Hydration Structure of CO2-Absorbed 2-Aminoethanol Studied by

Jan 13, 2014 - Advanced Technology Research Center, Mitsubishi Heavy Industries Ltd., 1-8-1 Sachiura, Kanazawa-ku, Yokohama 236-8515, Japan. ∥...
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Hydration Structure of CO2‑Absorbed 2‑Aminoethanol Studied by Neutron Diffraction with the 14N/15N Isotopic Substitution Method Yasuo Kameda,*,† Hiroshi Deguchi,‡ Hirotoshi Furukawa,‡ Yoshiyuki Kubota,‡ Yasuyuki Yagi,‡ Yoshihiro Imai,‡ Noriko Yamazaki,§ Noriko Watari,§ Takuya Hirata,∥ and Nobuyuki Matubayasi¶ †

Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan ‡ Power Engineering R & D Center, 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 ¶ Institute of Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan ABSTRACT: Neutron diffraction measurements were carried out for CO2-absorbed aqueous 11 mol % 2-aminoethanol (MEA) D2O solutions (corresponding to 30 wt % MEA solution) in order to obtain information on both the intramolecular structure and intermolecular hydration structure of the MEA carbamate molecule in the aqueous solution. Neutron scattering cross sections observed for (MEA)0.11(D2O)0.89, (MEA)0.11(D2O)0.89(CO2)0.06, and (MEA)0.11(D2O)0.89(DCl)0.11 solutions with different 14 N/15N ratios were used to derive the first-order difference function, ΔN(Q), which involves environmental structural information around the nitrogen atom of the MEA molecule. Intramolecular geometry and intermolecular hydration structure of MEA, protonated MEA (MEAD+), and MEA carbamate (MEA-CO2) molecules were obtained through the least-squares fitting of the observed ΔN(Q) in the high-Q region and the intermolecular difference function, ΔNinter(Q), respectively. In the aqueous solution, the MEA molecule takes the gauche conformation (dihedral angle, ∠NCCO = 45 ± 3°), suggesting that an intramolecular hydrogen bond is formed. On the other hand, values of the dihedral angle ∠NCCO determined for MEAD+ and MEA-CO2 molecules were 193 ± 4° and 214 ± 8°, respectively. These results imply that the intermolecular hydrogen bonds are dominated for MEAD+ and MEA-CO2 molecules. The intermolecular nearest neighbor N··· O(D2O) distance for the MEA molecule was determined to be 3.13 ± 0.01 Å, which suggests weak intermolecular interaction between the amino-nitrogen atom of MEA and water molecules in the first hydration shell. The nearest-neighbor N···O(D2O) distances for MEAD+ and MEA-CO2 molecules, 2.79 ± 0.03 and 2.87 ± 0.04 Å, clearly indicate strong hydrogen bonds are formed among the amino group of these molecules and neighboring water molecules.

■

H2O system has been investigated by 1H and 13C NMR methods and confirmed existence of the amine-carbamate in the solution.10−12 RISM-SCF-SEDD calculations for CO2absorbed aqueous MEA solutions have revealed that the hydration around oxygen atoms of the carbamate group drives stabilization of the MEA carbamate in the aqueous solution.13 Recently, the high-energy X-ray diffraction study was carried out for CO2-absorbed aqueous alkanolamine solutions.14,15 Information on the intramolecular structure and hydration structure around the amine-carbamate molecule was deduced from the observed difference function between scattering intensities from solutions with different CO2 content. However, structural information on molecular conformation of the amine carbamate and hydration environment around each functional group of amine carbamate in aqueous solution has not been

INTRODUCTION Increasing greenhouse gas emission is obviously an urgent problem which will cause drastic global climate change in the near future.1 In order to reduce the gas emission, the chemical absorption method is considered to be one of the most realistic solutions for the flue gas emitted from a large-scale power plant.2,3 Recently, this chemical absorption method has received considerable attention for application of capturing CO2 from the atmosphere, which was emitted from small-scale and mobile sources.4 The absorption media requires a good absorption rate of CO2 and a lower thermal energy for the recovery process. The use of an aqueous solution of alkanolamine has been proposed since the early 1930s.5 Aqueous 2-aminoethanol (NH2CH2CH2OH, MEA) solution has long been investigated as a high-performance absorption media.5,6 Theoretical studies by da Silva et al. indicated stability of amine-carbamate molecule in the aqueous solution.7−9 The liquid-phase composition of chemical species in the alkanolamine−CO2− © 2014 American Chemical Society

Received: December 1, 2013 Revised: January 13, 2014 Published: January 13, 2014 1403

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Table 1. Isotopic Composition, Mean Scattering Length, bN, of Nitrogen Atom, Total Cross Section, and Number Density of Sample Solutions Scaled in the Stoichiometric Unit, (*NH2CH2CH2OH)0.11(D2O)0.89(Z)z (Z = CO2 or DCl), σt, and ρ, Respectively sample I II III IV V VI a

14

( NH2CH2CH2OH)0.11(D2O)0.89 (15NH2CH2CH2OH)0.11(D2O)0.89 (14NH2CH2CH2OH)0.11(D2O)0.89(CO2)0.06 (15NH2CH2CH2OH)0.11(D2O)0.89(CO2)0.06 (14NH2CH2CH2OH)0.11(D2O)0.89(DCl)0.11 (15NH2CH2CH2OH)0.11(D2O)0.89(DCl)0.11

14

N (%) 99.6 0.3 99.6 0.3 99.6 0.3

15

N (%) 0.4 99.7 0.4 99.7 0.4 99.7

bN (10−12cm)

σt (barnsa)

ρ (Å−3)

0.936 0.645 0.936 0.645 0.936 0.645

40.322 39.506 41.163 40.404 44.554 43.750

0.02663 0.02646 0.02480

For incident neutron wavelength of 1.0 Å.

identified. In order to obtain more direct structural information on the intramolecular structure and hydration structure around amine carbamate, neutron diffraction with 14N/15N isotopic substitution technique is considered to be the most suitable experimental method. In the present paper, we describe results of intramolecular and intermolecular structures of MEA, protonated MEA (MEAD+), and MEA carbamate molecules in aqueous solution obtained from neutron diffraction measurements with the 14 N/15N isotopic substitution technique. Observed first-order difference function between 14N/15N isotopically substituted samples, ΔN(Q), was analyzed by the least-squares fitting procedure to obtain intramolecular parameters and intermolecular hydration structure around the amino group. In order to obtain the MEA structure, measurements were carried out on aqueous 11 mol % MEA solutions in D2O before CO2 loading with 14N/15N substitution. When CO2 is loaded into MEA solution, both MEAD+ and MEA carbamate molecules are generated. In order to obtain MEAD+ structure, measurements were performed on 14N/15N isotopically substituted aqueous 11 mol % MEA solutions in D2O before CO2 loading in which equimolar DCl was added. In this solution, MEAD+ is the dominant species. The ΔN(Q) value for analysis of the MEA carbamate molecule was extracted from subtraction between ΔN(Q) of 14N/15N substituted aqueous 11 mol % MEA solutions in D2O after CO2 loading and that of the MEA solutions before CO2 loading with DCl.

(15 NH 2CH 2CH 2OH)0.11(D2 O)0.89

(II)

(15 NH 2CH 2CH 2OH)0.11(D2 O)0.89 (CO2 )0.06

(IV)

(14 NH 2CH 2CH 2OH)0.11(D2 O)0.89 (DCl)0.11

(V)

(15 NH 2CH 2CH 2OH)0.11(D2 O)0.89 (DCl)0.11

(VI)

were prepared by mixing weighted amounts of MEA, D2O, and concentrated aqueous DCl solution in D2O (99% D, SigmaAldrich Co.). The molar ratio of MEAD+/MEA in the product solution was estimated to be ≈2500/1 from the pH value of the solution (∼6) and the pKa value of MEA molecule (9.4),16 which implies that the contribution from MEA in sample V and VI can be negligible. Sample parameters used in this study are listed in Table 1. Neutron Diffraction Measurements. The sample solution was sealed into a thin-walled cylindrical vanadium cell (6.0 mm in inner diameter and 0.1 mm in thickness). Time-offlight neutron diffraction measurements were carried out at 25 °C using the iMATERIA spectrometer17 equipped with an automatic sample changer18 installed at MLF facility of the JPARC, Tokai, Japan. Incident beam power of proton accelerator was 200 kW in measurements for samples I−IV and 120 kW for samples V and VI, respectively. An 8 GeV pulsed proton beam was injected to the circulating liquid mercury target.19 The thermalization of generated fast neutrons was achieved by the use of a temperature-controlled liquid parahydrogen moderator,20,21 and then incident neutrons were introduced to the sample position of the spectrometer. Scattered neutrons (neutron wave band of 0.18 ≤ λ ≤ 10.19 Å) were detected by ≈1500 3He position-sensitive proportional counters installed at 15°, 25°, 35°, 90°, and back scattering detector banks. Data accumulation time was ≈5 h for samples I−IV and ≈8 h for samples V and VI, respectively. Measurements were made in advance for the 10.0 mm ϕ vanadium rod, empty cell,and instrumental background. Data Reduction. Observed scattering intensities for the sample were corrected for instrumental background, absorption of sample and cell,22 multiple23 and incoherent scatterings. The coherent scattering lengths as well as the scattering and absorption cross sections for the constituent nuclei were referred to those tabulated by Sears.24 The wavelength dependence of the total cross sections for H and D nuclei was estimated from observed cross sections for liquid H2O and D2O, respectively.25 The corrected intensities were converted to an absolute scale using the corrected scattering intensities from the vanadium rod. The inelasticity correction was applied

EXPERIMENTAL SECTION Materials. Isotopically enriched and natural 2-aminoethanol, 15NH2CH2CH2OH (99.7% 15N, Isotec Co.), and 14 NH2CH2CH2OH (99.6% 14N, natural abundance, guaranteed grade, Tokyo Kasei Kogyo Co. Ltd.) were dissolved into D2O (99.9% D, Aldrich Co.) to prepare aqueous 11 mol % MEA solutions in D2O (I)

(III)

Acidified MEA solutions

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(14 NH 2CH 2CH 2OH)0.11(D2 O)0.89

(14 NH 2CH 2CH 2OH)0.11(D2 O)0.89 (CO2 )0.06

Because amino- and hydroxyl-hydrogen atoms of the MEA molecule are easily exchanged by solvent deuterium atoms, isotopic composition of the labile proton in these solutions is estimated to be H/D = 0.157/0.843. Absorption of CO2 into the solutions was performed by bubbling of the CO2 gas. The CO2 content of the solution was determined to be 0.52 times the molar quantity of constituent MEA molecules using a total organic carbon analyzer (TOC-VCSH, Shimadzu Co.), as follows 1404

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Table 2. Values of the Coefficients of aNj(Q) in Eq 1a

a

difference

A

B

C

D

E

F

I−II III−IV V−VI

0.03715 0.04161 0.03715

0.06789 0.06789 0.07143

−0.01053 −0.01053 −0.01053

0.00936 0.01191 0.00936

0 0 0.00674

0.00557 0.00557 0.00557

All measurements are denoted in barns.

by the use of the observed scattering intensities from the liquid null-H2O.26 The first-order difference function, ΔN(Q),27,28 is derived from the numerical difference between scattering cross sections observed for two solutions that are identical except for the scattering length of nitrogen atom has been changed. The ΔN(Q) can be written as a linear combination of partial structure factors, aNj(Q), involving contributions from the N−j pair: ΔN(Q ) = A[aNO(Q ) − 1] + B[aND(Q ) − 1] + C[aNH(Q ) − 1] + D[aNC(Q ) − 1] + E[aNC1(Q ) − 1] + F[aNN(Q ) − 1]

(1)

where A = 2cNcO(b14N − b15N)bO, B = 2cNcDex(b14N − b15N)bDex, C = 2cNcH(b14N − b15N)bH, D = 2cNcC(b14N − b15N)bC, E = 2cNcCl(b14N − b15N)bCl, and F = cN2(b14N2 − b15N2). The variables bj and cj denote the coherent scattering length of atom j and the number of j atom in the stoichiometric unit, (*MEA)x(D2O)1‑x(CO2)y(DCl)z, respectively. Dex stands for exchangeable hydrogen and deuterium atoms. Numerical values A − F for the respective difference functions are listed in Table 2. The observed ΔN(Q) from all detector banks were combined

Figure 1. Difference function, ΔN(Q), observed for aqueous 11 mol % (a) MEA, (b) MEAD+, and (c) MEA carbamate solutions in D2O (dots) and the best-fit of the intramolecular interference term INintra(Q) (solid lines).

at the Q-interval of 0.1 Å−1 and used for subsequent analyses. The distribution function, GN(r), is deduced from the Fourier transform of ΔN(Q):

G N(r ) = 1 + (A + B + C + D + E + F )−1(2π 2ρr )−1

∫0

Q max

Q ΔN(Q )sin(Qr )dQ

= [AgNO(r ) + BgND(r ) + CgNH(r ) + DgNC(r ) + EgNC1(r ) + FgNN(r )](A + B + C + D + E + F )−1

where ρ is the number density of sample. The upper limit of the integral, Qmax, was set to 30 Å−1 in the present study. Observed ΔN(Q) involves both contributions from intra- and intermolecular N-j interactions. Intramolecular interference term, INintra(Q), is evaluated by the following equation:

squares fitting of the high-Q region (Q > 7 Å−1) of the observed ΔN(Q). The fitting was performed using the SALS program.29 The intermolecular difference function, ΔNinter(Q), was obtained by subtracting INintra(Q) form the observed total ΔN(Q):

IN intra(Q ) = γ ∑ 2c Nbα(b14N − b15N)exp( − lNα 2Q 2/2)sin(QrNα)/(QrNα)

ΔN inter (Q ) = ΔN(Q ) − IN intra(Q )

(3)

where γ is the normalization factor, and lNα and rNα denote the root-mean-square amplitude and internuclear distance, respectively. Values rNα, lNα, and γ were determined through the leastΔN model (Q ) =

(2)

(4)

Structural parameters concerning the intermolecular N−j interaction were determined through the least-squares fitting procedure applying the following model function:30−32

∑ 2c NnNαbα(b14N − b15N)exp(−lNα 2Q 2/2)sin(QrNα)/(QrNα) + 4πρ(A + B + C + D + E + F )exp(−l02Q 2/2)[Qr0cos(Qr0) − sin(Qr0)]Q−3 (5)

where nNα is the coordination number of α atoms around N atom. The long-range parameter, r0, means the distance beyond which the continuous distribution of atoms around N atom can be assumed. The parameter, l0, describes the sharpness of the boundary at r0. Structural parameters nNα, lNα, rNα, l0, and r0 are determined from the least-squares fit to the observed ΔNinter(Q) in the range of 0.1 ≤ Q ≤ 30 Å−1. The fitting procedure was carried out using the SALS program.29 The intermolecular distribution function, GNinter(r), was obtained by a Fourier

transform of the ΔNinter(Q) using eq 2 with the upper limit of the integral, Qmax = 30 Å−1.

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RESULTS AND DISCUSSION Intramolecular Structure of MEA, MEAD+, and MEA Carbamate in the Aqueous Solution. The difference functions, ΔN(Q), observed for aqueous MEA solution are shown in Figure 1a. In the present study, ΔN(Q), the first diffraction peak located at Q ∼ 2 Å−1, and the interference 1405

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successfully eliminated in the smoothed ΔN(Q). In order to confirm the structural information involved in the present smoothed ΔN(Q) in the high-Q region, the smoothed ΔN(Q) was transformed applying various values of upper limit, Qmax, as indicated in Figure 4. The sharpness of the short-range peaks

feature extending to higher-Q region are obviously identified. The total distribution function, GN(r), was obtained by the Fourier transform of the observed ΔN(Q). Prior to the transform, a smoothing procedure of the ΔN(Q) was applied using the modified cubic spline method in order to avoid periodic high frequency ripples appearing in the GN(r), which are mainly arisen from statistical uncertainties involved in the observed ΔN(Q) in the high-Q region (Figure 2). The

Figure 4. Q-max dependence of the total distribution function, GN(r), observed for aqueous 11 mol % MEA solution.

appearing in the transformed GN(r) function is enhanced as Qmax increases up to 30 Å−1, implying that significant structural information is involved in the high-Q region of the present smoothed ΔN(Q). The first and second peaks appearing at r ∼ 1.0 and 1.5 Å in the GN(r) for aqueous 11 mol % MEA solution (Figure 5a) are attributable to intramolecular N−D and N−C

Figure 2. (a) Observed difference function, ΔN(Q), for aqueous 11 mol % MEA solution (dots) and smoothed ΔN(Q) (solid line). (b) The differences, δ(Q), between them are indicated below (dots).

distribution of deviations of the observed ΔN(Q) from the smoothed ΔN(Q) is well approximated by a Gaussian function with the full width at half-maximum of 0.0078 ± 2 barns (corresponding to the standard deviation of σ = 0.0033 ± 1 barns), as shown in Figure 3. The average statistical uncertainty

Figure 5. Distribution function, GN(r), observed for aqueous 11 mol % (a) MEA, (b) MEAD+, and (c) MEA carbamate solutions in D2O (solid lines) and corresponding intramolecular contributions (dotted lines).

Figure 3. Distribution of the difference between unsmoothed and smoothed ΔN(Q) functions for aqueous 11 mol % MEA solution (histograms) and the best-fit of the least-squares analysis employing the Gaussian function (solid line).

interactions within the MEA molecule, respectively. The integration value for the first peak indicates that the number of D atoms around the N atom is 2, which is consistent with the known molecular geometry of MEA. Structural parameters with respect to the MEA molecule were determined through the least-squares fitting analysis of the observed ΔN(Q) in the highQ region. Because oscillational amplitude of the intermolecular interference term diminishes much faster than the intramolecular one with increasing the Q-value, the oscillational feature appearing in the observed ΔN(Q) at high-Q region can be regarded as the intramolecular interference contribution. In the present study, the least-squares fitting procedure was

per one data point involved in the observed ΔN(Q) can be estimated to be ≈0.0033 barns from the counting statistics of average neutron counts. There are 6.2 × 106 counts/data points for sample solutions and ≈1.1 × 106 counts/data points for a vanadium rod, and the coherent scattering intensity of the sample solution involving MEA-natN is Σcibi2 = 2.99 barn. The agreement in values of the standard deviation estimated from these methods indicates that the statistical uncertainties involved in the observed difference function have been 1406

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applied in the Q-range of 7 ≤ Q ≤ 30 Å−1. In the fitting procedure, the following assumptions were employed: (a) Parameters for the intramolecular N-D interaction, rND and lND, were treated as independent parameters. The coordination number, nND, was fixed at 2. (b) The structural parameter, rNC, for intramolecular N−C interaction was allowed to vary independently, whereas the lNC was fixed to the value of 0.05 Å, which was estimated from related molecules.33−35 (c) The dihedral angle, ∠OCC−CCN, was treated as an independent parameter. (d) The other intramolecular nonbonded N···α distances applied in eq 3 were calculated by the following procedure: The structural parameters for the MEA molecule were first given in the Z-matrix form (represented by bond distance, bond angle, and dihedral angle) and then were transformed to the Cartesian coordinate (x, y, and z). Then intramolecular N···α distance was evaluated. In the present analysis, structural parameters, except for independent parameters described above, were fixed to the values determined from our previous X-ray study of pure liquid MEA36 and values from microwave studies.37,38 (e) The normalization factor, γ, was treated as an independent parameter. In the fitting procedure, the Marquardt method with the dynamical biweight estimation of the weight adjustment for the observed data, was employed for the nonlinear least-squares calculation.29 Standard deviations of independent parameters were determined from the error matrix in the condition, and the standard deviations of observed data were normalized to unity.29 The final results of independent parameters are summarized in Table 3. The present value of overall renormalization factor,

intermolecular hydrogen bonds between the MEA and neighboring water molecules. The intramolecular structure of protonated MEA, MEAD+, was determined from the least-squares fit of the observed ΔN(Q) obtained from the difference in observed scattering cross sections from samples V and VI (Figure 1b). The distribution function, GN(r) (Figure 5b), exhibits a large first peak at r = 1.0 Å, which can be assigned to the intramolecular N−D interaction within the MEAD+ molecule. The integral value of this first peak indicates that the coordination number of nND ∼ 3, which is consistent with the expected ionization state of the amino group, −ND3+. Final values of independent parameters are presented in Table 3. The present value, rND = 1.02 ± 0.01 Å, is slightly longer than that for neutral MEA molecule. The bond angle ∠NCC was determined to be 108 ± 8°. The value of rNC (= 1.46 ± 0.01 Å) is in good agreement within the experimental error with that for the neutral MEA. The present value of the dihedral angle ∠OCC−CCN = 193 ± 4° is significantly larger than that obtained for the neutral MEA, indicating the intramolecular hydrogen bond within the MEAD+ is not formed in the aqueous solution. Because the ΔN(Q) obtained from the difference between scattering cross sections for samples III and IV involves both contributions from MEA carbamate (40%) and MEAD+ (60%) molecules, where the mole fraction of constituent chemical species has already been determined from 1H and 13C NMR measurements, ΔN(Q) for the MEA carbamate can be approximately derived by the following equation: ΔN(Q )(for MEA carbamate) = ΔN (Q )(III − IV) − 0.60 × ΔN (Q )(V − VI)

Table 3. Intramolecular Parameters for MEA, MEAD+, and MEA Carbamate Molecules Determined from the LeastSquares Fitting Analysis of Observed ΔN(Q) in the Range of 7 ≤ Q ≤ 30 Å−1.a parameter

MEA

MEAD+

MEA carbamate

rND (Å) lND (Å) rNC (Å) ∠OCC−CCN (deg) ∠NCC (deg) rNCc (Åb) rCcO (Åc) ∠OCcO (deg) γ

1.01(2) 0.07(4) 1.47(2) 45(3) 107(fixed)

1.02(1) 0.08(1) 1.46(1) 193(4) 108(8)

1.0(2)

1.09(8)

1.00(2) 0.07(2) 1.47(2) 214(8) 107(fixed) 1.44(3) 1.25(5) 109(3) 1.1(2)

(6)

which is shown in Figure 1c. The distribution function GN(r) for MEA carbamate is obtained from the Fourier transform of the ΔN(Q)(for MEA carbamate) (Figure 5c). The integrated value of the first peak in the GN(r) was estimated to be ≈1, which agrees with the molecular structure expected for MEA carbamate, CO2−ND-CH2CH2OD. The second peak at r ∼ 1.5 Å in the GN(r) should involve both contributions from N−C and N−Cc (Cc: carbamate carbon atom) bonds. The final values for independent parameters are summarized in Table 3. The N−D distance of MEA carbamate was determined to be 1.00 ± 0.02 Å. The present value of the dihedral angle ∠OCC− CCN = 214 ± 8° suggests that the intramolecular hydrogen bond within the MEA carbamate is not formed in the aqueous solution. The Cc−O bond distance within the carbamate group is determined to be 1.25 ± 0.05 Å, which agrees well with that reported in the X-ray difference distribution function.14 The Cc−N bond length is determined to be 1.44 ± 0.03 Å. Intermolecular Hydrogen-Bonded Structure around the Amino Group. The intermolecular difference function, ΔNinter(Q), for each solution was deduced by subtracting the optimized intramolecular interference term, INintra(Q), from the observed total ΔN(Q), which is shown in Figure 6. Observed ΔNinter(Q) is characterized by a dominant first diffraction peak appearing at Q ∼ 2 Å−1 in all solutions. The intermolecular distribution function around the amino N atom, GNinter(r), is represented in Figure 7. The observed GNinter(r) seems rather featureless; however, contributions from the nearest neighbor N···OW and N···DW (OW and DW denote water oxygen and deuterium atoms, respectively) interactions should be involved in the broadened feature around r ∼ 3 Å as frequently observed in the intermolecular G N inter(r) for aqueous solutions

a

Estimated standard deviations are given in the parentheses. b Internuclear distance between the N atom and carbamate C atom (Cc). cDistance between carbamate carbon and oxygen atoms.

γ (1.0 ± 0.2) is very close to unity, which indicates that the correction and normalization procedures have been adequately applied. The present values of rND (1.01 ± 0.02 Å) and rNC (1.47 ± 0.02 Å) are in good agreement with those obtained from the gas phase microwave (rND = 1.017 ± 0.005 Å and rNC = 1.475 ± 0.023 Å)37 and high-energy X-ray studies for pure liquid MEA (rNC = 1.46 ± 0.02 Å).36 The dihedral angle ∠OCC−CCN was obtained to be 45 ± 3°, suggesting that an intramolecular hydrogen bond within the MEA should form in the aqueous solution. The present value of ∠OCC−CCN is slightly smaller than the value determined in the pure liquid MEA (53 ± 3°), 36 which may reflect the effect of 1407

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structure of D2O. (b) Contribution from the second hydration sphere of the N atom was also taken into account, where the contribution was assumed as a single interaction with the scattering length bα in eq 5 being 2bDex + bO. (c) Long-range structural parameters r0 and l0 were determined independently. The observed ΔNinter(Q) was successfully reproduced by calculated model function, ΔNmodel(Q), as shown in Figure 6. The final results of independent parameters are summarized in Table 4. Table 4. Results of the Least-Squares Refinement of ΔNinter(Q) Observed for Aqueous 11 mol % MEA, MEAD+, and MEA Carbamate Solutions in D2Oa ΔNinter(Q),

Figure 6. Intermolecular difference function, observed for aqueous 11 mol % (a) MEA, (b) MEAD+, and (c) MEA carbamate solutions in D2O (dots) and the best-fit of the intermolecular model function, ΔNinter(Q) (solid lines).

interaction

parameter (Å)

MEA

MEAD+

MEA carbamate

N···D2O(I)

rNO lNO nNO rND lND rN...D2O lN...D2O nN...D2O r0 l0

3.13(1) 0.16(1) 2.4(1) 3.56(1) 0.23(1) 4.0(7) 0.54(6) 3(1) 3.9(1) 0.5(1)

2.79(3) 0.20(2) 3.6(2) 3.27(4) 0.25(3) 5.28(6) 0.28(8) 0.8(1) 3.82(8) 0.5(1)

2.87(4) 0.13(1) 1.2(3) 3.34(7) 0.21(3) 4.2(1) 0.64(8) 10(4) 4.9(5) 0.8(2)

N···D2O(II)

long range a

Estimated standard deviations are given in the parentheses.

The present value of the nearest neighbor N···OW distance determined for neutral MEA solution (I and II), rNO = 3.13 ± 0.01 Å, is significantly larger than the average value for intermolecular hydrogen-bonded N(H)···O distance found in various organic crystals (2.89 Å),45 suggesting that the interaction between the amino nitrogen atom of the MEA molecule and the nearest neighbor water molecule is relatively weak. This result is consistent with the formation of the intramolecular hydrogen bond within the MEA molecule as mentioned in the previous section. On the other hand, the intermolecular N···OW distance obtained for MEAD+ and MEA carbamate molecules (2.79 ± 0.03 and 2.87 ± 0.04 Å, respectively) clearly suggests that the nearest neighbor water molecules around the amino group are strongly hydrogen bonded, which is consistent with the molecular conformation of MEAD+ and MEA carbamate without intramolecular hydrogen bond. The tilt angle, θ, defined as angle between the N···OW axis and the molecular plane of the water molecule, was evaluated from observed values of rNO and rND with known molecular geometry of heavy water in the liquid state (rOD = 0.983 and rDD = 1.55 Å46,47). The tilt angle, θ, for the nearest neighbor water molecule was determined to be 60 ± 3°, 67 ± 8°, and 56 ± 13° for MEA, MEAD+, and MEA carbamate molecules, respectively. An indication of the second hydration shell of the amino group can be seen in the GNinter(r) for the MEAD+ and MEA carbamate at r = 5.3 and 4.2 Å, respectively. According to previous X-ray diffraction study, water molecules in the first hydration shell of the carbamate group have been detected.15 The second peak in the present GNinter(r) for MEA carbamate may involve contribution from water molecules in the first hydration shell of the carbamate group of MEA carbamate molecule. In the present MEAD+ solution, the nearest neighbor interaction between the N atom and Cl− might be involved in the first hydration shell of the amino group. In order to deduce a more definitive conclusion concerning the orientation of the water molecule in the first-

Figure 7. Intermolecular distribution, GNinter(r), observed for aqueous 11 mol % (a) MEA, (b) MEAD+, and (c) MEA carbamate solutions in D2O (solid lines) and short- and long-range interactions indicated by broken and dotted lines, respectively.

containing ND4+,39,40 glycine,41,42 and alanine43 molecules. In the GNinter(r) for glycine molecule in aqueous alkaline solution in which the amino group takes the −ND2 form, an intermolecular hydrogen-bonded peak has been reported at rND = 1.97 Å,44 implying that lone pair electrons of the N atom forms intermolecular hydrogen bond of N···D−OD type. In the present GNinter(r) for aqueous MEA solution (Figure 7a), any indication of the peak cannot be found in the r range around 2 Å. This fact indicates that the N···D−OD type hydrogen bond is not formed in the aqueous MEA solution, which is probably due to formation of the intramolecular hydrogen bond of N··· D−O type within the MEA molecule. The quantitative analysis of the observed ΔNinter(Q) was performed by employing the least-squares fitting procedure. In the preliminary analysis, it was found that intermolecular interactions for the nearest neighbor N···OW, N···DW, the second nearest neighbor N···D2O, and the long-range contributions were required to reproduce the observed ΔNinter(Q). The fitting was carried out assuming the following conditions: (a) Structural parameters concerning the nearest neighbor N···D2O(I) interaction, rNO, lNO, nNO, rND, and lND, were treated as independent parameters. The value of nND was fixed to 2nNO during the fitting procedure for the molecular 1408

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(6) Romeo, L. M.; Bolea, I.; Escosa, J. M. Integration of Power Plant and Amine Scrubbing to Reduce CO2 Capture Costs. Appl. Therm. Eng. 2008, 28, 1039−1046. (7) da Silva, E. F.; Svendsen, H. F Ab Initio Study of the Reaction of Carbamate Formation from CO2 and Alkanolamine. Ind. Eng. Chem. Res. 2004, 43, 3413−3418. (8) da Silva, E. F.; Svendsen, H. F. Study of the Carbamate Stability of Amines Uing Ab Initio Methods and Free-energy Perturbations. Ind. Eng. Chem. Res. 2006, 45, 2497−2504. (9) da Silva, E. F.; Svendsen, H. F. Computational Chemistry Study of Reactions, Equilibrium and Kinetics of Chemical CO2 Absorption. Int. J. Greenhouse Gas Control 2007, 1, 151−157. (10) Suda, T.; Iwaki, T.; Mimura, T. Facile Determination of Dissolved Species in CO2-Amine-H2O System by NMR Spectroscopy. Chem. Lett. 1996, 777−778. (11) Jacobsen, J. P.; Krane, J.; Svendsen, H. F. Liquid-Phase Composition Determination in CO2-H2O-Alkanolamine Systems: An NMR Study. Ind. Eng. Chem. Res. 2005, 44, 9894−9903. (12) Bö ttinger, W.; Maiwald, M.; Hasse, H. Online NMR Spectroscopic Study of Species Distribution in MEA-H2O-CO2 and DEA-H2O-CO2. Fluid Phase Equilib. 2008, 263, 131−143. (13) Iida, K.; Sato, H. Proton Transfer Step in the Carbon Dioxide Capture by Monoethanol Amine: A Theoretical Study at the Molecular Level. J. Phys. Chem. 2012, 116, 2244−2248. (14) Deguchi, H.; Kubota, Y.; Yagi, Y.; Mitani, I.; Imai, Y.; Tatsumi, M.; Watari, N.; Hirata, T.; Kamrda, Y. Structure of Monoethanolamine and Diethanolamine Carbamates in Aueous Solutions Determined by High-Energy X-ray Scattering. Ind. Eng. Chem. Res. 2010, 49, 6−13. (15) Deguchi, H.; Kubota, Y.; Furukawa, H.; Yagi, Y.; Imai, Y.; Tatsumi, M.; Yamazaki, N.; Watari, N.; Hirata, T.; Matubayasi, N.; et al. Hydration Structure around CO2 Captured in Aqueous Amine Solutions Observed by High Energy X-ray Scattering. Int. J. Greenhouse Gas Control 2011, 5, 1533−1539. (16) The Merck Index, 12th ed.; BudavariS., Ed.; Merck and Co., Inc.: Whitehouse Station, NJ, 1996. (17) Ishigaki, T.; Hoshikawa, A.; Yonemura, M.; Morishima, T.; Kamiyama, T.; Oishi, R.; Aizawa, K.; Sakuma, T.; Tomota, Y.; Arai, M.; et al. IBARAKI Materials Design Diffractometer (iMATERIA) Versatile Neutron Diffractometer at J-PARC. Nucl. Instrum. Methods Phys. Res., Sect. A 2009, 600, 189−191. (18) Hoshikawa, A.; Ishigaki, T.; Nagai, M.; Kobayashi, Y.; Sagehashi, H.; Kamiyama, T.; Yonemura, M.; Aizawa, K.; Sakuma, T.; Tomota, M.; et al. Development of an Automatic Sample Changer for iMATERIA. Nucl. Instrum. Methods Phys. Res., Sect. A 2009, 600, 203−206. (19) Futakawa, M.; Haga, K.; Wakui, T.; Kogawa, H.; Naoe, T. Development of the Hg Target in the J-PARC Neutron Source. Nucl. Inst. Met. Phys. Res. A 2009, 600, 18−21. (20) Ikeda, Y. Current Status of 1MW Pulse Spallation Neutron Source (JSMS) of J-PARC. J. Nucl. Mater. 2005, 343, 7−13. (21) Arai, M. J-PARC and the Prospective Neutron Science. Pramana 2008, 71, 629−638. (22) Paalman, H. H.; Pings, C. J. Numerical Evaluation of X-ray Absorption Factors for Cylindrical Samples and Annular Sample Cells. J. Appl. Phys. 1962, 33, 2635−2639. (23) Blech, I. A.; Averbach, B. L. Multiple Scattering of Neutrons in Vanadium and Copper. Phys. Rev. 1965, 137, A1113−A1116. (24) Sears, V. F. Neutron Scattering Lengths and Cross Sections. Neutron News 1992, 3, 26−37. (25) Granada, J. R.; Gillette, V. H.; Mayer, R. E. Calculation of Neutron Cross Sections and Thermalization Parameters for Molecular Gases Using a Synthetic Scattering Function. II. Application to H2O, D2O, and C6H6. Phys. Rev. A 1987, 36, 5594−5605. (26) Kameda, Y.; Sasaki, M.; Usuki, T.; Otomo, T.; Itoh, K.; Suzuya, K.; Fukunaga, T. Inelasticity Effect on Neutron Scattering Intensity of the Null-H2O. J. Neutron Res. 2003, 11, 153−163. (27) Soper, A. K.; Neilson, G. W.; Enderby, J. E.; Howe, H. A. A Neutron Diffraction Study of Hydration Effect in Aqueous Solutions. J. Phys. C: Solid State Phys. 1977, 10, 1793−1801.

and second hydration shell of the amino group, it is necessary to obtain information on the N−O and N−H partial structure functions. This requires additional neutron diffraction measurements for 14N/15N and H/D isotopically substituted samples.

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CONCLUSIONS Structural information on intramolecular geometry and intermolecular hydrogen bonds around the amino group of MEA, MEAD+, and MEA carbamate molecules in aqueous solutions was successfully obtained from the neutron first-order difference function, ΔN(Q), between 14N/15N isotopically substituted samples. Experimental evidence was obtained for the structural change in intramolecular conformation of the MEA molecule before and after CO2 absorption. It has been revealed that the MEA molecule takes the gauche conformation with an intramolecular hydrogen bond in the aqueous solution. On the other hand, no indication of the intramolecular hydrogen bond has been detected for the MEAD+ and MEA carbamate molecules. Intermolecular interaction between the amino group of the MEA molecule and water molecules in the first hydration shell are relatively weak when compared with those for MEAD+ and MEA carbamate solutions in which strong hydrogen bonds are formed between the amino group and neighboring water molecules. The tilt angle between the N−OW axis and the molecular plane of the water molecule was evaluated from the nearest neighbor N−OW and N−DW distances determined from the least-squares fitting analyses of the observed intermolecular difference function, ΔNinter(Q).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-23-6284591. Tel.: +81-23-628-4581. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Prof. Toru Ishigaki (Ibaraki University) and members of iMATERIA instrumental group for their help during the course of neutron diffraction measurements. Neutron scattering experiments were approved by the Neutron Scattering Program Advisory Committee of MLF, JPARC (Proposal Nos. 2010BM0001 and 2011B0016).

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REFERENCES

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