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CO2 solubility and species distribution in aqueous solutions of 2-(isopropylamino)ethanol and its structural isomers
International Journal of Greenhouse Gas Control 17 (2013) 99–105
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CO2 solubility and species distribution in aqueous solutions of 2-(isopropylamino)ethanol and its structural isomers Hidetaka Yamada ∗ , Firoz A. Chowdhury, Kazuya Goto, Takayuki Higashii Chemical Research Group, Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 1 March 2013 Accepted 19 March 2013 Available online 31 May 2013 Keywords: Alkanolamine Carbon dioxide Chemical absorbent NMR Vapor–liquid equilibrium
a b s t r a c t The solubility of CO2 was measured in aqueous solutions of 2-(isopropylamino)ethanol (IPAE) and 2(propylamino)ethanol (PAE) at 313 K, 373 K and 393 K and CO2 partial pressures ranging from 5 kPa to 0.2 MPa. With different CO2 loadings, speciation analyses were conducted by accurate quantitative 13 C nuclear magnetic resonance spectroscopy using inverse-gated decoupling. IPAE had a larger capacity for CO2 than PAE because of the dominant formation of bicarbonate rather than carbamate. However, a significant amount of carbamate was also observed with low CO2 loadings in the IPAE solutions. The results for these secondary amine solutions were compared with those of the primary amines monoethanolamine and 5-aminopentanol. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Amines are the most widely used chemicals for CO2 capture, and are used in solvent- (Rochelle, 2009; Rochelle et al., 2011; Singh et al., 2011; Brúder et al., 2012; Sema et al., 2013), solid sorbent(Hiyoshi et al., 2005; Gray et al., 2008), and membrane-based systems (Kovvali et al., 2000; Duan et al., 2012). Amines react with CO2 to generate several ionic species, and these reactions are facilitated in polar environments and especially in aqueous solutions. Primary and secondary amines react with CO2 to form the carbamate anion and protonated amine as follows: 2R1 R2 NH + CO2 ↔ R1 R2 NCOO− + R1 R2 NH2 +
(1)
where R1 and R2 represent amino substituents. Another product of CO2 absorption in aqueous amine solutions is the bicarbonate anion (da Silva and Svendsen, 2007; Vaidya and Kenig, 2007). R1 R2 NH + CO2 + H2 O ↔ HCO3 − + R1 R2 NH2 +
(2)
Eqs. (1) and (2) show that various chemical species, including amine, protonated amine, carbamate, bicarbonate and water, will be present in amine solutions loaded with CO2 . Reactions (1) and (2) have different stoichiometries, reaction rates, and heat of reaction values. Therefore, the branching ratio between these reactions is highly relevant to the absorbent performance. In CO2 -loaded
∗ Corresponding author. Tel.: +81 774 75 2305; fax: +81 774 75 2318. E-mail address: [email protected] (H. Yamada). 1750-5836/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2013.03.027
aqueous amine solutions, the species distribution as determined by the branching ratio depends on the CO2 loading and the amine molecular structure (Yamada et al., 2010). Steric hindrance from amine substituents is a crucial factor in determining the CO2 absorption and desorption behavior (Sartori and Savage, 1983). Solvents containing sterically hindered amines such as 2-amino-2-methyl-1-propanol (AMP) and 2-piperidineethanol are possible alternatives to the conventional solvent with monoethanolamine (MEA). The application of sterically hindered amines will likely offer absorption capacity, absorption rate, and degradation resistance advantages over conventional amines for CO2 removal from gases (Bougie and Iliuta, 2012). We used screening tests to investigate CO2 absorption and desorption of single-component amine solvents, and found some amines had unique features (Goto et al., 2011). The moderately hindered amine 2-(isopropylamino)ethanol (IPAE) had a high absorption rate and large desorption capability. We formulated IPAE-based solvents and evaluated the solvents at a 1 ton CO2 /day test plant. The IPAE solvents are promising for reducing CO2 capture costs. In this work, solubility data for CO2 in aqueous solutions of alkanolamines, including IPAE and 2-(propylamino)ethanol (PAE), were measured at 313 K, 373 K and 393 K and at CO2 partial pressures between 5 kPa and 0.2 MPa to evaluate the CO2 capture potentials of these solvents. Furthermore, we investigated the species distributions in alkanolamine–H2 O–CO2 systems by 13 C nuclear magnetic resonance (NMR) spectroscopy using inverse-gated decoupling.
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OH
H2N
N H PAE
OH
H2N
MEA
AP
OH
N H
OH
IPAE
Fig. 1. Molecular structures of the investigated amines.
In 13 C NMR, the measurement of spin-lattice relaxation times is required for accurate quantification (Opella et al., 1976). However, little information is available on the relaxation times in alkanolamine–H2 O–CO2 systems. The relaxation times were measured to set an adequate time delay in the pulse sequence for the systems in this work. Based on the accurate quantification results, the species distribution dependence on the CO2 loading was compared among the structural isomers IPAE, PAE and 5aminopentanol (AP). The results were used to gain insight into the relationship between the absorption properties and molecular structures of alkanolamines. 2. Materials and methods The molecular structures of the alkanolamines investigated in this work are shown in Fig. 1. MEA, PAE and IPAE (purity 99%) were obtained from chemical companies and used without further purification. AP was purchased as a 50 wt% aqueous solution. Isothermal vapour–liquid equilibrium (VLE) data were measured at 313 K, 373 K and 393 K at CO2 partial pressures ranging from a few kPa to approximately 200 kPa. Approximately 700 mL of each 30 wt% aqueous amine solution was fed into an autoclave, followed by a N2 purge (99.99%, Ueno-gas, Nara, Japan). Each solution was agitated with a mechanical stirrer and the temperature was held constant during the experiment. The concentration of the gas mixture of CO2 (99.9%, Showa Denko, Kawasaki, Japan) and N2 was controlled using mass flow controllers, and then supplied to the autoclave after passing through a water saturator. The outlet gas was monitored by an infrared CO2 analyzer with the repeatability of ±0.5% and linearity of ±1.0% full-scale range (VA-3001, Horiba, Kyoto, Japan) after passing through a condenser. Equilibrium was obtained when the CO2 analyzer indicated a constant CO2 concentration (±0.01%). To analyze the equilibrium data, the total pressure in the gas phase was measured. The CO2 loading in the liquid phase was measured by a total organic carbon analyzer (TOC-VCSH , Shimadzu, Kyoto, Japan) after collecting a sample from the autoclave. The TOC analyzer had a measuring range of 0–30,000 mg/L for inorganic carbon and the coefficient of variation (reproducibility) was less than 1.5%. For 13 C NMR analyses, CO2 saturated solutions were prepared by passing the CO2 gas at a rate of 500 mL/min through 75 g of each 30 wt% aqueous alkanolamine solution at room temperature and atmospheric pressure for 15 min. Samples with lower CO2 loadings were obtained by mixing fresh 30 wt% aqueous alkanolamine solutions with the CO2 saturated solutions. The CO2 loadings of the sample solutions were quantified using the TOC analyzer. The pH in each sample solution was measured at 299 K by a pH meter with a resolution of 0.01 pH (D-51, Horiba) calibrated with pH 7 and pH 9 standard solutions. The density was measured at 298 K by a density/specific gravity meter with an accuracy of ±0.001 g/cm3 and a measuring range of 0–3 g/cm3 (DA-100, Kyoto Electronics, Kyoto, Japan). 13 C NMR spectra were recorded at 125.8 MHz with a NMR spectrometer (DRX-500, Bruker, Billerica, MA) at 303 K. A double-walled sample tube was used with C6 D6 as the lock solvent and Si(CH3 )4
Fig. 2. Equilibrium partial pressure of CO2 in 30 wt% aqueous solutions of MEA at 313 K, 373 K, and 393 K.
as the internal standard to retain the sample concentration. Quantitative spectra were obtained using inverse-gated decoupling with a pulse angle of 90◦ , a delay time of 60 s, and 512 scans. To determine the delay time, 13 C-spin-lattice relaxation times were measured by the inversion-recovery method.
3. Results and discussion 3.1. VLE The CO2 solubility in 30 wt% aqueous MEA, PAE and IPAE solutions are summarized in Table 1. In Table 1, P is the partial pressure of CO2 in the gas phase, and ˛ is the CO2 loading in the liquid phase, which is defined as a molar ratio of absorbed CO2 to total amine at equilibrium. Plots of P versus ˛ for MEA, PAE and IPAE are shown in Figs. 2–4, respectively. In Fig. 2, solubility data in 30 wt% aqueous MEA solutions measured at 393 K by Ma’mun et al. (2005), and at 313 K and 373 K by Shen and Li (1992) are shown for comparison. The literature data agree with the data obtained in this work.
Fig. 3. Equilibrium partial pressure of CO2 in 30 wt% aqueous solutions of PAE at 313 K, 373 K, and 393 K.
H. Yamada et al. / International Journal of Greenhouse Gas Control 17 (2013) 99–105
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Table 1 CO2 solubilities in 30 wt% aqueous amine solutions.a 313 K
MEA
k0 , k1 k2 , k3 PAE
k0 , k1 k2 , k3 IPAE
k0 , k1 k2 , k3 a
373 K
393 K
P (kPa)
˛
P (kPa)
˛
5.5 11.0 25.8 52.7 106.5 475.19 5004.7
0.498 0.524 0.547 0.567 0.599 −2681.7 −3070.5
4.5 11.0 25.6 60.2 105.6 −2.3237 −35.914
0.261 0.326 0.382 0.427 0.456 20.365 54.587
5.3 12.5 29.2 65.8 112.4 0.57793 46.452
˛ 0.151 0.216 0.282 0.336 0.381 1.5507 −57.514
4.8 11.5 26.9 62.4 113.9 226.2 −0.043008 −86.501
0.058 0.094 0.151 0.230 0.305 0.398 33.130 94.996
P (kPa)
5.7 12.3 26.8 57.8 107.8 213.4 −10.564 −39.506
0.579 0.646 0.724 0.794 0.845 0.902 37.736 19.133
4.4 10.6 25.1 59.5 108.7 218.4 −0.80624 −34.392
0.128 0.193 0.283 0.391 0.471 0.564 21.912 26.630
5.8 12.3 26.8 58.0 106.9 205.5 −65.106 −379.92
0.605 0.695 0.775 0.846 0.870 0.897 274.46 180.00
4.4 10.6 25.1 58.9 104.5 207.6 −0.31075 −48.977
0.085 0.131 0.209 0.329 0.419 0.564 25.526 38.123
4.8 11.7 27.3 62.6 111.2 214.3 −0.44744 −203.05
0.047 0.065 0.104 0.165 0.229 0.338 54.291 276.43
P, CO2 partial pressure; ˛, loading (mol CO2 /mol amine); k0 , k1 , k2 , k3 , the best-fit parameters for Eq. (3).
The curves shown in Figs. 2–4 were determined by the leastsquares fit of all data points to the following polynomial equation: ln P = k0 + k1 ˛ + k2 ˛2 + k3 ˛3
(3)
where k0 , k1 , k2 and k3 are fitting parameters. Eq. (3) satisfactorily fit the data in the studied CO2 partial pressure range as shown in Figs. 2–4. At the typical absorption temperature of 313 K, the aqueous solutions of IPAE and PAE had higher CO2 loadings than conventional aqueous MEA solutions in the CO2 partial pressure range 5 kPa to 0.2 MPa. Larger amounts of CO2 were desorbed from these secondary amine solutions than from MEA at the typical stripping temperature of 393 K. The moderately hindered amine, IPAE showed higher CO2 solubility at 313 K than the mildly hindered amine, PAE. However, this relationship reversed at higher temperatures (373 K and 393 K), which lead to different cyclic capacities for CO2 .
3.2. Cyclic CO2 capacity Based on the VLE data we evaluated the cyclic CO2 capacity, that is, the maximum loading difference between the CO2 -rich solution after absorption and the CO2 -lean solution after stripping. Assuming conditions of a 10–20 kPa CO2 partial pressure at 313 K for the absorption process and 5–100 kPa CO2 partial pressure at 393 K for the stripping process, we calculated the cyclic CO2 capacity as follows: ˛ = ˛313
K
− ˛393
(4)
K
The CO2 loadings in these conditions were obtained from the fitted regression equations. The evaluated cyclic CO2 capacities of 30 wt% aqueous MEA, PAE and IPAE are listed in Table 2. The aqueous IPAE solutions had a larger cyclic CO2 capacity than PAE or MEA because of high absorption of CO2 under the absorption conditions and the high desorption of CO2 under the stripping conditions. For example, the cyclic capacities calculated with 10 kPa CO2 and 313 K absorption and 100 kPa CO2 and 393 K stripping are 0.15 for MEA, 0.34 for PAE, and 0.44 for IPAE (mol CO2 /mol amine). It should be noted that 30 wt% aqueous MEA contains more amine molecules (4.9 mol amine/kg solvent) than 30 wt% aqueous PAE or IPAE (2.9 mol amine/kg solvent), which should be an advantage for MEA in terms of its CO2 capacity. However, IPAE still had a larger cyclic capacity in terms of CO2 capacity per mass of solvent (56 g CO2 /kg solvent) than MEA
Table 2 CO2 capacities of 30 wt% aqueous amine solutions.
MEA PAE IPAE Fig. 4. Equilibrium partial pressure of CO2 in 30 wt% aqueous solutions of IPAE at 313 K, 373 K, and 393 K.
Absorption, 313 K
Stripping, 393 K
Cyclic capacity,a ˛
˛10 kPa
˛20 kPa
˛ 5 kPa
˛ 100 kPa
˛10 kPa –˛ 100 kPa
˛20 kPa –˛ 5 kPa
0.52 0.63 0.66
0.54 0.69 0.75
0.15 0.06 0.04
0.37 0.29 0.22
0.15 0.34 0.44
0.39 0.63 0.71
a Difference between loadings under absorption (˛) and stripping (˛ ) conditions (mol CO2 /mol amine). The subscripts represent CO2 partial pressures.
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Fig. 5. Quantitative 13 C NMR spectrum of PAE–H2 O–CO2 at 303 K with peak interpretations and the relative values for peak areas.
(32 g CO2 /kg solvent). This trend was true at all CO2 pressures in the tested range for both absorption and stripping. IPAE shows a considerably large cyclic capacity when compared with a conventional secondary amine, diethanolamine (DEA). The CO2 solubility in 30 wt% aqueous DEA has been reported previously (Chowdhury et al., 2011): 67.8 at 20 kPa CO2 and 313 K, 25.9 at 100 kPa CO2 and 393 K, and the cyclic capacity of 41.9 (g CO2 /L solution). The cyclic capacities calculated from the corresponding conditions are 51.2 for PAE and 67.8 for IPAE (g CO2 /kg solvent) in this work. 3.3. Spectral analysis The CO2 loadings, densities and pH values of the samples prepared for 13 C NMR are summarized in Table 3. According to Eqs. (1) and (2), the density increased and the basicity decreased as the CO2 loading increased. Fig. 5 shows the quantitative 13 C NMR spectrum of the CO2 -loaded aqueous PAE solution (No. 3 in Table 3) with an interpretation of the peaks and their relative areas. Amine (R1 R2 NH/R1 R2 NH2 + ), carbamate (R1 R2 NCOO− ) and carbonate (HCO3 − /CO3 2− ) species were unambiguously identified in all the spectra, while no other species were detected except for trace impurities. One peak was detected for the carbon of the carbonate species, HCO3 − and CO3 2− , because of the fast exchange of protons. HCO3 − + H2 O ↔ CO3 2− + H3 O+
shifted to the shielded region as the ratio of HCO3 − increased. Therefore, the chemical shift of HCO3 − /CO3 2− monotonically increased with the increase of pH of the solution (Jakobsen et al., 2005). This trend was observed across the spectra of all the different amine systems, as shown in Fig. 6 in this work. The error in the peak areas of the 13 C NMR spectra was estimated to be less than 4% based on the standard deviations of peak areas from different carbons of the same amine species. However, for the carbonate species this is not sufficient to ensure accurate quantitation, because this species contains only one carbon atom. To acquire an accurate quantitative 13 C NMR spectrum, the delay time, which allows the system to return to thermal equilibrium in the standard inverse-gated decoupling sequence with a 90◦ pulse, should be at least five times longer than the spin-lattice relaxation time, T1 (Giraudeau et al., 2006). In this work, the relaxation times for all carbons in the spectra were determined by the inversionrecovery method, and for all the carbons except the carbamate and the carbonate carbons, the spin-lattice relaxation times were shorter than a few seconds in the amine–H2 O–CO2 systems. The carbamate carbon had the longest relaxation time in the system (approximately 10 s), which was slightly longer than that for the carbonate carbon (Table 4). Therefore, the delay time in the inversegated decoupling sequence was determined to be 60 s in this work. This ensured accurate quantitation with the 13 C NMR spectra, especially for comparing the yields of carbamate and carbonate.
(5)
It was not possible to distinguish between amines and protonated amines from the 13 C NMR spectra for this same reason. The HCO3 − /CO3 2− peak, which reflects the ratio of these two ions, Table 3 Densities and pH values of 30 wt% aqueous amine solutions at various CO2 loadings. Amine
˛a
b
pH
No.c
AP
0.00 0.35 0.68
0.990 1.033 1.068
13.0 10.9 8.6
– 1 2
PAE
0.00 0.34 0.66
0.985 1.030 1.065
12.5 10.3 9.0
– 3 4
IPAE
0.00 0.44 0.91
0.987 1.051 1.085
12.6 9.9 8.7
– 5 6
a b c
CO2 loading measured by TOC analysis (mol CO2 /mol amine). Density at 298 K (g/cm3 ). Sample number.
Fig. 6. 13 C chemical shifts of HCO3 − /CO3 2– at 303 K in several amine–H2 O–CO2 systems (Nos. 1–6 in Table 3).
H. Yamada et al. / International Journal of Greenhouse Gas Control 17 (2013) 99–105 Table 4 13 C spin-lattice relaxation times at 303 K in amine–H2 O–CO2 systems.a . Amine AP PAE IPAE
COHb 1.3 0.8 0.7
COO−c 9.1 8.1 10.5
HCO3 − /CO3 2−d –e 6.0 9.1
Relaxation time (s) for b the ˛-carbon in the alcohol group of amine, c the carbon of carbamate, and d the carbon of carbonate species measured using samples of Nos. 1, 3 and 5 in Table 3. e Carbonate species were not detected. a
3.4. Yields of carbamate and carbonate Fig. 7 shows the quantitative 13 C NMR peaks of carbamate and carbonate species in 30 wt% aqueous solutions of the unhindered primary amine (AP), mildly hindered secondary amine (PAE), and moderately hindered secondary amine (IPAE). The CO2 loadings, calculated from the peak areas of carbamate, carbonate, and the ˛-carbon in alcohol group of amine, agreed well with the values from the TOC analysis for sample Nos. 1–6 in Table 3. The chemical shifts of the carbamates were observed at almost the same position (164–165 ppm), while those of the carbonate species varied over a broad range (161–167 ppm) depending on the pH (Fig. 6).
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Fig. 7 also shows the yield of carbamate. =
[R1 R2 NCOO− ] 2− [R1 R2 NCOO− ] + [HCO− 3 ] + [CO3 ]
(6)
In aqueous solutions of the unhindered primary amine, AP, absorbed CO2 predominantly existed as the carbamate at low loading (0.4). By contrast, at high loading (0.7) the carbamate yield was comparable to that of carbonate (HCO3 − /CO3 2− ). The mildly hindered secondary amine, PAE, showed the same trend, but the carbamate yields were lower both at the low and high loadings. In the aqueous solutions of IPAE, a high CO2 loading (0.9) was attainable through a large reduction in carbamate formation. The carbamate yield in IPAE was the lowest among the three isomers. However, the data also showed that a substantial amount of carbamate was present at a loading of 0.4 in the aqueous IPAE solutions. 3.5. Structural factors Our previous studies using a quantum chemical method suggested that carbamate forms easily via a zwitterion intermediate, with relatively low activation energies even in the aqueous
Fig. 7. Quantitative 13 C NMR peaks for absorbed CO2 in 30 wt% aqueous solutions of AP, PAE and IPAE at 303 K. The CO2 loadings (˛) were calculated from the peak areas of HCO3 − /CO3 2– , COO− and COH. The yields of carbamate ( ) were determined by Eq. (6).
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Fig. 8. Carbamates of MEA (left) and AP (right) in their most stable conformations as determined with the Merck molecular force field (Francl, 1985). Dotted lines indicate the intramolecular hydrogen bonds.
solutions of hindered amines such as AMP and IPAE (Yamada et al., 2011a,b). R1 R2 NH + CO2 ↔ R1 R2 NH+ COO−
(7)
R1 R2 NH + R1 R2 NH+ COO− ↔ R1 R2 NCOO− + R1 R2 NH2 +
(8)
However, the carbamate decomposes because of its instability, which mainly arises from steric hindrance of amine substituents, and bicarbonate forms by reaction (2) as a final product. Because reaction (2) is more efficient than reaction (1) in terms of CO2 loading, both the AMP and IPAE solvents can provide high capacity. Recently, qualitative and quantitative 13 C NMR studies were performed on 30 wt% aqueous solutions of AMP for different CO2 loadings at 298 K, and the carbamate was observed (Ciftja et al., 2011). However, the yields of carbamate were less than 0.05 over the CO2 loading range 0.1–0.6. By comparison, as described above, aqueous IPAE solutions absorbed CO2 to yield a substantial amount of carbamate in the low loading range. This occurred because IPAE is moderately hindered, and explains why this solvent has favorable properties such as a moderately high absorption rate and a large capacity as reported by Goto et al. (2011). However, other factors beside the steric hindrance should be considered when interpreting experimental results (da Silva and Svendsen, 2007). Based on quantum mechanical calculations, Jhon et al. (2010) pointed out the importance of a proper combination of steric effects with global nucleophilicities of the amino group to predict the reactivity of amine with CO2 . Using 1 H NMR spectroscopy, McCann et al. (2011) revealed that both steric hindrance and the acid dissociation constant of the parent amine had a significant effect on the carbamate stability. In a screening study of the CO2 absorption performance, Puxty et al. (2009) found that amines with outstanding CO2 absorption capacities shared structural features, including steric hindrance and hydroxyl functionality two or three carbons from the amino nitrogen. Recently we showed that the number of carbons between the hydroxyl and amino groups (i.e., the alcohol chain length) influenced the stability of carbamate through intramolecular hydrogen bonding (Yamada et al., 2013). The amines investigated in this work, except for AP, have two carbons between the hydroxyl and amino groups (Fig. 1), and this possibly contributes to the stabilization of carbamate. Because it is an unhindered primary amine, AP showed the greatest tendency to form carbamate among the three structural isomers. Although it should be noted that the alcohol chain length also affects the basicity of the amino nitrogen and the aqueous solubility, we anticipate that the carbamate stability of AP will be less than that of MEA because of the different effects of intramolecular hydrogen bonding as shown in Fig. 8. In an earlier 13 C NMR study by Jakobsen et al. (2005), CO2 loadings of 0.6 and 0.8 and carbamate
yields of approximately 0.75 and 0.45, respectively, were achieved in aqueous 30 wt% MEA solutions at 293–323 K. Comparison of their results with ours suggests that the carbamate stability constants for MEA are likely to be higher than those for AP. 4. Conclusions VLE measurements and 13 C NMR analyses were performed on alkanolamine–H2 O–CO2 systems. The structural isomers AP, PAE and IPAE were compared to examine the structural factors underlying the CO2 absorption properties. Based on the 13 C spinlattice relaxation time measurements, accurate speciation was achieved especially for carbamate (R1 R2 NCOO− ) and carbonate (HCO3 − /CO3 2− ) species. The carbamate yield ( ) in the CO2 absorption reaction decreased with increasing steric hindrance of the amino group as follows: AP > PAE > IPAE . IPAE showed high CO2 solubility because of the predominant production of bicarbonate (HCO3 − ). However, a significant amount of carbamate ( IPAE = 0.3) was found in the aqueous IPAE solution with a low CO2 loading (0.4 mol CO2 /mol amine). The unique features of IPAE (i.e., the large CO2 capacity and relatively high absorption rate) previously reported by Goto et al. (2011) were explained by the moderate stability of carbamate in this work. The alcohol chain length of alkanolamine was another structural factor determining the absorption properties. Acknowledgment This work was financially supported by the Ministry of Economy, Trade and Industry, Japan. References Bougie, F., Iliuta, M.C., 2012. Sterically hindered amine-based absorbents for the removal of CO2 from gas streams. Journal of Chemical and Engineering Data 57, 635–669. Brúder, P., Owrang, F., Svendsen, H.F., 2012. Pilot study – CO2 capture into aqueous solutions of 3-methylaminopropylamine (MAPA) activated dimethylmonoethanolamine (DMMEA). International Journal of Greenhouse Gas Control 11, 98–109. Ciftja, A.F., Hartono, A., da Silva, E.F., Svendsen, H.F., 2011. Study on carbamate stability in the AMP/CO2 /H2 O system from 13 C-NMR spectroscopy. Energy Procedia 4, 614–620. Chowdhury, F.A., Okabe, H., Yamada, H., Onoda, M., Fujioka, Y., 2011. Synthesis and selection of hindered new amine absorbents for CO2 capture. Energy Procedia 4, 201–208. Duan, S., Taniguchi, I., Kai, T., Kazama, S., 2012. Poly(amidoamine) dendrimer/poly(vinyl alcohol) hybrid membranes for CO2 capture. Journal of Membrane Science 423–424, 107–112. Giraudeau, P., Wang, J.L., Baguet, É., 2006. Improvement of the inverse-gateddecoupling sequence for a faster quantitative analysis by 13 C NMR. Comptes Rendus Chimie 9, 525–529.
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
CO2 solubility and species distribution in aqueous solutions of 2-(isopropylamino)ethanol and its structural isomers Hidetaka Yamada ∗ , Firoz A. Chowdhury, Kazuya Goto, Takayuki Higashii Chemical Research Group, Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 1 March 2013 Accepted 19 March 2013 Available online 31 May 2013 Keywords: Alkanolamine Carbon dioxide Chemical absorbent NMR Vapor–liquid equilibrium
a b s t r a c t The solubility of CO2 was measured in aqueous solutions of 2-(isopropylamino)ethanol (IPAE) and 2(propylamino)ethanol (PAE) at 313 K, 373 K and 393 K and CO2 partial pressures ranging from 5 kPa to 0.2 MPa. With different CO2 loadings, speciation analyses were conducted by accurate quantitative 13 C nuclear magnetic resonance spectroscopy using inverse-gated decoupling. IPAE had a larger capacity for CO2 than PAE because of the dominant formation of bicarbonate rather than carbamate. However, a significant amount of carbamate was also observed with low CO2 loadings in the IPAE solutions. The results for these secondary amine solutions were compared with those of the primary amines monoethanolamine and 5-aminopentanol. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Amines are the most widely used chemicals for CO2 capture, and are used in solvent- (Rochelle, 2009; Rochelle et al., 2011; Singh et al., 2011; Brúder et al., 2012; Sema et al., 2013), solid sorbent(Hiyoshi et al., 2005; Gray et al., 2008), and membrane-based systems (Kovvali et al., 2000; Duan et al., 2012). Amines react with CO2 to generate several ionic species, and these reactions are facilitated in polar environments and especially in aqueous solutions. Primary and secondary amines react with CO2 to form the carbamate anion and protonated amine as follows: 2R1 R2 NH + CO2 ↔ R1 R2 NCOO− + R1 R2 NH2 +
(1)
where R1 and R2 represent amino substituents. Another product of CO2 absorption in aqueous amine solutions is the bicarbonate anion (da Silva and Svendsen, 2007; Vaidya and Kenig, 2007). R1 R2 NH + CO2 + H2 O ↔ HCO3 − + R1 R2 NH2 +
(2)
Eqs. (1) and (2) show that various chemical species, including amine, protonated amine, carbamate, bicarbonate and water, will be present in amine solutions loaded with CO2 . Reactions (1) and (2) have different stoichiometries, reaction rates, and heat of reaction values. Therefore, the branching ratio between these reactions is highly relevant to the absorbent performance. In CO2 -loaded
∗ Corresponding author. Tel.: +81 774 75 2305; fax: +81 774 75 2318. E-mail address: [email protected] (H. Yamada). 1750-5836/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2013.03.027
aqueous amine solutions, the species distribution as determined by the branching ratio depends on the CO2 loading and the amine molecular structure (Yamada et al., 2010). Steric hindrance from amine substituents is a crucial factor in determining the CO2 absorption and desorption behavior (Sartori and Savage, 1983). Solvents containing sterically hindered amines such as 2-amino-2-methyl-1-propanol (AMP) and 2-piperidineethanol are possible alternatives to the conventional solvent with monoethanolamine (MEA). The application of sterically hindered amines will likely offer absorption capacity, absorption rate, and degradation resistance advantages over conventional amines for CO2 removal from gases (Bougie and Iliuta, 2012). We used screening tests to investigate CO2 absorption and desorption of single-component amine solvents, and found some amines had unique features (Goto et al., 2011). The moderately hindered amine 2-(isopropylamino)ethanol (IPAE) had a high absorption rate and large desorption capability. We formulated IPAE-based solvents and evaluated the solvents at a 1 ton CO2 /day test plant. The IPAE solvents are promising for reducing CO2 capture costs. In this work, solubility data for CO2 in aqueous solutions of alkanolamines, including IPAE and 2-(propylamino)ethanol (PAE), were measured at 313 K, 373 K and 393 K and at CO2 partial pressures between 5 kPa and 0.2 MPa to evaluate the CO2 capture potentials of these solvents. Furthermore, we investigated the species distributions in alkanolamine–H2 O–CO2 systems by 13 C nuclear magnetic resonance (NMR) spectroscopy using inverse-gated decoupling.
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OH
H2N
N H PAE
OH
H2N
MEA
AP
OH
N H
OH
IPAE
Fig. 1. Molecular structures of the investigated amines.
In 13 C NMR, the measurement of spin-lattice relaxation times is required for accurate quantification (Opella et al., 1976). However, little information is available on the relaxation times in alkanolamine–H2 O–CO2 systems. The relaxation times were measured to set an adequate time delay in the pulse sequence for the systems in this work. Based on the accurate quantification results, the species distribution dependence on the CO2 loading was compared among the structural isomers IPAE, PAE and 5aminopentanol (AP). The results were used to gain insight into the relationship between the absorption properties and molecular structures of alkanolamines. 2. Materials and methods The molecular structures of the alkanolamines investigated in this work are shown in Fig. 1. MEA, PAE and IPAE (purity 99%) were obtained from chemical companies and used without further purification. AP was purchased as a 50 wt% aqueous solution. Isothermal vapour–liquid equilibrium (VLE) data were measured at 313 K, 373 K and 393 K at CO2 partial pressures ranging from a few kPa to approximately 200 kPa. Approximately 700 mL of each 30 wt% aqueous amine solution was fed into an autoclave, followed by a N2 purge (99.99%, Ueno-gas, Nara, Japan). Each solution was agitated with a mechanical stirrer and the temperature was held constant during the experiment. The concentration of the gas mixture of CO2 (99.9%, Showa Denko, Kawasaki, Japan) and N2 was controlled using mass flow controllers, and then supplied to the autoclave after passing through a water saturator. The outlet gas was monitored by an infrared CO2 analyzer with the repeatability of ±0.5% and linearity of ±1.0% full-scale range (VA-3001, Horiba, Kyoto, Japan) after passing through a condenser. Equilibrium was obtained when the CO2 analyzer indicated a constant CO2 concentration (±0.01%). To analyze the equilibrium data, the total pressure in the gas phase was measured. The CO2 loading in the liquid phase was measured by a total organic carbon analyzer (TOC-VCSH , Shimadzu, Kyoto, Japan) after collecting a sample from the autoclave. The TOC analyzer had a measuring range of 0–30,000 mg/L for inorganic carbon and the coefficient of variation (reproducibility) was less than 1.5%. For 13 C NMR analyses, CO2 saturated solutions were prepared by passing the CO2 gas at a rate of 500 mL/min through 75 g of each 30 wt% aqueous alkanolamine solution at room temperature and atmospheric pressure for 15 min. Samples with lower CO2 loadings were obtained by mixing fresh 30 wt% aqueous alkanolamine solutions with the CO2 saturated solutions. The CO2 loadings of the sample solutions were quantified using the TOC analyzer. The pH in each sample solution was measured at 299 K by a pH meter with a resolution of 0.01 pH (D-51, Horiba) calibrated with pH 7 and pH 9 standard solutions. The density was measured at 298 K by a density/specific gravity meter with an accuracy of ±0.001 g/cm3 and a measuring range of 0–3 g/cm3 (DA-100, Kyoto Electronics, Kyoto, Japan). 13 C NMR spectra were recorded at 125.8 MHz with a NMR spectrometer (DRX-500, Bruker, Billerica, MA) at 303 K. A double-walled sample tube was used with C6 D6 as the lock solvent and Si(CH3 )4
Fig. 2. Equilibrium partial pressure of CO2 in 30 wt% aqueous solutions of MEA at 313 K, 373 K, and 393 K.
as the internal standard to retain the sample concentration. Quantitative spectra were obtained using inverse-gated decoupling with a pulse angle of 90◦ , a delay time of 60 s, and 512 scans. To determine the delay time, 13 C-spin-lattice relaxation times were measured by the inversion-recovery method.
3. Results and discussion 3.1. VLE The CO2 solubility in 30 wt% aqueous MEA, PAE and IPAE solutions are summarized in Table 1. In Table 1, P is the partial pressure of CO2 in the gas phase, and ˛ is the CO2 loading in the liquid phase, which is defined as a molar ratio of absorbed CO2 to total amine at equilibrium. Plots of P versus ˛ for MEA, PAE and IPAE are shown in Figs. 2–4, respectively. In Fig. 2, solubility data in 30 wt% aqueous MEA solutions measured at 393 K by Ma’mun et al. (2005), and at 313 K and 373 K by Shen and Li (1992) are shown for comparison. The literature data agree with the data obtained in this work.
Fig. 3. Equilibrium partial pressure of CO2 in 30 wt% aqueous solutions of PAE at 313 K, 373 K, and 393 K.
H. Yamada et al. / International Journal of Greenhouse Gas Control 17 (2013) 99–105
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Table 1 CO2 solubilities in 30 wt% aqueous amine solutions.a 313 K
MEA
k0 , k1 k2 , k3 PAE
k0 , k1 k2 , k3 IPAE
k0 , k1 k2 , k3 a
373 K
393 K
P (kPa)
˛
P (kPa)
˛
5.5 11.0 25.8 52.7 106.5 475.19 5004.7
0.498 0.524 0.547 0.567 0.599 −2681.7 −3070.5
4.5 11.0 25.6 60.2 105.6 −2.3237 −35.914
0.261 0.326 0.382 0.427 0.456 20.365 54.587
5.3 12.5 29.2 65.8 112.4 0.57793 46.452
˛ 0.151 0.216 0.282 0.336 0.381 1.5507 −57.514
4.8 11.5 26.9 62.4 113.9 226.2 −0.043008 −86.501
0.058 0.094 0.151 0.230 0.305 0.398 33.130 94.996
P (kPa)
5.7 12.3 26.8 57.8 107.8 213.4 −10.564 −39.506
0.579 0.646 0.724 0.794 0.845 0.902 37.736 19.133
4.4 10.6 25.1 59.5 108.7 218.4 −0.80624 −34.392
0.128 0.193 0.283 0.391 0.471 0.564 21.912 26.630
5.8 12.3 26.8 58.0 106.9 205.5 −65.106 −379.92
0.605 0.695 0.775 0.846 0.870 0.897 274.46 180.00
4.4 10.6 25.1 58.9 104.5 207.6 −0.31075 −48.977
0.085 0.131 0.209 0.329 0.419 0.564 25.526 38.123
4.8 11.7 27.3 62.6 111.2 214.3 −0.44744 −203.05
0.047 0.065 0.104 0.165 0.229 0.338 54.291 276.43
P, CO2 partial pressure; ˛, loading (mol CO2 /mol amine); k0 , k1 , k2 , k3 , the best-fit parameters for Eq. (3).
The curves shown in Figs. 2–4 were determined by the leastsquares fit of all data points to the following polynomial equation: ln P = k0 + k1 ˛ + k2 ˛2 + k3 ˛3
(3)
where k0 , k1 , k2 and k3 are fitting parameters. Eq. (3) satisfactorily fit the data in the studied CO2 partial pressure range as shown in Figs. 2–4. At the typical absorption temperature of 313 K, the aqueous solutions of IPAE and PAE had higher CO2 loadings than conventional aqueous MEA solutions in the CO2 partial pressure range 5 kPa to 0.2 MPa. Larger amounts of CO2 were desorbed from these secondary amine solutions than from MEA at the typical stripping temperature of 393 K. The moderately hindered amine, IPAE showed higher CO2 solubility at 313 K than the mildly hindered amine, PAE. However, this relationship reversed at higher temperatures (373 K and 393 K), which lead to different cyclic capacities for CO2 .
3.2. Cyclic CO2 capacity Based on the VLE data we evaluated the cyclic CO2 capacity, that is, the maximum loading difference between the CO2 -rich solution after absorption and the CO2 -lean solution after stripping. Assuming conditions of a 10–20 kPa CO2 partial pressure at 313 K for the absorption process and 5–100 kPa CO2 partial pressure at 393 K for the stripping process, we calculated the cyclic CO2 capacity as follows: ˛ = ˛313
K
− ˛393
(4)
K
The CO2 loadings in these conditions were obtained from the fitted regression equations. The evaluated cyclic CO2 capacities of 30 wt% aqueous MEA, PAE and IPAE are listed in Table 2. The aqueous IPAE solutions had a larger cyclic CO2 capacity than PAE or MEA because of high absorption of CO2 under the absorption conditions and the high desorption of CO2 under the stripping conditions. For example, the cyclic capacities calculated with 10 kPa CO2 and 313 K absorption and 100 kPa CO2 and 393 K stripping are 0.15 for MEA, 0.34 for PAE, and 0.44 for IPAE (mol CO2 /mol amine). It should be noted that 30 wt% aqueous MEA contains more amine molecules (4.9 mol amine/kg solvent) than 30 wt% aqueous PAE or IPAE (2.9 mol amine/kg solvent), which should be an advantage for MEA in terms of its CO2 capacity. However, IPAE still had a larger cyclic capacity in terms of CO2 capacity per mass of solvent (56 g CO2 /kg solvent) than MEA
Table 2 CO2 capacities of 30 wt% aqueous amine solutions.
MEA PAE IPAE Fig. 4. Equilibrium partial pressure of CO2 in 30 wt% aqueous solutions of IPAE at 313 K, 373 K, and 393 K.
Absorption, 313 K
Stripping, 393 K
Cyclic capacity,a ˛
˛10 kPa
˛20 kPa
˛ 5 kPa
˛ 100 kPa
˛10 kPa –˛ 100 kPa
˛20 kPa –˛ 5 kPa
0.52 0.63 0.66
0.54 0.69 0.75
0.15 0.06 0.04
0.37 0.29 0.22
0.15 0.34 0.44
0.39 0.63 0.71
a Difference between loadings under absorption (˛) and stripping (˛ ) conditions (mol CO2 /mol amine). The subscripts represent CO2 partial pressures.
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Fig. 5. Quantitative 13 C NMR spectrum of PAE–H2 O–CO2 at 303 K with peak interpretations and the relative values for peak areas.
(32 g CO2 /kg solvent). This trend was true at all CO2 pressures in the tested range for both absorption and stripping. IPAE shows a considerably large cyclic capacity when compared with a conventional secondary amine, diethanolamine (DEA). The CO2 solubility in 30 wt% aqueous DEA has been reported previously (Chowdhury et al., 2011): 67.8 at 20 kPa CO2 and 313 K, 25.9 at 100 kPa CO2 and 393 K, and the cyclic capacity of 41.9 (g CO2 /L solution). The cyclic capacities calculated from the corresponding conditions are 51.2 for PAE and 67.8 for IPAE (g CO2 /kg solvent) in this work. 3.3. Spectral analysis The CO2 loadings, densities and pH values of the samples prepared for 13 C NMR are summarized in Table 3. According to Eqs. (1) and (2), the density increased and the basicity decreased as the CO2 loading increased. Fig. 5 shows the quantitative 13 C NMR spectrum of the CO2 -loaded aqueous PAE solution (No. 3 in Table 3) with an interpretation of the peaks and their relative areas. Amine (R1 R2 NH/R1 R2 NH2 + ), carbamate (R1 R2 NCOO− ) and carbonate (HCO3 − /CO3 2− ) species were unambiguously identified in all the spectra, while no other species were detected except for trace impurities. One peak was detected for the carbon of the carbonate species, HCO3 − and CO3 2− , because of the fast exchange of protons. HCO3 − + H2 O ↔ CO3 2− + H3 O+
shifted to the shielded region as the ratio of HCO3 − increased. Therefore, the chemical shift of HCO3 − /CO3 2− monotonically increased with the increase of pH of the solution (Jakobsen et al., 2005). This trend was observed across the spectra of all the different amine systems, as shown in Fig. 6 in this work. The error in the peak areas of the 13 C NMR spectra was estimated to be less than 4% based on the standard deviations of peak areas from different carbons of the same amine species. However, for the carbonate species this is not sufficient to ensure accurate quantitation, because this species contains only one carbon atom. To acquire an accurate quantitative 13 C NMR spectrum, the delay time, which allows the system to return to thermal equilibrium in the standard inverse-gated decoupling sequence with a 90◦ pulse, should be at least five times longer than the spin-lattice relaxation time, T1 (Giraudeau et al., 2006). In this work, the relaxation times for all carbons in the spectra were determined by the inversionrecovery method, and for all the carbons except the carbamate and the carbonate carbons, the spin-lattice relaxation times were shorter than a few seconds in the amine–H2 O–CO2 systems. The carbamate carbon had the longest relaxation time in the system (approximately 10 s), which was slightly longer than that for the carbonate carbon (Table 4). Therefore, the delay time in the inversegated decoupling sequence was determined to be 60 s in this work. This ensured accurate quantitation with the 13 C NMR spectra, especially for comparing the yields of carbamate and carbonate.
(5)
It was not possible to distinguish between amines and protonated amines from the 13 C NMR spectra for this same reason. The HCO3 − /CO3 2− peak, which reflects the ratio of these two ions, Table 3 Densities and pH values of 30 wt% aqueous amine solutions at various CO2 loadings. Amine
˛a
b
pH
No.c
AP
0.00 0.35 0.68
0.990 1.033 1.068
13.0 10.9 8.6
– 1 2
PAE
0.00 0.34 0.66
0.985 1.030 1.065
12.5 10.3 9.0
– 3 4
IPAE
0.00 0.44 0.91
0.987 1.051 1.085
12.6 9.9 8.7
– 5 6
a b c
CO2 loading measured by TOC analysis (mol CO2 /mol amine). Density at 298 K (g/cm3 ). Sample number.
Fig. 6. 13 C chemical shifts of HCO3 − /CO3 2– at 303 K in several amine–H2 O–CO2 systems (Nos. 1–6 in Table 3).
H. Yamada et al. / International Journal of Greenhouse Gas Control 17 (2013) 99–105 Table 4 13 C spin-lattice relaxation times at 303 K in amine–H2 O–CO2 systems.a . Amine AP PAE IPAE
COHb 1.3 0.8 0.7
COO−c 9.1 8.1 10.5
HCO3 − /CO3 2−d –e 6.0 9.1
Relaxation time (s) for b the ˛-carbon in the alcohol group of amine, c the carbon of carbamate, and d the carbon of carbonate species measured using samples of Nos. 1, 3 and 5 in Table 3. e Carbonate species were not detected. a
3.4. Yields of carbamate and carbonate Fig. 7 shows the quantitative 13 C NMR peaks of carbamate and carbonate species in 30 wt% aqueous solutions of the unhindered primary amine (AP), mildly hindered secondary amine (PAE), and moderately hindered secondary amine (IPAE). The CO2 loadings, calculated from the peak areas of carbamate, carbonate, and the ˛-carbon in alcohol group of amine, agreed well with the values from the TOC analysis for sample Nos. 1–6 in Table 3. The chemical shifts of the carbamates were observed at almost the same position (164–165 ppm), while those of the carbonate species varied over a broad range (161–167 ppm) depending on the pH (Fig. 6).
103
Fig. 7 also shows the yield of carbamate. =
[R1 R2 NCOO− ] 2− [R1 R2 NCOO− ] + [HCO− 3 ] + [CO3 ]
(6)
In aqueous solutions of the unhindered primary amine, AP, absorbed CO2 predominantly existed as the carbamate at low loading (0.4). By contrast, at high loading (0.7) the carbamate yield was comparable to that of carbonate (HCO3 − /CO3 2− ). The mildly hindered secondary amine, PAE, showed the same trend, but the carbamate yields were lower both at the low and high loadings. In the aqueous solutions of IPAE, a high CO2 loading (0.9) was attainable through a large reduction in carbamate formation. The carbamate yield in IPAE was the lowest among the three isomers. However, the data also showed that a substantial amount of carbamate was present at a loading of 0.4 in the aqueous IPAE solutions. 3.5. Structural factors Our previous studies using a quantum chemical method suggested that carbamate forms easily via a zwitterion intermediate, with relatively low activation energies even in the aqueous
Fig. 7. Quantitative 13 C NMR peaks for absorbed CO2 in 30 wt% aqueous solutions of AP, PAE and IPAE at 303 K. The CO2 loadings (˛) were calculated from the peak areas of HCO3 − /CO3 2– , COO− and COH. The yields of carbamate ( ) were determined by Eq. (6).
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Fig. 8. Carbamates of MEA (left) and AP (right) in their most stable conformations as determined with the Merck molecular force field (Francl, 1985). Dotted lines indicate the intramolecular hydrogen bonds.
solutions of hindered amines such as AMP and IPAE (Yamada et al., 2011a,b). R1 R2 NH + CO2 ↔ R1 R2 NH+ COO−
(7)
R1 R2 NH + R1 R2 NH+ COO− ↔ R1 R2 NCOO− + R1 R2 NH2 +
(8)
However, the carbamate decomposes because of its instability, which mainly arises from steric hindrance of amine substituents, and bicarbonate forms by reaction (2) as a final product. Because reaction (2) is more efficient than reaction (1) in terms of CO2 loading, both the AMP and IPAE solvents can provide high capacity. Recently, qualitative and quantitative 13 C NMR studies were performed on 30 wt% aqueous solutions of AMP for different CO2 loadings at 298 K, and the carbamate was observed (Ciftja et al., 2011). However, the yields of carbamate were less than 0.05 over the CO2 loading range 0.1–0.6. By comparison, as described above, aqueous IPAE solutions absorbed CO2 to yield a substantial amount of carbamate in the low loading range. This occurred because IPAE is moderately hindered, and explains why this solvent has favorable properties such as a moderately high absorption rate and a large capacity as reported by Goto et al. (2011). However, other factors beside the steric hindrance should be considered when interpreting experimental results (da Silva and Svendsen, 2007). Based on quantum mechanical calculations, Jhon et al. (2010) pointed out the importance of a proper combination of steric effects with global nucleophilicities of the amino group to predict the reactivity of amine with CO2 . Using 1 H NMR spectroscopy, McCann et al. (2011) revealed that both steric hindrance and the acid dissociation constant of the parent amine had a significant effect on the carbamate stability. In a screening study of the CO2 absorption performance, Puxty et al. (2009) found that amines with outstanding CO2 absorption capacities shared structural features, including steric hindrance and hydroxyl functionality two or three carbons from the amino nitrogen. Recently we showed that the number of carbons between the hydroxyl and amino groups (i.e., the alcohol chain length) influenced the stability of carbamate through intramolecular hydrogen bonding (Yamada et al., 2013). The amines investigated in this work, except for AP, have two carbons between the hydroxyl and amino groups (Fig. 1), and this possibly contributes to the stabilization of carbamate. Because it is an unhindered primary amine, AP showed the greatest tendency to form carbamate among the three structural isomers. Although it should be noted that the alcohol chain length also affects the basicity of the amino nitrogen and the aqueous solubility, we anticipate that the carbamate stability of AP will be less than that of MEA because of the different effects of intramolecular hydrogen bonding as shown in Fig. 8. In an earlier 13 C NMR study by Jakobsen et al. (2005), CO2 loadings of 0.6 and 0.8 and carbamate
yields of approximately 0.75 and 0.45, respectively, were achieved in aqueous 30 wt% MEA solutions at 293–323 K. Comparison of their results with ours suggests that the carbamate stability constants for MEA are likely to be higher than those for AP. 4. Conclusions VLE measurements and 13 C NMR analyses were performed on alkanolamine–H2 O–CO2 systems. The structural isomers AP, PAE and IPAE were compared to examine the structural factors underlying the CO2 absorption properties. Based on the 13 C spinlattice relaxation time measurements, accurate speciation was achieved especially for carbamate (R1 R2 NCOO− ) and carbonate (HCO3 − /CO3 2− ) species. The carbamate yield ( ) in the CO2 absorption reaction decreased with increasing steric hindrance of the amino group as follows: AP > PAE > IPAE . IPAE showed high CO2 solubility because of the predominant production of bicarbonate (HCO3 − ). However, a significant amount of carbamate ( IPAE = 0.3) was found in the aqueous IPAE solution with a low CO2 loading (0.4 mol CO2 /mol amine). The unique features of IPAE (i.e., the large CO2 capacity and relatively high absorption rate) previously reported by Goto et al. (2011) were explained by the moderate stability of carbamate in this work. The alcohol chain length of alkanolamine was another structural factor determining the absorption properties. Acknowledgment This work was financially supported by the Ministry of Economy, Trade and Industry, Japan. References Bougie, F., Iliuta, M.C., 2012. Sterically hindered amine-based absorbents for the removal of CO2 from gas streams. Journal of Chemical and Engineering Data 57, 635–669. Brúder, P., Owrang, F., Svendsen, H.F., 2012. Pilot study – CO2 capture into aqueous solutions of 3-methylaminopropylamine (MAPA) activated dimethylmonoethanolamine (DMMEA). International Journal of Greenhouse Gas Control 11, 98–109. Ciftja, A.F., Hartono, A., da Silva, E.F., Svendsen, H.F., 2011. Study on carbamate stability in the AMP/CO2 /H2 O system from 13 C-NMR spectroscopy. Energy Procedia 4, 614–620. Chowdhury, F.A., Okabe, H., Yamada, H., Onoda, M., Fujioka, Y., 2011. Synthesis and selection of hindered new amine absorbents for CO2 capture. Energy Procedia 4, 201–208. Duan, S., Taniguchi, I., Kai, T., Kazama, S., 2012. Poly(amidoamine) dendrimer/poly(vinyl alcohol) hybrid membranes for CO2 capture. Journal of Membrane Science 423–424, 107–112. Giraudeau, P., Wang, J.L., Baguet, É., 2006. Improvement of the inverse-gateddecoupling sequence for a faster quantitative analysis by 13 C NMR. Comptes Rendus Chimie 9, 525–529.
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