Quantitative Spectroscopic Study of Equilibrium in CO2-Loaded

Article pubs.acs.org/IECR

Quantitative Spectroscopic Study of Equilibrium in CO2‑Loaded Aqueous 2‑(Ethylamino)ethanol Solutions Hidetaka Yamada,*,† Yoichi Matsuzaki,‡ and Kazuya Goto† †

Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan Advanced Technology Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan

‡

ABSTRACT: The chemical absorption of CO2 in aqueous solutions of 2-(ethylamino)ethanol at 313 K was investigated by nuclear magnetic resonance (NMR) spectroscopy. Accurate 13C NMR quantitation by inverse-gated decoupling was achieved on the basis of 13C spin−lattice relaxation time measurements for all species in the system. The concentration of carbonate species (HCO3−/CO32−) monotonically increased with increasing CO2 loading, whereas that of carbamate anion [CH3CH2N(CH2CH2OH)COO−] showed a peak around 0.5 mol of CO2/mol of amine. Furthermore, we found that alkyl carbonate species (CH3CH2NHCH2CH2OCOO−) formed in the high-CO2 loading range, which was confirmed by 2D NMR spectroscopy and density functional theory calculations with a dielectric continuum solvation model at the B3LYP/aug-cc-pVDZ//B3LYP/6-311+ +G(d,p) level.

1. INTRODUCTION Amine scrubbing has been widely used to separate CO2 from gas streams in industrial processes.1 It is currently being tested on a larger scale for CO2 capture from large emission sources, such as coal-fired power plants, as a countermeasure to prevent global warming.2 In this context, the chemistry of CO2 absorption in aqueous amine solutions has been intensively studied in recent years to explore the properties required for efficient amine absorbents.3−15 One of the most valuable pieces of information on amine−H2O−CO2 chemistry is the species distribution in this system.16 Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for speciation and quantification. Because 13C NMR spectroscopy provides better quantitative information than 1H NMR spectroscopy, it was applied to various amine− H2O−CO2 solutions in earlier studies.16−27 However, there has been limited research on species distribution as a function of CO2 loading for secondary alkanolamine solutions,22,24 which is the subject of this study. Moreover, most of these studies have provided little information about parameters such as spin− lattice relaxation time, delay time, pulse duration, and pulse angle despite the importance of these parameters for accurate quantification. Very recently, Ciftja et al.28 discussed the main parameters to be considered for accurate measurements when using 13C NMR spectroscopy for the speciation of liquid-phase CO2 capture absorbents such as monoethanolamine (MEA), 2-amino-2methyl-1-propanol (AMP), diethanolamine (DEA), ethylenediamine (EDA), piperazine (PZ), N-methyldiethanolamine (MDEA), 2-(diethylamino)ethanol (DEEA), glycine, taurine, alanine, serine, proline, and sarcosine. They determined and reported the relaxation times for the species in absorbent− H2O−CO2 systems using the above absorbents. We also conducted accurate speciation studies in amine−H2O−CO2 solutions by 13C NMR spectroscopy based on relaxation time measurements for monopentanolamine (MPA), 2© 2014 American Chemical Society

(propylamino)ethanol (PAE), and 2-(isopropylamino)ethanol (IPAE).29 That study validated the significance of NMR spectroscopic analysis using appropriate experimental parameters. However, only two different CO2 loadings were prepared for each alkanolamine. In this study, we investigate CO2-loaded aqueous 2(ethylamino)ethanol (EAE) solutions by 13C NMR and 2D NMR spectroscopies. Secondary alkanolamine EAE, which can be prepared from renewable resources, is a promising absorbent for CO2 capture. A recent report demonstrated that the CO2 absorption rate in aqueous EAE is higher than that in the representative secondary alkanolamines DEA and diisopropanolamine (DIPA) at high amine concentration.30 More than 15 years ago, Mimura et al.31 also studied the kinetics of the reaction of CO2 with EAE. They concluded that this sterically hindered amine has excellent properties in terms of reaction rate under practical process conditions of a finite CO2 loading as compared to MEA. Two major pathways that contribute to CO2 absorption in aqueous EAE solutions include the formations of carbamate and bicarbonate anions32,33 2R1R2NH + CO2 ↔ R1R2NCOO− + R1R2NH 2+

(1)

R1R2NH + CO2 + H 2O ↔ HCO3− + R1R2NH 2+

(2)

where R1 and R2 represent −CH2CH3 and −CH2CH2OH, respectively. Consequently, dissolved CO2 molecules are present as carbamate anions (R1R2NCOO−) and carbonate species (bicarbonate and carbonate anions). Accurate 13C NMR quantitation based on relaxation time measurements in this study enabled the determination of the Received: Revised: Accepted: Published: 1617

October 16, 2013 January 8, 2014 January 9, 2014 January 9, 2014 dx.doi.org/10.1021/ie4034889 | Ind. Eng. Chem. Res. 2014, 53, 1617−1623

Industrial & Engineering Chemistry Research

Article

Figure 1. 13C NMR quantitative spectrum of a CO2-loaded aqueous EAE solution at 313 K with peak interpretations and relative values for peak areas.

electrostatics.36 Molecular geometries were optimized using a combination of the B3LYP hybrid functional with the 6-311+ +G(d,p) basis set in advance. 13C NMR chemical shifts were calculated at the B3LYP/aug-cc-pVDZ level of theory using the gauge-including atomic orbital (GIAO) method. The Gaussian 09 suite of programs37 was used for all calculations.

absolute concentrations of carbamate and carbonate species that varied depending on the CO2 loading. Furthermore, alkyl carbonate species was found to be present as a third form 2R1R2NH + CO2 ↔ R1NHCH 2CH 2OCOO− + R1R2NH 2+ (3)

This was confirmed by 1H−13C heteronuclear multiple-bond correlation (HMBC) experiments and quantum mechanical calculations.

3. RESULTS AND DISCUSSION 3.1. 13C NMR Quantitative Spectra. Figure 1 shows the 13 C NMR quantitative spectrum of a CO2-loaded aqueous EAE solution acquired by an inverse-gated decoupling sequence with a pulse angle of 30° and a delay time of 60 s. As discussed later, this pulse sequence is satisfactory for quantification in EAE− H2O−CO2 solutions. In the spectrum, methyl carbon (−CH3) peaks were observed at 13.61 ppm and 13.81 ppm for R1R2NH/R1R2NH2+ and R1R2NCOO−, respectively, whereas peaks for the methylene bridge (−CH2−) appeared at 43.00− 61.58 ppm. The peaks at 164.30 and 165.66 ppm at lower field were assigned to carbons of carbamate (R1R2NCOO−) and carbonate species (HCO3−/CO32−), respectively, where the latter peak represents HCO3−/CO32− overlap because of fast exchange on the NMR time scale through proton scrambling, and the peak is well-known to shift to higher field with decreasing pH of the solution.24,29 It is not possible to distinguish between R1R2NH and R1R2NH2+ from 13C NMR spectra for this same reason. The relative values for peak areas and the chemical shift assignments are also shown in Figure 1. The error in the peak areas was estimated to be 2% on the basis of the standard deviations of peak areas from different carbons of R1R2NH/ R1R2NH2+. The CO2 loading in the sample was calculated from the total peak area of CO2-containing spices (peaks e′ and f in Figure 1) and the peak area of the α-carbon in the alcohol group (peak d in Figure 1). The values for CO2 loading obtained from 13C NMR quantitative spectra of all of the samples are compared with those from TOC measurements in Table 1. The 13C NMR and TOC results agreed well, and the root-mean-square deviation of the loading obtained by the two methods was 0.016 mol of CO2/mol of amine. For the CO2saturated solution prepared with flowing CO2 gas, the value of CO2 loading obtained from 13C NMR spectra at 313 K was 0.80 mol of CO2/mol of amine. This value was close to the equilibrium solubility data of CO2 in 30 wt % aqueous EAE solution (0.84 mol of CO2/mol of amine) at 313 K and a CO2 partial pressure of 100 kPa.38 3.2. 13C Spin−Lattice Relaxation Times. The saturation of 1H resonance allows the observation of 13C without 1H−13C spin−spin coupling and results in the nuclear Overhauser effect

2. MATERIALS AND METHODS 2.1. Chemicals. EAE (>98% purity, Tokyo Chemical Industry, Tokyo, Japan) was purchased and used without further purification. CO2-saturated solutions were prepared by flowing CO2 gas at a rate of 500 mL/min into 100 g of 30 wt % (3.4 mol/L) aqueous EAE solutions at room temperature and atmospheric pressure for 30 min. The CO2 loading was lowered by mixing those solutions and fresh 30 wt % EAE solutions. The CO2 loadings of the prepared samples were measured with a total organic carbon analyzer (TOC-VCSH, Shimadzu, Kyoto, Japan). 2.2. NMR Spectroscopy. The sample solutions were analyzed using an NMR spectrometer (DRX-500, Bruker, Karlsruhe, Germany). To retain the sample concentration, a double-walled sample tube that was sealed during the measurements was used with C6D6 as the lock solvent and Si(CH3)4 as the internal standard. 13 C NMR spectra were recorded at 125.8 MHz. 13C spin− lattice relaxation times were measured by the inversion recovery method in which the time between the inversion (180°) and read (90°) pulses was varied from 0.2 to 16 s with a relaxation delay of 60 and 128 scans. The spin−lattice relaxation time for each chemical species was obtained using the Bloch equation.34 Quantitative spectra were acquired by an inverse-gated decoupling sequence and 1000 scans with a pulse angle of 30° and a delay time of 60 s. The effect of delay time was also examined in the range of 2−60 s. An HMBC spectrum was obtained using the standard pulse sequence.35 1H high-power pulse times for 90° and 180° (P1 and P2) were 13.4 and 26.8 ms, respectively. The 13C highpower pulse time for 90° (P3) was 9.0 ms. Delays for the creation of antiphase magnetization and evolution of the longrange couplings (D2 and D6) were 3.45 and 65.0 ms, respectively. The number of scans (NS) was 256. 2.3. Calculations. All calculations by density functional theory (DFT) were carried out in the aqueous phase using the SMD solvation model with the integral equation formalism polarizable continuum model (IEF-PCM) protocol for bulk 1618

dx.doi.org/10.1021/ie4034889 | Ind. Eng. Chem. Res. 2014, 53, 1617−1623


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Table 1. CO2 Loadings of 30 wt % (3.4 mol/L) Aqueous EAE Solution Samples loadinga sample no.

NMR spectroscopyb

TOC

1 2 3 4 5

0.18 0.33 0.50 0.66 0.80

0.16 0.33 0.50 0.66 0.83

a

CO2 loadings measured by 13C NMR and TOC analyses (moles of CO2 per mole of amine); bInverse-gated decoupling mode (30° pulse, 60-s delay).

by the interactions of 1H and 13C dipoles. 1H resonance has an essentially instantaneous effect on the spin−spin coupling, whereas 13C polarization resulting from the same irradiation develops with the time constant of spin−lattice relaxation. This difference in time scales enables a 1H-decoupled and quantitative 13C NMR spectrum to be acquired.39 In the inverse-gated decoupling method, accurate quantitation is ensured by a sufficient length of the delay time, which allows the system to return to thermal equilibrium. The delay time in a standard inverse-gated decoupling sequence with a 90° pulse should be at least five times longer than the 13C spin−lattice relaxation time.39,40 Table 2 lists the spin−lattice relaxation times determined for the EAE−H2O−CO2 system in this study.

Figure 2. 13C NMR quantitative peaks for absorbed CO2 in aqueous EAE solutions with different CO2 loadings at 313 K with peak interpretations (e′, carbamate; f, HCO3−/CO32−; g″, alkyl carbonate) and relative values for peak areas.

the solution with high CO2 loading (0.80 mol of CO2/mol of amine) is attributed to HCO3−, whereas the corresponding peaks for solutions with lower CO2 loadings represent mixed populations of HCO3− and CO32−. On the other hand, the chemical shift of EAE carbamate (peak e′) was slightly affected by CO2 loading, indicating that carbamate is present as an anion (R1R2NCOO−) and is not protonated in this pH range. Furthermore, our data reveal that a third form of dissolved CO2 emerges in aqueous EAE solutions with high CO2 loadings. This form (peak g″) was observed at around 158.5 ppm in Figure 2. Böttinger et al.22 studied the species distribution in solutions of CO2 in aqueous MEA by quantitative online NMR spectroscopy. In addition 13C peaks for MEA carbamate at 163.6 ppm and HCO3− at 159.9 ppm, they also observed a third peak of dissolved CO2 at 157.6 ppm in the spectrum of MEA−H2O−CO2 (0.70 mol of CO2/mol of amine) at 293 K. In their study, the 13C peak at 157.6 ppm was assigned to the carbonyl carbon of 2-oxazolidone based on both the 13C and 1H spectra. The formation of oxazolidone species is considered to be a key step in the degradation processes of CO2-rich aqueous alkanolamine solutions.41−46 Therefore, such information on minor products is of practical significance. As discussed later, the 13C peak at around 158.5 ppm found as a third form in this study was assigned to the carbon of alkyl carbonate species. Figure 3 and Table 3 show the species distribution in the EAE−H2O−CO2 system (samples in Table 1), which was determined from the 13C NMR spectra acquired by the quantitative pulse sequence (30° pulse, 60-s delay). Because the peaks of EAE/H+EAE shifted with increasing CO2 loading, some peaks (b and c) overlapped with those of carbamate (see Table 3). Therefore, the concentration ratios were calculated from 13C peak areas of the α-carbon in the alcohol group (−CH2OH) at around 60 ppm and dissolved CO2 between 158 and 166 ppm. The absolute concentrations of total amine in the sample solutions were confirmed by TOC measurements.

Table 2. 13C Spin−Lattice Relaxation Times at 313 K in the EAE−H2O−CO2 Systema EAE (H+EAE) carbon a b c d

carbamate

relaxation time carbon (s) 2.7 1.3 1.1 1.2

a′ b′ c′ d′ e′

relaxation time (s) 2.6 1.1 0.7 1.0 12.2

HCO3− (CO32−) carbon relaxation time (s) f

11.4

a

Relaxation times for carbons as shown in Figure 1 measured for a 30 wt % aqueous EAE solution with a loading of 0.8 mol of CO2/mol of amine.

Spin−lattice relaxation is mainly dominated by dipole− dipole interactions, and 13C spin−lattice relaxation time is affected by both directly bound and distant protons.28 As shown in Table 2, the spin−lattice relaxation times of methyl carbons were slightly longer (approximately 2 s) than those of methylene carbons (approximately 1 s). This is the same trend as reported by Ciftja et al.28 for DEEA−H2O−CO2. Compared with the methyl and methylene carbons, the carbamate and carbonate carbons showed considerably longer relaxation times (approximately 12 s). These measurements indicate that a delay time of 60 s in this study was sufficiently long for quantitation. 3.3. Chemical Forms of Dissolved CO2. All of 13C peaks arising from CO2 absorbed into aqueous EAE solutions were observed between 158 and 166 ppm in 13C NMR quantitative spectra acquired by the inverse-gated decoupling sequence. As shown in Figure 2, the chemical shift of HCO3−/CO32− overlap (peak f) decreased with increasing CO2 loading. A previous study reported that the chemical shifts in samples containing solely CO32− or HCO3− were 168.9 and 161.4 ppm, respectively.24 Therefore, the peak at 161.2 ppm observed for 1619



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It should be noted that eqs 1−3 represent overall reactions for CO2 absorption and do not show possible reaction pathways such as the interconversion of carbamate and bicarbonate R1R2NCOO− + H 2O ↔ HCO3− + R1R2NH

(4)

The species distribution in Figure 3, however, is independent of such reaction pathways, because the speciation in equilibrium was examined in this study. Recently, we investigated reaction pathways for the interconversion of carbamate and bicarbonate in primary amine solutions (MEA and AMP) using ab initio molecular orbital calculations combined with the polarizable continuum model formalism.48,49 Although a delineation of the mechanisms that underlie overall reactions 1−3 is beyond the scope of this article, the data shown in Figure 3 might be helpful in understanding them. An ab initio study for secondary amine solutions is currently underway. 3.4. Identification of Carbonyl Species. A 1H−13C HMBC spectral analysis provides structural information on a few bond order couplings between carbon and other neighboring protons. Therefore, it can be used as a reliable and acceptable technique for the determination of organic molecular structures.50 By using the HMBC technique, in this study, special attention was focused on the identification of the third form of dissolved CO2 (peak g″ in Figure 2) as alkyl carbonate, oxazolidone, or other species. As a result, the third form was assigned to the alkyl carbonate species, as shown in Figure 4. Surprisingly, we found that, in aqueous solutions with high CO2 loadings, a small amount of CO2 was covalently bounded to hydroxyl groups of both EAE and EAE carbamate. Except for the peak of HCO3−/CO32−, four peaks were obtained for the carbonyl carbons of carbamate (e′), alkyl carbonate (g″), and carbamate−carbonate (e* and g*) species in Figure 4. Figure 5 shows the molecular geometries of carbamate, alkyl carbonate, and carbamate−carbonate optimized at the SMD/ IEF-PCM/B3LYP/6-311++G(d,p) level of theory. The validity of this level of theory has been demonstrated for aqueous amine−CO2 systems in previous studies.49 For these optimized geometries, the four chemical shifts of carbamate (e′), alkyl carbonate (g″), and carbamate−carbonate (e* and g*) were calculated to be 167.11, 160.61, 167.05, and 162.42 ppm, respectively, at the B3LYP/aug-cc-pVDZ level of theory (Figure 5), and the corresponding experimentally observed chemical shifts were 164.13, 158.58, 163.75, and 159.65 ppm, respectively. The calculated and experimental chemical shifts exhibited a linear relationship with the coefficient of determination of 0.99. This agreement validates the above identification of carbonyl species in the EAE−H2O−CO2 solutions. The chemical shift of oxazolidone was predicted to be observed at lower field than that of alkyl carbonate species,

Figure 3. Species distribution in the EAE−H2O−CO2 system at 313 K determined from 13C NMR spectra obtained using an inverse-gated decoupling sequence with a pulse angle of 30°, a delay time of 60 s, and 1000 scans.

Böttinger et al.22 reported that the MEA−H2O−CO2 system exhibits a predominant formation of carbamate with no formation of HCO3−/CO32− below the loading of 0.46 mol of CO2/mol of amine (313 K, 30 wt % aqueous MEA). Above a loading of approximately 0.5 mol of CO2/mol of amine, the mole fraction of MEA carbamate decreased, whereas that of carbonate species (bicarbonate) monotonically increased with CO2 loading. Similar trends were observed for the EAE−H2O− CO2 system in Figure 3. However, it was found that comparable amounts of the carbamate and carbonate species were formed below the loading of 0.5 mol of CO2/mol of amine in the EAE−H2O−CO2 solutions. This can be attributed to the steric effect of the amino group of EAE, which is mildly hindered.31,47 It should be noted that not only the mole fraction of EAE carbamate but also its absolute concentration has a maximum at a loading of ∼0.5 mol of CO2/mol of amine. In addition, Figure 3 shows that, at higher CO2 loadings, a significant amount of the third form of dissolved CO2 is present in the aqueous EAE solutions. Despite the fact that 13C NMR spectroscopy was applied to various amine−H2O−CO2 solutions in earlier studies,16−29 reports of species distributions depending on CO2 loading for secondary alkanolamines have been limited to DEA and 2(butylamino)ethanol (BAE).22,24 Both DEA and BAE showed dependences similar to that in Figure 3. However, the third form in this study (peak g″ in Figure 2) was not observed in the BAE−H2O−CO2 system.24 For the DEA−H2O−CO2 system, the third form was provisionally assigned to 3-hydroxyethyl-2oxazolidone.22 Table 3. 13C NMR Peak Areas at 313 K for Samples in Table 1a

a

no.

a

b

c

d

a′

b′

c′

d′

e′

f

g″

1 2 3 4 5

2.88 2.61 2.53 2.57 2.97

2.93 2.51 2.47 −b −b

2.91 2.55 2.29 −b −b

2.98 2.66 2.53 2.66 3.00

0.39 0.74 0.88 0.66 0.39

0.41 0.61 0.92 −b −b

0.37 0.67 0.76 −b −b

0.41 0.73 0.84 0.68 0.46

0.42 0.74 0.87 0.74 0.40

0.19 0.39 0.80 1.45 2.23

0.000 0.000 0.021 0.065 0.099

Relative peak areas as shown in Figures 1 and 2. bNot determined because of the spectral overlap between EAE (H+EAE) and carbamate. 1620



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Figure 4. Expanded 2D HMBC spectrum of a 30 wt % aqueous EAE solution with a loading of 0.8 mol of CO2/mol of amine at 298 K with peak interpretations.

as shown in Figure 5. This prediction also supports our identification. 3.5. Pulse Parameters. Because 13C spin−lattice relaxation is described by a single exponential function with a time constant, the standard inverse-gated decoupling sequence for quantitative measurements requires a sufficient delay time, which is usually more than five times the longest relaxation time in the system.39,40 The relaxation times for the species in various amine−H2O−CO2 systems at 298 K reported by Ciftja et al.28 were 10 s at the longest. A delay time of 60 s is also considered to be appropriate for most amine−H2O−CO2 systems. In addition, we note that the pulse angle applied in this study was 30°, and the delay time could be reduced for such small flip angle pulses. As shown in Figure 6, the concentration ratio of carbamate to carbonate species in the CO2-loaded EAE solution was almost constant (±3%) regardless of the delay time between 15 and 60 s. This can be partly explained by the fact that the 13C spin−lattice relaxation times of carbonyl and carbonate carbons (e′ and f) are close to each other, as shown in Table 2. It is often the case

Figure 5. Molecular geometries of carbamate, alkyl carbonate, and oxazolidone species optimized at the SMD/IEF-PCM/B3LYP/6-311+ +G(d,p) level of theory with 13C chemical shifts calculated at the B3LYP/aug-cc-pVDZ level of theory.

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REFERENCES

(1) Rochelle, G. T. Amine scrubbing for CO2 capture. Science 2009, 325, 1652. (2) The Global Status of CCS: Update, January 2013; Global CCS Institute: Canberra, ACT, Australia, 2013. (3) Matin, N. S.; Remias, J. E.; Neathery, J. K.; Liu, K. The equilibrium solubility of carbon dioxide in aqueous solutions of morpholine: Experimental data and thermodynamic modeling. Ind. Eng. Chem. Res. 2013, 52, 5221. (4) Chen, X.; Rochelle, G. T. Thermodynamics of CO2/2methylpiperazine/water. Ind. Eng. Chem. Res. 2013, 52, 4229. (5) Gupta, M.; da Silva, E. F.; Hartono, A.; Svendsen, H. F. Theoretical study of differential enthalpy of absorption of CO2 with MEA and MDEA as a function of temperature. J. Phys. Chem. B 2013, 117, 9457. (6) Arshad, M. W.; Fosbøl, P. L.; von Solms, N.; Svendsen, H. F.; Thomsen, K. Heat of absorption of CO2 in phase change solvents: 2(Diethylamino)ethanol and 3-(methylamino)propylamine. J. Chem. Eng. Data 2013, 58, 1974. (7) Guido, C. A.; Pietrucci, F.; Gallet, G. A.; Andreoni, W. The fate of a zwitterion in water from ab initio molecular dynamics: Monoethanolamine (MEA)−CO2. J. Chem. Theory Compt. 2013, 9, 28. (8) Sumon, K. Z.; Henni, A.; East, A. L. L. Predicting pKa of amines for CO2 capture: Computer versus pencil-and-paper. Ind. Eng. Chem. Res. 2012, 51, 11924. (9) Naami, A.; Edali, M.; Sema, T.; Idem, R.; Tontiwachwuthikul, P. Mass transfer performance of CO2 absorption into aqueous solutions of 4-diethylamino-2-butanol, monoethanolamine, and N-methyldiethanolamine. Ind. Eng. Chem. Res. 2012, 51, 6470. (10) Chen, X.; Rochelle, G. T. Aqueous piperazine derivatives for CO2 capture: Accurate screening by a wetted wall column. Chem. Eng. Res. Des. 2011, 89, 1693. (11) Rochelle, G.; Chen, E.; Freeman, S.; Wagener, D.; Xu, Q.; Voice, A. Aqueous piperazine as the new standard for CO2 capture technology. Chem. Eng. J 2011, 171, 725. (12) Maiti, A.; Bourcier, W. L.; Aines, R. D. Atomistic modeling of CO2 capture in primary and tertiary aminesHeat of absorption and density changes. Chem. Phys. Lett. 2011, 509, 25. (13) Maneeintr, K.; Idem, R. O.; Tontiwachwuthikul, P.; Wee, A. G. H. Comparative mass transfer performance studies of CO2 absorption into aqueous solutions of DEAB and MEA. Ind. Eng. Chem. Res. 2010, 49, 2857. (14) McCann, N.; Phan, D.; Wang, X.; Conway, W.; Burns, R.; Attalla, M.; Puxty, G.; Maeder, M. Kinetics and mechanism of carbamate formation from CO2(aq), carbonate species, and monoethanolamine in aqueous solution. J. Phys. Chem. A 2009, 113, 5022. (15) Puxty, G.; Rowland, R.; Allport, A.; Yang, Q.; Bown, M.; Burns, R.; Maeder, M.; Attalla, M. Carbon dioxide postcombustion capture: A novel screening study of the carbon dioxide absorption performance of 76 amines. Environ. Sci. Technol. 2009, 43, 6427. (16) Yamada, H.; Shimizu, S.; Okabe, H.; Matsuzaki, Y.; Chowdhury, F. A.; Fujioka, Y. Prediction of the basicity of aqueous amine solutions and the species distribution in the amine−H2O−CO2 system using the COSMO-RS method. Ind. Eng. Chem. Res. 2010, 49, 2449. (17) Yamada, H.; Chowdhury, F. A.; Matsuzaki, Y.; Higashii, T. Computational investigation of carbon dioxide absorption in alkanolamine solutions. J. Mol. Model. 2013, 19, 4147. (18) García-Abuín, G.; Gómez-Díaz, D.; López, A. B.; Navaza, J. M.; Rumbo, A. NMR characterization of carbon dioxide chemical absorption with monoethanolamine, diethanolamine, and triethanolamine. Ind. Eng. Chem. Res. 2013, 52, 13432. (19) Shi, H.; Sema, T.; Naami, A.; Liang, Z.; Idem, R.; Tontiwachwuthikul, P. 13C NMR spectroscopy of a novel amine species in the DEAB−CO2−H2O system: VLE model. Ind. Eng. Chem. Res. 2012, 51, 8608. (20) Tomizaki, K.-y.; Kanakubo, M.; Nanjo, H.; Shimizu, S.; Onoda, M.; Fujioka, Y. 13C NMR studies on the dissolution mechanisms of carbon dioxide in amine-containing aqueous solvents at high pressures

Figure 6. Dependence on the delay time of the ratio of 13C NMR peak areas of carbamate and carbonate (HCO3−/CO32−) species, measured by inverse-gated decoupling with a pulse angle of 30° for a 30 wt % aqueous EAE solution with a loading of 0.8 mol of CO2/mol of amine.

that our interests are to measure ratios between carbamate and carbonate species in amine−H2O−CO2 systems and compare the properties between amines. For such purposes, a small flip angle pulse with a reduced delay time would still be effective.16

4. CONCLUSIONS To provide detailed information on CO2 capture reactions by aqueous solutions of a sterically hindered secondary amine, 2(ethylamino)ethanol (EAE), we performed nuclear magnetic resonance (NMR) spectroscopic analyses of the EAE−H2O− CO2 system. Based on the longest 13C spin−lattice relaxation times observed in this system (11−12 s), chemical species distributions at different CO2 loadings were determined by the inverse-gated decupling sequence with a delay time of 60 s. We also reported other parameters such as flip angle and its influence on quantitative measurements. Under low loading (0.5 mol of CO2/mol of amine), the content of carbamate anion decreased with increasing loading. On the other hand, the concentration of carbonate species increased monotonically with increasing CO2 loading, which enabled the EAE−H2O−CO2 system to exhibit a high CO2 loading. In addition to bicarbonate and carbamate, a third form of dissolved CO2 was detected for CO2-rich solutions. This form was analyzed for identification as alkyl carbonate, oxazolidone, or other species by 2D NMR spectroscopy and density functional theory calculations. As a result, we conclude that the hydroxyl group of EAE can be directly bound to CO2 to form alkyl carbonate species in the EAE−H2O−CO2 system under ambient temperature and pressure conditions, although it is a minor product.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-774-75-2305. Fax: +81-774-75-2318. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the COURSE 50 project founded by the New Energy and Industrial Technology Development Organization (NEDO), Kawasaki, Kanagawa, Japan. 1622



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Quantitative Spectroscopic Study of Equilibrium in CO2-Loaded

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