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. 2024 Jan 3;146(2):1612–1618. doi: 10.1021/jacs.3c11808

On the Key Influence of Amino Acid Ionic Liquid Anions on CO2 Capture

Bohak Yoon 1, Sijia Chen 1, Gregory A Voth 1,*
PMCID: PMC10798249  PMID: 38170906

Abstract

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Amino acid ionic liquids (AAILs) are promising green materials for CO2 capture and conversion due to their large chemical structural tunability. However, the structural understanding of the AAILs underlying the CO2 reaction dynamics remains uncertain. Herein, we examine the steric effects of AAIL anions with various chemical structures on CO2 capture behavior. Based on ab initio free-energy sampling, we assess reaction mechanisms for carbamate formation via a two-step reaction pathway with a zwitterion intermediate undergoing dynamic proton transfer. Our results show that free-energy barriers for carbamate formation can be significantly reduced as the degree of steric hindrance of the anions decreases. Further analyses reveal that reduced steric hindrance of anions causes markedly stronger intermolecular interactions between zwitterion and anions, leading to an increased kinetically favorable intermolecular proton transfer for carbamate production. We also describe the correlation strength between intramolecular interactions within the zwitterion and intermolecular interactions between the zwitterion and anions. We conclude that the favored structural flexibility due to the less steric hindrance of the zwitterion leads to enhanced intermolecular interactions, facilitating proton transfer to nearby AAIL anions for carbamate formation. Our study provides invaluable insight into the influence of various degrees of steric hindrance of the AAIL anions governing CO2 chemisorption. These findings may aid in the design of optimal AAIL solvents for the CO2 capture process.

Introduction

Global warming is mainly caused by the increasing presence of CO2 emissions in the atmosphere.1,2 Due to large fossil fuel consumption, extensive efforts are urgently needed to reduce CO2 release into the environment as well as direct removal from the atmosphere.3,4 The most extensively employed incumbent technology for postcombustion CO2 capture is understood to be amine scrubbing involving a chemical absorption process with aqueous amines.5 Despite the widespread utilization of this technology, it has several critical drawbacks, including degradation of amine solvents6−9 and corrosion10 of the operating equipment, leading to the release of hazardous chemicals into the environment11−14 and requiring high recovery costs for energy utilization.12,14 Amino acid ionic liquids (AAILs) are greener alternative solvents for both direct air and postcombustion CO2 capture owing to their biocompatibility, cost-effectiveness, synthesizability, and chemical tunability.15−19 In particular, AAILs have a large window for structural flexibility,16 allowing for their optimal design for the CO2 capture process. As AAILs are known to undergo chemical reactions with CO2 to form carbamate products through proton transfer of zwitterion intermediate,20 similar to aqueous amines,21 a detailed understanding of the effects of structural variation of AAILs on the CO2 reaction mechanisms and dynamics is needed.

A few theoretical papers based on static density functional theory (DFT) calculations have attempted to reveal the effects of structural variations, specifically the degree of steric hindrance, of AAIL anions during CO2 chemisorption.22−25 However, the influence of the chemical structural tunability with various degrees of steric hindrance of AAIL anions on reaction dynamics and the kinetics of carbamate formation governing the capture of CO2 by AAIL cannot be explained by static calculations, with only a few molecules in an implicit solvent model or in the gas phase. It is therefore important to obtain a deeper fundamental understanding of the structural, kinetic, and dynamic features in the explicit condensed phase of AAILs; this includes the degree of steric hindrance as the absorption reaction of CO2 by AAILs is a highly dynamic process that can be primarily affected by steric effects.

In this study, we investigate the influence of AAIL anions with a particular focus on their steric effects on the reaction mechanisms and overall performance in the CO2 chemisorption process. Specifically, we examine AAIL anions of glycine, alanine, and valine with various degrees of steric hindrance due to the presence of hydrophobic methyl or isopropyl groups. We compute explicit ab initio molecular dynamics (AIMD) combined with an enhanced sampling technique of well-tempered metadynamics to assess free-energy surfaces and corresponding barriers, and thus the kinetic preference, for carbamate formation in condensed phases of CO2-loaded cholinium cations and glycine, alanine, and valine AAIL anions having a diverse set of steric hindrances. We assess the structural and dynamic features responsible for the preferred reaction kinetics of dynamic proton transfer from the zwitterion intermediate during carbamate formation. We further analyze the critical influence of the relative intramolecular flexibility of the anions and zwitterions toward their intermolecular interactions that may predominantly affect the overall CO2 capture.

Experimental Section

Computational Methods

Born–Oppenheimer AIMD simulations based on DFT were prepared from all-atom classical MD simulations computed with the LAMMPS software.26 A cubic-periodic simulation box containing 15 AAIL pairs with cholinium cations and glycine, alanine, or valine anions and with five CO2 molecules was set up using PACKMOL.27 The MD system was simulated for 20 ns under an isobaric–isothermal (constant NPT) ensemble to obtain the equilibrated density and box dimensions. The system was then equilibrated under the canonical (constant NVT) ensemble for 15 ns before a production run of 50 ns. The final structures from the MD production runs were used as the initial configurations for the AIMD simulations. All AIMD runs were computed with the CP2K package28 using the strongly constrained and appropriately normed (SCAN)29 semilocal density functional at the meta-generalized gradient approximation theory level (meta-GGA). The sensitivity to the density functional choice was confirmed by comparing free-energetics with other functional choices (Supporting Information); the conclusions drawn were found to be largely unchanged by the functional selection. Norm-conserving Goedecker–Teter–Hutter (GTH) pseudopotentials30 were used to represent interactions between ionic cores and valence electrons. A hybrid Gaussian and plane-waves (GPW)31 method was employed in the QUICKSTEP32 module using a triple-ζ Gaussian basis set with two polarization function sets (TZV2P) with a plane-wave cutoff kinetic energy of 460 Ry. Only the Γ-point was implemented to sample the Brillouin zone due to the lack of structural symmetry in ionic liquids, while the selection of k-point sets was also tested. The van der Waals (long-range) interactions were treated using the DFT-D3 semiempirical dispersion corrections developed by Grimme et al.33 with Becke-Johnson (BJ) damping.34 The temperature of the system was controlled with canonical sampling through velocity rescaling (CSVR).35 The equations of motion were integrated with a velocity-Verlet algorithm36,37 using a time step of 0.3 fs. All AIMD simulations with a cubic system box were computed for 50 ps in the constant NVT ensemble prior to the production runs of 100 ps. After equilibration by the production runs, metadynamics simulations38 with a well-tempered algorithm39,40 were implemented using PLUMED41 for computing free-energy surfaces for the reaction steps. Ten replicas were used for each reaction pathway during well-tempered metadynamics simulations, while each replica was computed for 100 ps for full convergence. Further details on the metadynamics runs are available in the Supporting Information.

Results and Discussion

The molecular mechanism of CO2 capture by AAIL through the chemisorption process is predominantly determined by carbamate formation via proton transfer from a zwitterion intermediate formed from the reaction between CO2 and the AAIL anion.20,42 Various studies based on static quantum mechanical (QM) calculations in the gas phase or an implicit solvent model reveal that CO2 absorption capacity, indicative of the relative favorability of carbamate formation, can be largely affected by various chemical structures of the AAIL anions, highlighting the importance of steric effects during the CO2 absorption process.22−25 The intermolecular interactions of AAIL anions and zwitterions may become substantially affected by the degree to which hydrophobic methyl or isopropyl groups induce various steric hindrance features, similar to the cases of aqueous amine solvents.43 However, important dynamics and configurational and conformational entropic effects during the CO2 capture process that may largely govern the overall performance in fully condensed phases of ionic liquids cannot be described with the static QM methods. It is thus critical to have a deeper fundamental understanding of the important dynamic and entropic effects induced by various degrees of steric hindrance of AAILs during the CO2 chemisorption process.

In this work, AAIL anions of glycine, alanine, and valine with various degrees of steric hindrance were investigated to assess their steric effects on the CO2 reaction mechanism. From the perspective of the chemical structure (shown in Figure 1a), alanine (valine) has one methyl (isopropyl) group on the α-carbon compared to glycine. Thus, the degree of steric hindrance due to the presence of a methyl or isopropyl group increases in the order of glycine, alanine, and valine (glycine < alanine < valine). The overall reaction pathways for the three cases are illustrated in Figure 1. Specifically, the reaction mechanism of CO2 chemisorption by AAIL anions involves a two-step reaction route: a nucleophilic attack by a lone pair of a N atom in the anion to an electrophilic C atom of CO2, leading to zwitterion intermediate formation, followed by H+ transfer to a nearby anion, thereby forming a product pair of carbamate and a neutral amino acid.20,42 Here, we compare this particular reaction path involving intermolecular proton transfer to specifically understand the various degrees of steric hindrance of AAIL on the overall CO2 capture reaction mechanism. Other possible reaction pathways are also investigated (see Supporting Information); however, we conclude that investigating the above reaction pathway is adequate for understanding the intermolecular features involving dynamics. The complete reaction routes for glycine (in blue), alanine (in orange), and valine (in green) are shown in Figure 1b–d, respectively.

Figure 1.

Figure 1

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(a) Chemical structures of AAIL anions of glycine (in blue), alanine (in orange), and valine (in green), with various degrees of steric hindrance caused by the methyl or isopropyl groups studied in this work. Two-step reaction pathways for carbamate formation are also shown. The first step involves zwitterion formation from the reaction of CO2 with the AAIL anion of (b) glycine, (c) alanine, and (d) valine, while the second step depicts intermolecular proton transfer from the zwitterion to the O atom in AAIL anions of (b–d) cases, respectively.

To understand the steric effects of AAIL anions on the CO2 reaction mechanism and its reaction kinetics, we first carried out AIMD-metadynamics simulations at 300 K to obtain the free-energy surface (FES), as shown in Figure 2, of the two-step reaction pathways (as illustrated in Figure 1) for carbamate formation from the reaction between CO2 and AAIL anions with various degrees of steric hindrance (i.e., glycine, alanine, and valine). The initial reactions between CO2 and AAIL anions of glycine, alanine, and valine lead to zwitterion intermediate formation, which is described by a collective variable (CV) of ξ3 involving a bond distance between N in AAIL anions (glycine, alanine, and valine) and C in CO2. A comparatively larger ξ3 illustrates the initial state where CO2 and the anion are separated with a negligible interaction; a comparatively smaller ξ3 indicates a state where a zwitterion intermediate is formed through a bond formation between the two molecules. Thus, the first step following the minimum reaction path on the FES [i.e., (i) → (ii) → (iii) shown in Figure 2] describes the zwitterion formation. Thereafter, intermolecular H+ transfer from the zwitterion to the O atom in nearby AAIL anions of glycine, alanine, and valine occurs to form carbamate products. The intermolecular H+ transfer reaction routes are depicted by a CV of ξ1–ξ2—a linear combination of the coordination numbers describing the protonation state of both the zwitterion (ξ1) and the nearby anion (ξ2). Here, the two coordination numbers with switching functions are described as follows:

graphic file with name ja3c11808_m001.jpg 1
graphic file with name ja3c11808_m002.jpg 2

where in eqs 1 and 2, rO,zwitterion-H (rO,anion-H) is the instantaneous distance between the O atom in zwitterion (anion) and proton (H); rcutoff describes the distance beyond which the bond breaks and is set to 1.3 Å, which is determined from the first peak positions of the radial distribution functions from the average of five independent canonical ensemble AIMD runs. The CV ξ1 (ξ2) has a value close to unity if the proton is bonded to the O atom of the zwitterion (anion) and a value of zero if the proton is transferred. Thus, the CV of ξ1–ξ2 with a positive (negative) value close to 1 (−1) denotes a state in which the proton is bonded (transferred) to the zwitterion (the nearby anion) prior to (after) carbamate formation.

Figure 2.

Figure 2

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Free-energy surface, FES (in kcal mol–1), for the formation of carbamate from the reaction of CO2 with the AAIL anion of (d) glycine, (e) alanine, and (f) valine via zwitterion intermediate formation, computed from AIMD-metadynamics shown in the middle. The reaction path [(i) → (ii) → (iii) → (iv) → (v)] with the minimum free-energy path (in red empty circle) involves the first step [(i) → (ii) → (iii)] with zwitterion formation, followed by the second step [(iii) → (iv) → (v)] with intermolecular proton transfer to the O atom in AAIL anions of (a,d,g)-glycine, (b,e,h)-alanine, and (c,f,i)-valine cases, respectively. 1D free-energy profiles along the minimum free-energy path for the three cases are depicted on the right (g–i). The corresponding molecular configurations for the three cases are shown on the left (a–c), respectively, while N, O, C, and H atoms are depicted in the blue, red, cyan, and white balls. The statistical errors in the free-energy surfaces are all within <4 × 10–1 kcal mol–1.

Here, the second step along the minimum reaction path on the FES [i.e., (iii) → (iv) → (v) shown in Figure 2] illustrates intermolecular proton transfer from the zwitterion to the nearby anion to form carbamate. Thus, the FES describes the overall two-step reaction pathway [i.e., (i) → (ii) → (iii) → (iv) → (v) shown in Figure 2] for carbamate formation through intermolecular proton transfer from the zwitterion intermediate formed from the reaction between CO2 and AAIL anions. More details on the collective variables are given in the Supporting Information. From our AIMD-metadynamics for the glycine, alanine, and valine cases, 2D FES are shown in Figure 2d–f, while 1D free-energy profiles along the minimum free-energy path are shown in Figure 2g–i, respectively. The predicted free-energy barriers from AIMD-metadynamics simulations are 18.5 ± 0.2, 24.1 ± 0.3, and 29.9 ± 0.4 kcal mol–1 for the glycine, alanine, and valine cases, as shown in Figure 2d,g; e,h; and f,i, respectively. The molecular configurations for the three cases are also shown in Figure 2a–c, respectively. We also calculated equilibrium coefficients based on the computed free energies and compared them to the experimental values22 to confirm the validity of our results; more details on the equilibrium coefficients can be found in the Supporting Information. Our FES results reveal that nucleophilic reactions and subsequent proton transfer for carbamate formation become kinetically less facile, as seen by the higher barrier heights, as steric hindrance increases–that is, the least sterically hindered anion, glycine, yields the lowest free-energy barrier, followed by alanine and valine. Thus, the kinetic favorability of carbamate formation follows the order of glycine > alanine > valine, consistent with the order of CO2 absorption capacity reported in other work.22 In other words, the overall CO2 capture performance may become suppressed as the degree of hydrophobicity (steric hindrance) in the AAIL anions is enhanced. It is important to note that the free energies from our AIMD-metadynamics simulations explicitly incorporate a sufficient number of surrounding AAIL molecules with various effects. These include significant configurational and conformational entropic contributions and temperature fluctuations based on statistical mechanics and therefore provide a detailed mechanistic insight into the kinetics of CO2 reaction behavior in the condensed phases of ionic liquids, which cannot be described with static QM calculations with only a few molecules in the gas phase.22−25

To investigate relative structural favorability due to steric effects from dynamic simulations, we computed the radial distribution function (RDF), gzwitterion-anion (r), between the zwitterion and the three AAIL anions (glycine, alanine, and valine) from five independent AIMD simulations in the constant NVT ensemble at 300 K for 40 ps each (200 ps total). The RDF, as shown in Figure 3, illustrates uniquely different first-peak intensities for the intermolecular interactions of Hzwitterion(anion)–Oanion, describing the intermolecular interactions between the proton (Hzwitterion) and the oxygen atom (Oanion) in the anion. The intensity of the first peak around 2.0–2.1 Å increases with decreasing steric hindrance—that is, glycine with the least steric hindrance has the highest and sharpest first peak, followed by alanine and valine. The enhanced first-peak intensity in the RDF indicates stronger intermolecular interactions between the zwitterion and anion, thereby facilitating more facile proton transfer. In other words, intermolecular H transfer becomes more favored with a decreasing degree of steric hindrance in the order of glycine > alanine > valine. Our findings are consistent with our FES results (in Figure 2) that the free-energy barrier for intermolecular proton transfer from the zwitterion to the O atom in the anions is of the same order, thereby depicting its various kinetic favorabilities facilitating carbamate formation. It is worth noting that these important structural features evident from the RDF can highlight important steric effects on the relative kinetic tendency of the CO2 capture process by AAIL in the explicit condensed phase, which, as stated before, cannot be explained with a few molecules in the gas phase by static QM methods.

Figure 3.

Figure 3

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Radial distribution function, gzwitterion-anion (r), with respect to the distance r (in Å), between proton (H) in the zwitterion and the O atom (O) in AAIL anions of glycine (in blue solid line), alanine (in orange dashed line), and valine (in green dotted line) at 300 K from AIMD simulations. The statistical errors in the radial distribution function are all within <2 × 10–2.

To further investigate the influence of intramolecular structural flexibility (due to various degrees of steric hindrance) on the intermolecular features facilitating proton transfer to form carbamate products, we calculated two-dimensional combined distribution functions (2D-CDF) from five independent AIMD simulations in the canonical ensemble at 300 K for 50 ps each (250 ps total). Here, the 2D-CDF illustrates the correlation between the dihedral angles of the zwitterions (ψO2–C1–C2–N), representing their intramolecular structural flexibility, and the intermolecular interaction strength (rH–O) between the proton (H) in the zwitterion and the O atom in anions, thus illustrating the favorability of proton transfer for carbamate formation. As shown in Figure 4, the strength in intermolecular interactions of the zwitterion–anion weakens, evident by a smaller distribution with reduced intensity on 2D-CDF, as the structural flexibility decreases, and thus, the steric hindrance increases in the order glycine > alanine > valine. This suggests that the higher structural flexibility due to a lesser degree of steric hindrance allows zwitterions to interact more freely and strongly with anions due to enhanced intermolecular interactions; thus, carbamate formation can become more kinetically facile. Our 2D-CDF analyses are consistent with our AIMD free-energy sampling results in Figure 2, which show that carbamate formation becomes kinetically more favorable as the steric hindrance reduces. Our analyses demonstrate that the steric effects of AAIL anions can significantly affect the reaction mechanism as well as the structural, dynamic, and kinetic favorability of the CO2 capture process.

Figure 4.

Figure 4

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Combined distribution functions of dihedral angle distribution of zwitterions, ψO2–C1–C2–N (in radian), and radial distribution function, rH–O (in Å), between proton (H) in the zwitterion and the O atom in AAIL anions of (a) glycine, (b) alanine, and (c) valine, at 300 K from AIMD simulations. The statistical errors in the combined distribution functions are all within <2 × 10–2.

Conclusions

This study elucidates the influence of AAIL anions on CO2 capture behavior. Specifically, we investigated various degrees of steric hindrance by the anions of glycine, alanine, and valine due to the presence of methyl/isopropyl groups and their influence on the preferred reaction mechanism, dynamics, and kinetics of carbamate formation. Based on AIMD combined with free-energy sampling with well-tempered metadynamics, we first examined the molecular mechanism for carbamate formation via a two-step reaction pathway involving intermolecular proton transfer from the zwitterion formed from the CO2 reaction with AAIL anions. Our results reveal that carbamate formation can become kinetically more facile (due to a lower barrier height) as the degree of steric hindrance of anions decreases (in the order valine > alanine > glycine). We further show that various degrees of steric hindrance cause markedly different strengths in intermolecular interactions between the zwitterion and anions, directly affecting the energetics and structural favorability of proton transfer from the zwitterion to nearby anions, forming carbamate products. Additionally, we analyzed the relative correlation strength between intramolecular interactions of zwitterion and intermolecular interactions of zwitterion–anions and concluded that higher structural flexibility due to weaker steric hindrance in the anion leads to enhanced intermolecular interaction strength, thereby leading to kinetically more favorable proton transfer, which facilitates carbamate formation. This study therefore provides invaluable insight into the steric effects of the AAILs governing the CO2 reaction mechanism. The present work may also aid in designing structurally optimal ionic liquids for both the direct air capture and postcombustion CO2 capture processes.

Acknowledgments

This research was supported by the Air Force Office of Scientific Research, under award no. FA9550-21-1-0380. The computational resources for this research were provided by the University of Chicago Research Computing Center (RCC).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c11808.

  • Detailed description of the sensitivity of the choice of DFT functionals, calculation and comparison of equilibrium coefficients, and metadynamics simulation setup and convergence tests (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja3c11808_si_001.pdf (866.5KB, pdf)

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