Ab Initio Study of CO2 Capture Mechanisms in Aqueous

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Ab Initio Study of CO2 Capture Mechanisms in Aqueous Monoethanolamine: Reaction Pathways for the Direct Interconversion of Carbamate and Bicarbonate Yoichi Matsuzaki,*,† Hidetaka Yamada,‡ Firoz A. Chowdhury,‡ Takayuki Higashii,‡ and Masami Onoda† †

Advanced Technology Research Laboratories, Nippon Steel and Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan ‡ Chemical Research Group, Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan S Supporting Information *

ABSTRACT: Ab initio molecular orbital calculations combined with the polarizable continuum model (PCM) formalism have been carried out for a comprehensive understanding of the mechanism of carbon dioxide (CO2) absorption by aqueous amine solutions. CO2 is captured by amines to generate carbamates and bicarbonate. We have examined the direct interconversion pathways of these two species (collectively represented by a reversible hydrolysis of carbamate) with the prototypical amine, monoethanolamine (MEA). We evaluate both a concerted and a stepwise mechanism for the neutral hydrolysis of MEA carbamate. Large activation energies (ca. 36 kcal/ mol) and lack of increase in catalytic efficiency with the inclusion of additional water molecules are predicted in both the mechanisms. We also examined the mechanism of alkaline hydrolysis of MEA carbamate at high concentrations of amine (high pH). The addition of OH− ion to carbamate anion was theoretically not allowed due to the reduction in the nucleophilicity of the former as a result of microsolvation. We propose an alternative pathway for hydrolysis: a proton transfer from protonated MEA to carbamate to generate the carbamic acid that initially undergoes a nucleophilic addition of OH− and subsequent low-barrier reactions leading to the formation of bicarbonate and free MEA. On the basis of the calculated activation energies, this pathway would be the most efficient route for the direct interconversion of carbamate and bicarbonate without the intermediacy of the free CO2, while the actual contributions will be dependent on the concentrations of protonated MEA and OH− ions.

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where −R stands for the −CH2CH2OH group. There have been several experimental3−11 and theoretical12−17 studies on the mechanism represented by eq 1. The two-step mechanism3,4 suggests that the reaction proceeds through a zwitterion intermediate (1a, Figure 1) which undergoes deprotonation by another amine to form the carbamate (1b, Figure 1). Recent theoretical studies on aqueous MEA/CO2 system provides support for this mechanism, revealing further that formation of the zwitterion (1a, Figure 1) is the rate-determining step.16 In addition to the carbamate, CO2 is also captured as a bicarbonate. Such bicarbonates are the dominant products for sterically hindered amines, such as 2-amino-2-methyl-1propanol (AMP).18,19 The bicarbonate is formed via the following reactions

INTRODUCTION

It is widely known that CO2 is responsible for global warming and problems associated with climate change.1 Chemical absorption using an aqueous amine solution is currently one of the most promising technologies to capture CO2 from sources of large postcombustion gas streams.2 Practical applications of amine-based CO2 capture technology are restricted due to high costs associated with the technology. A large portion (>50%) of the cost arises from absorbent regeneration. It is therefore essential to reduce absorbent regeneration energy consumption by developing new absorbents with desired features such as high absorption rate, high absorption capacity, and low heat of reaction. To that end, it is crucial to comprehend in extensive detail the reaction mechanisms underlying the CO2 capture process. Monoethanolamine (MEA) has been extensively evaluated for CO2 capture. Possible reaction pathways relevant to the CO2 capture by aqueous MEA are illustrated in Figure 1. The direct reaction of dissolved CO2 with MEA produces a carbamate anion (eq 1) −

CO2 + 2RNH 2 ⇄ RNHCOO + RNH3

+

© 2013 American Chemical Society

CO2 + H 2O + RNH 2 ⇄ HCO3− + RNH3+

(2)

CO2 + OH− ⇄ HCO3−

(3)

Received: July 5, 2013 Revised: September 2, 2013 Published: September 4, 2013

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Figure 1. Reactions involved in the CO2 capture by MEA and its further modifications. Reactions 1a−3 describe the process of CO2 capture while reactions 4−8 are relevant to the direct interconversion of carbamate and bicarbonate. Reactions with dashed arrows indicate the involvement of the hydroxide ion.

structural resemblance to amine carbamates. Amide hydrolysis has received considerable attention due to its relevance to various biochemical processes and extensive theoretical studies have been devoted to elucidating mechanisms, including the acid- and base-catalyzed pathways.24−31 In the present study, we focus on the prototypical amine used in CO2 capture, MEA, and investigate in detail the reaction pathways for the direct interconversion of carbamate and bicarbonate as represented by eq 4. We consider neutral as well as alkaline conditions, taking the rather high concentration of amine (high pH) in practical absorbents into account.

In eq 2, amine acts as the base in the hydration of CO2 as proposed for the reaction mechanism of tertiary amines.20 This mechanism has been also suggested for MEA/CO221 and AMP/CO222 systems on the basis of the transition structures determined by quantum chemical calculations. The contribution of reaction in eq 3 to the overall reaction rate is dependent on the pH. In addition to eqs 1−3, the interconversion of carbamate and bicarbonate is commonly adopted in the kinetic analyses of CO2 absorption processes (eq 4)7,9,11 RNHCOO− + H 2O ⇄ HCO3− + RNH 2

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(4)

COMPUTATIONAL DETAILS All calculations were carried out using the Gaussian 09 suite.32 The molecular geometries along reaction pathways were optimized by Møller−Plesset second-order perturbation (MP2) theory combined with the 6-311++G(d,p) basis set. The optimized transition-state (TS) geometries were confirmed to be connected with designated reactants and products by intrinsic reaction coordinate (IRC) calculations. The geometries of reactant complexes (RC) and product complexes (PC) obtained via the IRC calculations were further optimized. The optimized geometries were confirmed to be located as stationary points on the potential energy surfaces by performing normal vibration analyses. On the basis of the optimized geometries, single-point energy calculations were performed using the coupled-cluster theory with single, double, and noniterative triple excitation (CCSD(T)) combined with the 6311++G(2df,2p) basis set. Energy values were corrected for zero-point vibrational energies (ZPE). Activation energies were the energy difference between RC and TS. Solvent polarization effects were incorporated by using the PCM33 with the dielectric constant of water (78.39). We used Pauling atomic radii to construct the PCM solvent cavity, since such a cavity provides a reliable performance on the MEA/CO2 system.16 In this study, we are not concerned with the free energy since a continuum model lacks the ability to assess the free energy of solvents.

Equation 4 essentially formalizes the combination of eqs 1 and 2 with the free CO2(aq) as an intermediate. However, recent 1H NMR investigation11 on the dissociation of MEA carbamate into bicarbonate has indicated a direct contribution from the hydrolysis of MEA carbamate as described by eq 4 in addition to the contributions from the reverse reaction in eq 1. In the case of AMP, the formation of the dominant bicarbonate product was accounted for by the initial formation of AMP carbamate, followed by its subsequent hydrolysis to bicarbonate via eq 4.23 Such a mechanistic consideration can be extended to various secondary amines that produce a significant quantity of bicarbonate in addition to carbamate.19 Therefore, it is essential to obtain a clear picture of the mechanism of eq 4 for a comprehensive understanding of CO2 absorption, and hence, the development of high-performance absorbents. The mechanism of carbamate−bicarbonate interconversion (eq 4) has not been investigated in great detail. In the case of aqueous AMP/CO2 system, we employed the B3LYP densityfunctional theory combined with the SMD solvation model to suggest the concerted hydrolysis of AMP carbamate as a plausible reaction mechanism.22 The calculated energy barrier for the water-assisted dissociation of AMP carbamate is high (37 kcal/mol), suggesting an insignificant contribution from this pathway to the formation of bicarbonate. To take this a step further, the mechanistic investigations performed on the hydrolysis of amide compounds will be helpful because of their 9275

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The conformational variations of MEA and its derivatives such as the carbamate anion and the protonated cation may have significant effects on the kinetics and thermodynamics of CO2 absorption reactions. Although some conformers of alkanolamines are expected to be stabilized via the intramolecular hydrogen bonds, a preference for such conformers depends on the competition between intra- and intermolecular interactions. Despite the detailed analyses of the conformational distributions,34,35 there has been no consensus on this issue.17 Since the purpose of the present study is to elucidate a general reaction mechanism, we have discarded the conformers that form rigid intramolecular hydrogen bonds. Within such a restriction, we have examined several conformers for the zwitterion, carbamic acid, and carbamate as well as free MEA and selected the gGg conformer (for the definition, see ref 35) throughout the present study because it was found to be a major conformer in those species.

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RESULTS AND DISCUSSION Concerted Neutral Hydrolysis of MEA Carbamate. We have investigated the mechanism of neutral hydrolysis of MEA carbamate pursuant to the theoretical insights obtained for amide compounds. There have been a number of theoretical studies on the neutral hydrolysis of formamide in which both concerted24,25 and stepwise26,27 mechanisms have been proposed taking the catalytic effects of including additional water molecules into account. Herein, we describe our computational results on the concerted hydrolysis of MEA carbamate in which one or two explicit water molecules are incorporated. The reaction geometries and the corresponding energetics are shown in Figure 2. Hereafter, we adopt the notation of “(wn)” to indicate the number of explicit water molecules n in our computational model. In both systems, the IRC calculations obtained the designated reactants (MEA carbamate) and the products (bicarbonate) without detecting any stable intermediates. Therefore, both TS4(w1) and TS4(w2) represent the transition structures in a concerted mechanism for the hydrolysis of MEA carbamate described by eq 4. The calculated energy barriers to the carbamate decomposition are 41.1 and 37.3 kcal/mol for the nonassisted and water-assisted reactions, respectively. These values are close to those predicted for AMP/CO2 system by the B3LYP densityfunctional theory combined with the SMD solvent model.22 Catalytic effects of additional water molecules have been extensively investigated for various hydrolysis reactions.25,36,37 In the case of MEA carbamate, the activation energy is reduced by 3.8 kcal/mol with an assistance from one water molecule. This is much smaller than the corresponding value of 26.1 kcal/ mol36 predicted by the MP2/SCRF/6-311++G** calculations for the neutral hydration of CO2. Notice that in both the transition structures shown in Figure 2, the formation of the new N−H bond is nearly complete while the new C−O bond is still weak. These transition structures can be roughly considered as a complex of MEA carbamate zwitterion and OH−. Such an asynchronous nature of the reaction could be partly ascribed to the polarization of the bulk solvent that favors charge separation in the solute. In the hydration of CO2,36 the charge separation in the transition state is significantly increased in the presence of catalytic water molecule leading to the remarkable reduction in the energy barrier. In contrast, the charge separation at the transition state of the present system is hardly influenced by the presence of the water

Figure 2. PCM-MP2/6-311++G(d,p) optimized reaction geometries for the neutral hydrolysis of MEA carbamate (eq 4) with (a) one and (b) two water molecules. Bond distances are given in angstroms. (c) PCM-CCSD(T)/6-311++G(2df,2p) calculated potential energy profiles on the basis of the optimized geometries shown in panels (a) and (b). The energy levels are plotted relative to the separated reactants: MEA carbamate and H2O for the nonassisted case, MEA carbamate and 2H2O for the water-assisted case. The values in italics indicate activation energies.

molecule (the dipole moments of transition structure are 15.9 and 14.6 D for the nonassisted and assisted cases, respectively), thus accounting for the insignificant reduction in energy barrier. The asynchronous nature of the considered MEA/CO2 reaction becomes more evident when an additional water molecule is incorporated. We attempted to optimize the expected eight-membered cyclic transition structure for the concerted mechanism, TS4b(w3) in Figure 3. The IRC calculation terminated by finding a stationary-point geometry that is comprised of a zwitterion and OH− (RC4b(w3), Figure 3). We found that this intermediate derives from the MEA carbamate and water molecules via TS4a(w3) as shown in Figure 3. Accordingly, the hydrolysis proceeds via a two-step mechanism in this case. In the first step, MEA zwitterion and OH− are formed via a Grotthuss-type proton transfer through the hydrogen-bond network with the energy barrier of 14.3 kcal/mol. The second step represents the base-catalyzed hydration of zwitterion leading to the formation of bicarbonate and free MEA that presents an energy barrier of 21.4 kcal/mol. Although each energy barrier in the reaction involving three water molecules is much smaller than those for the TS4(w1) and TS4(w2), it should be noted that the product complex in the first step (PC4a(w3)) is higher in energy than TS4a(w3) after ZPE correction, indicating the metastable nature of the former. Therefore, an effective energy barrier for the overall reaction should be evaluated by the energy difference between TS4b(w3) and RC4a(w3), which amounts to 38.7 kcal/mol, displaying no reduction in the energy barrier. Moreover, due to the stepwise nature of the reaction emerged from these studies, 9276

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molecule hydrates the C−O moiety of carbamate to produce a tetrahedral intermediate RNHCO(OH)2− (INT1) as described by eq 5 RNHCOO− + H 2O ⇄ RNHCO(OH)2−

(5)

The reaction geometries for the stepwise hydrolysis of the MEA carbamate with the assistance of one water molecule are displayed in Figure 4a; Despite extensive efforts, we could not find a transition structure for the nonassisted reaction. Because the IRC calculations found no reaction intermediates, TS5(w2) describes a concerted reaction. Furthermore, it also demonstrates the asynchronous nature of the reaction, similar to that observed for the reaction represented by eq 4. The new O−H bond is almost entirely formed while the new C−O bond is still long. Notice that the MEA species in TS5(w2) emerges as a carbamic acid instead of the zwitterion identified in eq 4. The activation energy of 35.9 kcal/mol (Figure 4c) required for the generation of TS5(w2) is smaller than that required for the TS4(w2) by 1.4 kcal/mol. Because the present calculations predict that the MEA carbamic acid is more stable than the MEA zwitterion by 4.2 kcal/mol, the difference in the barrier height is consistent with the relative stability of these species. The second step is a proton transfer from one of the hydroxyl groups to the nitrogen atom leading to dissociation and release of free MEA and bicarbonate

Figure 3. (a) PCM-MP2/6-311++G(d,p) optimized reaction geometries for the neutral hydrolysis of MEA carbamate (eq 4) involving three water molecules. Bond distances are given in angstroms. (b) PCM-CCSD(T)/6-311++G(2df,2p) calculated potential energy profile on the basis of the optimized geometries shown in panel (a). The energy levels are plotted relative to the separated reactants consisting of MEA carbamate and 3H2O. The values in italics indicate activation energies.

RNHCO(OH)2− ⇄ RNH 2 + HCO3−

(6)

We found that the energy barrier to the direct proton transfer is 24.4 kcal/mol, which is dramatically reduced with the assistance of a single water molecule. As shown in Figure 4b, the reaction represented in eq 6 turns out to be a two-step reaction as in the case of N-methyl acetamide.28 The corresponding energy profile is shown in Figure 4c (in conjunction with that of the reaction in eq 5). In the first

further inclusion of water molecules will no longer affect the reaction rate. Stepwise Neutral Hydrolysis of MEA Carbamate. By analogy with the mechanism of amide hydrolysis in a neutral reaction,26,27 we also considered a stepwise mechanism for the neutral hydrolysis of MEA carbamate. In the first step, a water

Figure 4. PCM-MP2/6-311++G(d,p) optimized reaction geometries for (a) the first step in the neutral hydrolysis of MEA carbamate (eq 5) and (b) the second step in the neutral hydrolysis of MEA carbamate (eq 6). Bond distances are given in angstroms. (c) PCM-CCSD(T)/6-311++G(2df,2p) calculated potential energy profile on the basis of the optimized geometries shown in panels (a) and (b). The energy levels are plotted relative to the separated reactants consisting of MEA carbamate and 2H2O. The values in italics indicate activation energies. 9277

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Alkaline Hydrolysis of MEA Carbamate. Since the high concentration of amine in practical CO2 absorbents leads to a relatively high concentration of OH− ion, it is worthwhile to consider a mechanism for base-promoted hydrolysis of MEA carbamate. We first examined the OH− addition to MEA carbamate to give the dianion product RNHCO2(OH)2−. Using a computational model in which the OH− ion is not explicitly solvated, we could optimize the minimum-energy structure for the designated product, as well as determine the overall reaction pathway resulting in the formation of bicarbonate and free MEA (Supporting Information Figure S1). In this case, the formation of dianion is rate-determining with the predicted activation energy of 28.8 kcal/mol. However, in the presence of a single water molecule, the solvated OH− ion was repelled by the MEA carbamate probably due to the electrostatic repulsion between negative charges and the reduced nucleophilicity of the solvated OH− ion. This indicates that the relevant dianion is not stable in aqueous solution, and hence, we exclude such pathways. On the basis of these considerations, it is likely that carbamate becomes neutral by receiving a proton prior to the addition of OH−. In this regard, it is noted that the abovementioned two-step pathway for the reactions of eqs 4 and 5 (Figures 3 and 5) are consistent with such a consideration. In the first step, a proton is transferred from H2O to MEA carbamate with energy demands of 15.7 (for the formation of the zwitterion) and 12.8 kcal/mol (for the formation of the carbamic acid). These energies are responsible for the large energy barrier associated with the reactions of eqs 4 and 5. Nevertheless, in practical absorbents the protonation of MEA carbamate may take place readily if the protonated MEA acts as a proton donor, because it is a much stronger acid when compared to H2O (pKa = 9.5319 for protonated MEA vs pKa = 15.74 for H2O). A significant concentration of the protonated MEA is generated via the direct CO2 absorption reaction described in eq 1. We have investigated the energies associated with the formation of both the zwitterion (back-reaction of 1b, Figure 1) and the carbamic acid (eq 7) starting from the MEA carbamate and the protonated MEA acting as the proton donor

step, INT1, a relatively strong base, abstracts a proton with an activation energy of only 5.4 kcal/mol from the nearby water molecule.28 The resulting product (INT2) can be informally considered as a zwitterion consisting of MEA and carbonate (H2CO3). In the second step, the hydroxide ion abstracts a proton from INT2 leading to the formation of bicarbonate and free MEA. This is an essentially barrierless process; the negative activation energy comes from the ZPE correction. Therefore, INT1 can be efficiently converted into bicarbonate via the fast reaction represented in eq 6, even when PC5(w2) is close in energy to TS5(w2). As observed for the formation of bicarbonate from the carbamate (eq 4), the formation of the RNHCO(OH)2− species from the precursor carbamate (eq 5) also turns out to be a two-step reaction when three water molecules are incorporated. The first step represents a proton transfer from a H2O molecule to MEA carbamate producing the MEA carbamic acid and OH− (Figure 5a). The energy required to

RNHCOO− + RNH3+ ⇄ RNHCOOH + RNH 2

(7)

The reaction energies for the formation of zwitterion and carbamic acid are 4.5 and 1.9 kcal/mol, respectively (Figure 6). In comparison with the relevant reactions in which H2O acts as a proton donor (initial reactions in Figures 3 and 5), reaction energies for the formation of the zwitterion and the carbamic acid are reduced by 11.2 and 10.9 kcal/mol, respectively, when protonated MEA is the proton source. Corresponding reduction in the reaction free energy (ΔΔG) is determined to be 8.5 kcal/mol from the experimental pKa values of protonated MEA and H2O. In addition, the ΔG for the backreaction of 1b (Figure 1) was predicted to be 8.3 kcal/mol by QM/MM simulations,16 which is larger than the present value of 4.5 kcal/mol. Although our reaction energies cannot be directly compared with the free energies, it is likely that our computations underestimate the reaction energies by ∼4 kcal/ mol for both the formation of the zwitterion (back-reaction of 1b, Figure 1) and the carbamic acid (eq 7). By analogy with the hydrolysis of amides,28−30 the generated carbamic acid (eq 7) can undergo an attack by the nucleophilic OH− leading to the formation of the tetrahedral intermediate RNHCO(OH)2− (INT1, eq 8)

Figure 5. (a) PCM-MP2/6-311++G(d,p) optimized reaction geometries for the first-step in the neutral hydrolysis of MEA carbamate (eq 5) involving three water molecules. Bond distances are given in angstroms. (b) PCM-CCSD(T)/6-311++G(2df,2p) calculated potential energy profile on the basis of the optimized geometries shown in panel (a). The energy levels are plotted relative to the separated reactants consisting of MEA carbamate and 3H2O. The values in italics indicate activation energies.

form the carbamic acid (12.8 kcal/mol, Figure 5b) is smaller than that required for the zwitterion (15.7 kcal/mol, Figure 3b) and is consistent with the relative stabilities of these species. The second step represents a base-catalyzed hydration of carbamic acid. Because PC5a(w3) is higher in energy than TS5a(w3) after ZPE correction, we estimate the effective energy barrier for the overall reaction as an energy difference between TS5b(w3) and RC5a(w3) that amounts to 37.1 kcal/ mol (Figure 5b). Thus, incorporation of even three molecules of water does not lead to an improved catalytic effect, consistent with similar observation made for the formation of bicarbonate (eq 4). 9278

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Figure 7. (a) PCM-MP2/6-311++G(d,p) optimized reaction geometries for the addition of OH− to MEA carbamic acid (eq 8) in which the OH− is solvated by two water molecules. Bond distances are given in angstroms. (b) Potential energy profiles obtained at the PCMCCSD(T)/6-311++G(2df,2p) level on the basis of the optimized geometries shown in panel (a) together with the corresponding diagram for the case of bare OH−. The energy levels are plotted relative to the separated reactants consisting of MEA carbamic acid and either OH− or OH−(H2O)2. The values in italics indicate activation energies.

mol. Even when the above-mentioned correction for the reaction in eq 7 is considered, this pathway would potentially be the most effective route connecting carbamate and bicarbonate without calling for the intermediacy of a free CO2. However, it should be noted that the contribution of this pathway is dependent on the concentration of carbamic acid in alkaline solutions as the OH− ion will transform it back to the carbamate. Pathways for the Interconversion of Carbamate and Bicarbonate. Pathways for the direct conversion between carbamate and bicarbonate as suggested by the present calculations are summarized in Table 1 with the activation

Figure 6. PCM-MP2/6-311++G(d,p) optimized reaction geometries for (a) the back-reaction of 1b (Figure 1) and (b) the reaction of eq 7. Bond distances are given in angstroms. (c) PCM-CCSD(T)/6-311+ +G(2df,2p) calculated potential energy profiles on the basis of the optimized geometries shown in panels (a,b). The energy levels are plotted relative to the separated reactants consisting of MEA carbamate and protonated MEA. The values in italics indicate activation energies.

RNHCOOH + OH− ⇄ RNHCO(OH)2−

Table 1. Pathways for the Direct Interconversion of Carbamate and Bicarbonate with the Activation Energies for the Rate-Determining Steps Calculated by the CCSD(T)/6311++G(2df,2p)//MP2/6-311++G(d,p) Method Combined with the PCM Solvation Model

(8)

In our computational model (Figure 7a), the OH− ion forms hydrogen bonds with two water molecules, a scenario that is similar to the second-step of the reaction shown in Figure 5 (TS5b(w3)). We have also calculated the corresponding reaction energies for the naked OH− and found that the solvation of OH− increases the energy barrier by 3.7 kcal/mol. In this regard, we observe an increase in the desolvation of OH− with the progress in reaction, a feature observed in the theoretical studies of reaction in eq 3.38−40 On the basis of the calculated activation energies, the direct addition of OH− (TS8(w2), 18.1 kcal/mol, Figure 7) to the carbamic acid is more effective than the base-catalyzed hydration (TS5b(w3), 23.6 kcal/mol, Figure 5). INT1 can be readily transformed into bicarbonate via the two-step reaction of eq 6 (Figure 4b). As a consequence, a new pathway for the decomposition of carbamate into bicarbonate in alkaline condition emerges as a combination of the reactions represented by eqs 6−8. Because of the metastable nature of the carbamic acid (PC7 is higher in energy than TS7, Figure 6), we evaluate the effective energy barrier for the present pathway as a sum of the reaction energy of the reaction in eq 7 (1.9 kcal/mol) and the activation energy of reaction in eq 8 (18.1 kcal/mol) that amounts to 20.0 kcal/

activation energies for the rate-determining steps (kcal/mol) pathways

reactions

a b c

eq 4 eqs 5, 6 eqs 7, 8, 6

RNHCOO− → HCO3− TS4(w2): 37.3 TS5(w2): 35.9 20.0 (PC7: 1.9 + TS8(w2): 18.1)

HCO3− → RNHCOO− TS4(w2): 30.9 TS6b(w1): 29.8 TS6b(w1): 29.8

energies predicted for each rate-determining step. The dissociation of MEA carbamate via neutral hydrolysis is associated with a rather large activation energy (paths a and b). The alkaline hydrolysis (path c) proceeds more quickly if MEA carbamate could be effectively converted into carbamic acid by accepting a proton from protonated MEA. The reverse process (formation of carbamate from bicarbonate and MEA) is predicted to be slightly faster than the neutral hydrolysis of MEA carbamate. 9279

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dissociation and is consistent with the experimental rate constants: k9 = 6.0 × 10−3 (M−1s−1) and k−9 = 8.3 × 10−5 (s−1). The contribution of pathway c to the dissociation process of MEA carbamate depends on the concentration of protonated MEA (which in turn can transfer a proton to MEA carbamate, thereby, producing carbamic acid). The high pH and low initial concentration of MEA (0.035 M)11 would make the pathway c less probable than pathways a and b. However, for absorbents used in practice, the concentration of MEA is much higher and a significant amount of protonated MEA is produced via the fast CO2 absorption (eq 1). Under these conditions and a moderately high pH, the pathway c would be more efficient than the pathways a and b. On the other hand, the reactions listed in Table 2 constitute indirect conversion pathways between carbamate and bicarbonate that proceed via free CO2(aq) with much smaller activation energies than the direct pathways (a−c). Due to the intrinsically low solubility of CO2, the contributions from indirect pathways would be prominent at high CO2 pressures, while at low pressure, the contributions of direct pathways would be larger than those at high pressure.

For a comprehensive analysis of the interconversion pathways, the intermediacy of free CO2 should also be considered. As described above, the back-reaction of 1b (Figure 1) produces the zwitterion which may dissociate into CO2 and MEA via the back-reaction of 1a (Figure 1). Since our computational results for the reaction represented by 1a are essentially the same as those reported in the literature,15−17,22 we have only reported the activation energies (Table 2). Table 2. Reactions of Equations 1, 2, and 3 That Constitute Indirect Interconversion Pathways of Carbamate and Bicarbonate with the Activation Energies for the RateDetermining Steps Calculated by the CCSD(T)/6-311+ +G(2df,2p)//MP2/6-311++G(d,p) Method Combined with the PCM Solvation Model activation energies for the rate-determining steps (kcal/mol) reactions

CO2 absorption

CO2 desorption

eq 1 eq 2 eq 3

TS1a: 6.6 TS2: 15.3 TS3(w2): 4.9

9.9 (RC1b: 4.5 + TS1a: 5.4) TS2: 15.5 TS3(w2): 17.0

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CONCLUSIONS Using ab initio methods combined with the PCM solvation model, we have investigated the direct interconversion of carbamate and bicarbonate for the MEA/CO 2 system represented collectively as a reversible hydrolysis of carbamate (eq 4). We have evaluated both concerted and stepwise mechanisms for the neutral hydrolysis of MEA carbamate and predicted rather large activation energies (∼36 kcal/mol) and insignificant catalytic enhancements with the addition of excess water molecules in both mechanisms. We have also examined the alkaline hydrolysis of MEA carbamate, taking the high concentration of amine (high pH) in practical absorbents into account. In our calculations, the addition of OH− ion to carbamate anion was not theoretically allowed when the nucleophilicity of OH− was reduced by microsolvation. We suggest an alternative pathway: a proton transfer from the protonated MEA to carbamate generates carbamic acid, which then undergoes an attack by OH− and subsequent low-barrier reactions lead to the formation of bicarbonate and free MEA. On the basis of the calculated activation energies, this pathway would be the most efficient route for the interconversion of carbamate and bicarbonate without invoking the intermediacy of free CO2, while the actual contributions are dependent on the concentrations of protonated MEA and OH− ions. The reaction mechanisms explored in the present study contribute to a comprehensive understanding of CO2 capture processes in the context of the concentration of the amine, pH, and partial pressure of CO2.

Complete results are provided in the Supporting Information (Figure S2). Because of the metastable nature of the MEA zwitterion (RC1b is higher in energy than TS1b, Figure 6), an effective barrier for the decomposition of MEA carbamate into free CO2 is evaluated as a sum of the reaction energy of the reverse reaction represented by 1b and the activation energy for the reverse reaction in 1a: 4.5 + 5.4 = 9.9 kcal/mol (Figure 1). As mentioned above, the former would be underestimated by ∼4 kcal/mol on the basis of the comparison with the QM/MM free energy determinations.16 The free CO2 produced via the reverse reaction of eq 1 may undergo a formation of bicarbonate via the reactions described by eqs 2 and 3. The predicted activation energies for these reactions are listed in Table 2, while the complete results are provided in the Supporting Information (Figures S3 and S4). As can be seen in Table 2, the energy barrier for the reaction in eq 3 is much lower than that for the reaction in eq 2. Although the concentration of OH− is much lower than that of amine, the reaction in eq 3 will have a pronounced effect on the kinetics of the bicarbonate formation at high pH (high amine concentration). The reverse reactions of eqs 2 and 3 followed by the direct CO2 absorption (eq 1) constitute pathways that transform bicarbonate into carbamate. The kinetics for the dissociation of MEA carbamate into bicarbonate and the reverse process, that is, formation of MEA carbamate from bicarbonate and MEA, have been experimentally investigated.11 In the reduced model, the formation and dissociation of MEA carbamate was described by the forward and reverse reactions of eq 1, respectively, with the rate constants k7 and k−7 (the notations used in ref 11 have been retained in the current discussion). However, the measured rates of formation and dissociation of MEA carbamate could not be simulated with the reduced model. A refined model that further incorporated the reversible reactions of eq 4 with the rate constants denoted by k9 and k−9 for the formation and dissociation of MEA carbamate, respectively, satisfactorily reproduced the measured data, highlighting the significance of the reactions in eq 4. We attribute the reactions associated with k9 and k−9 rate constant to either the path a or b (especially b for MEA). In both the pathways, the calculated energy barrier for the formation of MEA carbamate is lower than that for its

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4 and complete reference 32. 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. 9280

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ACKNOWLEDGMENTS This work was financially supported by the COURSE50 project founded by the New Energy and Industrial Technology Development Organization, Japan.

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dx.doi.org/10.1021/jp406636a | J. Phys. Chem. A 2013, 117, 9274−9281