CO2 Activation Within a Superalkali-Doped Fullerene

Introduction

In 1985, Kroto and co-workers discovered an extremely stable cluster consisting of 60 carbon atoms during a study of long-chain carbon molecules. (Kroto et al., 1985). This cluster, called fullerene, has a football shape with 12 pentagonal and 20 hexagonal rings. (Kroto et al., 1985). Shortly after, a study presenting successful formation of fullerenes with a lanthanum atom trapped in the cavity of C₆₀, called endofullerene, C₆₀La, was published. (Heath et al., 1985). Another consequent experiment proved the stability of C₆₀La⁺ against H₂, O₂, NO, and NH₃. (Weiss et al., 1988). This suggested that the lanthanum atom can be “protected” by being encapsulated in the fullerene. Since then, a number of studies focusing on novel properties of C₆₀ and its interactions with other species have been carried out. (Ruoff et al., 1993; Diederich and Gómez-López, 1999; Bakry et al., 2007; Mignolet et al., 2013; Wang et al., 2014; Elliott et al., 2018; Kouřil et al., 2018) Some investigations concentrated on medical applications, hydrogen storage, and various endofullerene. (Wang et al., 2013; Srivastava et al., 2016; Srivastava et al., 2017; Elliott et al., 2018; Kouřil et al., 2018). Due to the fullerene's unique cage-like cavity, a procedure called molecular surgery can be performed to entrap an atom or molecule. (Murata et al., 2006; Krachmalnicoff et al., 2016). Utilizing this technique, encapsulation of molecular hydrogen and HF were successfully achieved. (Murata et al., 2006; Krachmalnicoff et al., 2016). In addition, a recent paper by Jana and Chattaraj (2020) describes the effects of a molecular reaction environment, dodecahedrane, on the He dimer bonding. Also, theoretical studies of a new type of endofullerenes with superalkali have been performed. (Srivastava et al., 2016; Srivastava et al., 2017) Superalkalis are clusters with very low adiabatic ionization energies. (Gutsev and Boldyrev, 1982; Gutsev and Boldyrev, 1983; Gutsev and Boldyrev, 1987; Lia et al., 1988; Gutsev and Boldyrev, 1990; Tong et al., 2013a; Tong et al., 2013b). The first and most common superalkalis have the formula M_k+1L, where M is an alkali atom with valence k and L is an electronegative atom. (Gutsev and Boldyrev, 1985; Gutsev and Boldyrev, 1987; Zhao et al., 2017). Nambiar et al. (2021) showed the importance of these compounds through a density functional computational study to improve the efficacy of redox reactions.

The concentration of carbon dioxide (CO₂) in the atmosphere has been increasing constantly since the early 20^th century with rapid industrialization. (D’Amato and Akdis, 2020; Kuzovkin and Semenov, 2020). This is a globally recognized issue as CO₂ contributes significantly to the greenhouse effect and the acidification of the oceans. (Crowley and Berner, 2001; Hönisch et al., 2012). Increment of atmospheric temperature causes climate change that threatens the overall ecosystem. To capture CO₂ molecules present in the air, various methods have been employed such as packed column of monoethanolamine and metal-organic frameworks. (Lv et al., 2015; Li et al., 2019; Li et al., 2020). The next step is activating CO₂ molecules and converting them to value-added chemicals such as hydrocarbon fuels. (Hu et al., 2013). The activation of CO₂ is extremely complex due to its stability and much efforts have been made by researchers to directly convert it into liquid hydrocarbons, useful for the aviation sector as novel jet fuels (Boreriboon et al., 2018; Vogt et al., 2019; Yao et al., 2020) or into oxygenates, such as ethanol. (Song et al., 2016; Bai et al., 2017; Wang et al., 2018). This conversion, whether it involves a direct CO₂ hydrogenation route or not, entails the usage of metal-based catalysts to ensure an overall reasonable efficiency. (Yao et al., 2020).

In our previous studies, successful activation of CO₂ with a superalkali species, Li₃F₂, were presented. (Park and Meloni, 2017). The computational study showed charge transfer from Li₃F₂ to CO₂, which indicates migration of the unpaired electron from Li₃F₂ to CO₂. The activated CO₂ showed geometric change such as bent $\widehat{OCO}$ angle. The activated CO₂ then can be transformed to other organic molecules with catalysis. (Liu et al., 2016; Luc et al., 2017). Removing the unpaired electron from [Li₃F₂⋅CO₂] cluster weakens the interaction between Li₃F₂ and CO₂ and geometry of CO₂ returns back to the linear form. (Park and Meloni, 2017). The superalkali Li₃F₂ was observed and characterized experimentally. (Yokoyama et al., 2000; Haketa et al., 2002). They also confirmed three stable Li₃F₂ structures through a computational density functional approach. (Haketa et al., 2002). In this investigation, the Li₃F₂-doped fullerene and its endo-reaction with CO₂ has been characterized using the B3LYP/6-31G* level of theory. These results are explained in terms of energetics and molecular orbitals of the species involved. In addition, these findings will be beneficial in providing insights for CO₂ reduction and in helping the exploration of new materials with tailored properties.

Computational Methods

Geometries and total electronic energies of the investigated species were calculated at the B3LYP/6-31G* level of theory (Becke, 1988; Lee et al., 1988) using the computational software Gaussian09. (Frisch et al., 2016). B3LYP is one of the most commonly used density functional theory (DFT) methods that employs a three-parameter exchange functional developed by Becke (1992) and Becke (1993) with a correlational functional proposed by Lee, Yang, and Parr (LYP) Becke (1988) to approximate the exchange-correlation energy. The B3LYP/6-31G* level has been employed to study endofullerene systems because it yields reliable geometries and energies. (Wang et al., 2013; Srivastava et al., 2016; Srivastava et al., 2017). Partial atomic charges are calculated based on the Mulliken population analysis (Mulliken, 1955) and natural bond orbital (NBO) population analysis. (Reed et al., 1985).

The adiabatic ionization energy (AIE) is calculated by taking the zero-point energy corrected electronic energy difference between the optimized neutral and cation, whereas the adiabatic electron affinity (AEA) is obtained by subtracting the zero-point-energy corrected electronic energy of the optimized anion and neutral. All the optimized structures have real vibrational frequencies and their Cartesian coordinates have been reported in the Supplementary Material.

Results and Discussion

The main intent of this computational investigation is to study the interactions relevant to the reduction of CO₂ by the superalkali Li₃F₂ inside our molecular reaction vessel, i.e., C₆₀, and see how this environment affects the CO₂ activation. The system is fairly large and, therefore, computationally challenging to investigate. We have analyzed the possible interactions between the fullerene and the two reactants, CO₂ and Li₃F₂. All the computed energetics are reported in Table 1 together with the available literature (experimental and computed) values.

TABLE 1

TABLE 1. Energetics of all the species relevant to this study calculated at the B3LYP/6-31G* level of theory. The zero-point-energy corrected total electronic energy (E₀) is in Hartree, the adiabatic ionization energy (AIE) and adiabatic electron affinity are in eV, and the binding energy at 0 K is in kJ mol⁻¹. [SA⋅CO₂] stays for the endo superalkali⋅CO₂ complex.

Figure 1 reports the optimized geometries for CO₂ and CO₂⁻, Li₃F₂(C_2v), Li₃F₂(D_3h) and their cations. The structures of CO₂ and CO₂⁻ reproduce well the literature experimental values for both bond distances and bond angles. In fact, for CO₂ we have r_C-O = 1.17 Å (1.16 Å) (Herzberg, 1966) and for CO₂⁻ we have r_C-O = 1.25 Å (1.25 Å) (Hartman and Hisatsune, 1966) and ∠OCO^° = 134^° (127 ± 7^°) . (Hartman and Hisatsune, 1966). Both the geometries of the lowest energy Li₃F₂(C_2v) isomer, trigonal bipyramidal Li₃F₂(D_3h), and their cations are in agreement with our previous work. (Park and Meloni, 2017; Cochran and Meloni, 2014). Figure 2 shows the two superalkali isomers reducing the CO₂. The geometry for the previously studied C_2v isomer interacting with CO₂ is in agreement with our previous results, (Park and Meloni, 2017), whereas the D_3h⋅CO₂ species is reported for the first time. When the D_3h structure reacts with CO₂, the trigonal bipyramidal geometry is distorted by increasing two “equatorial” Li-Li distances, maintaining only the Li(1)-Li(2) distance of 2.30 Å, with Li(3) being closer to the CO₂, Li(3)-O(7) = Li(3)-O(8) = 2.03 Å and increasing the axial F-F distance from 2.40 to 2.68 Å. The ∠OCO^° bond angle is 128^° and the C-O bond length is 1.26 Å. The Li₃F₂ isomers have similar binding energy with CO₂, with the C_2v isomer presenting a stronger interaction of 184 kJ mol⁻¹. These clusters can be defined as “free” or “naked” because they are isolated in the gas phase. The presented energy values are calculated at the B3LYP/6-31G* level and are within 10% from the literature reported quantities, whether they are experimental or computed at very high level of theory.

FIGURE 1

FIGURE 1. B3LYP/6-31G* optimized geometries of (A) CO₂, (B) CO₂⁻, (C) Li₃F₂(C_2v), (D) Li₃F₂⁺(C_2v), (E) Li₃F₂(D_3h), (F) Li₃F₂⁺(D_3h).

FIGURE 2

FIGURE 2. B3LYP/6-31G* optimized geometries of (A) Li₃F₂(C_2v), and (B) Li₃F₂(D_3h) reducing CO₂.

When a molecule is inserted in the fullerene (yielding an endofullerene), the chemical system is not free, but it will be subjected to the interactions with the carbon cage (“solvation effects”). In Figure 3, the two endofullerenes with CO₂ and Li₃F₂ are shown. In the case of carbon dioxide, it is clear from the energetics presented in Table 1 that CO₂ is destabilized by C₆₀ having a negative binding energy at 0 K of −147 kJ mol⁻¹ or a solvation energy at 0 K of 147 kJ mol⁻¹, calculated as E₀(CO₂) + E₀(C₆₀)—E₀(C₆₀⋅CO₂). The CO₂ occupies the center of the C₆₀, aligned with the C₃ axis passing through a hexagonal face, minimizing its interactions with the C cage. The solvation energy is more properly defined as the Gibbs free energy change associated with the transfer of a molecule from the gas phase into a solvent, i.e., it provides the relative equilibrium populations of a species between gas phase and the solvent. Therefore, we should also know the entropy change connected with this process. The values that we are reporting in this investigation are at 0 K, so that ${\Delta }_{solv}{H}_0^o={\Delta }_{solv}{G}_0^o=- \mathrm{B}\mathrm{E}\left(\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{t}-\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{s}\right)$, from which we can see that negative binding energies correspond to positive solvation energies (destabilizing effect). For the encapsulated superalkali two main findings can be noticed. First, the superalkali inside the fullerene is “forced” to assume a D_3h geometry, a structure almost identical to the free D_3h cluster but less stable than the free C_2v cluster. Despite having started the Li₃F₂ geometry optimization from different initial configurations, the optimized structure inside the fullerene resulted in the trigonal bipyramidal geometry. The Li-F distances are shortened in C₆₀ from 1.83 (free superalkali) to 1.77 Å, which corresponds to a compression along the F-F distance from 2.40 to 2.21 Å, and the two Li-Li bonds elongate to 2.40 Å. The second result is that C₆₀ interacts strongly with Li₃F₂ with a binding energy at 0 K of 119 kJ mol⁻¹ or solvation energy at 0 K of −119 kJ mol⁻¹, calculated as E₀(Li₃F₂(D_3h)) + E₀(C₆₀)—E₀(C₆₀⋅Li₃F₂(D_3h)). This interaction is not a reduction of C₆₀, where the electron from the superalkali is transferred to the fullerene. In fact, upon ionization of C₆₀·Li₃F₂, the encapsulated Li₃F₂ retains its trigonal bipyramidal structure, just slightly distorted (as described above) due to the interactions with C₆₀. In addition, looking at the Mulliken population and the natural orbital population neither the endo-Li₃F₂(D_3h) nor the C₆₀ show an increase or change of electron charges. The C₆₀·Li₃F₂ HOMO, the main contribution of which is given by C 2p AO’s, is delocalized almost entirely on the fullerene (Figure 4). The fact that the C₆₀·Li₃F₂ AIE is much lower than C₆₀ AIE, 5.64 vs. 7.08 eV, respectively, can be explained using a molecular orbital character argument. The interaction of C₆₀ with the superalkali makes the HOMO of fullerene less bonding, and consequently, most of the C-C bonds are elongated by 0.1–0.2 Å. Upon ionization, the C-C bonds in C₆₀·Li₃F₂ are shortened on average by 0.1 Å, which can be interpreted as the removal of an electron from a HOMO with antibonding character.

FIGURE 3

FIGURE 3. Two different views of B3LYP/6-31G* optimized structures of (A)-(B) C₆₀ · CO₂ and (C)-(D) C₆₀ · Li₃F₂(D_3h).

FIGURE 4

FIGURE 4. HOMO of C₆₀ · Li₃F₂(D_3h).

The insertion of CO₂ within C₆₀⋅Li₃F₂(D_3h) produces an unexpected result (Figure 5). In previous computational studies, (Zhao et al., 2017; Park and Meloni, 2017; Sikorska and Gaston, 2020), naked superalkali have been shown to be capable of reducing carbon dioxide by transferring an electron and yielding an activated bent CO₂⁻. In this investigation, we show that our molecular vessel C₆₀ forces the reaction to proceed in a different way (Figure 6). From an analysis of the optimized geometries inside fullerene, it is clear that CO₂ is activated by showing a ∠OCO^° bond angle of 132^° and C-O bond lengths of 1.20 Å, 0.03 Å longer than r_C-O in CO₂ but 0.06 Å shorter than r_C-O in the free Li₃F₂(D_3h)⋅CO₂ species. The activation of CO₂ is achieved by a F transfer from Li₃F₂ to CO₂ with the formation of a C-F bond of 1.38 Å, almost identical to the r_C-F of 1.382 Å in CH₃F. (Demaison et al., 1999). This moiety FCO₂ does not resemble either the fluorocarboxyl radical FCO₂ for which r_C-O is 1.234 Å, r_C-F is 1.310 Å, and ∠OCO^° bond angle is 118.8^°, (Zelinger et al., 2003), or the fluoroformate ion FCO₂⁻ for which r_C-O is 1.234 Å, r_C-F is 1.46 Å, and ∠OCO^° bond angle is 135.9^°. (Arnold et al., 1995; Thomas et al., 2018). In addition, both fluorocarboxyl radical and fluoroformate ion are planar, whereas the endo-reaction species (Figure 6) resembles a non-planar (trigonal pyramidal) FCO₂ that interacts with what it looks like a FLi₃ species. All the attempts to optimize this structure outside C₆₀ as free endo-Li₃F₂⋅CO₂ returned a Li₃F₂(D_3h)⋅CO₂ geometry. Unfortunately, this prevents us from quantifying the interaction of Li₃F₂(D_3h) with CO₂ inside C₆₀. In fact, the reaction we need is

${\text{C}}_{60}\cdot \left[\text{SA}\cdot {\text{CO}}_2\right]+{\mathrm{C}}_{60}\to {\mathrm{C}}_{60}\cdot \mathrm{L}{\mathrm{i}}_3{\mathrm{F}}_2\left({\mathrm{D}}_{3\mathrm{h}}\right)+{\mathrm{C}}_{60}\cdot \mathrm{C}{\mathrm{O}}_2(1)$

from which the interaction of Li₃F₂(D_3h) with CO₂ can be derived if we were able to find the [SA⋅CO₂] reaction product as a free species and then its binding energy (or negative solvation energy) with C₆₀, i.e.,

${\text{C}}_{60}\cdot \left[\text{SA}\cdot {\text{CO}}_2\right]\to {\text{C}}_{60}+\left[\text{SA}\cdot {\text{CO}}_2\right](2)$

FIGURE 5

FIGURE 5. Two different views of B3LYP/6-31G* optimized geometry of C₆₀ · Li₃F₂(D_3h) · CO₂.

FIGURE 6

FIGURE 6. B3LYP/6-31G* optimized geometry of the Li₃F₂^.CO₂ complex inside C₆₀.

In fact, the interaction of Li₃F₂(D_3h) with CO₂ inside fullerene can be calculated as:

${\Delta }_{\mathrm{r}}{\mathrm{H}}^{\mathrm{o}}\left(1\right)-\mathrm{B}\mathrm{E}\left({\mathrm{C}}_{60}\cdot \mathrm{C}{\mathrm{O}}_2\right)-\mathrm{B}\mathrm{E}\left({\mathrm{C}}_{60}\cdot \mathrm{L}{\mathrm{i}}_2{\mathrm{F}}_3\left({\mathrm{D}}_{3\mathrm{h}}\right)\right)+\mathrm{B}\mathrm{E}\left({\mathrm{C}}_{60}\cdot \left[\mathrm{S}\mathrm{A}\cdot \mathrm{C}{\mathrm{O}}_2\right]\right)(3)$

In other words, this expression tells us that the interaction between endo-Li₃F₂(D_3h) and endo-CO₂, i.e., the BE of superalkali-CO₂ in fullerene, is equal to the enthalpy of reaction (1) plus the solvation energies of CO₂ and Li₃F₂(D_3h) minus the solvation energy of [SA⋅CO₂]. Because we cannot derive this last solvation energy absolute value due to the impossibility of optimizing the free endo-[SA⋅CO₂] species, we can estimate this interaction by performing a single-point energy calculation of the free [SA⋅CO₂] optimized inside C₆₀. This structure necessarily represents a higher energy structure than a real minimum and, therefore, the estimated BE of superalkali-CO₂ in fullerene would denote an upper bound providing us some insights on this interaction. From this computation we get an upper bound for $\text{B}\text{E}\left({\mathrm{C}}_{60}\cdot \left[\mathrm{S}\mathrm{A}\cdot \mathrm{C}{\mathrm{O}}_2\right]\right)$ of −965 kJ mol⁻¹, which tells us that this endo-product is highly destabilized by C₆₀!

Conclusion

The activation of CO₂ by the Li₃F₂ superalkali within C₆₀ has been investigated at the B3LYP/6-31G* level of theory. C₆₀ has been utilized as a reaction vessel and its interaction with the reactants, superalkali and carbon dioxide, have been computed. C₆₀ is capable of forcing a superalkali geometry, which does not present the global minimum in the gas phase. Specifically, Li₃F₂ takes the D_3h structure. C₆₀ has a stabilizing effect on the superalkali but a destabilizing effect on the CO₂, as it can be deduced by the binding energies of these two systems, BE(C₆₀⋅CO₂) = −147 kJ mol⁻¹ and BE(C₆₀⋅Li₃F₂) = 119 kJ mol⁻¹. Upon interaction of Li₃F₂(D_3h) with CO₂ inside fullerene, CO₂ is clearly activated showing a ∠OCO^° bond angle of 132^° and C-O bond lengths of 1.20 Å, 0.03 Å longer than r_C-O in CO₂ but 0.06 Å shorter than r_C-O in the free Li₃F₂(D_3h)⋅CO₂ species. The activation of CO₂ is achieved by a F transfer from Li₃F₂ to CO₂ with the formation of a C-F bond of 1.38 Å. Due to the impossibility of optimizing a free superalkali-CO₂ complex, [SA⋅CO₂], resembling the one optimized within the C₆₀, a single-point energy calculation has been performed on the free [SA⋅CO₂]. This energy has been utilized to provide an upper bound for the binding energy of Li₃F₂(D_3h) with CO₂ within C₆₀ of −965 kJ mol⁻¹, showing that C₆₀ destabilizes the reaction product.

Data Availability Statement

The original contribution presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author Contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

Funding

This research was supported by the American Chemical Society Petroleum Research under the fund grant number #56067-UR6.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2021.712960/full#supplementary-material

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