Theoretical Investigation on the Different Reaction Mechanisms of

Feb 12, 2014 - Theoretical Investigation on the Different Reaction Mechanisms of Aqueous 2-Amino-2-methyl-1-propanol and Monoethanolamine with CO2...
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Theoretical Investigation on the Different Reaction Mechanisms of Aqueous 2‑Amino-2-methyl-1-propanol and Monoethanolamine with CO2 Hong-Bin Xie,† Ning He,‡ Zhiquan Song,† Jingwen Chen,*,† and Xuehua Li† †

Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China ‡ State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China S Supporting Information *

ABSTRACT: A fundamental understanding on the inherent mechanisms leading to the different reactivity of the representative systems for unhindered and hindered families of amines, monoethanolamine (MEA) and 2-amino-2-methyl-1-propanol (AMP) in CO2 capture, is important for the rational design of novel amines. In this work, a comparative study on the reaction mechanisms between AMP + CO2 and MEA + CO2 was investigated at the B3LYP/6-311++G(d,p) level with the appropriate treatment of solvent effects. Two channels that lead to carbamate and bicarbonate were considered. The total reaction rate coefficients were computed using a microkinetic model. The results indicate that the formation of bicarbonate for AMP + CO2 is more favorable in kinetics than the formation of carbamate. However, the case is reversed for MEA + CO2. This explains well the experimental observation of different product distributions between the AMP + CO2 and MEA + CO2 reactions. The complementary results from QM/MM molecular dynamics simulations with umbrella sampling confirm the mechanistic difference between AMP + CO2 and MEA + CO2. In addition, it is found that the difference in electrostatic potential distribution between Cα of AMP and MEA is a possible reason leading to their different reaction mechanisms.

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INTRODUCTION The capture (separation) and sequestration (long-term storage) of CO2 is seen as a critical near-term strategy for mitigating the effects of greenhouse gas emissions. Generation of electricity from fossil fuels accounts for approximately 25% of the global CO2 emissions.1 Moreover, this fraction could increase drastically in the next 25 years.2 Therefore, the development of technologies that facilitate the cost-effective and energy-efficient capture of CO2 from power plant flue gas is of paramount importance. Postcombustion capture (PCC) could be retrofitted with relative ease to the back end of an existing power station, which is considered to be the current frontline technology for CO2 capture from flue gas.3,4 However, this technology requires efficient materials that are able to quickly separate CO2 from flue gas and can be easily regenerated.4 Alkanolamines have been studied as promising materials for the PCC application.1,3−10 Especially, as the representative systems for unhindered and hindered families of amines, monoethanolamine (MEA)9,11−20 and 2-amino-2-methyl-1propanol (AMP)21−32 aqueous solutions have been extensively studied. MEA is able to quickly capture 90% of CO2 in flue gas with a 2:1 amine:CO2 reaction stoichiometry to form carbamate.17,18 However, its reaction with CO2 is highly exothermic, which results in high energy consumption in the solvent regeneration step.17,18 Compared with MEA, AMP has higher CO2 loading capacities due to its 1:1 amine:CO2 reaction stoichiometry to form bicarbonate and lower regeneration temperature.24,33 However, a significant disadvantage of AMP for CO2 capture is its slow kinetics in the reaction. © 2014 American Chemical Society

Therefore, extensive efforts have been made to develop aminebased absorbents that exhibit enhanced CO2-absorbing capacities with faster kinetics and are capable of regeneration at low temperature.1,3,7 To advance the design of ideal amine systems, a fundamental understanding on the inherent mechanisms leading to the different reactivity between AMP and MEA toward CO2, e.g., thermodynamics, reaction products, and kinetics, is important. In view of molecular structures, the only difference between MEA and AMP is their substituents on Cα site of the −NH2 group, i.e., −H and −CH3 for MEA and AMP, respectively. Therefore, −CH3 should play an important role in differentiating the reaction mechanism of AMP from MEA toward CO2. Although there are many individual studies on the reaction mechanism for AMP22,31 and MEA13,15,16,18,19,34,35 with CO2, the role of −CH3 on the different reaction mechanisms between AMP and MEA toward CO2 is still not clearly revealed. A main reason can be ascribed to the controversy in the theoretical study over the reaction mechanism of AMP with CO2.22,31 Ismael et al. first investigated the reaction mechanism of AMP + CO2 at LDA/DNP level, and proposed that the reaction pathway to form carbamate (eq 1) is the most favorable, and the formed unstable carbamate can further react with water to form bicarbonate.22 Received: Revised: Accepted: Published: 3363

October 2, 2013 January 25, 2014 February 3, 2014 February 12, 2014 dx.doi.org/10.1021/ie403280h | Ind. Eng. Chem. Res. 2014, 53, 3363−3372

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energy calculations with vibrational frequency calculations. The Gibbs free energy (G) is given by

(1)

where “B” represents an amine molecule. However, a recent study by Yamada et al. indicated that the reaction of carbamate with water cannot occur due to its high reaction energy barrier, and the reaction of AMP with CO2 was thermodynamically controlled.31 Yamada et al. concluded that although the formation of bicarbonate via the following pathway is kinetically unfavorable compared with that of carbamate, it is thermodynamically favorable. B + CO2 + H 2O → BH+ + HCO−3

G = ESCF + Gcorr

(3)

where ESCF is the total energy including solvation free energy and Gcorr is a correction term. Gcorr is given by Gcorr = E + kBT − TS

(4)

where E is the internal thermal energy computed from the molecular partition function (translational, rotational, and vibrational degrees of freedom), kB is the Boltzmann constant, T is the absolute temperature, and S is the entropy, also computed from the molecular partition function. The G value for complexes formed from reactants was set as the reference state for the relative G calculation of elemental reactions. We employed a two-step approach and considered conformationally averaged enthalpy for each species based on the reactions (5) and (6) to calculate the heat of reaction (ΔH) of carbamate and bicarbonate products as in our previous study.17 Yamada et al. calculated the ΔH values based on one single configuration for reactant and product.31 Thus, the computational scheme in this study should be more reliable.

(2)

It is worth mentioning that the thermodynamic stability of the carbamate/bicarbonate product was evaluated by the energy difference between the product and its corresponding transition state in the study by Yamada et al. However, in principle, the thermodynamic stability of a product should only depend on the enthalpy difference between reactants and products, regardless of the reaction pathways. Thus, the conclusion that the reaction of AMP with CO2 is thermodynamically controlled could be questionable. A further investigation on the reaction mechanism of AMP with CO2 and role of −CH3 is necessary. In contrast to the previously individual studies on the reaction mechanisms of AMP + CO2, we performed a comparative study at a same theoretical level on the reaction mechanisms of AMP/MEA with CO2. This can provide a better way to disclose the role of −CH3. We focused on the two most possible reaction pathways to form carbamate and bicarbonate in eqs 1 and 2. A detailed kinetic calculation was performed using microkinetic modeling. Moreover, a reliable computational scheme was used to calculate the thermodynamic stability of carbamate and bicarbonate products.

2RNH 2 + CO2 → RNH3+ + RNHCO2−

(5)

2RNH 2 + CO2 + H 2O → RNH3+ + HCO3−

(6)

All of the computational details were the same as those in our previous study17 except for the theoretical method used to calculate the enthalpy in solutions. Here, the enthalpy for each species in aqueous solutions was calculated at the B3LYP/6311++G(d,p) level with the SMD model, the same as that used in the investigation of the reaction mechanisms. In addition, the experimental reaction enthalpy for carbamate from MEA + CO2 is available,17 which enable us to verify the reliability of the computational scheme. This is also a reason why we calculated the reaction enthalpy, rather than the reaction free energy. Potential of Mean Force Calculations. Perhaps the most questionable part in the ab initio electronic structure calculations above is the use of implicit solvent model (or with one explict water) to account for solvent effects. A better approach would use a large system containing many water molecules. Here, to complement our ab initio calculation with implict solvent model, mixed quantum and molecular mechanics (PM3-PDDG/MD) simulations with the umbrellasampling method were carried out using AMBER 12 program39 to estimate the difference in the activation free energy for the rate-determining step of MEA/AMP + CO2 in the carbamate channel. PM3-PDDG has been extensively tested for gas-phase structures and energetics,40 and has given excellent results in solution-phase QM/MM studies for a wide variety of organic and enzymatic reactions.41−47 Also, we made a comparison for the potential of CO2 approaching MEA/AMP in the gas phase (Figure S1) between B3LYP/6-311++G(d,p) and PM3-PDDG, and the results indicated that the PM3-PDDG/MM method is reliable to reveal the difference in activation free energy for the rate-determining step in the carbamate channel. MEA/AMP + CO2 were immersed in 2238 TIP3P water molecules with periodic boundary conditions. The simulation was carried out in two steps. In the first step, each system was equilibrated in the NPT ensemble at 298 K for 200 ps. Production molecular dynamics (MD) simulations were then run in the NVT ensemble. In MD simulations, the time step was 1 fs. A 10 Å cutoff was used for all nonbonded interactions,

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COMPUTATIONAL DETAILS Ab Initio Electronic Structure Calculation. The Gaussian 09 package was used for the DFT calculations.36 Previous studies indicate that solvent effects are of critical importance in the study of the reaction of amines with CO2.18,37 Accordingly, solvent effects were carefully taken into account by two schemes: one was the implicit SMD38 solvent model within the polarizable continuum model (PCM) formalism (Mod_1) and the other is the combination of implicit SMD solvation model with one explicit H2O molecule (Mod_2). In Mod_2, one explicit H2O molecule was first built to form a complex with AMP/MEA for the carbamate channel and then the reaction of the complex with CO2 was investigated under the SMD model. For the bicarbonate channel, the reaction itself already needs one explicit H2O. Therefore, we did not consider the effect of another explicit H2O molecule in Mod_2 for bicarbonate channel. In other words, for the carbamate channel, Mod_1 and Mod_2 were used, but for the bicarbonate channel, only Mod_1 was used. In this way, reactants in the carbamate channel in Mod_2 are the same as those in the bicarbonate channel, which allows a thorough comparison of reaction process between the carbamate and bicarbonate channels. The optimized geometries and harmonic frequencies of reactants, products, isomers, and transition states were obtained at the B3LYP/6-311++G(d,p) level. Connections of the transition states between designated local minima were confirmed by intrinsic reaction coordinate (IRC) calculations at the B3LYP/6-311++G(d,p) level. We have estimated the Gibbs free energy for each species at 298 K by combining total 3364

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(MEAC and AMPC) were considered for the critical steps to investigate conformation effects of the reactants. In addition, in the aqueous solutions, before the reaction occurs, the reactive center N atom of AMP and MEA toward CO2 would form HB with water molecules, due to its strong electron donor ability. The formation of such HB could influence the attack of CO2 on the N atom of AMP and MEA, and the proton transfer from water to N atom, which could be the most direct effect of explicit waters on the reaction to form carbamate. Therefore, when the effect of explicit water was considered, one explicit water molecule was exerted to the reactants to form HB with the N atoms of AMP and MEA. The conformations of complexes AMP−H2O and MEA−H2O are presened in Figure 2. The AMP/MEA and AMP−H2O/MEA−H2O may form

and the electrostatic energy was evaluated using the particlemesh Ewald summation. One ns simulation per window was run with harmonic-restraint potentials imposed on the reaction coordinate (r) between the N atom of MEA/AMP and C atom of CO2.

V (r ) = k(r − r0)2

(7) −1

−2

where the force constant is k = 20 kcal mol Å and the “equilibrium” reaction coordinate (r0) for a given window is set every 0.1 Å from 5 to 1.5 Å. The points from the simulations were saved every 0.1 ps. A total number of 10 000 points were collected for each windows. To determine the potential of mean force (PMF) for the systems studied, we processed the results from all the windows of PM3-PDDG/MD simulations with the umbrella sampling method using weighted histogram analysis method (WHAM). The bin dimension applied in the WHAM calculations of the PMF was 0.05 Å. Conformer Selection. AMP and MEA have conformational degrees of freedom that may influence their reactivity. For MEA, previous studies on the most stable conformer were controversial.35,48−51 The studies with implicit solvent model indicated that the conformer MEAA (Figure 1) involving a

Figure 2. Three conformations of the complex of AMP and MEA with H2O optimized at the B3LYP/6-311++G(d,p) level with SMD model. The red balls stand for O atoms, the blue ones for N atoms, the gray ones for C atoms, and white ones for H atoms.

several complexes via different interactions with CO2. Here, only the complexes that can directly lead to chemical reactions were considered. From technical point of view, the complexes were confirmed by IRC to connect the corresponding transition state and the product. Calculating Reaction Rate Coefficients (k). The reaction rate constant (k) for elementary reactions was calculated by the transition state theory:

Figure 1. Three conformations of AMP and MEA optimized at the B3LYP/6-311++G(d,p) level with SMD model. The red balls stand for O atoms, the blue ones for N atoms, the gray ones for C atoms, and white ones for H atoms.

hydrogen-bonding (HB) interaction of O−H···N is the most stable.48,49 However, molecular dynamics modeling (with explicit water molecules) employing empirical force fields found that the conformer MEAB without HB (Figure 1) is the most stable.50 A recent ab initio study with two explict waters by Han et al. argued that MEA prefers to form HB with explict water and therefore MEA should take a more stretched form (MEAC in Figure 1).35,51 For AMP, only the studies with implict solvent model have been performed. These studies showed the conformer AMPA with HB (Figure 1),31,52 corresponding to the conformer MEAA, was the most stable. Performing a long-time ab initio molecular dynamics (MD) calculation involving many explict water molecules would be a more reliable approach to determine the most stable conformer for MEA and AMP, but at present this is not computationally feasible. Based on the current status, we proposed a compromised scheme to present AMP and MEA molecules for the mechanistic study. The conformer A (MEAA and AMPA) was used for the whole study. The other two representative conformers B (MEAB and AMPB) and C

k = (c 0)Δn

⎛ ΔG 0 ⎞ kBT exp⎜ ⎟ h ⎝ RT ⎠

(8)

where c0 is the standard-state concentration (1 mol L−1), Δn is the change of the number of moles from reactants to the transition states, h is the Planck constant, ΔG° (activation free energies) is the difference in the Gibbs free energy between the reactants and the transition states, and R is the gas constant. For the carbamate channel, we start from the same assumed reaction scheme as Ali et al.9 and ours:18 k2

amine + CO2 XooY zwitterion k −1

kBase

zwitterion + amine ⎯⎯⎯→ carbamate

(9) (10)

Invoking the pseudo steady-state approximation, the zwitterion concentration can be expressed as 3365

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Figure 3. Schematic free energy surfaces for the amine + CO2 reactions to form carbamate and amine + H2O + CO2 to bicarbonate: (a) for AMP; (b) for MEA. The calculations were performed at the B3LYP/6-311++G(d,p) level using solvent model Mod_1.

Figure 4. Conformations of prereactive complexes AMP/MEA + CO2 and AMP/MEA + CO2 + H2O at the B3LYP/6-311++G(d,p) level with Mod_1 and Mod_2.

[zwitterion] =

k 2[amine][CO2 ] k −1 + k base[amine]

k3

amine + CO2 + H 2O → bicarbonate

The overall rate of the consumption of CO2 is therefore

The overall rate of consumption of CO2 is therefore rCO2 =

k basek 2[amine]2 [CO2 ] k −1 + k base[amine]

rCO2 = k 3[amine][CO2 ][H 2O] (12)

(16)

[H2O] can be considered as the constant in the process of reactions because of [H2O] ≫[amine] and [CO2] in the aqueous solution of amine. Therefore, corresponding to kc of the carbamate channel, we can define the reaction rate coefficient kb for the bicarbonate channel as

We can rewrite eq 12 as rCO2 = kc[CO2 ]

(15)

(11)

(13)

where the pseudo-first-order reaction rate coefficient kc is defined as

k b = k 3[amine]

(17)

2

k basek 2[amine] kc = k −1 + k base[amine]

In eqs 12 and 16, the elementary reaction rate coefficients k2, k−1, and k3 can be calculated using the transition state theory as given in eq 8.

(14)

For the bicarbonate channel, we assume the reaction scheme as 3366

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Figure 5. Geometries of transition states for the reaction AMP/MEA + CO2 and AMP/MEA + CO2 + H2O at the B3LYP/6-311++G(d,p) level with Mod_1 and Mod_2.

Figure 6. Schematic free energy surfaces for AMP/MEA + CO2 + H2O reaction to form carbamate or bicarbonate: (a) for AMP; (b) for MEA. The calculations were performed at the B3LYP/6-311++G(d,p) level. 3367

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Figure 9. Potential of mean force plot for CO2 approaching AMP/ MEA obtained at the PM3-PDDG/MM level of theory. The distance between the N atom of MEA/AMP and the C atom of CO2 is taken as the reaction coordinate.

carbamate than that of MEA carbamate. Previous studies also found that introduction of CH3 on α-site of MEA can decrease the stability of carbamate.53 Moreover, the computed ΔH value of carbamate from AMP + CO2 is lower than that of bicarbonate as product (−12.4 kcal mol−1), clearly indicating that the formation of carbamate from AMP + CO2 is more thermodynamically favorable. This finding is different from that of Yamada et al., who concluded that the reaction of AMP with CO2 can form bicarbonate via a thermodynamically favorable way.31 The difference lies in the different ways for evaluating the thermodynamical stability of carbamate and bicarbonate. In this study, the thermodynamical stability of the products was evaluated by calculating the enthalpy difference between reactants and products. While Yamada et al. evaluated the thermodynamic stability by calculating the energy difference between the product and its corresponding transition state.31 Reaction Mechanism of AMP/MEA + CO 2. The schematic free energy surfaces of reactions AMP/MEA+CO2 computed with Mod_1 are presented in Figure 3. Figures 4 and 5 show the conformations of prereactive complexes and transition states for the reaction AMP/MEA + CO2 , respectively. For the carbamate channel, the first step is the formation of zwitterions with ΔG of 6.9 and 6.7 kcal mol−1 for AMP and MEA, respectively. In the second step, the zwitterions react with AMP or MEA to form carbamates. The ΔG values are 0.7 and 1.1 kcal mol−1 in going from the complexes (ComAMP‑Zwitterion and ComMEA‑Zwitterion) to their corresponding carbamates for AMP and MEA, respectively. Therefore, for the carbamate channel, the first step forming zwitterions is the ratedetermining step. For the channel of bicarbonate, ΔG values for AMP and MEA are 11.7 and 11.8 kcal mol−1, respectively, which are 4.8 and 5.1 kcal mol−1 higher than those of the rate-determining step in the corresponding carbamate channel, respectively. From Figure 3, it can be found that ΔG values for each step and relative free energies of products for the reaction AMP + CO2 are almost the same as those of the corresponding reaction of MEA + CO2. In addition, the calculated ΔG value for the hydrolysis process of carbamate of AMP (40.8 kcal mol−1) is also almost the same as that of MEA (40.5 kcal mol−1), indicating that the hydrolysis of carbamate of AMP and MEA is unfeasible due to their high ΔG values. Yamada et al. also found that the hydrolysis of carbamate from AMP is impossible because of high activation energy.31 To sum up, we cannot find any differences in the reaction mechanisms between AMP and MEA with CO2 when just considering the implicit solvent model (Mod_1). It is worth mentioning that for the both reactions AMP + CO2 and MEA + CO2, the ΔG values of the second step in the

Figure 7. Conformation changes of the transition states (TS-1AMP/MEA and TS-1AMP/MEA‑1H2O) in the rate-determining step in carbamate channel of AMP/MEA + CO2 from Mod_1 to Mod_2. The key distances and dihedrals CCCO (dCO) and CHCO (dHO) (along the direction of arrow) are presented. Unit for distance and dihedrals are angstrom and degree, respectively.

Figure 8. Electrostatic potential surfaces of AMP, MEA, and CO2 at the B3LYP/6-311++G(d,p) level with solvent model Mod_1.

Table 1. Elementary Reaction Rate Coefficients, kb and kc, for MEA and AMP with Mod_1 and Mod_2 Mod_1 MEA k2a k−1b kBasea k3a kbb kcb a

7.65 1.02 3.80 1.40 7.01 6.99

× × × × × ×

107 1010 109 104 102 104

Mod_2 AMP

5.46 1.02 2.40 1.66 8.30 3.17

× × × × × ×

107 1010 109 104 102 104

MEA

AMP

× × × × × ×

5.74 × 105 1.02 × 1010 2.4 × 109 1.66 × 104 8.30 × 102 3.34 × 102

4.61 3.93 3.80 1.40 7.01 1.11

107 1010 109 104 102 104

Unit is L mol−1 s−1. bUnit is s−1.

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RESULTS AND DISCUSSION Thermodynamics. The calculated ΔH is −16.6 kcal mol−1 for the MEA + CO2 reaction to form carbamate, which is consistent with the corresponding experimental value of −17.3 kcal mol−1.17 The consistence further verifies the reliability of the computational scheme employed in the current study. For the reaction MEA + CO2 to form carbamate, the ΔH value is lower that the corresponding value for AMP+CO2 (−15.2 kcal mol−1), indicating the lower thermodynamic stability of AMP 3368

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Figure 10. Transition-state geometries and activation energies for the formation of zwitterion and bicarbonate at the B3LYP/6-311++G(d,p) level for conformer B of AMP and MEA.

Figure 11. Transition-state geometries and activation energies the formation of zwitterion and bicarbonate at the B3LYP/6-311++G(d,p) level for conformer C of AMP and MEA.

presents the schematic free energy surfaces for the reaction AMP/MEA + CO2 + H2O. In the ComAMP/MEA‑H2O complex (Figure 2), the N-site of AMP/MEA forms hydrogen bonds with H2O molecule. Therefore, to form zwitterion intermediate in the carbamate channel, CO2 has to first destroy the H-bond of ComAMP/MEA‑H2O to form a prereactive complex Com2AMP/MEA‑CO2‑H2O (Figure 4). The G values for these prereactive complexes Com-2AMP/MEA‑CO2‑H2O are 1.8 and 1.9 kcal mol−1 higher than those of complexes Com-1AMP/MEA‑CO2‑H2O in the bicarbonate channel for MEA and AMP, respectively (Figure 6). The concentration ratio between Com-2AMP/MEA‑CO2‑H2O and Com-1AMP/MEA‑CO2‑H2O should follow Boltzmann distribution in the reaction, and therefore the concentration of Com2 AMP/MEA‑CO2‑H2O is much lower than that of Com1AMP/MEA‑CO2‑H2O. To clearly consider this factor, the G value of Com-1AMP/MEA‑CO2‑H2O was set to zero in the free energy surfaces for AMP/MEA + CO2 + H2O reaction. From Figure 6, it is seen that the ΔG value of the rate-determining step in the carbamate channel for AMP is 2.6 kcal mol−1 higher than that for MEA. This is different from the case when Mod_1 was used, where ΔG (6.9 kcal mol−1) of AMP is almost the same as that of MEA (6.7 kcal mol−1). As will be discussed in section 3.4, the increase of ΔG in the carbamate channel for AMP

carbamate channels are very low, and therefore, the reactions should proceed fast. The fast chemical kinetics may make the reaction of the second step become diffusion-controlled. The measured diffusion coefficients of 1 mol L−1 AMP and MEA in aqueous solution are 0.63 × 10−9 and 1.0 × 10−9 m2 s−1 at 298 K, respectively.54,55 Based on the diffusion coefficient of AMP and MEA, and the approach of Shoup et al.56 for the calculation of diffusion rate, free diffusion rates for AMP and MEA in aqueous solution were calculated to be 2.4 × 109 and 3.8 × 109 L mol−1 s−1, respectively. These values can be reasonably taken as approximate values for the diffusion rates of AMP and MEA to their corresponding zwitterion intermediates in the reaction. However, the elemental reaction rates for the second step are calculated to be 1.9 × 1012 and 1.1 × 1012 L mol−1 s−1 (the concentration of AMP/MEA is 1 mol L−1) for AMP and MEA using eq 8, respectively, which are much faster than those of diffusion rates of AMP and MEA. Therefore, the second step in the carbamate channel is diffusion-controlled for both reactions. To the best of our knowledge, this is the first report that the second step for forming carbamate is diffusion-controlled. Reaction Mechanism of Complexes ComAMP/MEA‑H2O with CO2. Figure 4 shows the conformations of prereactive complexes of the reaction AMP/MEA + CO2 + H2O. Figure 6 3369

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in good agreement with the experimental observation that carbamate is a main product. However, for the reaction of AMP + CO2, the reaction rate coefficient kc > kb cannot explain the experimental observation that bicarbonate is a main product. When one explicit H2O was involved, the formation rate coefficient of bicarbonate kb for the reaction MEA + CO2 is still lower than that of carbamate kc. However, for the reaction AMP + CO2, the formation rate coefficient of bicarbonate kb is higher than that of carbamate kc. Thus, when one explicit H2O molecule was involved, the theoretical results can reasonably explain the experimental observation of product distribution for both reactions.32 In addition, the reaction rate coefficient kc of MEA + CO2 is higher than kb of AMP + CO2, which is also in good agreement with the experimental observation that MEA + CO2 proceeds faster than AMP + CO2. It indicates that Mod_2 is reasonable to treat the AMP/MEA + CO2 system. It was found that a decrease in k2 for AMP from Mod_1 to Mod_2 is the main reason that the formation rate coefficient of bicarbonate kb is higher than that kc of carbamate by comparing kb, kc and the components k2, k−1 and kBase of kc between Mod_1 and Mod_2. The decrease in k2 for AMP from Mod_1 to Mod_2 results from the increase of ΔG. Therefore, our study indicates that the involvement of explicit H2O is critical to study the reaction mechanism of AMP + CO2. Potential of Mean Force Calculations. As discussed above, the difference (2.6 kcal mol−1) in the activation free energy for zwitterion formation between AMP + CO2 and MEA + CO2 is the reason leading to their difference in reaction mechanism. Here, the potential of mean force calculations with PM3-PDDG/MM method were performed to estimate the difference in activation free energy of zwitterion formation between AMP + CO2 and MEA + CO2 to verify the conclusion from B3LYP/6-311++G(d,p) with Mod-2. The PMF for CO2 approaching MEA/AMP from 5 to 1.5 Å is plotted in Figure 9. It can be observed that the free energy increases with the decrease of C−N distances from 5 to 1.987 and 1.970 Å for MEA and AMP, respectively, where the free energy reaches a maximum. This maximum corresponds to the transition state structure. The free energy further decreases from the transition state to a local minimum with the C−N distance of 1.625 and 1.609 Å for MEA and AMP, respectively, and the local minimum corresponds to the zwitterion intermediate. The C− N distances for the transition state are a little shorter than those predicted by the DFT calculations (Figure 5). However, the C−N distances for the zwitterion from the PMF calculations are a little longer than those predicted by the DFT calculations (1.587 and 1.576 Å of zwitterion for MEA and AMP, respectively). The difference in activation free energy between MEA + CO2 and AMP + CO2 is 1.9 kcal mol−1, which well agree with the results (2.6 kcal mol−1) from the B3LYP/6-311+ +G(d,p) level. This indicates that the conclusion from the B3LYP/6-311++G(d,p) level with Mod-2 is reliable. Conformation Effect. The transition-state geometries and activation energies for the formation of the zwitterions and bicarbonate from conformers B (MEAB and AMPB) and C (MEAC and AMPC) were presented in Figures 10 and Figure 11, respectively. The results show that for the conformers AMPB and AMPC the involvement of one explicit water increases the activation free energy for the formation of zwitterions by about 2 kcal mol −1 . Nevertheless, the involvement of one explicit water has little effects on the activation free energy for the corresponding process of MEAB/ MEAC. Thus, the role of explicit water in differentiating the

results in the total reaction rate coefficient for the carbamate channel being lower than that of the bicarbonate channel. The ΔG values for the reactions, in which zwitterion intermediate comes back to reactants, are 3.0 and 3.8 kcal mol−1 for MEA and AMP, respectively, which are comparable to those (3.8 and 3.8 kcal mol−1 for MEA and AMP, respectively) with Mod_1. For the second step in the carbamate channel, although ΔG for MEA and AMP change a little from Mod_1 to Mod_2, the reaction rate is still faster than the corresponding diffusion rate of AMP and MEA. Therefore, it has no effect on the total reaction rate coefficients. For the carbamate channel, a possible reason that leads to the ΔG difference between AMP/MEA + CO2 in the ratedetermining step could be the structure change of the corresponding transition states before and after the involvement of one explicit H2O. As shown in Figure 7, an obvious change in the structure is the orientation of CO2. The dihedral CCCO (dCO) and CHCO (dHO) (Figure 7) in the transition states for AMP and MEA systems were used as parameters to describe the change, respectively. The dCO and dHO are 117.1 and 77.8 degree in Mod_1. The dCO and dHO change to 177.3 and 177.6 degree in Mod_2. The change shortens the distance between the O atom of CO2 and C atom/H atom on the Cα site of AMP/MEA, i.e., the distance between the O atom of CO2 and the C/H atom of AMP/MEA is changed from 3.93 and 4.26 Å to 3.30 and 2.78 Å for AMP and MEA, respectively. Moreover, it is found that an obvious difference of electrostatic potential distribution between −CH3 on Cα atom of NH2 of AMP and −H on Cα atom of NH2 of MEA. As shown in Figure 8, the electrostatic potential on the H-site of MEA is positive. However, the electrostatic potential on the CH3-site of AMP is negative. While, electrostatic potential on the O-site of CO2 is negative. Therefore, the decreased distance between the O atom of CO2 and the C atom/H atom of MEA/AMP would increase the electrostatic attraction between MEA and CO2, while increase the electrostatic repulsion between AMP and CO2. Accordingly, ΔG for intramolecular conversion in the rate-determining step for MEA decreased from 6.7 kcal mol−1 without explicit H2O to 5.1 kcal mol−1 with one explicit H2O, while for AMP increased from 6.9 to 7.8 kcal mol−1. It is well explained why ΔG in the rate-determining step for AMP is 2.6 kcal mol−1 higher than that for MEA, when Mod_2 was used. Moreover, we found that the solvation free energy has almost no contribution to the difference in the activation free energies for the formation of zwitterions between MEA and AMP (Table S1). Reaction Rate Coefficients for Carbamate and Bicarbonate Channel. We have computed k2, k−1, and k3 using ΔG from B3LYP/6-311++G(d,p) calculations with Mod_1 and Mod_2, respectively. As mentioned above, the second step is predicted to be diffusion-controlled reaction, kBase is approximately equal to free diffusion rate of amine in aqueous solution. All the calculated elementary reaction rate coefficients are presented in Table 1. Based on the elementary reaction rate coefficients, we have computed the values of k c and k b with AMP/MEA concentration of 0.05 mol L−1 at 298 K, which is in the concentration range that Li et al.57 and Ali et al.9 used in their experiments. As shown in Table 1, the reaction rate coefficients kb is lower than kc for AMP + CO2 and MEA + CO2 when Mod_1 was used. Thus, the formation rate of bicarbonate for AMP and MEA is slower than that of carbamate. For the reaction of MEA + CO2, the reaction rate coefficient kc > kb is 3370

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activation free energy for the rate-determining step (the formation of zwitterions) of the carbamate channel can be independent of the conformations of AMP and MEA.

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CONCLUSION In this work, a comparative study on the reaction mechanism between AMP + CO2 and MEA + CO2 was investigated at the B3LYP/6-311++G(d,p) level with the appropriate treatment of solvent effect. Two channels that lead to the formation of carbamate and bicarbonate were considered. The total reaction rate coefficients were computed using a microkinetic model. The results indicate that the formation of bicarbonate for AMP + CO2 is more favorable in kinetics than the formation of carbamate; however, for MEA + CO2 the case is reversed. This explains well the experimental observation of different product distributions for AMP + CO2 and MEA + CO2 reactions. In addition, it is found that the difference in electrostatic potential distribution between CH3-site on Cα of AMP and its corresponding H-site of MEA could be a reason, leading to their different reaction mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

Potential energy curve for CO2 approaching MEA/AMP at PM3-PDDG and B3LYP/6-311++G(d,p) level. Cartesian coordinates and absolute free energies of optimized geometries and corresponding transition states involved in the reaction of AMP/MEA + CO2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was supported by the National Natural Science Foundation of China (21207016, 21137001, and 21325729), the Fundamental Research Funds for the Central Universities (DUT12RC(3)07), Liaoning Provincial Education Department (L2012021), and Key Laboratory of Industrial Ecology and Environmental Engineering (MOE). We thank Professor Alan Grossfield for providing the code of the weighted histogram analysis method.

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