Atmospheric Chemical Reactions of Monoethanolamine Initiated by

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Atmospheric Chemical Reactions of Monoethanolamine Initiated by OH Radical: Mechanistic and Kinetic Study Hong-Bin Xie,† Chao Li,† Ning He,‡ Cheng Wang,† Shaowen Zhang,§ and Jingwen Chen*,† †

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China ‡ State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China § Key Laboratory of Cluster Science (Ministry of Education), School of Chemistry, Beijing Institute of Technology, Beijing 100081, China S Supporting Information *

ABSTRACT: Monoethanolamine (MEA) is a benchmark and widely utilized solvent in amine-based postcombustion CO2 capture (PCCC), a leading technology for reducing CO2 emission from fossil fuel power plants. The large-scale implementation of PCCC would lead to inevitable discharges of amines to the atmosphere. Therefore, understanding the kinetics and mechanisms of the transformation of representative amine MEA in the atmosphere is of great significance for risk assessment of the amine-based PCCC. In this study, the H-abstraction reaction of MEA with ·OH, and ensuing reactions of produced MEA-radicals, including isomerization, dissociation, and bimolecular reaction MEA-radicals+O2, were investigated by quantum chemical calculation [M06-2X/aug-cc-pVTZ//M06-2X/6-311++G(d,p)] and kinetic modeling. The calculated overall rate constant [(7.27 × 10−11) cm3 molecule−1 s−1] for H-abstraction is in excellent agreement with the experimental value [(7.02 ± 0.46) × 10−11 cm3 molecule−1 s−1]. The results show that the product branching ratio of NH2CH2· CHOH (MEA-β) (43%) is higher than that of NH2·CHCH2OH (MEA-α) (39%), clarifying that MEA-α is not an exclusive product. On the basis of the unveiled reaction mechanisms of MEA-radicals + O2, the proton transfer reaction mass spectrometry signal (m/z 60.044), not recognized in the experiment, was identified.

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INTRODUCTION Amine-based CO2 capture has been considered to be a leading candidate for postcombustion CO2 capture (PCCC) technology.1−3 It was emphasized in a viewpoint paper that aminebased PCCC technology is sufficiently developed to be implemented on a global scale to significantly reduce CO2 emission.4 Monoethanolamine (MEA) is a benchmark and most widely utilized solvent in amine-based PCCC technology.1,3,5 Given the possible large-scale implementation of amine-based PCCC, it is likely that there will be relatively significant discharges of MEA or other alkanolamines to the atmosphere from PCCC unit because of their relatively high vapor pressure.6 It has been estimated that a CO2 capture plant using MEA as solvent, which removes 1 million tons CO2 per year from flue gas, could potentially emit 80 tons MEA into the atmosphere.7,8 Such quantities of MEA may cause potential risks to the environment.9 Therefore, to assess the environmental risk of PCCC, a sound understanding of the kinetics and mechanisms of the atmospheric transformation of representative amine MEA is indispensable. In the atmosphere, the likely fate of MEA is gas-phase oxidation. Among the various possible atmospheric oxidation reactions, reactions initiated by hydroxyl radicals (·OH) are of © 2014 American Chemical Society

paramount importance. To date, several experimental studies have been performed to determine the rate constants (kOH) of the MEA + ·OH reaction and identify the products of ·OHinitiated oxidation of MEA in the atmosphere.10−12 The kOH values at room temperatures for the reaction of MEA with ·OH were determined to be (7.02 ± 0.46) × 10−11 by Borduas et al.,12 (7.61 ± 0.76) × 10−11 by Onel et al.,11 and (9.2 ± 1.1) × 10−11 cm3 molecule−1 s−1 by Karl et al.,10 indicating a fast reaction of MEA with ·OH. Nielsen et al. identified ·OHinitiated oxidation products of MEA in the atmosphere and found that formamide and formaldehyde were major products.13 Borduas et al. found HNCO species, a potential hazardous compound to human health, in the experiment of · OH-initiated oxidation of MEA besides formamide and formaldehyde. All the above experimental studies suggested that the initial reaction MEA + ·OH and the subsequent reaction of MEA-radicals with O2 should play an essential role in the ·OH-initiated oxidation of MEA in the atmosphere. Received: Revised: Accepted: Published: 1700

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constants for the reaction MEA + ·OH. However, the POLYRATE program is not suited for the reaction of MEAradicals with O2, as such reactions involve multiple-wells in one reaction pathway. We applied the master equation method to calculate the reaction rate constant for MEA-radical + O2. The master equation method is a powerful tool for calculating the time-dependent, temperature-dependent, and pressure-dependent kinetics of multiple-channel and multiple-well reactions. We have successfully used it previously to investigate the reaction kinetics of NCO + C2H2 and HCNO + CN.19,20 Details for the master equation rate constant calculation are provided in the Supporting Information (SI). Conformation Effect. The reaction ·OH + MEA proceeds via hydrogen atom abstraction from either C−H, N−H, or O− H bonds in MEA. In the MEA molecule, the H-atoms of the N−H and O−H bonds, and the lone pair electrons on the O and N atoms physically satisfy the condition to form intermolecular hydrogen bonds with ·OH. However, the possibility of forming such H-bonds depends on the interacting sites (e.g., H-atom at α-C or β-C, O or N sites) between ·OH and MEA, and conformations of MEA. In general, the formation of intermolecular H-bonds influences the activation energy barrier (Ea) of the reaction. Thus, it is necessary to consider the conformation effects. MEA has 13 possible nondegenerate conformers.21,22 To account for the different conformations in a physically reasonable way, we generated a Boltzmann-weighted distribution to compute the conformationally averaged energy of prereactive complex (ECR), reaction enthalpy (ΔH), Ea, Γ, and kOH

However, detailed mechanistic information such as the product branching ratio (Γ) of the reaction of MEA and ·OH, as well as the reactions of MEA-radicals with O2, is limited. On the basis of the product distributions in the MEA atmospheric photooxidation experiments carried out at the EUPHORE atmospheric chamber facility, Nielsen et al. proposed that the Γ value of the initial H-abstraction between MEA and ·OH are 8% of ·NHCH2CH2OH, 84% of NH2·CHCH2OH, and 8% of NH2CH2·CHOH.13 However, according to the Evans−Polanyi relationship,14 the C−H bond enthalpy9 in −CH2− (394 kJ mol−1) should be nearly identical with that in −CH2OH (395 kJ mol−1) of MEA, suggesting that the Γ value of NH2· CHCH2OH and NH 2CH 2·CHOH could be of similar magnitude. The experimental speculation could be called into question if the Evans−Polanyi relationship14 is applicable to the reaction of MEA with ·OH. In addition, among the reactions of MEA-radicals with O2, only the reaction of the speculated main product NH2·CHCH2OH with O2 was reported.15 The uncertain or fragmental mechanistic information regarding the ·OH-initiated oxidation of MEA could present an obstacle for understanding the fate of MEA, and even for further evaluating the potential environmental risk for the PCCC. It became the purpose of this study to investigate the reaction mechanism and product branching ratio of the critical steps in the ·OH-initiated oxidation of MEA using a combination of both quantum chemistry calculations and kinetic modeling. This study includes the reaction mechanism and kinetics of MEA + ·OH, and the isomerization and dissociation of main MEA-radicals and their corresponding reactions with O2.

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A̅ =

COMPUTATIONAL DETAILS Ab Initio Electronic Structure and Kinetics Calculations. All the structure and energy calculations were performed using the GAUSSIAN 09 program package.16 The geometry optimizations and harmonic frequency calculations of the reactants, products, isomers and transition states were performed at the M06-2X/6-311++G(d,p) level.17 The connections of the transition states between designated local minima were confirmed by intrinsic reaction coordinate (IRC) calculations at the M06-2X/6-311++G(d,p) level. Single-point energy calculations were performed at the M06-2X/aug-ccpVTZ level based on the geometries at the M06-2X/6-311+ +G(d,p) level. Zero-point energy at M06-2X/6-311++G(d,p) level was used to correct the single-point energy. Zhao and Truhlar have demonstrated that the M06-2X functional of the density functional theory (DFT) has the best performance for predicting the barrier height without increasing the computational cost.17 The mean signed error of the barrier height for hydrogen transfer reaction is −0.5 kcal mol−1, and for unimolecular and association reaction 0.3 kcal mol−1.17 This study mainly focuses on the process of hydrogen transfer and bond association or dissociation. Therefore, the M06-2X method should be sufficiently reliable to predict the reaction mechanism and kinetics of the titled reaction. In addition, to further verify the reliability of the M06-2X method, the singlepoint energy was calculated with a highly cost-expensive CCSD(T)/6-311++G(3df,2pd) method based on the geometries at the M06-2X/6-311++G(d,p) level for some selected species. The improved canonical variational transition state theory (ICVT) with small-curvature tunneling (SCT) within the POLYRATE 2010-A program18 was used to calculate the rate

∑ wA i i i

(1)

where Ai and wi stand for the different parameters associated with the reaction process and the Boltzmann weight for conformer i, respectively. wi is given by wi =

e−ΔGi / kBT ∑i e−ΔGi / kBT

(2)

where ΔGi is the relative free energy of conformer i (the free energy of global minimum is set as a reference state), kB is Boltzmann’s constant, and T is the absolute temperature. The structures of the 13 MEA conformers and their respective Boltzmann weights are presented in SI Figure S1 and Table S1. The notation for categorizing the conformation structure of MEA is the same as that used by Radom et al.23 A general conformer is represented as xYz, where x designates the value of the lpN−N−C−C (lpN = lone-pair electrons of N-atom) dihedral angle, Y is the O−C−C−N dihedral angle, and z designates the C−C−O−H dihedral angle. A one-letter abbreviation is used for dihedral angle: G or g for gauche(+) (+60°), G′ or g′ for gauche(−) (−60°), T or t for trans (180°).

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RESULTS AND DISCUSSION Initial Reactions with ·OH. The possible H-abstraction pathways are depicted in Figure 1 for the reaction of MEA + ·OH. The Most Favorable Pathways. The H-abstraction reaction of MEA + ·OH leads to four different MEA radicals (MEA-N, MEA-α, MEA-β, and MEA-O) and H2O via the seven Habstraction transition states (Figure 1). Each reaction pathway proceeds through a prereactive complex. The conformationally 1701

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the species, which is in the range of quantum chemical computational errors.19 More importantly, the order of Ea for the six H-abstraction processes predicted by the M06-2X/augcc-pVTZ method is consistent with that predicted using the CCSD(T)/6-311++G(3df,2pd) method. Therefore, the costefficient M06-2X/aug-cc-pVTZ//M06-2X/6-311++G(d,p) method is sufficiently reliable to predict the reaction mechanism and kinetics. Conformation Effect. As can be seen from SI Table S3 that lists the Ea values of the seven H-abstraction processes for the 13 conformers of MEA, Ea varies greatly with the different conformers. For the conformers g′Gg′, gGg′, gGg, and tTt, the H-abstraction occurring at the β site is the most favorable; for the conformers gGt, g′Tt, gTg, tTg, g′Gt, and gTg′, the Habstraction occurring at the α site is the most favorable; and for the conformers tGt, tGg, and tGg′, the H-abstraction occurring at the N site is the most favorable. The great variations in Ea with conformers could be ascribed to the different intermolecular interactions (e.g., H-bonds and dispersive forces) between the different conformers and ·OH. Taking the conformers tGt, tGg, and tGg′ as examples, the H-abstraction occurring at the N site is the most favorable, even though the bond enthalpy of N−H (442 kJ mol−1) is much higher than that of C−H (395 kJ mol−1). As can be seen from Figure 3, intermolecular H-bonds are formed between ·OH and tGt, tGg, and tGg′ at the N-site of their transition states. The formation of the H-bonds stabilizes the transition states and thus leads to a lower Ea. Kinetics. The calculated conformationally averaged kOH and Γ for the reaction MEA + ·OH at 298 K are listed in Table 1. The kOH values for each pathway of the 13 conformers at temperatures ranging from 200 to 376 K are listed in SI Table S4. The calculated conformationally averaged kOH at 298 K is 7.27 × 10−11 cm3 molecule−1 s−1, which agrees well with the experimental values 7.02 ± 0.46,12 7.61 ± 0.76,11 and 9.2 ± 1.1 × 10−11 cm3 molecule−1 s−1.10 Moreover, over the temperature range from 200 to 376 K, the calculated overall kOH decreases with increasing temperature (Figure 4), also agrees with the experimental results.11,12 This consistency further proves that the current computational scheme is reliable. It is worth mentioning that we have also considered the variational and tunneling effects in the calculation of kOH, which together with the discussion of the recrossing effect are detailed in the SI. The calculated Γ values for the formation of P1−1, P1−2, P1−3, and P1−4 are 17%, 39%, 43%, and 1%, respectively, suggesting a strong preference for the α and β H-abstraction, which qualitatively complies with the Evans−Polanyi relationship14 (bond enthalpy of O−H > N−H > Cα-H ≈ Cβ−H). However,

Figure 1. Possible pathways for the reaction of MEA with ·OH, The symbols “TS1‑m and P1‑m” denote the transition states and products involved in the reaction and m denotes different species.

averaged ECR, Ea, and ΔH of the possible H-abstraction pathways are listed in Table 1. It can be concluded from the conformationally averaged Ea values that the H-abstraction occurring at the β site of MEA via transition state TS1−3 is the most favorable, followed by those occurring at the α site via TS1−2′ and TS1−2, and the N site via TS1−1. However, the Habstraction process occurring at the O site is less favorable than the other sites. The geometries of the H-abstraction pathways occurring at the α site and β site for the most stable g′Gg′ are presented in Figure 2. As indicated by their ΔH values, the stability of P1−2 is similar to P1−3, both of which are thermodynamically more stable than P1−1 and P1−4. Therefore, in view of thermodynamics, the Habstraction reaction occurring at the α and β site of MEA is comparable and is more favorable than those at other sites. We also compared the Ea values from the M06-2X functional with those calculated by the computationally expensive CCSD(T) method for the six H-abstraction processes (occurring at the N, α, and β site of MEA) for four conformers, including the most stable g′Gg′. The Ea values from the M062X/aug-cc-pVTZ//M06-2X/6-311++G(d,p) method and the CCSD(T)/6-311++G(3df,2pd)//M06- 2X/6-311++G(d,p) method are very close (SI Table S2). The deviation in Ea between these two methods is ≤1.0 kcal mol−1 for almost all

Table 1. Conformationally Averaged ECR, Ea, ΔH, kOH, and Γ for the Reaction of MEA with ·OHa species P1−1 P1−2 P1−3 P1−4 kOH (overall)

TS

ECR (kcal mol−1)

Ea (kcal mol−1)

ΔH (kcal mol−1)

kOH × 1011 (cm3 molecule−1 s−1)

Γ

TS1−1 TS1−1′ TS1−2 TS1−2′ TS1−3 TS1−3′ TS1−4

−3.1 −3.2 −4.0 −6.0 −5.8 −3.4 −5.5

−1.4 −1.2 −1.4 −2.2 −3.0 −0.3 −0.8

−17.7 −17.9 −25.8 −25.7 −23.8 −24.4 −11.3

1.22

0.17

2.85

0.39

3.09

0.43

0.11 7.27

0.01

The species and the transitional states (TS) correspond to Figure 1. ECR, Ea, and ΔH were calculated at the M06-2X/aug-cc-pVTZ//M06-2X/6311++G(d,p) level. ECR and Ea was calculated at 0 K, and ΔH at 298 K. kOH and Γ were calculated at 298 K. a

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Figure 2. M06-2X/6-311++G(d,p)-optimized geometries for some important intermediates and transition states involved in the reactions of MEA(g′Gg′) + ·OH and MEA-β +O2. The distances are in angstroms.

employed in the current study, we conclude that both MEA-β and MEA-α are main products for the initial reaction of MEA + ·OH, and MEA-β plays a more important role in the subsequent reaction than MEA-α due to a higher Γ value of MEA-β. This is the first time to point out that MEA-β and MEA-α are main products for the reaction MEA + ·OH, clarifying that MEA-α is not an exclusive product. Subsequent Reactions for Primary Intermediates. The chemically activated intermediates produced in the initial reaction can subsequently isomerize, dissociate or react with atmospheric O2. As Silva et al. have investigated the reaction mechanism of MEA-α + O2,15 here we focus primarily on the reaction of MEA-β. Isomerization and Dissociation. The possible isomerization and dissociation pathways of MEA-β and their corresponding Ea and ΔH are presented in Figure 5. The activation energy (Ea) for all the pathways are greater than 24 kcal mol−1, indicating that the isomerization and dissociation reactions of the MEA-β proceed slowly. Therefore, the MEA-β radical has a great chance to react with atmospheric O2. Reaction Mechanism of MEA-β with O2. The calculated schematic potential energy surface of the MEA-β + O2 reaction is shown in Figure 6 and the corresponding ΔH values are listed in SI Table S5. The geometries of some important intermediates and transition states are presented in Figure 2. As can be seen from Figure 6, the triplet 3O2 can barrierlessly add to the β-C site of the MEA-β radical to form the peroxy radical IM3−1. As shown in SI Figure S2, depending on the attacking direction of O2 on MEA-β, the low-lying IM3−1 has three isomeric forms, which can easily interconvert with the barriers around or less than 4 kcal mol−1. For IM3−1, four types of Htransfer pathways, from the β-site of NH2, the α-site of NH2, the N-site and the O-site of OH, to the O-site of peroxy radical via the transition states TS3−4, TS3−3, TS3−2, and TS3−1, respectively, were identified. Among these pathways, the Htransfer from the O-site and β-site of NH2 simultaneously initiates the breaking of CO and OO bonds to form CP3−1 (finally to P3−3) and P3−1, respectively. The two intermediates IM3−2 and IM3−3 formed in the H-transfer processes can further

Figure 3. Conformations of transition states for H-abstraction reaction occurring at the N site of MEA for the tGt, tGg, and tGg′ conformers.

Figure 4. Calculated reaction rate constants over the temperature range from 200 to 376 K for the reaction MEA + ·OH.

this is inconsistent with the previous experimental expectation (8% P1−1, 84% P1−2, and 8% P1−3) of Nielsen et al.13 On the basis of a main product formamide (NH2CHO) identified in a chamber experiment under pseudo-natural conditions, Nielsen et al. speculated that branching ratio of P1−2 is 84%.13 They assumed that P1−2 can react with O2 and NO via peroxy radical intermediate to produce NH2CHO. However, a recent theoretical study has indicated that the reaction of P1−2 with O2 mainly forms 2-iminoethanol, rather than a collisionally deactivated peroxy radical intermediate.15 Thus, the speculated formation pathway of NH2CHO from P1−2 may not be realistic, and the speculated branching ratio of P1−2 can be questionable. Under the complicated experimental conditions that involved photoradiation and various reactants (e.g., O3, O2, NO, and NO2),13 it is not easy to speculate precursors from the products. Thus, based on the reliable computational scheme 1703

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dissociate to different products via bond breaking processes. By comparing the overall reaction energy barriers (Figure 6) and ΔH of various reaction pathways (SI Table S5), we conclude that the pathway proceeding via the intermediate IM3−1 to form P3−3 (amino acetaldehyde NH2CH2CHO and ·HO2) is the most favorable, followed by the pathway to form P3−4 (amino ethenol NH2CHCHOH and ·HO2). The other pathways contribute very little to the final products due to their high Ea or ΔH values. Thus, in the following kinetics calculation, we concentrate on the pathways to form P3−3 and P3−4. Kinetics and Branching Ratio. When using the master equation to calculate the rate constants, we focused only on the two favorable pathways, that is, the pathways to form P3−3 and P3−4. In addition, the two pathways were treated in a single master equation. The calculated rate constant at 298 K and 760 Torr (the condition is relevant to the troposphere) is 2.89 × 10−11 cm3 molecule−1 s−1, which is comparable to that (2.98 × 10−11 cm3 molecule−1 s−1) of the reaction MEA-α + O2.15 The conformations of IM3−1 almost have no effects on the rate constant (SI Figure S3). As can be seen from Figure 7, in the temperature range of 200−350 K and 760 Torr, the calculated total rate constants increase with increasing temperature. Our branching ratio analysis (Figure 8) indicates that in the temperature range of 200−350 K, the reactants predominantly form P3−3 (amino acetaldehyde NH2CH2CHO and ·HO2) with Γ > 88% and the Γ values for the other species (P3−4, IM3−2, CP3−1, and CP3−2) are negligibly small. Note that effective

Figure 5. Possible pathways, Ea (kcal mol−1) and ΔH (kcal mol−1) values calculated at the M06-2X/aug-cc-pVTZ//M06-2X/6-311+ +G(d,p) level for isomerization and dissociation reactions of the MEA-β radical. The symbols “TS2‑m and P2‑m” stand for the transition states and products involved in the reaction and m denotes different species. Ea and ΔH were calculated at 0 and 298 K, respectively.

Figure 6. Schematic potential energy surface for the MEA-β + O2 reaction calculated at the M06-2X/aug-cc-pVTZ//M06-2X/6-311++G(d,p) level. The total energy of the reactant MEA-β + O2 is set as zero (reference state). The symbols R3, CP3‑m, IM3‑m, TS3‑m and P3‑m stand for reactants, complexes, intermediates, transition states, and products involved in the reaction, respectively, and m denotes different species. ΔE was calculated at 0 K. 1704

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spectrometry signal of m/z 60.044 (C2H6NO+), for which the identification was failed in the previous experimental study,12,13 corresponds to a mixture of NHCHCH 2 OH and NH2CH2CHO, rather than one of them. The formed NH2CH2CHO itself and its transformation products in the atmosphere may increase the environmental risks of MEA emission. NH2CH2CHO may photolyze as other aliphatic aldehydes to formamide NH2CHO,25 which could further react with ·OH to produce isocyanic acid HNCO that is a potential hazardous compound to human health.26,27 Further study is necessary to investigate atmospheric photochemistry of NH2CH2CHO. Since N-centered radicals typically react slowly with O2,28 the formed MEA-N radicals can be removed from the atmosphere primarily through bimolecular reactions with other trace compounds (e.g., NO and NO2).29 It is reasonable to believe that the reaction of the MEA-N radical (Γ = 16%) with NO produce the carcinogenic nitrosamines based on a recent study on CH3NH + NO.29 This also increases the environmental risk of MEA emission from PCCC units.

Figure 7. Calculated reaction rate constants for the reaction MEA-β + O2 over the temperature ranging from 200 to 350 K and at 760 Torr.

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ASSOCIATED CONTENT

S Supporting Information *

Details on master equation rate constant calculation, conformations, Boltzmann weight of MEA conformers, and variational, tunneling and recrossing effect; Ea, ΔH, and kOH values for the 13 conformers of MEA; conformational effects of IM3−1 on the reaction rate constant of MEA-β + O2; schematic potential energy surfaces for interconversion of three IM3−1 isomers and the reaction NO + IM3−1. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Branching ratios (Γ) for the formation of P3−3, P3−4, IM3−1, IM3−2, CP3−1, and P3−2 at 760 Torr as a function of temperature (200−350 K) for the reaction MEA-β + O2.

stabilization takes place because of collisional deactivation at low temperatures, making the branching ratio of IM3−1 as high as 11.6% and 6% at 200 and 225 K, respectively. Therefore, at low temperatures or under the high-pressure limit conditions, the intermediate IM3−1 can be available for further bimolecular reactions or thermal unimolecular decomposition. The reaction with NO can also be a removal pathway for the deactivated peroxy radical intermediate to form alkoxyl radicals and NO2.24 We further evaluated the competition between the unimolecular decomposition and the bimolecular reaction with NO for the deactivated peroxy radical IM3−1. Our calculation shows that the deactivated IM3−1 can thermally decompose to P3−3 with a rate constant of 0.95 s−1 at 298 K, and the calculated bimolecular reaction rate constant for the reaction of IM3−1 with NO is 6.2 × 10−12 cm3 molecule−1 s−1 (SI Figure S4). Therefore, the bimolecular reaction NO + IM3−1 would begin to dominate at NO level around 6 ppb and above. Nevertheless, it can be inferred that the activated IM3−1 should decay very fast as the overall reaction of MEA-β + O2 is barrierless and the bimolecular reaction would not compete with it. Implications. Our calculated conformationally averaged total rate constant (7.27 × 10−11 cm3 molecule−1 s−1) of MEA + ·OH is in excellent agreement with the experimental results. More importantly, this study reveals that not only MEA-α, which was speculated as the exclusive product in the previous experimental studies,13 but also MEA-β is a main product from the reaction MEA + ·OH and Γ value of MEA-β is higher than that of MEA-α. Therefore, both MEA-α and MEA-β should be considered in the tropospheric chemistry of MEA. This study predicts that the reaction of MEA-β + O2 leads to the product NH2CH2CHO, which has a same mass to charge ratio as NHCHCH2OH formed in the reaction MEA-α + O2.15 Thus, our study first reveals that a proton transfer reaction mass

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AUTHOR INFORMATION

Corresponding Author

*Phone/fax: +86-411-84706269. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Brian. B. Laird (University of Kansas) for improving the composition and Prof. Donald G. Truhlar (University of Minnesota) for providing the POLYRATE 2010A program. The study was supported by the High-tech Research and Development Program of China (2012AA06A301), the National Natural Science Foundation of China (21207016, 21325729), the Fundamental Research Funds for the Central Universities (DUT12RC(3)07) and the Liaoning Provincial Education Department (L2012021).

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REFERENCES

(1) Veawab, A.; Tontiwachwuthikul, P.; Chakma, A. Corrosion Behavior of Carbon Steel in the CO2 Absorption Process Using Aqueous Amine Solutions. Ind. Eng. Chem. Res. 1999, 38, 3917−3924. (2) Dai, N.; Shah, A. D.; Hu, L.; Plewa, M. J.; McKague, B.; Mitch, W. A. Measurement of Nitrosamine and Nitramine Formation from NOx Reactions with Amines during Amine-Based Carbon Dioxide Capture for Postcombustion Carbon Sequestration. Environ. Sci. Technol. 2012, 46, 9793−9801. (3) Liu, Y.; Zhang, L.; Watanasiri, S. Representing Vapor−Liquid Equilibrium for an Aqueous MEA−CO2 System Using the Electrolyte Nonrandom-Two-Liquid Model. Ind. Eng. Chem. Res. 1999, 38, 2080− 2090. (4) da Silva, E. F.; Booth, A. M. Emissions from Postcombustion CO2 Capture Plants. Environ. Sci. Technol. 2013, 47, 659−660. 1705

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dx.doi.org/10.1021/es405110t | Environ. Sci. Technol. 2014, 48, 1700−1706