Carbon Dioxide Absorption by the ImidazoliumâAmino Acid Ionic

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Carbon Dioxide Absorption by the Imidazolium-Amino Acid Ionic Liquids, Kinetics and Mechanism Approach Mojtaba Rezaeian, Mohammad Izadyar, and Ali Nakhaei Pour J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03152 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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Carbon Dioxide Absorption by the Imidazolium-Amino Acid Ionic Liquids, Kinetics and Mechanism Approach Mojtaba Rezaeian, Mohammad Izadyar*, Ali Nakhaeipour Computational Chemistry Research Lab., Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran * Correspondence to: Mohammad Izadyar. Tel: ++05138805533 (E-mail: [email protected])

Abstract The kinetics and mechanism of CO2 absorption by ionic liquids (ILs) were studied,

theoretically.

The

studied

ILs

are

composed

of

1-ethyl-3-

methylimidazolium [Emim]+ as the cation with a general formula of the [Emim][X] (X=Gly-, Ala-, Lys-, Arg-). To investigate the alkyl chain length and the number of the amine group effects on the CO2 absorption, different amino acid anions were chosen. On the basis of the enthalpy changes during CO2 capture, a chemisorption nature is confirmed. An increase in the number of amine (-NH2) groups in the ILs structures, facilitates the CO2 absorption. According to kinetic results, the rate of CO2 absorption by [Emim][Gly] is higher than that of [Emim][Ala]. This can be interpreted by a higher steric hindrance in [Emim][Ala] due to an additional methyl group in the amino acid chain. Donor-acceptor interactions and C-N bond formation were investigated by natural bond orbital analysis. Moreover, topological studies show a covalent nature for the C-N bond critical point that showing CO2 capture is a chemisorption process. Finally, on the basis of kinetic energy results, donoracceptor interaction and topological analysis, [Emim][Arg] is proposed as the best candidate for CO2 absorption from the kinetic and thermodynamic viewpoints.

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1. Introduction In recent years, Ionic liquids (ILs) have been revolutionized in the research centers and chemical industries.1 These compounds, as green solvents, have an important role in reducing the use of hazardous materials effect.2,3 ILs have different properties, including high chemical and physical stability, inherent conductivity, high polarity, high liquid range and the ability of dissolving a large range of different compounds.4 For example, 1-Butyl-3-methylimidazolium trifluromethanesolfonate ([Bmim][TFO]) is a suitable IL in the separation of aromatic hydrocarbons from an aliphatic mixture.5 ILs have various applications in catalysis,6 green chemistry,7 quantitative analysis,8 enzyme and biocatalysis,9,10 and chemical industries.11 ILs are also applied in acidic gas absorption processes.12,13 For example, ILs with a polyacrylamide anionic part show a high tendency for acidic gas absorption, because of the active site of the polyacrylamide anion. Within a joint experimental and theoretical study, SO2 absorption of ILs was investigated by Cui and coworker's.14 Acrylamide anion has a nitrogen atom and C=O group for possible interaction with SO2 and the obtained results show a chemical interaction between N atoms and C=O functional groups of the ILs and S atom of the SO2. Calculated absorption energy show that these types of ILs are good candidates for acidic gas absorption. Coal and oil are important materials in energy production. During this transformation, CO2 is produced, therefore, CO2 is considered as a byproduct of the modern industries.15 This gas is produced by burning coal, oil and natural gas.16 CO2 creates environmental problems and may cause climate change.17 Based on reports in recent years, the concentration of CO2 in the atmosphere increased to about 400 ppm, because of an increase in the use of fossil fuels in industrial processes.18,19



Carbon capture and storage technology is a very effective method to reduce the amount of CO2 released from the burning of fossil fuels.20 Generally, amine solutions are recognized as absorbents for CO2 capture.21 Although, these solutions are applied for gas absorption, their corrosive and toxic nature have restricted their wide applications.22 Therefore, designing and utilization of the new solvents having a high biodegradability (low toxicity) and low energy requirements for CO2 absorption, is of great interest.23,24 ILs are a group of solvents that are used for CO2 capture.25 In this process, there are some physical and chemical interactions between the CO2 molecule and absorbent active sites. The number of the active sites and the absorption capacity of the anions and cations are important factors in CO2 absorption.26 Brenneck and coworkers investigated the solubility and CO2 absorption capacity by the phosphonium-based ILs.27 The effect of pressure and temperature on the CO2 absorption show that the CO2 solubility decreases by temperature increment and pressure reduction. Also, an increase in the alkyl chain length increases the capacity of CO2 absorption. The chemical nature of the anion plays a dramatic role on their gas absorption energies. For example, it was reported that the affinity of [NTF2]- toward CO2 absorption is more than [NO3]- and [DCA]- (Dicyanamide).28 Amine functionalized ILs are another class of the absorbents that are identified for CO2 capture.29 For example, Bates and colleagues introduced a group of ILs composed of an amine group in the cationic part, having a high rate of CO2 absorption.30 Also, amino acid functionalized ILs are a new generation of ILs, having a high absorption ability for CO2 capture.31,32 These ILs were initially reported by Fukumoto in 2005.33 Low viscosity34 and high resistance to oxidative degradation and high biodegradability are their specific properties, which make them superior to other ILs.35 The performance of this type of ILs in CO2 absorption process can be 3 ACS Paragon Plus Environment

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related to less vapor pressure in comparison to amine-based ILs, which reduces the amount of solvent evaporated during the separation process.36 Kinetics and mechanism of a CO2 absorption by amino acid ILs (AAILs) were investigated by Guo et al.37 Guo reported that glycine-based AAILs as the solvent interacts with CO2 in the industrial conditions of temperature and concentration. For these purposes, Guo et al. considered the natural and the base forms of glycine for their investigation on the mechanism of CO2 absorption. The interaction occurred between amine group of glycine and gaseous CO2. The results show, CO2 capture occurs through a barrier energy of 45.2 ± 2.2 (kJ.mol-1) for base form of glycine which is lower than that of the natural form (71.2 ± 9.6 kJ.mol-1). In determining the nature of the amino acid moiety on the rate of CO2 absorption, Guo applied an alanine-based AAIL reporting the CO2 absorption activation energy to be smaller than that of glycine.37 Diethanol amine glycinate ([DEA][Gly]) protic ILs was applied for CO2 absorption, experimentally, by Lu and coworkers.38 Since, this IL is composed of the ammonium and amine groups, [DEA][Gly] can be considered as an amino acid IL. Lu claimed that the partial pressure of CO2 and IL concentration have an important effect on the absorption efficiency. Lu further investigated the structural properties of [DEA][Gly] with IR and 1HNMR spectroscopy. A separate study was conducted on the performance of [DEA][Gly] IL, [DEA] and [bmim][PF6]. According to the results, CO2 solubility in [DEA][Gly] IL is more than amine-based solvents and non-amino acids ILs.38 They concluded that [DEA][Gly] has a high potential as an efficient solvent to be commercially used for CO2 capture. CO2 absorption by amino acid ILs composed of the imidazolium cation and glycine amino acid anions immobilized into porous poly (methyl methacrylate) was reported by Wang et al.39 They proposed a mechanism, in which 0.5-1.5 mole CO2 4 ACS Paragon Plus Environment


is absorbed per one mole of the IL through the amine group of the amino acid. Kinetic studies showed that the absorption of CO2 by this IL obeys the first-order law. The changes in the capacity of the CO2 absorption at the different temperatures were also investigated and the obtained result shows a decrease in capacity by temperature increment. To investigate the effect of the anion part on CO2 absorption, different anions such as arginine, alanine and glycine were applied. Theses ILs have different absorption capacities, which are in agreement with the maximum theoretical values. 39 Based on the results, CO2 absorption capacity of the amino acid ILs decreases from glycine to arginine anions. Aparcio and colleagues studied the CO2 absorption by Choline-based ILs that contain choline benzoate and choline salicylate ILs.40 They investigated the short range anion-cation and ion-CO2 interactions using the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) methods. These analyses showed that the anion groups of the salicylate have an important role in the stability of the ILs. In the case of benzoate, CO2-ion interaction is stronger than salicylate. Molecular dynamic (MD) simulations confirm that IL structures are stable during the CO2 absorption and the aromatic-based ILs have a higher viscosity than nonaromatic ones, which decreases during CO2 capture. Also, a change in the mole fraction of CO2 in the mixture alters the properties of the ILs and CO2 absorption capacity. According to the theoretical results, the ILs having an aromatic substitution in their anion part, are good candidates for CO2 capture. A density functional theory (DFT) study was done by Wang and coworkers on the polymeric ammonium ILs (PILs) as the absorbent for CO2.41 The electrostatic maps show a higher ability of the anions for CO2 absorption than its counter cation. In the presence of water molecules, due to the interaction of the anion part of the PILs (CO3-) and CO2, bicarbonate anion (HCO3-) is formed. They also proposed that 5 ACS Paragon Plus Environment

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H2O addition to the IL systems increases the absorption rate of CO2. By addition of water, the absorption/desorption equilibrium are shifted to a desorption hydrated state and the activation energy barrier decreases. Firaha and coworkers investigated CO2 absorption with ILs containing [Emim] cation and amino acids as the anions.42 They proposed a mechanism for CO2 absorption in a joint ab initio/molecular dynamics study and analyzed the thermodynamics and kinetics aspects of CO2 absorption. They claimed stoichiometry ratios of 1:1 and 1:2 (IL/CO2), in which Gibbs energy of the reaction increases in the presence of the aromatic group, long alkyl chains, halogens and hydroxyl group functionalized on the [Emim] cation. They concluded that carbamic acid formation is more favorable than carbamate. Mercy et.al studied CO2 absorption with ILs that have a phosphonium cation and four super-base as anion by DFT method.43 CO2 absorption occurs on the anions part through a covalent bond and the presence of cation decreases the cohesive energy of ILs. In [N111][L-Ala] ILs, CO2 is absorbed on the alanine anion through a monomolecular mechanism and a proton is transferred between the aminate anion and carboxylate group, yielding carbamic acid. Saravanamurgan and co-workers investigated CO2 absorption by AAILs having the following anions: lysine, histidine asparagine and glutamine with ammonium cation.44 The results show that these types of ILs have a high capacity for CO2 absorption via a chemisorption process, in which carbamic acid is formed. Amino acid ILs have amine (NH2) group for interaction with CO2, which makes them ideal for CO2 absorption. Moreover, amino acids have a high biodegradability that makes them safe for the environment. Because of these reasons, amino acids ILs were proposed as a good candidate for CO2 absorption. 6 ACS Paragon Plus Environment


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Here, we studied the thermodynamic and kinetic aspects of CO2 absorption by the amino acid ILs, using quantum mechanics calculations. For this purpose, we investigated the most probable configurations of the anions against the cation in the ILs. In this procedure, we proposed two paths for CO2 absorption by the ILs. To investigate the most possible interactions between the anions, cation and CO2, NBO analysis was applied for investigation of the bond properties and donor-acceptor interactions. Finally, QTAIM analysis was used to evaluate the topological parameters at the bond critical points (BCPs) (a point between two atoms, defined by maximum electron density) at the transition states (TSs) of CO2 absorption.

2. Computational details DFT calculations were applied to investigate the mechanism of CO2 absorption by the amino acids ILs, using the Gussian09 program.45 Dispersion energy is an important part of interactions in liquids.46 Since M06-2X functional and 6-311++G(d,p) basis set are sufficient to include the dispersion energy and diffusion/polarization effects, respectively, the application of this combination is preferred to generate accompanying reasonable results.47,48 Initially, different configurations of the modeled ILs were considered based on the partial charge distribution and analyzed. Then, the most stable structures were chosen to investigate the CO2 absorption. Finally, different mechanisms of CO2 absorption were proposed and discussed from various molecular viewpoints. To determine the most probable configurations of the anions and cations in the ILs, MP2 calculations were performed on the structures and the corresponding interaction energies (∆G) using equation 1: ∆G = 𝐺[𝐸𝑚𝑖𝑚][𝐴𝐴] − 𝐺[𝐸𝑚𝑖𝑚]+ − 𝐺[𝐴𝐴]−

(eq.1) 7

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Where G[Emim][AA] is the IL energy and G[Emim]+ and G[AA]- are the energy of the isolated cation and the anions of the ILs, respectively. ILs and the corresponding cation and anions were considered in the gas phase. IL structures were optimized in three probable directions (as shown in Figure 1) at the M06-2X/6-311++G(d,P) level of theory. To differentiate between the obtained structures completely, MP2 calculations were applied on the optimized structures of ILs at the M06-2X level. Theoretical energies were obtained according to eq. 1, in which the complex of cation and anion (IL) at the most probable sites of interaction were initially considered at a distance of about 2 Ǻ All structures were fully optimized and their stabilities were confirmed through vibrational frequency analysis. The TS structures were obtained by the Synchronous Transit-Guided Quasi-Newton (STQN) method.49 NBO analysis was applied on the stationary points along the reaction path to describe the electronic charge distribution on the atoms and charge transfer between the frontier molecular orbitals.50 Through this analysis, it is possible to calculate the donor-acceptor interaction energies at the center of the reaction which are important in the reaction process. Since, the second-order perturbation energies were calculated in this procedure, further calculation of the counterpoise correction is not necessary. Insight into the topological properties and the nature of the interactions, different segments at the TS, molecular topology, electron localization function (ELF) and localized orbital locator (LOL) analyses were performed at the BCPs, using the Multi wfn3.1 program.51,52 ELF and LOL analyses are applied to investigate the strength of the interactions between the atoms at the center of the reaction. These analyses produce some important quantitative parameters which are useful in the description of the critical bonds nature between the ILs and CO2. 3. Results and Discussion 8 ACS Paragon Plus Environment


3.1. Research procedure

For determining the most probable sites of the interactions between the cations and anions of the ILs, three regions were considered and shown in Figure 1. There are two N atoms in the imidazolium ring, N10 and N11, with a temporary positive charge having a resonance stability. Carboxylate anion (COO-) can interact with imidazolium ring from these positions e.g. C1 site of the A position, attached to H8 atom having an acidic character, which is the most probable site for interaction relative to C13 and C14 atoms. After optimization of the ILs and evaluation of their relative stabilities, at the defined regions (Figure S1), the most stable structures were determined and depicted in Figure 2.

Figure 1. The most probable sites of interaction, proposed for [Emim][Gly]

After determining the most stable configurations, the mechanism of CO2 absorption by these ILs was investigated. For this purpose, two probable paths were proposed. Figure 3 shows the molecular and zwitter-ionic paths of the CO2 absorption. The final product of the reaction is an acidic compound (P), carbamic acid, passing through two different paths, paths A and B. In contrast to path A, Path 9 ACS Paragon Plus Environment

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B is a two-step zwitter-ionic mechanism, yielding the product through the TSB1, intermediate and TSB2.

[Emim][Gly]

[Emim][Ala]

[Emim][Lys]

[Emim][Arg]

Figure 2. The optimized most stable structures of the studied ILs

3.2. Structural Analysis

Atoms numbering of the studied ILs is show in Figure 1. The carboxylate anion of all ILs has a high affinity toward H8, which induces strong interactions between O2…H8 or O3…H8 pairs (A position). Because of the acidic character of H8 and less steric hindrance in A direction in comparison to B and C positions, the interactions between O2, O3 and H8 are important. Geometrical parameters of the studied ILs 10 ACS Paragon Plus Environment


are represented in Table S1. According to the obtained results in all structures, a strong interaction between H8 and O2/O3 is predicted (e.g. theoretical distance of O2….H8 in [Emim][Gly] is 2.26 Å and that of O3….H8 is 2.31 Å). Since, CO2 absorption occurs on the nitrogen atom of the amino acid chain, the distance changes of N5, C12, O13, H7 are more important than other atoms. The results of the structural analysis in the presence of CO2 are reported in Table 1. All amino acid ILs have a similar behavior and interaction, which occurs between the equivalent atoms. On the basis of this data, during the absorption process in path A of the proposed mechanism, O13-H7 and C12-N5 bond distances decrease, confirming the corresponding bond formation, while the bond length of the N5-H7 increases, showing the bond cleavage. These geometrical changes confirm that CO2 absorption takes place by the ILs. 3.3. Energy analysis In order to differentiate between the interaction energies of the ion pairs of the studied ILs, MP2/6-311++G(d,P) level of theory was applied. Thermodynamic results of these ILs in three directions are represented in Table S2. The results of the interaction energies are represented in Table 2. On the basis of the interaction energy analysis, corresponding conformation of the ion pairs at the interaction (structure A) are the most stable configurations than other proposed structures and [Emim][Arg]


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Figure 3. The proposed mechanism for CO2 absorption by the AAILs.


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Table 1. The results of structural analysis of CO2 absorption by the studied ILs, bond length are in (Ǻ). [Emim][Gly] Bond length

R*

N5-C12 O13-H6 O14-H6 O13-H7 O14-H7 N5-H6 N5-H7 O2-H8 O3-H8 C1-H8 C12-O13 C12-O14 * ≠ ∆r =rTS–rR

4.06 1.38 1.51 4.47 3.16 2.67 2.68 2.42 2.83 4.96 1.05 1.28 3.79 3.07 2.79 1.01 1.01 1.02 1.01 2.35 1.31 1.99 2.75 4.01 2.31 2.37 2.89 2.41 2.70 3.79 1.16 1.31 1.28 1.16 1.22 1.20 R*= Reactant

P

TS

[Emim][Ala]

[Emim][Lys]

[Emim][Arg]

*∆r≠

R

P

TS

*∆r≠

R

P

TS

∆r≠

R

P

TS

∆r≠

-2.55 -1.80 -0.15 -3.68 -1.00 0.01 0.30 2.02 0.58 1.38 0.12 0.04

5.05 5.96 5.32 5.35 4.35 1.02 1.02 2.16 2.08 2.33 1.16 1.15

1.38 3.14 2.38 1.08 3.09 1.01 2.34 2.44 3.92 3.35 1.31 1.22

1.51 2.67 2.84 1.26 2.80 1.02 1.31 1.89 3.75 2.97 1.28 1.20

-3.54 -3.24 -2.48 -4.09 -1.55 0.00 0.29 -0.27 1.67 0.64 0.12 0.05

2.75 3.35 2.66 3.96 3.66 1.02 1.01 2.27 2.69 2.59 1.16 1.15

1.37 3.16 2.42 1.05 3.05 1.01 2.36 4.76 3.25 4.28 1.32 1.22

1.51 2.75 2.81 1.27 2.80 1.02 1.31 2.36 2.15 2.40 1.27 1.20

-1.24 -0.60 0.15 -2.69 -0.86 0.00 0.30 0.09 -0.54 -0.19 0.09 0.05

4.07 3.60 4.99 4.09 5.36 1.02 1.01 1.77 2.44 2.36 1.15 1.16

1.39 2.37 3.15 1.01 3.03 1.01 2.31 2.22 2.41 2.59 1.32 1.21

1.51 2.79 2.76 1.26 2.81 1.02 1.30 2.39 2.85 2.60 1.28 1.20

-2.56 -0.81 -2.23 -2.83 -2.55 0.00 0.29 0.52 0.37 0.17 0.13 0.04


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is the most stable IL, from the interaction energy view point. Since, structure A of the ion pair is preferred to structures B and C, energetically, in the next step of this research, these configurations were considered as the IL structures for possible CO2 absorption. Table 2. The calculated interaction energies between the cation and anion pairs of the ILs (kJ.mol-1) ILs Structure A Structure B Structure C [Emim][Gly] -388.68 -380.44 -368.99 [Emim][Ala]

-399.40

-346.55

-378.29

[Emim][Lys]

-382.07

-361.97

-357.14

[Emim][Arg]

-369.55

-362.08

-343.02

Calculated thermodynamic and kinetics parameters of the studied ILs are reported in Table S3. Thermodynamic and kinetic properties in pathways A of the CO2 absorption were calculated and reported in Table 3. On the basis of the Gibbs energies, Path 2 is not thermodynamically favorable (∆G>0), therefore this path was ignored. Thermodynamic parameters for this path is listed in Table S4. Considering the enthalpy and entropy changes during CO2 capture, a chemisorption nature and a reduction in the degree of freedom of the studied systems are confirmed, respectively. On the basis of the activation Gibbs energy values, [Emim][Lys] is the most efficient ILs for CO2 absorption in comparison to other ILs. Having insight into the reaction pathway from an energy view point, potential energy diagrams of CO2 absorption by ILs are depicted in Figure 4. On the basis of this figure, the theoretical trend in the reactivity of the studied ILs toward CO2 absorption is as follows: [Emim][Arg]> [Emim][Lys]> [Emim][Gly]> [Emim][Ala].



The number of amine groups and alkyl chain length are two important parameters that affect the CO2 absorption process. Kinetics analysis shows that, the effect of the number of the amine groups on the energy of CO2 absorption is more important than the alkyl chain length. [Emim][Gly] and [Emim][Ala] have equivalent amine groups, but the CO2 absorption rate by [Emim][Ala] is less than [Emim][Gly]. This behavior originates from a longer alkyl chain length of the [Emim][Ala], which elevates its steric hindrance against the CO2 absorption. On the basis of the calculated activation energies, [Emim][Arg] is a better IL candidate for CO2 absorption, in agreement with the experimental results.53 According to the kinetic results, an increase in the amine group of the amino acid chain increases the rate of the reaction, which potentially makes [Emim][Arg] the best candidate. 3.3. DFT reactivity indices Fukui frontier molecular orbital theory,54 including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), was used to investigate the interactions of the anion and cation pairs of the ILs. DFT reactivity indices such as the electronic chemical potential (𝜇), electronic chemical hardness (𝜂) and electrophilicity index (𝜔) were calculated and reported in Table 4. The [Emim] cation shows less chemical potential than the anions, while the electrophilicity index of the anions is more than the cation. Therefore, it can be concluded that [Emim] cation acts as an electrophile, electron acceptor, and amino acid anions act as a nucleophile, electron donor, in the ILs. The chemical potential of the Lys- and Arg- is less than Ala- and Gly-, while, the electrophilicity index of these anions is more than Ala- and Gly-. These different behaviors show that Lysand Arg- are the stronger nucleophile than other anions. Moreover, Lys - and Arghave the most chemical activity, on the basis of the electronic chemical hardness. 15 ACS Paragon Plus Environment

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Table 3. Thermodynamic and kinetic parameters of the CO2 absorption by different ILs ΔG≠ (kJ.mol-1) ΔH≠ (kJ.mol-1)

ΔS≠ (J.mol-1.K-1)

Ea (kJ.mol-1)

133.39

-61.91

138.35

154.25

136.60

-59.22

141.55

-52.84

137.22

125.53

-39.02

130.49

-111.40

144.37

120.76

-79.18

125.72

ΔG (kJ.mol-1)

ΔH (kJ.mol-1)

ΔS (J.mol-1.K-1)

[Emim][Gly]

-15.58

-37.10

-71.25

151.84

[Emim][Ala]

-11.81

-38.66

-90.10

[Emim][Lys]

-24.99

-40.74

[Emim][Arg]

-13.70

-46.91

130

Emim Gly Emim Ala

TSs

100

Ea (kJ.mol-1)

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


Emim Lys Emim Arg

70 40

Rs 10

Ps -20 -50

Reaction coordinate Figure 4. The potential energy diagram for CO2 absorption by the ILs



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According to interaction energy, (∆E), of [Emim][X] (X=Gly-, Ala-, Lys-), a strong interaction is predicted between the cation and anion pairs, while in the case of [Emim][Arg], the interaction is weaker. Finally, it is reasonable to conclude that [Emim][X] (X=Gly-, Ala-, Lys-, Arg-) act as a nucleophile, electron donor, in the CO2 absorption process. Table 4. the HOMO and LUMO energies and the DFT reactivity indices for the amino acids ILs EHOMO(a.u.)

ELUMO(a.u.)

𝜇 (a.u.)

𝜂 (a.u.)

𝜔 (a.u.)

[Emim][Gly]

-0.2475

-0.0192

-0.1333

0.2283

0.0390

[Emim][Ala]

-0.2485

-0.0228

-0.1356

0.2257

0.0410

[Emim][Lys]

-0.2502

-0.0214

-0.1358

0.2288

0.0403

[Emim][Arg]

-0.2575

-0.0225

-0.1399

0.2340

0.0420

[Emim]+

-0.4879

-0.1470

-0.3170

0.3409

0.1474

[Gly]-

-0.1038

0.1166

0.0064

0.2209

0.0009

[Ala]-

-0.1046

0.1088

0.0021

0.2135

0.000

[Lys]-

-0.1122

0.0701

-0.0210

0.1823

0.0012

[Arg]-

-0.1209

0.0665

-0.0271

0.1875

0.0010

CO2

-0.4541

-0.0014

-0.2278

0.4556

0.0570

Compound

3.4. Charge transfer analysis Through NBO analysis, using the second-order perturbation theory, it is possible to calculate the stabilization energy, E(2), including information about the donoracceptor interactions in describing the amine-CO2 reaction. Moreover, the study of the atomic charge changes during the reaction is useful to describe the charge transfer effects related to the bond formation and bond cleavage. 17 ACS Paragon Plus Environment

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NBO analysis was performed by using the M06-2X functional by employing the 6-311++G (d,P) basis set. Within the first part of this analysis, natural atomic charges of the atoms in the reactants (R), TSs and products were analyzed. Then, the charge transfer process during the reaction was investigated as represented in Table S5. On the basis of Table S5, atomic charge changes, especially at the TSs may be the driving force of the CO2 absorption process. The maximum changes in the atomic charges at the TSs are related to C12, N5, O13 and H7 atoms. Table 5 shows the charge transfer and the stabilization energies of the donor-acceptor interactions in the reactants and TSs. In the TS structures, the strongest interactions or the largest stabilizing effect are seen between the N5 atoms of the amine group of the anions and C12 atom of CO2. Lone pair electrons of N5 interact with the antibonding orbital of C12 (lp N5→π* C12-O13),

in the cases of [Emim][Ala] and [Emim][Arg], while [Emim][Gly]

and [Emim][Lys] show the interaction types of σN5-H7→π*C12-O13 and lp O13→σ* N5H7,

respectively. These types of the donor-acceptor interactions confirm N5-C12

bond formation, during CO2 absorption by the amino acid anions. Figure 5 shows the correlation of the activation energies and sum of the stabilization energies at the TSs. Since the stabilization energy of the [Emim][Arg] is higher than that of other ILs, it is reasonable to conclude that an increase in the number of the amine group of amino acid anions, increases the possibility of CO 2 absorption and decreases the activation energy of CO2 absorption, which is in accordance with the obtained results from the energy analysis.


The Journal of Physical Chemistry 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

Page 20 of 32

Table 5. Main donor-acceptor interaction energies in the reactants, TSs and product at the M06-2X level. Donor → Acceptor (Reactant)

[Emim][Gly]

[Emim][Ala]

[Emim][Lys]

[Emim][Arg]

lp O13→π*C12-O14 lp O3→π* C9-H8 lp O13→π*C12-O14 ∑E(2)R lp O13→σ* C12 lp O14→π* C12-O13 lp O13→π* C12-O14 ∑E(2)R lp N5→π*C12-O14 Lp O3→π* C1-O2 Lp O13→π* C12-O14 ∑E(2)R lp O13→π* C12-O14 lp O14→σ* C9-H8 lp O14→π* C12-O13 ∑E(2)R

E(2)R* (kcal.mol-1)

15.70 1.97 9.95 27.62 3.41 16.04 37.77 57.22 3.06 21.37 15.61 40.04 3.09 20.96 15.80 39.85

Donor →Acceptor (TS)

E(2)R* (kcal.mol-1)

Donor →Acceptor (Product)

E(2)P* (kcal.mol-1)

σ N5-H7→π* C12-O13 σ C12-O14→σ* N5-H7 σ N5-H7→π* C12-O14 ∑E(2)TS lp O2→σ* C9-H8 lp N5→π* C12-O13 σ N5-C12→σ* O13-H7 ∑E(2)TS lp O13→σ*N5-H7 σ O13→π* N5-C12 lp O13→σ* N5-H7 ∑E(2)TS lp N5→σ*C12-O13 lp O14→π* N5-C12 lp N5→π* C12-O13 ∑E(2)TS

10.59 6.54 3.60 20.73 7.92 3.84 5.56 17.32 10.19 11.06 8.79 30.04 15.20 7.74 14.39 37.33

lp O13→σ* C12-O14 lp O14→σ* C12-O13 lp N5→σ* C12-O14 ∑E(2)P lp O13→σ* C12-O14 lp O14→σ* C12-O13 lp N5→σ* C12-O14 ∑E(2)P lp O13→σ* C12-O14 lp O14→σ* C12-O13 lp N5→σ* C12-O14 ∑E(2)P lp O13→σ* C12-O14 lp O14→σ* C12-O13 lp N5→σ* C12-O14 ∑E(2)P

51.88 30.91 64.02 146.81 59.70 29.68 61.99 151.37 32.64 25.44 61.42 119.5 28.46 32.75 15.20 76.41

*E(2)R=E(2) reactant, E(2)TS=E(2) TS and E(2)P=E(2) Product


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


Ea (kJ.mol-1)

Page 21 of 32

144 142 140 138 136 134 132 130 128 126 124

R² = 0.9963

15

20

25

30

35

40

ΣE(2)TS (kJ.mol-1) Figure 5. Correlation diagram between the activation energies and sum of the stabilization energies at the TSs.

4.4. QTAIM Analysis QTAIM analysis is a powerful method for investigation of the nature of interactions at the bond critical points (BCPs). This analysis is performed by calculation of the topological properties such as electron density (𝜌(𝑟)), Laplacian (∇2 𝜌(𝑟)), kinetic (G(r)) and potential (V(r)) energy densities and –G/V ratios. These parameters are related to the strength of a given bond. For example, in the case of a covalent interaction, 𝜌(𝑟) is greater than 0.20 au. and ∇2 𝜌(𝑟) value is negative. In the case of hydrogen and van der Waals interactions, the values of electron density (𝜌(𝑟)), is ~10-2 and 10-3 au, respectively. The Laplacian (∇2 𝜌(𝑟)), is positive and its normal value for hydrogen bond is in the range of 0.02-0.139 au.55 The results of the molecular topology of the BCPs at the TSs structure were represented in Table 6. The electron density of the C12-N5 BCP increases from the reactants toward the TSs, while the Laplacian of the electron density decreases. These changes confirm the bond formation of C12-N5 during the CO2 absorption by the ILs. In contrast to these changes, N5-H7 BCP shows an electron density 20 ACS Paragon Plus Environment


decrement at the TS, which is according to the N5-H7 bond cleavage. Also, in the TS structures, because of negative values of ∇2 𝜌(𝑟), the interaction of O13 and H7 leads to O13-H7 bond formation. This type of interaction was verified in the studied ILs, but in the cases of [Emim][Lys] and [Emim][Arg], the strength of this bond is higher than [Emim][Ala] and [Emim][Gly]. For better insight about the TS structure, ring critical points (RCPs) of the TS structures are represented in Table S6. Table 6. Topological parameters of the TSs in CO2 absorption by the ILs Bond Parameters [Emim][Gly] [Emim][Ala] [Emim][Lys] [Emim][Arg] 0.148 0.146 0.144 0.149 𝜌(𝑟) 2 N5-H7 -0.168 -0.151 -0.151 -0.155 ∇ 𝜌 V(r) -0.180 -0.178 -0.175 -0.183 -G/V 0.380 0.393 0.392 0.393 0.0246 0.274 0.247 0.249 𝜌(𝑟) 2 C12-N5 -0.571 -0.569 -0.568 -0.573 ∇ 𝜌 V(r) -0.343 -0.339 -0.342 -0.341 -G/V 0.291 0.290 0.292 0.290 0.145 0.097 0.147 0.153 𝜌(𝑟) 2 O13-H7 -0.077 0.409 -0.101 -0.114 ∇ 𝜌 V(r) -0.193 -0.149 -0.197 -0.201 -G/V 0.450 0.145 0.437 0.432

Theoretical average values of –G/V ratios of the C12-N5 and O13-H7 bonds are 0.291 and 0.450 respectively, confirming their covalent nature in the CO 2 absorption process. A pictorial representation of the ELF and LOL in the critical bond region of the N5…C12 and O13…H7 at the TS is shown in the Figure 6. According to Figure 6, higher values of ELF show a covalent interaction between these atoms. Then, there is considerable interaction between the C12 atom of CO2 and N5 atom of amino acid chain. Also, there is another strong interaction between


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[Emim][Gly]

[Emim][Ala]

TS( N5….C12-O13....H7)

TS( N5….C12-O13....H7)

Emim][Lys]

[Emim] [Arg]

TS( N5….C12-O13....H7)

TS( N5….C12-O13....H7)

Figure 6. ELF and LOL of the TS in the CO2 absorption process by the ILs. 22 ACS Paragon Plus Environment


O13 atom of the CO2 and H7 atom of the amine group in the amino acid chain, which can be considered as the driving force of the carbamic acid formation. The RDG isosurface provides significant information about interaction. These surfaces were plotted at the TSs and shown in Figure 7. According to Figure 7, we have a weak interaction between the cation and anion in the ILs (green and light brown region). In all ILs, there are steric effects in the imidazolium ring (red point in rings). The hydrogen bonds are observed in all structures and the strongest one is related to [Emim][Arg]. In all ILs, N5…H7…O13 interaction is the strongest, which leads to carbamic acid formation as the final product.

Figure 7. The plot of RDG iso-surface for showing the weak interactions in the TS structures.


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Conclusion In this research, we investigated the thermodynamic, kinetic and mechanism aspects of CO2 absorption by [Emim][X] (X=Gly-, Ala-, Lys-, Arg-) . Calculations were done, using the M06-2X/6-311++G(d, p) level of theory. At the first step of this research, the most probable configurations of the cations against the anions were investigated. Thermodynamic studies showed that the CO2 absorption by the [Emim][Lys] is more favorable than other ILs, and ∆H values of CO2 absorption shows a chemisorption nature, in which the amine group of the amino acid anions is the active site for CO2 absorption. On the basis of these results, the number of amine groups of the amino acid is directly correlated with the rate of CO2 absorption. The rate of CO2 absorption by [Emim][Arg] is faster than other ILs (Ea =125.72 kJ.mol-1) which is in agreement with the experimental results. In the case of [Emim][Ala], the rate of CO2 absorption is less than [Emim][Gly] because of a longer alkyl chain. NBO results show an effective charge transfer from the amine group of the anions and CO2 (Lp N5→σ*C12-O13), according to N5-C12 bond formation. Topological parameters at the TSs show a covalent nature for the new bond formation of the N-C between the ILs and CO2, which confirms the possibility of CO2 absorption. Finally, [Emim][Arg] is the best candidate for the absorption of CO2, because of a higher rate than other ILs. Supporting Information Supplementary tables and figures of the molecules. This material is available free of charge via… Acknowledgment



Research Council of Ferdowsi University of Mashhad is acknowledged for financial supports (Grant No. 3/32435). We hereby acknowledge that part of this computation was performed on the HPC center of Ferdowsi University of Mashhad.

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Carbon Dioxide Absorption by the ImidazoliumâAmino Acid Ionic

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