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Review 2

Fundamental Understanding of CO Capture and Regeneration in Aqueous Amines from First-Principles Studies: Recent Progress and Remaining Challenges Haley M. Stowe, and Gyeong S Hwang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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Industrial & Engineering Chemistry Research

Fundamental Understanding of CO2 Capture and Regeneration in Aqueous Amines from First-Principles Studies: Recent Progress and Remaining Challenges Haley M. Stowe a and Gyeong S. Hwanga,b,* a

Materials Science and Engineering Program and bMcKetta Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, USA.

Aqueous amine-based chemical scrubbing has been considered the most promising near-term solution for CO2 capture from flue gas. However, its widespread implementation is hindered by the high cost associated with the parasitic energy consumption during solvent regeneration, along with degradation and corrosion problems. Computer simulations have been widely used to improve our fundamental understanding of CO2 absorption materials and processes in efforts to design and develop high-performance, cost-effective solvents. Here, we review recent progress in first-principles studies on molecular mechanisms underlying CO2 absorption into aqueous amines and solvent regeneration. We also briefly discuss aspects which remain unclear such as degradation and corrosion mechanisms, and the reaction-diffusion behavior of CO2 at the solvent-gas interface. This review highlights the increasingly significant role of first-principlesbased atomistic modeling in exploring the function and properties of candidate materials as well as the complex physicochemical phenomena underlying CO2 capture, solvent degradation, and corrosion, especially when direct experimental characterization at the atomic level may be difficult.

*Author to whom correspondence should be addressed: Tel: 1-512-471-4847, Fax: 1-512-471-7060, E-mail: [email protected]

Keywords: atomistic modeling, quantum mechanics, molecular mechanism, solution structure and dynamics, degradation and corrosion

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I.

INTRODUCTION The observed warming of the Earth’s climate since the mid-20th century is largely attributed

to the rapid growth of anthropogenic CO2 emissions. Global warming is expected to have enormous negative societal impacts such as reduced potable water availability and agricultural production, in addition to sea level rise.1 Since it is predicted that most of the increase in greenhouse gas emissions over the next few decades will be from growing electricity use worldwide, of which coal is expected to remain a dominant fuel, there have been ongoing efforts to reduce CO2 emissions from fossil fuel power plants.2–4

Figure 1. CO2 capture from flue gas by aqueous amine in the absorber and solvent regeneration in the desorber (stripper). Chemical absorption by aqueous amine-based solvents appears the most promising nearterm solution for CO2 capture from flue gas because of their ability to capture CO2 at low pressure with adequate absorption/desorption kinetics.5,6

In particular, monoethanolamine

(MEA) is considered the benchmark solvent and has been the most extensively studied. Figure 1 illustrates the CO2 capture process using amine scrubbing. The flue gas flows countercurrently to the aqueous amine solvent in the absorber, where CO2 is removed via chemical reaction into


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the liquid stream at about 40 °C. This liquid stream then goes to a stripper using water vapor and operating at 100 to 120 °C; here, the solvent is regenerated as CO2 is released into the gas stream. Then, pure CO2 can be produced when water is condensed out of the gas stream, followed by CO2 compression and storage. Unfortunately, widespread implementation of these CO2 capture systems has been limited due to the high cost associated with the high parasitic energy consumption during regeneration, solvent degradation, and corrosion.2,5–9 Some fundamental aspects underlying the CO2 reaction with amine-based aqueous solvents remain unclear, even though an accurate description of these chemical processes and relevant physical properties would be essential in efforts to optimize their performance. This is largely due to the difficulty of direct experimental characterization of the complex reaction-diffusion behavior on the atomic scale. First-principles-based molecular modeling can play a significant role in assisting efforts to design better solvents and improve the efficiency of commercial-scale applications.

A theoretical approach can be used to elucidate the molecular mechanisms

underlying CO2 capture and solvent regeneration.10–12 An improved fundamental understanding of these reaction mechanisms and intermolecular interactions can be used to provide explanations for experimental observations, and may provide valuable hints on how to improve the performance of existing solvents and in the rational design and synthesis of more energy- and cost-efficient solvent materials. It can also be used to provide fundamental data and improve kinetic and thermodynamic models for process optimization. In this paper, we review recent theoretical studies on the molecular mechanisms underlying CO2 capture by aqueous amines and solvent regeneration. First, we present a fundamental understanding obtained from first-principles calculations regarding the elementary reaction steps and intermediates during CO2 capture to form carbamate and bicarbonate as well as solvent



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regeneration, emphasizing that an explicit description of the solution structure and dynamics is crucial.

We also present quantum chemical studies of the effects of substituents on

thermodynamic properties such as pKa values, reaction enthalpies, and carbamate stabilities. Thereafter, we discuss the role of classical force field simulations in evaluating transport properties of amines and CO2 in aqueous solution and the dynamic processes during CO2 capture/regeneration. Opportunities of first-principles based studies in improving understanding of solvent degradation and corrosion mechanisms are also briefly discussed.

This review

highlights the value of a first-principles-based computational approach in improving fundamental understanding of the complex phenomena underlying CO2 reaction-diffusion behavior in aqueous amine-based solvents, and its complementary role in efforts to improve performance of these systems. II.

CO2 CAPTURE REACTION MECHANISMS

2.1. Carbamate Formation

Figure 2. solution

Proposed two-step zwitterion mechanism during CO2 capture in aqueous amine


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During CO2 capture in aqueous solution by most primary and secondary amines, two amine molecules react with one CO2 molecule to form a carbamate ion and a protonated amine (i.e. 2MEA + CO2 → MEACOO- + MEAH+), limiting the loading capacity to 0.5 mol CO2 per mol amine. This has been proposed to occur via a single-step termolecular (direct) or two-step zwitterion mechanism. Figure 2 shows the two-step process, whereby a zwitterion intermediate is formed, followed by deprotonation to another amine.13,14 Figure 3 illustrates the proposed single-step termolecular mechanism, in which the amine, CO2 and base molecules form a complex and the amine-CO2 and proton transfer reactions occur simultaneously.15

Figure 3. Proposed one-step termolecular mechanism for CO2 capture in aqueous amine solution To evaluate the most likely reaction mechanism of CO2 with MEA, a static approach based on quantum mechanical (QM) calculations with implicit solvent has been typically used. However, predicted activation barriers and reaction energetics tend to vary widely depending on the initial configurations used.

Table 1 displays predicted free energy barriers (∆G‡) for

carbamate formation from MEA using the implicit solvent approach, which vary from essentially barrierless (0.10 kcal/mol) to about 13 kcal/mol.16–25 Furthermore, even when using the same basis set and theory level (for example, B3LYP/6-311++G(d,p) with the SMD solvation model), ∆G‡ varies from 3.2 to 7.0 kcal/mol. In these studies, the origin of the barrier was typically the interaction between CO2 and the amine, while direct proton transfer to the N site of the other amine is predicted to be virtually barrierless.16,17,20,22,24



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Table 1. Free energy barriers (∆G‡, in kcal/mol) predicted from static QM calculations for carbamate formation from 2 MEA and 1 CO2 molecules (2MEA + CO2 → MEACOO- + MEAH+). ∆G‡ (kcal/mol) Basis Set 4.6 6-311++G(d,p) 12.9 6-311+G(2d,2p) 6.5 6-31G** 9.6 6-31G** 5.3 6-311++G(d,p) 4.1 6-311++G(d,p) 4.1 6-311++G(d,p) 6.9 6-311++G(d,p) 10.5 6-311++G(d,p) 5.97 6-31G+(d) 3.91 6-311G+(d,p) 3.56 6-31G+(d) 2.36 6-311G+(d,p) 5.67 6-31G+(d) 1.88 6-311G+(d,p) 2.70 AUG-cc-pVTZ 0.10 AUG-cc-pVTZ 1.45 AUG-cc-pVTZ 0.90 AUG-cc-pVTZ 1.23 AUG-cc-pVTZ 2.05 AUG-cc-pVTZ 7.0 6-311++G(d,p) 3.2 6-311++G(d,p) 12.0 6-311++G(d,p) 7.2 6-311++G(d,p) *PCM with several explicit water models20

Theory B3LYP SCS-MP2 B3LYP MP2 MPW1K M06-2X M08-HX BB1K BMK B3LYP B3LYP M06-2X M06-2X MP2 MP2 MP2 M06-HF M05-2X B3LYP-GD2 B3LYP-GD3 M06-2X B3LYP B3LYP B3LYP B3LYP

Solvent Model IEF-PCM SMD IPCM IPCM SM8 SM8 SM8 SM8 SM8 PCM* PCM* PCM* PCM* PCM* PCM* PCM PCM PCM PCM PCM PCM SMD SMD CPCM SMD

Reference 17 18 16 16 19 19 19 19 19 20 20 20 20 20 20 21 21 21 21 21 21 22 23 24 25

Da Silva and Svendsen determined the zwitterion to be unstable, leading them to speculate that either CO2 capture follows the termolecular mechanism or the zwitterion is a very short-lived transition state.26,27 However, Xie et al. predicted all vibrational frequencies of the zwitterion to be positive, suggesting it is a stable intermediate.24 Sumon et al. found that the addition of several explicit water molecules to the implicit solvent model tends to decrease the free energy barriers and the carbamate energies relative to the reactants. This implies that water


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molecules act to stabilize the zwitterion as well as the carbamate, and that the hydrogen-bonding interactions between the solutes and surrounding water may not be adequately represented by the implicit solvent model.28

Figure 4. AIMD snapshots showing the elementary reaction steps during CO2 capture in aqueous MEA (a-b) and MEA regeneration (c).10 System contains 20 H2O, 4 MEA, and 2 CO2 molecules in a cubic simulation box of edge length 10.0 Å, representing approximately 40 wt% aqueous MEA solution. Blue, red, gray and white balls represent N, O, C and H, respectively. Solvent is represented by white isosurface. Blue dotted lines highlight hydrogen bonds prior to proton transfer. More recently, ab initio molecular dynamics (AIMD) simulations based on density functional theory (DFT) with explicit solvent molecules has been used to identify likely reaction steps and intermediates. These simulations consistently demonstrate that reaction between MEA and CO2 proceeds via the two-step mechanism with a zwitterion intermediate, as illustrated in Figure 4.10,29–31 After the CO2 approaches N in MEA and reacts to form the zwitterionic adduct



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(MEA+COO-) [Fig 4(a)], the solvated proton undergoes rapid migration following a Grotthuslike mechanism until it is abstracted by N in another MEA to form carbamate (MEACOO-) and protonated amine (MEAH+) [Fig 4(b)]. These suggest the deprotonation process to have an insignificant barrier, provided the proton in the zwitterion is well-connected through the hydrogen-bonded water network to another available base site.10 Otherwise, the zwitterion intermediate can remain in aqueous solution on the order of 100s of ps.31 If another MEA is nearby the zwitterion, direct proton transfer can occur.29 However, solvation of the zwitterion is thought to be more likely in typical amine concentrations used (~ 30 wt%).10,32 These results suggest that AIMD may better evaluate the effects of the solution structure and dynamics on progression of the amine-CO2 reaction in aqueous solution. Furthermore, including the solvent environment explicitly is crucial to describe proton transfer through the hydrogenbonded water network. AIMD simulations have also elucidated the reaction mechanisms underlying CO2 removal (i.e., solvent regeneration).10,30,32 As shown in Fig. 4(c), we first observe formation of carbamic acid (MEACOOH) due to proton transfer from N of MEAH+ to O of MEACOO-. As O sites tend to be less basic than N atoms,12 deprotonation will subsequently occur until the proton reaches N of MEACOO- to form the zwitterion. If the zwitterion is not well-connected to another base site through the water network, deprotonation may not occur and the zwitterion can be relatively stable until CO2 is released. These results suggest the arrangement and availability of water molecules around the zwitterion may affect the relative probability between the competitive CO2 desorption and deprotonation routes.10 Enhanced sampling methodology (AIMD and metadynamics) has also been used to construct the free energy profiles describing CO2 capture and release from the zwitterion.30,32,33


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Guido et al. predicted the free energy barriers between CO2 release (solvent regeneration) and deprotonation (CO2 capture) from the zwitterion to be between 6 and 8 kcal/mol.30 The similar barriers imply the two routes are competitive. The authors also stated that inclusion of explicit solvent is crucial as the water molecules beyond the first solvation shell also participate in the reactions.

Furthermore, entropic effects such as rearrangement of the water molecules to

stabilize the intermediate are predicted to lower the free energy barrier.30 Ma et al. used a similar approach to evaluate the free energy barriers during the entire CO2 capture and release processes including proton transfer, and determined zwitterion formation to have an energy barrier of 12 ± 3 kcal/mol.32 Note that this tends to be larger than the predicted barriers from static QM calculations in Table 1, which may be attributed to breaking of hydrogen bonds of water surrounding solvated CO2 prior to it approaching the N site that would not be present in a model using implicit solvent.27,32 The free energy barriers for CO2 release and deprotonation from the zwitterion were again predicted to be between 7 and 10 kcal/mol, and these values were predicted to not change significantly from dilute (3 wt% MEA) to 30 wt% MEA aqueous solution.32

Based on predicted free energy barriers for each step in the CO2 capture and

regeneration processes, the authors claimed the rate-limiting steps to be zwitterion formation from solvated CO2 and MEA (14-15 kcal/mol) and deprotonation of MEAH+ (16 kcal/mol), respectively.32 Static QM calculations have also been used to predict the basicity (pKa) of various alkanolamines and amines, as the pKa values are thought to be directly related to absorption rate. As shown in Figure 5, Versteeg et al. found linear correlations between the logarithm of the absorption rate (k, in m3/mol/s) and the pKa values for aqueous amines which predominantly form carbamate.34 However, some amines such as the diamines piperazine (PZ) and hydrazine



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(HZ) have much higher absorption rates than predicted from their basicities. These systems will be reviewed next.

Figure 5. Observed Bronsted relation between logarithm of the CO2 absorption rate (k, in m3/mol/s) and basicity (pKa) for amines which predominantly form carbamates. DIPA = diisopropanolamine,34 MIPA = monoisopropanolamine,34 MEA = monoethanolamine,35 PZ = piperazine,36 HZ = hydrazine.35 Dotted line is correlation generated by Versteeg (1988).34 2.2 CO2 Capture in Aqueous Diamines Aqueous PZ, a cyclic diamine with two secondary amine groups, has been proposed as an effective solvent for CO2 capture due to its many advantages such as low regeneration energy, low rates of thermal and oxidative degradation, and low corrosivity relative to other widely used amines including MEA.37–39 In addition, PZ exhibits a relatively high absorption rate. However, due to its low solubility in aqueous solution, its typically used in small concentrations (~5-10 wt%) in aqueous amine blends as a rate promoter.40,41 Recent studies have also proposed concentrated PZ (~30 wt%) as a solvent due to the decrease in risk of solids precipitation under CO2-loaded conditions.37,38


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AIMD simulations have demonstrated CO2 capture by aqueous PZ occurs via the two-step zwitterion mechanism to form piperazine carbamate (PZCOO-) and protonated PZ (PZH+), as described earlier for aqueous MEA.12,31 Since there are two basic N atoms in PZ, PZCOO- and PZH+ may further participate in CO2 capture, increasing the absorption capacity to 1 mol CO2 per mol amine. They may react with CO2 to form dicarbamate (COO-PZCOO-) or protonated carbamate (H+PZCOO-), respectively. Likewise, PZCOO- and PZH+ may participate in the protonation/deprotonation processes.

While some studies have assessed PZ’s basicity and

carbamate/bicarbonate formation energies using static QM calculations with implicit solvent, an understanding of the relative roles played by PZH+ and PZCOO- were lacking.27,42–46

Figure 6. Reaction routes for absorption into aqueous PZ confirmed or predicted.12 Very recently, we used combined DFT and classical force field calculations to elucidate the molecular mechanisms underlying CO2 absorption into aqueous PZ, including the roles of PZCOO- and PZH+.12

The probable routes that we proposed based on our findings are

summarized in Figure 6. From AIMD simulations combined with radial distribution analysis, we



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confirmed that PZ and PZCOO- may be protonated, while protonation of PZH+ hardly occurs. As the available basic sites are well-linked via the hydrogen-bonded water network, the relative concentrations of protonated/deprotonated amine species can be predicted from their relative basicities. We also find the probability of H+PZCOO- formation to increase with CO2 loading. This is not only because of the greater availability of PZ relative to PZCOO-, but also because of the thermodynamic favorability of proton transfer from PZH+ to PZCOO- (i.e., PZH+ + PZCOO→ PZ + H+PZCOO-). Regarding the tendency of PZH+ and PZCOO- to react with CO2, we found that while PZCOO- can easily react with CO2 forming COO-PZCOO-, the direct CO2 reaction with PZH+ hardly occurs. Rather, our AIMD simulations consistently demonstrate that the proton is first released from PZH+, followed by the PZ + CO2 reaction forming PZCOO-, which can then easily abstract a proton to form H+PZCOO-. This behavior has largely been attributed to the low pKa of PZH+. However, our spatial distribution analysis using classical molecular dynamics (MD) simulations suggests that H2O tends to more densely pack around PZH+, which may hinder CO2 accessibility relative to PZ/PZCOO-, further suppressing the PZH+ + CO2 reaction.12 While CO2 capture by PZCOO- appears to be kinetically possible, our free energy calculations predict COO-PZCOO- formation to be highly unlikely at thermodynamic equilibrium relative to the monocarbamates.12 However, the formation of COO-PZCOO- at high CO2 loadings has been reported from nuclear magnetic resonance (NMR) measurements, albeit in small concentrations.36,47 Analysis of the distributions of PZ/PZCOO-/COO-PZCOO- near the gas-liquid surface shows that although PZ accumulates at the gas-solvent interface, PZCOO- also remains near the surface.12 This hints that while PZ may predominantly capture CO2, the PZCOO- + CO2 reaction to form COO-PZCOO- may be more likely to occur with increasing CO2


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loading as PZ is depleted. Based on our results, we speculate that COO-PZCOO- is mostly produced from the PZCOO- + CO2 reaction near the gas-solvent interface, rather than interconversion with the monocarbamates (PZCOO-/H+PZCOO-) in bulk solution. We also suggest that composition at the gas-liquid interface may be highly important in modeling and predicting the performance of aqueous amines, especially in cases where multiple amines are involved in CO2 capture. Typical two-film models employed to describe CO2 reaction and diffusion at the gas-solvent interface in modeling of CO2 capture by aqueous amines are described in Section IV. Lee et al. have recently demonstrated that aqueous hydrazine (NH2NH2), the simplest diamine, can be a promising solvent for CO2 capture due to its high absorption rate, large working capacity, relatively easy regeneration, and high thermal stability.48

Moreover,

hydrazine (HZ) has a high boiling point and is completely miscible with water; it is also relatively safe in aqueous solution. Similarly to PZ, HZ has two available basic N sites that can participate in CO2 capture. However, the reaction mechanisms were not well established until very recently, largely due to difficulties in identifying reaction products as they are in dynamic equilibrium. Using first-principles calculations combined with 13C and 15N NMR spectroscopy, we have elucidated the reaction mechanisms of CO2 absorption by aqueous HZ.49 The study revealed important CO2 capture behavior which appears to be unique relative to other diamines at typical concentrations used. While both mono-carbamates and di-carbamate are major products, the latter is found to be the dominant product at relatively high CO2 loadings while the former are predominant at low CO2 loadings; note that this has not been observed for aqueous PZ, as



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discussed above.

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In addition, reversible precipitation of mono-carbamate becomes more

probable with increasing CO2 loading. Our first-principles calculations predicted similar thermodynamic favorabilities for the hydrazine mono- and di-carbamates, and two-dimensional exchange NMR and isotopic labeling experiments demonstrated facile interconversion between them.49 These results suggest that all products are in dynamic equilibrium under typical operating conditions. Thus, their relative concentrations are primarily determined by the CO2 loading, or the availability of free (unprotonated) HZ relative to mono-carbamate. Moreover, both mono- and di-carbamates may release CO2 with relatively low energy use, which is essential to reducing the cost of aminebased CO2 capture. This work also emphasizes the value of the combined first-principles and experimental NMR approach as an effective means for exploring the complex reaction dynamics involved in CO2 capture by aqueous diamines.49 2.3 Bicarbonate Formation Bicarbonate (HCO3-) is primarily formed during CO2 capture in aqueous solution by tertiary amines such as methyldiethanolamine (MDEA), and is thought to be formed via basecatalyzed hydration of CO2, i.e. MDEA + H2O + CO2 → MDEAH+ + HCO3-, as proposed by Donaldson and Nguyen.50 This increases the theoretical maximum capacity to 1 mol CO2 per mol amine, but the absorption rate tends to be much slower than in aqueous amines which predominantly form carbamates.51,52 Sterically hindered amines, including 2-amino-2-methyl-1propanol (AMP) and 2-piperidineethanol (2-PE), also tend to form bicarbonate rather than carbamate.53,54 However, as shown in Figure 7, they exhibit absorption rates which are orders of magnitude higher than tertiary amines of similar pKa values.


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Figure 7. Bronsted relation observed between logarithm of the absorption rate (k, in m3/mol/s) to basicity (pKa) for amines which predominantly form bicarbonate. TEA = triethanolamine,55 MDEA = methyldiethanolamine,56 DMMEA = dimethylethanolamine,56 DEEA = diethylethanolamine,57 AMP = 2-amino-2-methyl-1-propanol,35 2-PE = 2-piperidinethanol,58 TREA = triethylamine.56 Dotted line is correlation generated by Versteeg (1988).56 On the basis of the much higher absorption rate of AMP relative to tertiary amines, it has been speculated that first AMP and CO2 react to form carbamate (AMPCOO-), which undergoes hydrolysis to form bicarbonate due to its possible instability (i.e., AMPCOO- + H2O → HCO3- + AMPH+).59,60 Several studies have attempted to predict the thermodynamic favorability of bicarbonate formation for AMP (as compared to MEA) using static QM calculations in implicit solvent. As shown in Table 2, these results are widely scattered, but carbamate formation is typically predicted to be more thermodynamically favorable than bicarbonate formation for both MEA and AMP.11,17–19,61 Furthermore, free energy barriers for the hydrolysis of both AMPCOOand MEACOO- to HCO3- are typically predicted to be high and also comparable, as shown in Table 3, implying that not only MEACOO- but also AMPCOO- are relatively stable and may not



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easily undergo hydrolysis to form HCO3-.17,22,61,62 Rather, it would be more likely that the carbamates would revert back to the amine and CO2.27 Table 2. Free energy changes (∆Gaq, in kcal/mol) for formation of bicarbonate relative to carbamate (also referred to as carbamate stabilities) for AMP and MEA predicted from static QM calculations in implicit solvent. AMPCOO- + H2O → AMP + HCO3∆Gaq (kcal/mol) 1.10 -2.54 10.5 11.1 -2.28

MEACOO- + H2O → MEA + HCO3∆Gaq (kcal/mol) 2.10 2.08 9.80 12.9 2.35

Basis Set

Theory

Solvent Model

6-311++G(d,p) 6-311++G(d,p) 6-311++G(d,p) 6-311++G(2d,2p) 6-311++G(d,p)

B3LYP B3LYP M06-2X SCS-MP2 B3LYP

SM8T IEF-PCM SM8 SMD SMD

Reference 61 17 19 18 11

Table 3. Free energy barriers (∆G‡aq, in kcal/mol) for bicarbonate formation via carbamate hydrolysis for AMP and MEA predicted from static QM calculations in implicit solvent AMPCOO- + H2O → AMP + HCO3∆G‡aq (kcal/mol) 41.0 40.0 40.8 36.4

MEACOO- + H2O → MEA + HCO3∆G‡aq (kcal/mol)

40.5 37.6

Basis Set

Theory

Solvent Model

6-31G(d,p) 6-311++G(d,p) 6-311++G(d,p) 6-311++G(d,p)

B3LYP B3LYP B3LYP B3LYP

SMD/IEF-PCM SMD/IEF-PCM SMD/PCM IEF-PCM

Reference 62 62 22 17

AIMD simulations also demonstrate that AMPCOO- can remain intact for ~100 ps at extremely high temperatures.11

As the simulation time is limited, this does not indicate

AMPCOO- cannot convert to bicarbonate, but it at least suggests AMPCOO- is not so unstable. Moreover, since the energy barriers for the interconversion between carbamate and bicarbonate are predicted to be quite similar in aqueous AMP and MEA (see Table 3), it does not appear that AMPCOO- would undergo hydrolysis more quickly than MEACOO-. These results suggest that although the carbamates may eventually convert to bicarbonate via hydrolysis, thermodynamic


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equilibrium may not be easily reached at typical absorber operating conditions. Therefore, the relative concentrations of bicarbonate and carbamate may not be predicted only by their thermodynamic favorabilities.

Figure 8. AIMD snapshots demonstrating base-catalyzed hydration of CO2 by AMP in aqueous solution (AMP + H2O + CO2 → AMPH+ + HCO3-). System contains 21 H2O, 2 AMP, and 2 CO2 molecules in a cubic simulation box of edge length 9.91 Å. Blue, red, gray and white balls represent N, O, C and H, respectively. Solvent is represented by white isosurface. Our AIMD simulations demonstrate frequent occurrence of the AMP + H2O → AMPH+ + OH- reaction, rather than formation of carbamate (i.e., 2AMP + CO2 → AMPCOO- + AMPH+).10,11 If OH- does not abstract a proton to form H2O, bicarbonate can be formed via the OH- + CO2 → HCO3- reaction.11,63 This can also occur in one step, via the amine-catalyzed hydration of CO2 proposed by Donaldson and Nguyen50, as shown in Figure 8 (i.e., AMP + H2O + CO2 → AMPH+ + HCO3-). These results suggest that bicarbonate formation may be more kinetically favorable relative to carbamate formation in aqueous AMP. Radial/spatial distribution analyses indicate that H2O tends to have a more localized distribution around N of AMP (NAMP) than N of MEA (NMEA).11 This may be attributed to the stronger interaction between NAMP and H of H2O (HH2O), as well as the reduction in available volume for H2O to occupy near NAMP due to the presence of the bulky methyl groups. As shown in Figure 9, the enhanced packing of H2O suppresses the accessibility of CO2 to NAMP, relative to the MEA case. This implies that CO2 may more easily approach NMEA than NAMP, thereby enabling the 2 MEA + CO2 → MEACOO- + MEAH+ reaction, whereas this reaction may be



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significantly impeded by solvation of N in aqueous AMP.

Page 18 of 39

Others have also predicted

accessibility of CO2 to be reduced for AMP relative to MEA in aqueous solution from classical MD simulations.64,65 These analyses demonstrate that the solvent structure may significantly influence the CO2 capture behavior in aqueous amines.

Figure 9. Radial distribution functions between N in MEA (NMEA) or N in AMP (NAMP) and C in CO2 (CCO2), along with spatial distribution functions of HH2O (red) and CCO2 (yellow) around NAMP/NMEA from classical MD simulations performed at 323 K. Systems contain 1530 H2O, 34 CO2, 170 AMP(MEA) molecules in a cubic periodic simulation box with edge length 42.04 Å (40.41 Å).11 Based on these results, preferential formation of bicarbonate during CO2 absorption into aqueous AMP may be largely attributed to kinetic factors. The strong NAMP-HH2O hydrogen bonding interaction suppresses the reaction of NAMP with CO2 to form carbamate and facilitates


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the bicarbonate reaction route, while thermodynamic favorability of bicarbonate and carbamate are predicted to be similar.11

This highlights that kinetic factors, besides thermodynamic

favorabilities, should be considered when evaluating CO2 capture into aqueous amines. While our study suggests bicarbonate formation tends to be kinetically more probable for AMP relative to MEA in aqueous solution, these analyses do not demonstrate why the rate of bicarbonate formation tends to be at least an order of magnitude higher than tertiary amines of similar basicities.

In a forthcoming work, we evaluate how a greater ease of water

rearrangement around the N site may enhance the bicarbonate formation rate in sterically hindered amines relative to tertiary amines. III.

SUBSTITUENT EFFECTS In efforts to improve fundamental understanding of the relationship between the amine

structure and the CO2 capture performance, several studies exist which evaluate how the electronic and steric properties of various substituents affect thermodynamic properties such as pKa values and reaction energetics, in addition to free energy activation barriers. Static QM calculations generally predict that electron-withdrawing (electron-donating) groups tend to decrease (increase) basicity.27,46,66,67 This has also been observed experimentally, where alcohol groups and alkyl groups tend to decrease and increase pKa values, respectively. For example, ethylamine has a higher pKa than MEA (which contains an alcohol group).27 Gangarapu et al. found that methyl groups on the α-carbon of MEA exhibit little influence on basicity, but substitution of F-containing groups on the α- and β-carbons (see Figure 10 for the αand β-carbon positions) tend to decrease basicity, which is attributed to their electronwithdrawing effects.67 Similarly, electron-withdrawing groups such as CH2F, COCH3, and CN were predicted to decrease pKa values of substituted piperazines, while electron-donating groups



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such as CH3, and CH2NH2, were predicted to increase pKa values, although hydrogen bonding effects could complicate these trends.46

Figure 10. Predicted trends when hydrogens α- and β-carbons in MEA are substituted with electron-withdrawing and electron-donating groups. 66–69 Increasing basicity has also been predicted to correlate with increasing exothermicities (∆H) for MEA-derivatives with various substituent groups.68 Electron-withdrawing substituents, such as F-containing groups, have been predicted to decrease ∆H (less exothermic) for carbamate formation, corresponding with their decreasing basicity.

Similarly, electron-donating alkyl

groups were predicted to increase exothermicity, albeit with a smaller influence;66 this was predicted in absence of implicit solvent. Lee and Kitchen predicted electron-withdrawing groups tend to decrease the magnitude of the enthalpy changes in bicarbonate formation (in addition to carbamate formation), while electron-donating groups and intramolecular hydrogen bond formation were found to increase the exothermicities of these reactions; the bicarbonate


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formation energy was also shown to correlate with electronegativity of the amine.69 Based on these results, it was suggested that solvent regeneration would require less energy as basicity (and correspondingly exothermicity) decreases.66,67 The Gibbs free energy for carbamate hydrolysis (i.e., ∆G for XCOO- + H2O → X + HCO3where X is an amine) was predicted to decrease substantially with F-containing substituents on MEA, especially when they were placed on the β-carbon instead of the α-carbon.67 Similarly, substitution of electron-withdrawing CH2F, COCH3, and CN groups and electron-donating CH3 and CH2NH2 groups tends to decrease and increase carbamate stability, respectively, in piperazines.46 In addition to electron-withdrawing/donating effects, the carbamate stability was also shown to decrease due to the increase in steric hindrance induced by some substituents, as exhibited by increasing N-C length.46,70 Robinson et al. compared measured absorption capacities and IR spectral data with predicted charge and exposed area on the N atom, as well as N-C bond lengths in the corresponding carbamates to evaluate their stabilities relative to bicarbonate for several functionalized piperidines.71,72 While they also did not identify a single descriptor to predict which amines predominantly form bicarbonate, they suggested the tendency of 2-alkyl and 2-hydroxylalkysubstituted piperidines to form bicarbonate may partly be from a reduction in the exposed area of the N atom as well as lower stabilization of the carbamates due to electronic redistribution.71 Similarly, they also found methyl substitution on piperazines to increase absorption capacity, which was attributed to increased partial negative charge and reduced exposed area on N.72 As discussed previously, not only thermodynamic factors, but also kinetic factors, should be considered when predicting CO2 capture by aqueous amines. Xie et al. evaluated the effects of – CH3, -NH2, -OH, -OCH3 and -F substituents on α- and β-carbons of MEA on the free energy



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barriers (∆G‡) for bicarbonate and carbamate formation. In all cases, ∆G‡ was predicted to be lower for carbamate formation relative to bicarbonate formation.73 However, the difference between the two was shown to decrease with increasing basicity. For both bicarbonate and carbamate formation, ∆G‡ values were predicted to be lower for amines with lower pKa values, albeit with some outliers attributed to hydrogen bonding interactions during the transition state.73 Gangarapu et al. predicted that substitution of alkyl groups on both the α- and β-carbons of MEA were predicted to have little effect on ∆G‡ for carbamate formation, while a substantial decrease was predicted for ∆G‡ of bicarbonate formation.18 While these studies provide valuable insight into the effect of substituents on the relative stabilities of reaction intermediates and free energy barriers, there are several less studied aspects like the structure of the solvent that may have a critical influence on CO2 capture. For example, Jhon et al. found that while predicted global nucleophilicities from static QM calculations correlated with pKa values, accessibility of CO2 to the N sites estimated using classical MD simulations in aqueous solution did not.74 IV. CO2 SOLUBILITY AND DIFFUSIVITY Since CO2 reacts in aqueous amine solution, the diffusivity (DCO2) and Henry’s law solubility constants (HCO2) cannot be experimentally measured despite their importance in development of kinetic and thermodynamic models for predicting/modeling CO2 absorption into aqueous amines. Instead, these properties have been typically estimated using the CO2/N2O analogy, where the diffusivity and Henry’s constant of N2O (DN2O, HN2O) are measured in aqueous amine solution (as it does not react and has a similar molecular structure as CO2). Then, HCO2 and DCO2 are estimated from [HCO2/HN2O]aqueous


amine

= [HCO2/HN2O]water and

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[DCO2/DN2O]aqueous amine = [DCO2/DN2O]water, where it is assumed that the ratio of the properties of CO2 and N2O in water is equivalent to that in aqueous amine.75,76 Diffusion coefficients of CO2 as well as self-diffusivity of MEA can be predicted using classical MD simulations, as chemical reactions will not occur.77,78 Yu et al. estimated DCO2 and viscosities for several ternary and quaternary aqueous amine blends to optimize the transport properties, although they speculate that reaction kinetics should also be considered in improving performance of amine blends.79 Chen et al. predicted HCO2 values using continuous fractional component Monte Carlo simulations, and computed self-diffusivity coefficients from classical MD simulations of CO2 and N2O in 30 wt% aqueous MEA and water to evaluate the validity of the CO2/N2O analogy.80 Their findings suggest the ratios of these properties are indeed similar between water and aqueous amine solution, although individual diffusivity coefficients tended to be predicted as too high.80 This may be expected, as diffusion coefficients and viscosities of water tend to be predicted as higher and lower, respectively, than experimental values, which has typically been attributed to the inaccurate description of polarizability in classical force fields.81 In addition, HCO2 and DCO2 values between the unreacted aqueous MEA and when it has reacted to 0.5 CO2/MEA loading were predicted to have an insignificant difference (~17% and less than 10%, respectively).80 Modeling of CO2 absorption into aqueous amine solutions is typically based on a two-film model where stagnant gas and liquid films are assumed to exist on either side of the gas-solvent interface, as illustrated in Figure 11(a). CO2 diffuses through the gas film and dissolves into the liquid according to Henry’s law (i.e. the gas-liquid interface is at equilibrium), and then reacts with amines in the liquid film. 38,82–85



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Figure 11. (a) Two-film model typically used to describe CO2 diffusion and reaction at the gasliquid interface where HCO2 is the Henry’s gas constant, and PCO2(PCO2*) and CCO2(CCO2*) are the partial pressures and concentrations, respectively, of CO2 in the bulk (at the interface). (b) Predicted near-surface distributions of N of PZ (NPZ), N of PZCOO- (NPZCOO-) and N of PZH+ (NPZH+) from classical MD simulations at 323 K (absorber conditions) with a slab system representing 30 wt% PZ at about 0.5 CO2 loading in bulk solution. The distribution of C of CO2 (CCO2,shaded gray) included in (b) has been adjusted to represent PCO2 = 1 bar Absorption models typically used including the two-film model do not account for a composition change in the solvent near the surface. However, amines are thought to accumulate at the gas-solvent interface, as predicted from classical MD simulations and observed from photoelectron spectroscopy measurements.12,86,87 This is shown by the atomic distributions of N


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in PZ species near the solvent surface in Figure 11(b) predicted from classical MD simulations.12 This in turn implies that the diffusion-reaction behavior near the interface may differ from that in solution. Moreover, thermodynamic quantities typically used in models such as HCO2 may vary due to the near-surface composition change. Therefore, it would be necessary to assess the reaction and diffusion processes at the gas-solvent interface at the molecular level using firstprinciples based atomistic simulations, as it may be difficult to characterize experimentally.

V.

SOLVENT DEGRADATION AND CORROSION Solvent degradation and corrosion are estimated to contribute about (~10 %) to the total cost

of CO2 capture, and some products of degradation may be hazardous if released to the environment.88–90 Several degradation and corrosion inhibitors have been tested in CO2-loaded aqueous amine systems, but the molecular mechanisms underlying their performance remain unclear.90,91 Solvent degradation occurring in CO2-loaded aqueous amines has been classified into two types: thermal degradation and oxidative degradation. Thermal degradation has typically been defined as degradation occurring due to high temperature and high CO2 loading in the stripper.92,93 The products of thermal degradation of primary and secondary amines in aqueous solution are typically oxazolidones and imidazolidones, which are thought to form via cyclization of carbamates.94 Other reported products include diamines, which are thought to form from a reaction between the amines and oxazolidones/imidazolidones, and urea, which is thought to form from a reaction between carbamate and the amine.92–94 Tertiary amines such as MDEA can undergo (de)methylation to form various primary and secondary amines, which may then undergo the degradation mechanisms discussed above.93,95 During thermal degradation of



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piperazine, formation of formic, acetic, and glycolic acids, diamines, imidazolidones, and piperazine derivatives have been reported.93 Oxidative degradation is typically defined as solvent degradation that occurs due to the presence of O2 in the flue gas (~ 3-4%) entering the absorber.93,96 degradation

products

include

ammonia/ammonium,

aldehydes,

Reported oxidative formate/formic

acid,

glycolate/glycolid acid, acetone, piperazinones and oxazolidones.93,97-100 Similar to thermal degradation, oxidative degradation of PZ can form other piperazine derivatives, including some with carbonyl groups, and MDEA can undergo oxidative degradation to form other various amines and alkanolamines.93 Vevelstad et al. predicted the free energy of formation of various oxidative and thermal degradation products of MEA using static QM calculations, and reported most to be thermodynamically favorable.94 Gupta et al. estimated the activation barrier for several possible pathways during MEA degradation, and reported many of them to be kinetically feasible (~17-50 kcal/mol).101 A metal ion, typically Fe2+, which may leach into solution during carbon steel or stainless steel corrosion, must be present for significant oxidative degradation to occur.102-104 Other possible metal ions in solution could be Cr3+, Ni2+, and Mn2+ from stainless steel corrosion, and Cu2+ and V ions from corrosion inhibitors.105 Proposed initiation mechanisms for oxidative degradation include electron abstraction and hydrogen abstraction, whereby a radical abstracts either an electron from the lone pair of N or hydrogen from the amine, respectively.93,97,100 The process underlying both leaching of metal ions into solution due to corrosion, as well as the mechanisms occurring during oxidative degradation, are not well understood. Thermal degradation of CO2-loaded aqueous amines can be mitigated by reducing the operating temperature of the stripper, although lowering the temperature typically increases the


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overall energy required for regeneration.106,107 However, oxidative degradation and corrosion may be difficult to avoid due to the presence of O2 in flue gas. Furthermore, carbon steel is prone to corrosion in CO2-loaded aqueous amine-based solutions, while stainless steel may be prohibitively expensive.93,104 First-principles studies can play a significant role in assessing the reaction mechanisms and intermediates underlying corrosion and degradation, while it would be difficult to experimentally isolate and characterize short-lived reactive species involved. An improved understanding of corrosion and oxidative degradation processes at the molecular level can aid in the design of inhibitors in efforts to improve corrosion and degradation resistance in aqueous amine-based CO2 capture systems.

VI. SUMMARY AND OUTLOOK Recent first-principles studies have provided significant insight into the molecular mechanisms underlying CO2 capture and regeneration in aqueous amine solutions. Static QM calculations with implicit solvent models have been widely used to evaluate the relative stabilities of intermediates and predict thermodynamic quantities such as reaction energetics, including the influence of substituents.

Recently with advances in computing power and

computational methodology, explicit solvent AIMD simulations based on DFT have been applied to explore the reaction dynamics of CO2 in aqueous amines. The explicit solvent approach can better account for the effects of the solution structure and dynamics. Especially, including the solvent environment explicitly is crucial to describe the CO2-amine reactions involving proton transfer through the hydrogen-bonded water network. In addition, the recent development of enhanced MD sampling techniques such as metadynamics allows for precise evaluation of the free energy surfaces for the CO2 capture and regeneration processes. Classical MD simulations can be also effective for predicting the molecular structures of the bulk solution



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and the solvent-gas interface, and for approximating the solubility and diffusivity of CO2 in aqueous amine solutions Despite recent progress, there are several aspects of these systems which remain unclear. Commonly used approximations regarding the reaction and diffusion processes near the solvent surface in modeling of CO2 absorption into aqueous amine solutions may need to be further evaluated.

In particular, the possible effect of near-surface composition variation on

thermodynamic and transport properties, which has been commonly ignored, may warrant investigation. Solvent degradation is estimated to account for about 10% of the total cost of CO2 capture, yet its mechanisms remain uncertain.87,93

Similarly, the mechanisms underlying

corrosion remain unclear. Several types of corrosion and degradation inhibitors have been tested, but their roles are not yet well understood.90 First-principles-based computer simulations will continuously play a key role in the detailed mechanistic study of existing and new solvents for CO2 capture, and the underlying mechanisms of solvent degradation and corrosion. The improved understanding gained from computational studies combined with experiments will also greatly help the design and development of new solvents and inhibitors in efforts to further reduce cost.

ACKNOWLEDGEMENTS This contribution was identified by Prof. Yun Hu (Michigan State University, USA) as the Best Presentation in the symposium “USA-China Symposium on Energy”) of the 2016 ACS Fall National Meeting in Philadelphia, PA. The authors acknowledge support from the Korea CCS R&D Center (KCRC) grant (2016M1A8A1925491) funded by the Korea government (Ministry of Science, ICT & Future Planning) and the R.A. Welch Foundation (No. F-1535)


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Bishnoi,

S.;

Rochelle,

G.

T.

Absorption

of

Carbon

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in

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