation Analysis. Computer conformation analysis was carried out in order to obtain the stable conformations of the amine carbamate molecules. Obtained conformations were used for calculation of theoretical difference distribution functions. In this analysis, CONFLEX version 6.223 and the MMFF94s24 force field were used. To take the solvation effect into account, a generalized Born/surface area model (GB/SA model)25 was used. The optimized conformations were selected with Boltzmann distribution probability of more than 1%. Calculation of Theoretical Difference Distribution Functions. In order to confirm the assignment of the observed peaks in the difference distribution functions, theoretical difference intramolecular distribution functions were calculated. To evaluate the theoretical difference interference term, information on the mole fraction of coexisting chemical species in the solution and conformation of the amine carbamate molecule should be needed. We used the mole fraction measured by the NMR measurements and the conformations obtained by the computer conformation analysis. The difference intramolecular interference term of the solumodel tion, ∆iR-β (Q), can be calculated by the sum of the terms of the existing species which include the captured CO2. model ∆iR-β (Q) ) xCO2

∑ {c ∆i k

model (Q)} k

(5)

k

where xCO2 is the number of CO2 molecule in the stoichiometric unit. ck is the mole fraction of the kth species in the solution. ∆imodel (Q) is the intramolecular interference term of the kth k species, which can be calculated by

(Q) ) ∆imodel k

〈f〉R2

1 - 〈f〉β2

kth species

∑

fi(Q)fj(Q) ×

i*j

sin(Qrij) 1 exp - Q2lij2 2 Qrij

(

)

(6)

where rij and lij are the interatomic distance and the root-meansquare amplitude for the i-j pair, respectively. The theoretical difference distribution function, ∆gmodel R-β (r), can model (Q). be calculated by the Fourier transform of ∆iR-β model ∆gR-β (r) )

1 2π2Fr

∫

Qmax

0

model Q∆iR-β (Q) sin(Qr) dQ

(7)

The upper limit of the integral, Qmax, in eq 7 was set to the same value used to evaluate the total g(r). Results and Discussion Experimental Difference Distribution Function. Figures 1a and b show the interference terms, i(Q), for the MEA and the DEA solutions, respectively. Oscillational features arising from the molecular structure are clearly observed in the high-Q region of the observed i(Q). Although signal-to-noise ratio of the DEA0 solution is not so good in the high-Q region, the oscillation in the observed i(Q) can be identified. The total distribution functions, g(r), for the MEA and the DEA solutions are shown in Figure 2a and b, respectively. Three dominant peaks are observable in the present g(r) functions. The first peak at r ) 0.1 nm can be ascribed to the sum of contributions from intramolecular interactions of O-H within water molecule and O-H, C-H, and N-H within the amine (carbamate) molecule. The second peak appearing at r ) 0.15 nm is attributable to the sum of contributions from intramolecular interactions (mainly C-C, C-N, and C-O) within the

Figure 1. Interference terms observed for (a) MEA0-2 and (b) DEA0-2 solutions.

Figure 2. Distribution functions observed for (a) MEA0-2 and (b) DEA0-2 solutions.

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010

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Figure 3. Difference distribution functions for (a) MEA and (b) DEA solutions.

amine (carbamate) molecule. The third peak at r ) 0.28 nm can be attributed mainly to the intermolecular hydrogen-bonded O · · · O interaction between the nearest neighbor H2O molecules. The position of this peak agrees well with that reported for pure liquid water.16 Although the overall feature of the g(r) for solutions with different CO2 content looks very similar, the difference in the magnitude of the g(r) can be noted at r ) 0.12 and 0.22 nm in both the MEA and the DEA solutions. The intensity of the g(r) at r ) 0.12 nm becomes higher with increasing CO2 concentration, indicating that a peak originated from captured CO2 is involved at r ) 0.12 nm. A new small peak grows at r ) 0.22 nm with increasing CO2 content. Since these variations in the intensity occur with increasing the CO2 concentration, these variations are certainly attributable to interatomic correlations related with the captured CO2. In order to distinguish the interatomic correlation of the captured CO2, the difference distribution function, ∆g(r), was extracted by eq 4. The ∆g(r) for the MEA and the DEA solutions are shown in Figure 3a and b, respectively. The peaks at r ) 0.12 and 0.22 nm can be observed in the ∆g(r) for both the MEA and the DEA solutions. These peaks should correspond to the variations found in the total g(r) function. The peak at r ) 0.12 nm can be assigned to the C-O interaction of the captured CO2 molecule, however, the observed bond length seems slightly longer than that expected for the C-O bond length of the CO2 molecule (0.1164 nm).26 Assuming that the observed peak at r ) 0.22 nm is mainly attributable to the nonbonding O · · · O interaction within the captured CO2 molecule, the present value of the O · · · O distance obtained from the peak position of the ∆g(r) (0.22 nm) seems to be too short in comparison with that for the linear CO2 molecule (0.232 nm) in the gas phase.26 On the other hand, the C-O and O · · · O distances are very close to those reported for the glycine carbamate (rO · · · O ) 0.224 nm) in which the O-C-O bond angle is found to be 124.9°.27 The C-O and O · · · O distances for the glycine carbamate molecule are reported to be 0.126 and 0.224 nm, respectively.27 These values are very close to those obtained from the present ∆g(r). These results indicate that the captured CO2 molecule forms the carbamate with the amine molecules. Species in MEA2 and DEA2 Solutions. We next calculated theoretical difference distribution functions to confirm the conclusions described above. In order to evaluate the theoretical difference interference term, information on the mole fraction of coexisting chemical species in the solution and the molecular structure of the amine carbamate is needed. The mole fraction and the molecular structure were evaluated by NMR measurements and computer conformation analysis, respectively. Spectra obtained for the MEA2 and the DEA2 solutions are shown in Figure 4a and b, respectively. In the 1H NMR spectra of each solution, the peaks of H3 and H4 correspond to amine carbamate. The peaks of H2 and H5 correspond to amine +

protonated amine. It is not possible for 1H NMR study to distinguish between amine and protonated amine because of fast exchange of the proton. In the 13C NMR spectra of each solution, the peak of C1 corresponds to amine carbamate, and the peak of C2 corresponds to HCO3- + CO32-, which are indistinguishable due to fast exchange of the proton. Peaks in Figure 4 and their origin atoms are summarized in Table 1. NMR spectra indicate that existing species are mainly amine + protonated amine, amine carbamate, and HCO3- + CO32- both for MEA2 and DEA2 solutions. The molar ratio of these species was evaluated from the corresponding peak area. We used the peaks of H2 and H3 for the mole ratio of amine + protonated amine and amine carbamate, and the peaks of C1 and C2 for amine carbamate and HCO3- + CO32-, respectively. By combining the results of 1H and 13C NMR, the molar ratio of these species was determined. The results were summarized in Table 2. The molar ratio in Table 2 is normalized in such a way that the sum of the species originated from the captured CO2 equals one. MEA solution has a tendency to store CO2 as amine carbamate than DEA solution. This tendency agrees with the composition of the 30 wt % MEA-H2O-CO2 system and the BEA-H2O-CO2 system reported by Jakobsen et al.10 The difference in carbamate ratio between MEA and DEA solutions can be considered to arise from the difference in chemical properties between primary amine (MEA) and secondary amine (DEA). Jakobsen et al. distinguished between HCO3- and CO32using calibration measurements on variation in the peak shift as a function of the ratio of the existing chemical species and reported that few CO32- exist in the MEA system. Hence, we can ignore the contribution from CO32- in the present analysis of the X-ray data for the MEA system. The CO2 concentrations calculated from Table 2 were 0.51 for MEA2 and 0.46 for DEA2 in the units of moles CO2 per mole amine. These values agree within ca. 2% with those by the total organic carbon analyses that were performed just after CO2 absorption. Since the sample amine solutions for X-ray scattering measurements were treated in the same manner as NMR measurements, variation in CO2 concentration during X-ray scattering measurements can also be ignored. Structure of Amine Carbamate by Computer Conformation Analysis. We next performed the computer conformation analysis of amine carbamate molecule. For MEA carbamate, 5 conformers were selected within Boltzmann distribution probability of more than 1%. The Boltzmann distribution probability of the most stable conformer was 62%, and that of the second one was 10%. Because of the large difference of the probability between the most and the second stable conformer, we used the atomic coordinates from the most stable one for the evaluation of the X-ray difference interference terms. As for DEA carbamate, 15 conformers were obtained. The probability of the most stable conformer was 16%, and followed by 15%, 14%, and 11%. This result shows that there are more conformers

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Figure 4. NMR spectra of (a) MEA2 and (b) DEA2 solutions. Table 1. Assignment of Absorption Peaks in Figure 4 and Their Origin Atomsa NMR 1

H

peak

MEA2

DEA2

H1 H2

H2O, HO-CH2-CH2-NH2 and other exchangable protons HO-CH2-CH2-NH2 HO-CH2-CH2-NH2H+ HO-CH2-CH2-NH-COO HO-CH2-CH2-NH-COO HO-CH2-CH2-NH2 HO-CH2-CH2-NH2H+

H2O, (HO-CH2-CH2)2-NH and other exchangable protons (HO-CH2-CH2)2-NH (HO-CH2-CH2)2-NHH+ (HO-CH2-CH2)2-N-COO (HO-CH2-CH2)2-N-COO (HO-CH2-CH2)2-NH (HO-CH2-CH2)2-NHH+

HO-CH2-CH2-NH-COO HCO3-, CO32HO-CH2-CH2-NH-COO HO-CH2-CH2-NH2 HO-CH2-CH2-NH2H+ HO-CH2-CH2-NH-COO HO-CH2-CH2-NH2 HO-CH2-CH2-NH2H+

(HO-CH2-CH2)2-N-COO HCO3-, CO32(HO-CH2-CH2)2-N-COO (HO-CH2-CH2)2-NH (HO-CH2-CH2)2-NHH+ (HO-CH2-CH2)2-N-COO (HO-CH2-CH2)2-NH (HO-CH2-CH2)2-NHH+

H3 H4 H5 13

C

C1 C2 C3 C4 C5 C6

a

amine protonated amine amine carbamate amine carbamate amine protonate amine amine carbamate amine carbamate amine protonated amine amine carbamate amine protonated amine

Atoms in italics in chemical formulae indicate the origin atoms.

Table 2. Molar Ratio of Species Evaluated by NMR MEA2 NMR 1

H

13

corresponding peak peak area number of origin protons

C

a

corresponding peak peak area number of origin carbon atoms molar ratiob

amine + amineH+ a

carbamate

H2 99.9 2

H3 78.5 2

1.10

C1 105.0 1 0.86

DEA2 HCO3- + CO32-

C2 16.4 1 0.14

amine + amineH+ a

carbamate HCO3- + CO32-

H2 100 4

H3 47.2 4

1.48

C1 25.4 1 0.70

C2 10.9 1 0.30

AmineH+ means a protonated amine. b Molar ratio is normalized in such a way that the sum of the species including the captured CO2 equals one.

for DEA carbamate than for MEA carbamate. This can be considered to arise from the difference in number of constituent atoms in the carbamate molecule. Although the difference of the probabilities among the conformers was not so large for DEA carbamate, we used the atomic coordinates only from the most stable one for simplicity. The structures of the amine carbamates used for the calculation of difference interference terms were schematically depicted in Figure 5. Atoms, C(9),

O(10), and O(11), and C(18), O(19), and O(20) represent the CO2 captured by MEA and DEA, respectively. Theoretical Difference Distribution Function. By combining the molar ratio by NMR and the structure of amine carbamate molecule by the computer conformation analysis, the theoretical difference intramolecular distribution functions were calculated. Since the contribution from the CO32- can be ignored in the MEA2 solution as mentioned above, contributions from

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010

Figure 5. Molecular geometry of (a) MEA and (b) DEA carbamate molecules used for the calculation of intramolecular difference interference terms.

MEA carbamate and HCO3- were taken into account. As for the DEA2 solution, although the mole fraction of the CO32was unknown, no contribution from the CO32- was assumed. Then, eq 5 can be replaced by model model model ∆iR-β (Q) ) xCO2(ccarbamate∆icarbamate (Q) + cHCO3-∆iHCO -(Q)) 3 (8)

where ccarbamate and cHCO3- are mole fractions of amine carbamate and HCO3- determined from the NMR measurements, respectively. The values employed are ccarbamate ) 0.86 and cHCO3- ) 0.14 for the MEA2 and ccarbamate ) 0.70, cHCO3- ) 0.30 for the DEA2 solutions, respectively. In eq 6, the interatomic distances, rij, and the root-mean-square amplitude, lij, are needed. For HCO3-, rij determined by the single crystal X-ray diffraction study28 was employed. For amine carbamate molecule, rij were calculated from the atomic coordinates obtained by the computer conformation analysis. Atom pairs with the interatomic distance up to 0.3 nm were taken into account in the evaluation of the theoretical intramolecular interference terms. lij were assumed to those proposed for various organic molecules.29 Numerical values of rij and lij used in the calculation for the amine carbamate molecules are summarized in Table 3. Comparison between Experimental and Calculated Difference Distribution Functions. Figure 6 shows the calculated difference distribution functions for amine carbamate, HCO3-, and their sum (amine carbamate + HCO3-). The experimental ∆g(r) curves in Figure 3 are also shown for comparison. Good agreement was obtained between the experimental and calculated ∆g(r) for the MEA2 solution in the range of r < 0.25 nm. The positions and heights of the peaks at r ) 0.12 and 0.22 nm are well-reproduced. The calculated peak at

r ) 0.12 nm consists of the sum of contributions from the C(9)-O(10)/O(11) interactions within the MEA carbamate and C-O interactions within the HCO3-. The peak at r ) 0.22 nm is described as the sum of contributions from nonbonding O(10) · · · O(11) and O(10)/O(11) · · · N(7) interactions in the carbamate molecule. Contribution from nonbonding O · · · O interaction within the HCO3- is also involved in this peak. These agreements between observed and calculated ∆g(r) indicate that the difference distribution function derived from the present X-ray scattering method is sufficiently reliable for the structure analysis of the amine-H2O-CO2 system and that the molecular geometry analyzed using the GB/SA model reproduces the real structure well. The calculated ∆g(r) for the DEA2 solution did not reproduce the experimental ∆g(r) well. We think that peaks at r ) 0.05, 0.09, and 0.16 nm are unphysical peaks caused by the low signal-to-noise ratio in the interference term of the DEA0 solution (Figure 1b). However, peaks at 0.12 and 0.22 nm can be confirmable. Considering that these peaks appeared at almost the same position between the MEA carbamate and the DEA carbamate, the local structure of CO2 in the primary amine carbamate is almost the same as that in the secondary amine carbamate. The calculation indicates that a new C-N bond (C(9)-N(7) interaction in Figure 5 in the case of MEA) emerges when the carbamate molecule is formed. On the basis of the calculated structure, a peak emerged from this interaction should appear at r ) 0.145 nm in the difference distribution function. However, no C-N peak can be observed in both experimental and calculated curves in Figure 6. One reason for this could be explained by the truncation effect associated with the Fourier transform. Figure 7 shows the calculated difference distribution functions for the C(9)-N(7) and the C(9)-O(10)/O(11) interactions in the MEA carbamate solution. As can be seen in Figure 7, the curve of C(9)-O(10)/O(11) interaction has a negative value around r ) 0.15 nm due to the truncation effect. Furthermore, the intensity of the C(9)-N(7) peak at r ) 0.15 nm is not so high compared to that for the C(9)-O(10)/O(11) one. Then, the C(9)-N(7) interaction cannot be apparently observed in the present ∆g(r). Another reason for the invisibility of the peak at r ) 0.145 nm might be that our computer conformation analysis does not have enough accuracy and C-N peak in actual carbamate lies at slightly different distance. In order to verify this hypothesis, more strict simulation will be required.

Table 3. Atom Pairs, Interatomic Distance (rij), and Root-Mean-Square Amplitude (lij) Used for the Calculation of Difference Interference Termsa MEA carbamate

a

11

DEA carbamate

atom pair

rij/nm

lij/nm

atom pair

rij/nm

lij/nm

C(9) · · · C(3) C(9) · · · H(4) C(9)-N(7) C(9) · · · H(8) C(9)-O(10)/O(11) C(9) · · · H(12) O(10) · · · N(7) O(10) · · · H(8) O(10) · · · O(11) O(11) · · · O(2) O(11) · · · C(3) O(11) · · · H(4) O(11) · · · N(7) O(11) · · · H(12)

0.258 0.250 0.145 0.203 0.126 0.286 0.229 0.233 0.224 0.265 0.290 0.168 0.233 0.296

0.007 0.015 0.005 0.010 0.005 0.014 0.007 0.014 0.007 0.015 0.007 0.014 0.007 0.015

C(18) · · · C(3) C(18)-N(7) C(18) · · · C(8) C(18) · · · H(11) C(18) · · · H(13) C(18) · · · H(17) C(18)-O(19)/O(29) O(19) · · · C(3) O(19) · · · N(7) O(19) · · · H(17) O(19) · · · O(20) O(20) · · · N(7) O(20) · · · C(8) O(20) · · · H(11) O(20) · · · O(12) O(20) · · · H(13)

0.255 0.148 0.257 0.281 0.250 0.266 0.126 0.281 0.236 0.248 0.223 0.235 0.282 0.276 0.264 0.168

0.007 0.005 0.007 0.014 0.015 0.014 0.005 0.007 0.007 0.015 0.007 0.007 0.007 0.015 0.015 0.014

Atom numbers in parentheses in the atom pair column correspond to the notation used in Figure 5.

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Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010

Figure 6. Calculated difference distribution functions, ∆g(r), for (a) MEA2 and (b) DEA2 solutions. The ∆g(r) was evaluated by intramolecular contributions from carbamate + HCO3-, carbamate only, and HCO3-. The experimental difference distribution functions of MEA2 and DEA2 solutions in Figure 3 are also shown at upper sides for comparison.

a good agreement in the range of r ) 0-0.25 nm. We think that the local structure of CO2 in the primary amine carbamate is almost the same as that in the secondary carbamate, since the positions of peaks attributable to captured CO2 in MEA carbamate are quite close to those in DEA carbamate. The present results indicate that X-ray scattering method is a valuable tool to analyze the structure of the amine carbamate molecule in the amine-H2O-CO2 system. Figure 7. Calculated difference distribution functions of C(9)-N(7) and C(9)-O(10)/O(11) interactions within the MEA carbamate molecule.

In the region of r > 0.25 nm, agreement is not good between experimental and calculated ∆g(r) functions as seen in Figure 6. We can note three reasons for this disagreement. First, the calculated ∆g(r) in the present study includes atom pairs whose interatomic distance is only up to 0.3 nm. The calculated ∆g(r) curves in Figure 6 have no peak in the region of r > 0.3 nm. Second, the model employed in the computer conformation analysis may be too simplified. Disagreement about the second reason might be also found in the shorter-r region if compared carefully. The third reason can be that the calculated ∆g(r) involves only intramolecular contributions. In the solution state, it can be supposed that solvent molecules are surrounding the -COO group of the carbamate molecule as well as the HCO3-. In fact, the experimental ∆g(r) function includes the intermolecular contribution. A broad peak around r ) 0.28 nm appearing in Figure 6a could be attributed to the nearest neighbor hydrogen-bonded O(10)/O(11) · · · H2O and O(HCO3-) · · · H2O interactions. It is of considerable interest because these hydrogen bonds may increase the recovering energy. This requires better statistical accuracies of the observed scattering data, and it will be a near future subject. Conclusions We applied the high-energy X-ray scattering method to analyze the structure of the MEA-H2O-CO2 and the DEAH2O-CO2 systems. Difference distribution functions were successfully obtained from the observed scattering intensities. Intramolecular peaks arising from the captured CO2 were obviously observed in the difference distribution functions. The molar ratio of chemical species in the amine-H2O-CO2 system was determined from the NMR measurements. The structures of amine carbamates were calculated using the CONFLEX version 6.2 and the MMFF94s force field with GB/SA model. By combining the results obtained from the NMR and the computer conformation analysis, theoretical difference intramolecular distribution functions were evaluated. Comparison between the experimental and the calculated functions showed

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ReceiVed for reView June 12, 2009 ReVised manuscript receiVed October 23, 2009 Accepted November 2, 2009 IE9009556