Comparison of Solvation Effects on CO2 Capture with Aqueous Amine

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Comparison of Solvation Effects on CO Capture with Aqueous Amine Solutions and Amine-Functionalized Ionic Liquids Hidetaka Yamada J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b07860 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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The Journal of Physical Chemistry

Comparison of Solvation Effects on CO2 Capture with Aqueous Amine Solutions and AmineFunctionalized Ionic Liquids Hidetaka Yamada* Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan ∗To whom correspondence should be addressed. E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: Amines are the most widely utilized chemicals for post-combustion CO2 capture, because the reversible reactions between amines and CO2 through their moderate interaction allow effective “catch and release.” Usually, CO2 is dissolved in the form of an anion such as carbamate or bicarbonate. Therefore, the reaction energy diagram is potentially governed to a large extent by the polarity of the surrounding solvent. Herein, we compared aqueous amine solutions and amine-functionalized ionic liquids by investigating their dielectric constants and performing an intrinsic reaction coordinate analysis for the CO2 absorption process. Quantum mechanical calculations at the CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level within the continuum solvation model (SMD/IEF-PCM) revealed contrasting dependencies of C−N bond formation on the dielectric constant in those solutions. Amines react with CO2 on an energy surface that is significantly affected by the dielectric constant in conventional aqueous amine solutions, whereas amine-functionalized anions and CO2 form stable C−N bonds with a comparatively lower activation energy regardless of the dielectric constant.


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INTRODUCTION Most scenarios consistent with average global warming of less than 2°C above pre-industrial levels substantially depend on CO2 capture and storage (CCS).1 Amine scrubbing with alkanolamines such as monoethanolamine (MEA) is a mature CO2 capture technology used for industrial processes and has been well demonstrated for larger emission sources.2 However, conventional amine scrubbing is energy intensive and expensive, which is one of the obstacles to global CCS deployment.3-6 In this context, advanced amine solvent technologies7,8 and transformational materials including ionic liquids (ILs)9-11 have been extensively studied over the last decade. Chemical absorption with aqueous amine solutions is suitable for low-concentration CO2, e.g., post-combustion flue gas (ca. 10−15% CO2) because of the moderate reactivity between CO2 and the amino group. Primary and secondary amines react with CO2 to produce a carbamate anion and a protonated amine as follows:12 2R1R2NH + CO2 ⇄ R1R2NCOO− + R1R2NH2+

(1)

The bicarbonate anion (HCO3−) also forms in aqueous amine solutions. The thermodynamics and kinetics of these reactions can be tuned by changing the substituents R1 and R2 when searching for new amines to develop advanced amine solvents. Quantum mechanical (QM) continuum solvation models provide useful insights into the relationship between substituents and solvent properties.12-24 Usually amine−CO2−H2O systems are embedded in a cavity surrounded by a continuum with the dielectric constant of water (εr = 78).14-29 In practice, attention should be paid to the dielectric constant of the solvent (εr < 78) and its dependence on solvation energy.


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ILs are promising in terms of their negligible vapor pressure, high thermal stability, and structural tunability.9-11 For CO2 capture, amine-functionalized anion-tethered ILs have attracted attention because of their favored stoichiometry30-41 R1R2R3N− + CO2 ⇄ R1R2R3NCOO−

(2)

Gurkan et al. reported that phosphonium-based amino acid ILs can react with CO2 in a 1:1 stoichiometry, achieving higher molar capacities than those of aqueous amine solvents.30,31 Using first-principles G3 calculations, they designed a new class of ILs based on aprotic heterocyclic anions, among which 2-cyanopyrrolide (2-CNpyr−) showed a moderate CO2 affinity and a viscosity suitable for the CO2 capture process.31 Tang and Wo studied azole anion–CO2 interactions using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level and developed an absorption model consistent with experimental measurements.36 However, note that they performed gas phase calculations (εr = 1). Various other studies also applied gas phase calculations to IL−CO2 systems for simplicity.30,32,33,38 This study compares aqueous amine solutions and amine-functionalized anion-tethered ILs to highlight the intrinsic difference between their CO2 absorption reactions. Certainly, the stoichiometry differs as shown in eqs 1 and 2. However, it bears no immediate relationship to the reaction energy diagram nor the dependence on solvation energy. Herein, we identify the distinctly different responses of C−N bond formation to the dielectric constant in the first elementary steps of eqs 1 and 2. We also analyze the CO2 absorption reactions in terms of enthalpies. CO2 absorption includes physical solution-diffusion and chemical reaction. This study focuses on the latter, i.e., CO2 binding. More recently, QM molecular dynamics (QM/MD) simulations in explicit solvents showed great facility in describing the detailed mechanism of


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elementary reactions in amine−CO2−H2O systems.42-47 However, QM continuum solvation models, which implicitly treat the solvent molecules, are better suited to the above-mentioned intent and purpose in this study. Hence, we do not consider the explicit involvement of the solvent to show its implicit role as a reaction field.

METHODS Transition state optimizations and intrinsic reaction coordinate (IRC) calculations were performed for the aqueous phase by DFT at the B3LYP/6-311++G(d,p) level with the SMD solvation model using the integral equation formalism polarizable continuum model (IEF-PCM) protocol for bulk electrostatics.27,48 The reactants and products obtained from the IRC calculations were optimized at the same level. To provide the energy diagram, we used coupledcluster theory with single, double and noniterative triple excitations (CCSD(T)) within the continuum solvation model (SMD/IEF-PCM)48 and carried out single-point energy calculations at the 6-311++G(d,p) basis set. To examine the dependence of solvent dielectric constant, water (εr = 78) and 4-mepyridine (εr = 12) solvents were employed to represent aqueous solutions and ILs, respectively. To obtain reaction enthalpies, the enthalpies of each species were calculated based on the DFT geometry optimized structures followed by a frequency analysis for thermal and zero point energy corrections at the B3LYP/6-311++G(d,p) level of theory in vacuum or within the continuum solvation model. The most stable conformation in vacuum as determined using molecular mechanics with the MMFF force field,49 was selected as the initial geometry for the DFT optimizations.


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The DFT and CCSD(T) calculations were performed using the Gaussian09 software package.50 The Spartan08 software package was used for the molecular mechanics simulations. The employed levels of theory, which were previously examined for alkanolamine−CO2−H2O systems,20,27,28 are valid for the objectives of this study. For the purpose of checking the basis set dependence, the 6-311++G(2df, 2p) basis set was also applied to a subset of single-point energy calculations. MEA (99% purity, Wako Pure Chemical, Japan) was purchased and used without further purification. 1-Ethyl-3-methylimidazolium glycine ([emim+][Gly−], 0.90 % water, Katayama Seiyaku, Japan) was used for measurements immediately after breaking its seal. Dielectric measurements were carried out with an open-ended coax line technique51 at frequencies up to 6 GHz and a temperature of 297 K using a vector network analyzer (37225C, Anritsu, Japan).

RESULTS AND DISCUSSION The transition states of C−N bond formation between CO2 and (a) MEA, (b) Gly−, and (c) 2CNpyr− optimized at the SMD/IEF-PCM/B3LYP/6-311++G(d,p) level of theory in water (εr = 78) are shown in Figure 1 along with the IRC calculation results. The C−N bond lengths at the transition states are (a) 2.18 Å, (b) 2.23 Å, and (c) 2.23 Å, and decrease to (a) 1.59 Å, 1.58 Å, and 1.47 Å, respectively, in the product geometries optimized at the same level of theory followed by the IRC calculations. Figure 1 clearly shows that compared with the neutral amine MEA, the negatively charged amines Gly− and 2-CNpyr− covalently bind to CO2 with relatively


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low activation energies for the forward reactions and high activation energies for the reverse reactions.

Figure 1.

Transition state geometry and electronic energy profile along the IRC for the

transition states of (a) MEA, (b) Gly−, and (c) 2-CNpyr− during C−N bond formation with CO2 calculated at the SMD/IEF-PCM/B3LYP/6-311++G(d,p) level in water (εr = 78).

According to the Born formula, the solvation energy of a sphere of radius a with charge Ze depends on the dielectric constant εr A 1 solv 1 8πε εr

(3)


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where ε0 is the permittivity of free space and NA is the Avogadro number.52 The dielectric constants measured for 30 wt.% aqueous MEA solution and [Emim+][Gly−] IL were ca. 75 and 10, respectively, as shown in Figure 2. There are no reports of the dielectric constants of aminefunctionalized anion-tethered ILs, and the available εr-values for other ILs in the literature are also limited.53,54 Wakai et al. determined the dielectric constants of five 1-alkyl-3methylimidazoliums, and the observed εr-values at 298 K fall between 15.2 and 8.853, which are low values similar to those measured for [Emim+][Gly−] in the present study. Compared with ILs, aqueous MEA solutions showed a high relative permittivity close to that of water. To the best of our knowledge, this is the first report of a dielectric constant for an amine solution relevant to CO2 capture. The result confirms that the QM continuum solvation model of water is a valid description for MEA−CO2−H2O systems,14-19,21,22,25.28 provided that the CO2 loading is low.

Figure 2. Dielectric constants measured at 297 K for aqueous amine solution (30 wt.% MEA) and IL ([Emim+][Gly−]) alongside the data for water as a reference.


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Recent studies on MEA−CO2−H2O systems support a zwitterion intermediate, i.e., product in Figure 1a undergoes deprotonation by another MEA to form the carbamate, revealing that the formation of the zwitterion is the rate-determining step.26,44 Similarly, a proton transfer from the amino group to the carboxylate easily follows the C−N bond formation between CO2 and Gly− in Figure 1b.41 NH2CH2COO− + CO2 ⇄ (OOCNH2CH2COO)− ⇄ −OOCNHCH2COOH

(4)

Such proton transfer processes stabilize the final products further in polarized environments. The reaction enthalpies for MEA carbamate formation (eq 1) calculated for the dielectric constants of 78, 12, and 1 are listed in Table 1, where the three constants employed represent water, IL, and vacuum as surrounding environments, respectively. As is well known, MEA carbamate formation is exothermic in aqueous solutions. However, it becomes endothermic with a decrease in the dielectric constant. The calculation results show that the enthalpy for the MEA carbamate formation is dramatically affected by solvation effects, varying from ca. −50 kJ/mol in water to ca. 500 kJ/mol in vacuum, where the variation is an order of magnitude greater than the errors in the calculations. Table 1 indicates that, in contrast to MEA carbamate formation, the enthalpy for C−N bond formation between CO2 and the amine-functionalized anion is hardly affected by the dielectric constant of the IL. Grukan et al. evaluated the solvation effect on the free energies of reaction between CO2 and pyrrolide (Pyr−)-based ILs by calculations at the PCM/B3LYP/6-311++G(d,p) level.31 They compared the effects of three different solvent models corresponding to aniline (εr = 6.9), isoquinoline (εr = 11), and acetonitrile (εr = 20), and confirmed that the choice of solvent


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has little influence on the results. Our findings extend this work to cover a wide range of εrvalues. As shown in Table 1, for both 2-CNpyr− and Pyr−, C−N bond formation with CO2 is similarly exothermic either in water (εr = 78) or vacuum (εr = 1), and the cyano substituent on Pyr− significantly affects the reaction enthalpies through its electron-withdrawing nature, regardless of the εr-value. Further, the calculated enthalpies of reaction between CO2 and Gly− (eq 4) are also comparative among different εr-values. In Table 1, the reaction enthalpies calculated using the eclectic energies at B3LYP/6-311++G(d,p) show similar dependence on the dielectric constant to those at CCSD(T)/6-311++G(2df,2p), supporting the above-mentioned results.

Table 1. Calculated Reaction Enthalpies for CO2 Absorption a ∆H (kJ/mol) Reaction \ εr 2MEA + CO2 → Carbamate + H+MEA Gry− + CO2 → −OOCNHCH2COOH 2-CNpyr− + CO2 → 2-CNpyrCOO− Pyr− + CO2 → PyrCOO−

78

12

1

−46 (−46)

43 (40)

483 (484)

−39

−45

−69

−53 (−66)

−22 (−33)

−34 (−45)

−99

−65

−99

a

Calculated at SMD/IEF-PCM/B3LYP/6-311++G(d,p) or B3LYP/6-311++G(d,p) and at 298 K; reaction enthalpies corrected by the electronic energies at CCSD(T)/6311++G(2df,2p) are shown in parentheses.

Figures 3-5 show electronic energies of transition states and products relative to those of reactants during C−N bond formation for various dielectric constants, which were obtained by


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single-point energy calculations at the CCSD(T)/6-311++G(d,p) level of theory within the SMD model on the optimized geometric structures followed by the IRC calculations shown in Figure 1. The calculated activation energies for zwitterion formation in Figure 3 (> 30 kJ/mol) are close to the experimentally determined value for CO2 absorption in aqueous MEA solutions (41 kJ/mol),7,55 in line with the fact that zwitterion formation is rate-determining, as mentioned above. However, Figure 3 clearly suggests that the zwitterion product is destabilized in low polarity and non-polar environments. In vacuum, virtually no reaction occurs between MEA and CO2, because the zwitterion is remarkably unstable. The single-point energy calculations at CCSD(T)/6-311++G(2df,2p) gave similar results to those at CCSD(T)/6-311++G(d,p) as shown in Figure 3, supporting the validity of the 6-311++G(d,p) basis set for the present study.

Figure 3. Energy diagram of the MEA + CO2 reaction for various dielectric constants calculated at the CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level of theory using the SMD model. Results at the CCSD(T)/6-311++G(2df,2p)//B3LYP/6-311++G(d,p) level of theory are shown in parentheses.


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The calculated activation energies for C−N bond formation between CO2 and the aminefunctionalized anion are relatively low as shown in Figures 4 and 5. Interestingly, unlike MEA, the lower solvent polarity lowers the reaction barrier. Furthermore, the products in Figures 4 and 5 are moderately stabilized regardless of the dielectric constant, partly supporting the validity of previous studies employing gas phase calculations for ILs.30-33,38 These results indicate that amine-functionalized anions relevant to CO2 capture tend to show relatively low barriers and low heats of absorption.

Figure 4. Energy diagram of the Gly− + CO2 reaction depending for various dielectric constants calculated at the CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level of theory using the SMD model.


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Figure 5. Energy diagram of the 2-CNpyr− + CO2 reaction for various dielectric constants calculated at the CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level of theory using the SMD model.

CONCLUSIONS In this study, we focused on the effect of the solvent reaction field on the CO2 binding steps in amine-based solvents. Based on εr measurements for solvents, we used εr as a parameter that implicitly represents the reaction field. This approach allowed for a comparison between aqueous solutions and ILs to extract clear differences regarding the field dependence, which are hidden behind the complicated factors governing the thermodynamics and kinetics of CO2 absorption, such as the basicity of the amine and mass transfer. The CO2 absorption rate of aqueous amine solutions decreases with increasing CO2 loading.56 This behavior is usually explained in terms of the unreacted amine concentration and the solvent viscosity. In addition, our results suggest that the decreased dielectric constant of the solvent due


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to CO2 loading and/or use of a carbon-rich amine possibly contribute to the decrease in the CO2 absorption rate. Very recently, Cantu et al. estimated solvent dielectric constants from classical MD simulations for a CO2 binding organic liquid, namely 1-((1,3-dimethylimidazolidin-2ylidene)-amino)-propan-2-ol at different CO2 loadings and found that as the number of charged species increases, inhomogeneity in the liquid leads to a decrease in the dielectric constant.57 In contrast to aqueous amine solutions, the energy diagram of the reaction between CO2 and amine-functionalized anions barely depends on the solvation effect. The formed CO2-bound anion is likely to be stabilized in ILs regardless of the dielectric constant. Therefore, a trade-off in the CO2 capture properties of amine solutions between absorption rate and heat of absorption56 does not necessarily apply to amine-functionalized ILs.


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ASSOCIATED CONTENT Supporting Information A PDF file listing the Cartesian coordinates for the transition states optimized at the SMD/IEFPCM/B3LYP/6-311++G(d,p) level shown in Figure 1, information on conversion from IRC to C−N distance in Figure 1, and calculated dipole moments are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Phone: +81-774-75-2305. Notes The author declares no competing financial interests. ACKNOWLEDGMENT This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP26420773.


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TOC GRAPHICS


23


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(a)

(b)


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(c)

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Pro

HO(CH2)2NH2 + CO2  HO(CH2)2NH2+COO r = 78

E (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

0 Rea

30 (28)

32 (30)

32 (27)

TS

TS

Pro

1 (7) Pro

45 (44)

 TS

r = 1

r = 12 0

0

Rea

Rea




107 (99)

Page 27 of 29

NH2CH2COO + CO2  (OOCNH2CH2COO) r = 78 E (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21

r = 12

TS

13 TS

0 Rea

r = 1

0 10 Pro

Rea

9 Pro


0

0

Rea

TS 12 Pro


+ CO2  E (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

17 12

TS

TS

0

0

0

1

Rea

Rea

Rea

TS

r = 78

 55 Pro

13

r = 12

21 Pro


r = 1

Pro

Page 29 of 29


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Comparison of Solvation Effects on CO2 Capture with Aqueous Amine

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