Enthalpic Driving Force for the Selective Absorption of CO2 by an Ionic

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Enthalpic Driving Force for the Selective Absorption of CO by an Ionic Liquid Clyde A. Daly, Thomas Brinzer, Cecelia Allison, Sean Garrett-Roe, and Steven A. Corcelli J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00347 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018

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The Journal of Physical Chemistry Letters

Enthalpic Driving Force for the Selective Absorption of CO2 by an Ionic Liquid Clyde A. Daly Jr.,1 Thomas Brinzer,2,3 Cecelia Allison,1 Sean Garrett-Roe,2,3 and Steven A. Corcelli*,1 1

Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46656

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Department of Chemistry, University of Pittsburgh, 219 Parkman Ave., Pittsburgh, Pennsylvania 15260

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Pittsburgh Quantum Institute, University of Pittsburgh, 3943 O'Hara St., Pittsburgh, Pennsylvania 15260 Abstract Molecular dynamics (MD) simulations validated against two-dimensional infrared (2D IR) measurements of CO2 in an imidazolium-based ionic liquid have revealed new insights

into the mechanism of CO2 solvation. The first solvation shell around CO2 has a distinctly quadrupolar structure, with strong negative charge density around the CO2 carbon atom and positive charge density near the CO2 oxygen atoms. When CO2 is modeled without atomic charges (thus, removing its strong quadrupole moment), its solvation shell weakens and changes significantly, into a structure that is similar to that of N2 in the same liquid. The solvation shell of CO2 evolves more quickly when its quadrupole is removed, and we find evidence that solvent cage dynamics is measured by 2D IR spectroscopy. We also find that the solvent cage evolution of N2 is similar to that of CO2 with no atomic charges, implying that the weaker quadrupole of N2 is responsible for its higher diffusion and lower absorption in ionic liquids.

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Table of Contents Image

Ionic liquids (ILs) have attracted tremendous attention for their potential use as materials for carbon capture and sequestration.1–6 The excitement for the use of ILs for carbon capture is motivated by the fact that CO2 has greater solubility in common ionic liquids than other small-molecule gases common in air, such as N2.7,8 A wealth of computational and experimental studies have examined the structure and dynamics of solutions of CO2 in ionic liquids.3,9–19 In particular, Huang et al. published a seminal paper in 2005 on the structure and dynamics of CO2 dissolved in the 1-butyl-3-methylimidizolium hexafluorophosphate [bmim][PF6] IL.11 This study found that CO2 reorganizes free spaces already present in the liquid, which would normally not be large enough to accommodate CO2, into voids that it can occupy.11,20 Thus, when CO2 is dissolved into the IL its partial molar volume is smaller than in neat supercritical CO2 at the same thermodynamic conditions because no new space has been created within the liquid. Here, using molecular dynamics (MD) simulations, we have investigated CO2 solvation in [bmim][PF6] from the perspective of the solute and we find that the local solvation 2


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structure is distinctly quadrupolar in character. We also find that the dynamical evolution of the solvent cage is captured by 2D IR measurements of the spectral diffusion dynamics of the asymmetric stretch of CO2. Taken together, these results reveal an enthalpic mechanism for the selectivity of CO2 dissolution in ILs that complements the entropic model of cavity reformation from the work of Huang et al. In particular, we find a mechanism for this cavity reformation – the CO2 uses its charges to move the ions into its preferred solvation structure. Using the same protocols and IL force fields developed in prior work,21–23 a 1 µs simulation of a single solute molecule in 256 ion pairs of [bmim][PF6] was collected with a 250 fs time resolution. The length of the trajectory was chosen to ensure adequate sampling of the solvent around the solutes, and the resolution is sufficient to capture all but inertial contributions to the dynamics. The solutes investigated were CO2, N2, and CO2 with its atomic charges set to zero (uncharged CO2). The solutes were described with the transferable potentials for phase equilibria (TraPPE) model.24 Figure 1 compares the charge density in the immediate vicinity of the solutes over time. The charge density is computed in cylindrical coordinates with respect to the bond axis of the linear solutes, and a two-dimensional slice is shown in Figure 1 (see SI for more details). Atomic charges of the solvent are binned if the position of the solvent atom satisfies |𝑧𝑧| ≤ 5.0 Å and 𝑟𝑟 ≤ 5.0 Å at both 𝑡𝑡 = 0 and 𝑡𝑡 = 𝜏𝜏, where 𝑧𝑧 is the height of the

considered atom above the plane passing through the center of the molecule and 𝑟𝑟 is the distance of the atom in that plane. Thus, Figure 1 captures the structure and dynamics of the initial solvent cage around the solutes.

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ILs made from ions that have non-polar moieties organize themselves in two major ways.25 One major structure is an alternation between more highly and less highly charged regions. Inside of the charged regions, positive and negative charges alternate. The cation tails of [bmim][PF6] are not large enough to create strong polarity alternation, though there will still be regions of higher and lower absolute charge density.25 Figure 1 shows that CO2 is preferentially located in the charged regions of the liquid, while N2 and uncharged CO2 are more indifferent to the IL moieties by which they are surrounded. N2 and uncharged CO2 also escape from their local solvation environment more quickly. There is a clear quadrupolar solvation pattern in the case of CO2 (Figure 1 A, left panel). Near the positive carbon (+0.700 e), there is a high density of negative charge due to preferential interactions with the fluorine atoms on the anions. Near the negative oxygens (-0.350 e) there is a weaker but still significant concentration of positive charge. The positive hydrogens on the [bmim] rings dominate this region. As one moves away from the CO2, the sign of the charge density oscillates and the magnitude decreases. The uncharged CO2 is solvated very differently (Figure 1A, right panel). On average, the absolute value of the charge density is much lower. Additionally, the pattern of charge alternation is different. Instead of distinct lobes of charge, alternating rings of charge surround the uncharged CO2. The solvation of N2 (Figure 1A, middle column) lies between these two extremes, but is more qualitatively similar to the uncharged CO2. Near the N2 solute, there are rings of charge, similar to the uncharged CO2, but there is also a modest increase in the amount of negative charge near the center of the molecule and the amount of positive charge at the poles of the molecule. The absolute value of the charge in these rings is small, near

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the values for the uncharged CO2. Thus, the smaller quadrupole moment of N2 is not sufficient to reorganize the nearby solvent into the distinctive quadrupolar pattern observed for CO2. As we move through time, the basic features of these charge patterns stay roughly the same, but decrease in intensity. However, the charge density around CO2 decreases less quickly. In particular, the charge density present near the CO2 carbon atom at long times indicates substantial anion persistence, extending to 1 ns. Meanwhile, uncharged CO2 and N2 have no remaining excess of charge from their first snapshots 1 ns later (Figure 1E). The TraPPE force field models N2 with 3 “atomic” locations – two equal negative charges (-0.482 e) are at the locations of the nitrogen atoms, and a fictitious positive charge (+0.964 e) is placed in the center of the bond. This is done to properly represent the quadrupole moment of N2. CO2’s TraPPE model has a similar arrangement, but with slightly weaker charges (in both cases, the solutes are also modeled with atomic-centered Lennard-Jones interactions). Because these models are so similar, differences between the solvation shell of N2 and that of CO2 are strongly influenced by the difference in the total strength of their quadrupoles. Experimentally, the quadrupole moment of CO2 is 4.017 B and of N2 is -1.415 B.26,27 Even though it has smaller atomic charges, the twotimes longer bond axis of CO2 gives it a three-times larger quadrupole moment than N2. Other molecules with no overall charge and large multipole moments, such as SO2 and water, are known to associate preferentially with charged moieties when dissolved in ILs, and AIMD simulations support that CO2 interacts more strongly and specifically with anions and cation charge centers than uncharged moieties.20,22,28–33 This suggests CO2

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will have stronger and longer-lived interactions with charged particles, such as the ions that ionic liquids are made of, than N2. The average electronic contribution to the solvation energy of CO2, calculated by taking the appropriate integral over the charge density (with a spherical cutoff of 20 Å; see SI), is -1.84 kcal mol-1. For N2, it is -0.26 kcal mol-1. By definition, it is 0.00 kcal mol-1 for the uncharged CO2. The frequency fluctuation auto-correlation function (FFCF) is an observable that is accessible to theory through direct calculation from a vibrational frequency trajectory, and to experiment through examination of 2D-IR spectra with varied waiting times. It tracks the randomization of the vibrational frequency and is generally sensitive to local structure and dynamics. We used 2D-IR spectroscopy experiments as done in prior work to obtain the experimental FFCF (experimental details are contained in the SI), and applied a previously developed spectroscopic map to our 1 𝜇𝜇s simulations to calculate theoretical

FFCFs for CO2 and uncharged CO2.21,23,34–36 The experimental methods specific to this work will be elaborated in detail in a follow up paper. These functions are quantified by finding the best fits to a multi-exponential function, 𝐶𝐶(𝑡𝑡) = 𝑎𝑎1 𝑒𝑒

𝑡𝑡 −𝜏𝜏 1

+ 𝑎𝑎2 𝑒𝑒

𝑡𝑡 −𝜏𝜏

2

+ 𝑐𝑐

(1)

from 𝑡𝑡 = 1 ps to 𝑡𝑡 = 200 ps. The experiment is unable to capture dynamics in the fast modulation limit that extends to several hundred femtoseconds for CO2.21 Thus, we start our fits of the calculated FFCF at 1 ps for consistency. 200 ps is the maximum waiting time accessible to experiment. The time constants (𝜏𝜏𝑖𝑖 ) are shown in Table 1 and the amplitudes and offset are presented in the SI. Because the experiment cannot capture this initial fast decay, a comparison of the amplitudes and offset of the calculated and

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experimental FFCF is not meaningful. However, the time constants are directly comparable. In our case, the calculated time constants are in good agreement with the experimental time constants for CO2, but are dissimilar to those for the uncharged CO2 (Table 1). The agreement validates the theoretical model underlying the FFCF calculation, and shows that the quadrupole moment of the CO2 and the resulting solvent rearrangement is an important contributor to the CO2 solvation dynamics. In order to quantify the dynamics of the solvating ions, we calculate solvent cage correlation functions (SCCFs, 𝐶𝐶𝑆𝑆𝑆𝑆 (𝑡𝑡)). In these calculations, the entire set of anions and

cation rings within the first solvation cylinder (|𝑧𝑧| ≤ 5.0 Å and 𝑟𝑟 ≤ 5.0 Å) is considered the

solvent cage of the solute. Otherwise, the correlation functions are defined identically to

the ion cage correlation functions of Zhang et al.37 The SCCFs are shown in Figure 2, and their time constants are in Table 1. The SCCFs were fit under an identical protocol to the FFCFs, using Eq. 1 and the 𝑡𝑡 = 1 ps to 𝑡𝑡 = 200 ps range.

In a previous study, Brinzer et al. found a strong correlation between experimental

FFCF time constants for small molecule probes and bulk IL viscosity across a number of ILs.34 Using simulations, Kohagen et al.38 first found and Zhang et al.37,39 later confirmed that ion pair and ion cage lifetimes are correlated to IL transport properties such as diffusion and conductivity and bulk properties such as viscosity. Corroborating these observations, Araque et al. found that the dynamics of small neutral solutes are gated by the motions of the ionic liquid, while charged solutes incorporate themselves into the ion network.40 An interesting correspondence we find is that the time constants for the SCCF are nearly the same as those for the FFCF for CO2. This result confirms the idea that the FFCF for small probe molecules in an IL solvent reports on the evolution of local IL

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structure, particularly the breakup of the local solvation environment on the experimentally accessible timescales. However, this relationship depends on the strength of the interactions between the probe and the liquid. The uncharged CO2 has quite different dynamics from the experiment and CO2. The correspondence between the timescales of the FFCF and the SCCF remains but is weaker, since the dynamics of the uncharged CO2 are less strongly linked to the dynamics of the surrounding solvent. This shows that the frequency fluctuation dynamics of the solute are determined by the strength of the interaction between it and the ions and whether or not the probe can successfully integrate itself into the ionic network. The charged CO2 does this well (Figure 1) and thus experiences slower dynamics. The uncharged CO2 has weaker interactions, and thus experiences faster dynamics. The FFCF of N2 is not examined because its small infrared activity precludes 2D IR measurements. However, we are able to compare its SCCF to those of the CO2 models. The SCCF for N2 is remarkably similar to that of the uncharged CO2 model (Figure 2), but its time constants are similar to those of the charged CO2 (Table 1). Thus, both the structure and dynamics of N2 are somewhere between those of the two CO2 models, but with a stronger similarity to the uncharged CO2. This is also supported by the simulated diffusion constants of the three models: 1.5 ± 0.1 x 10-10 m2 s-1 for charged CO2 (compare to 0.6 ± 0.2 x 10-10 m2 s-1 from experiment),41 1.9 ± 0.2 x 10-10 m2 s-1 for N2, and 2.3 ± 0.2 x 10-10 m2 s-1 for uncharged CO2. By comparison to N2 and uncharged CO2, CO2 is stuck within its local cages. This suggests that N2 has a quadrupole moment below the threshold for being incorporated strongly into the more highly charged IL regions, but is still affected in a stronger way by the IL dynamics than uncharged CO2.

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In conclusion, we find that the charge distribution of a solute strongly influences its local structure and dynamics in ILs. We also find that even molecules without overall charges or dipoles may be incorporated into the IL ion cages if the multipole moment that they do have is strong enough, as is the case for CO2 and is not the case for N2. This reduces the diffusion and promotes the solubility of CO2 over the apparently similar N2. Finally, the FFCFs of probe molecules of various charge distributions are directly connected to the both the dynamics of ions near the probe and how strongly the probe interacts with those ions.

Supporting Information Additional details concerning the MD simulations and associated calculations are contained in the Supporting Information.

AUTHOR INFORMATION Corresponding Author Steven Corcelli: [email protected] Funding Sources No competing financial interests have been declared. This research was supported by the National Science Foundation: CHE-1565471 to S.A.C. and CHE1454105 To S.G.-R.

ACKNOWLEDGMENT The authors would like to thank the Center for Computing at the University of Notre Dame for computational resources and support.

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Table 1. Time constants of multi-exponential fits to the FFCFs and SCCFs for CO2, uncharged CO2, N2 (SCCF only) and experiment.

Experiment CO2

FFCF

𝜏𝜏1 (ps) 4.49

𝜏𝜏2 (ps)

FFCF

4.46

73.1

SCCF

5.76

79.2

77.7

Uncharged CO2

FFCF

2.37

51.1

SCCF

4.57

66.9

N2

SCCF

6.29

75.9

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Figure 1. Charge density of IL solvent around CO2, N2, and uncharged CO2 over time. The time separations are (A) 𝛕𝛕 = 𝟎𝟎 ps, (B) 𝛕𝛕 = 𝟏𝟏 𝐩𝐩𝐩𝐩, (C) 𝛕𝛕 = 𝟏𝟏𝟏𝟏 𝐩𝐩𝐩𝐩, (D) 𝛕𝛕 = 𝟏𝟏𝟏𝟏𝟏𝟏 𝐩𝐩𝐩𝐩, (E) 𝛕𝛕 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏 𝐩𝐩𝐩𝐩.

Figure 2. Solvent cage correlation functions for CO2, N2, and uncharged CO2. Uncharged CO2 loses memory of its initial cage more quickly than N2, which loses memory more quickly more quickly than CO2.

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Enthalpic Driving Force for the Selective Absorption of CO2 by an Ionic

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