Superalkalis for the Activation of Carbon Dioxide: A Review

The activation of carbon dioxide is essential not only for global carbon balance but also for its conversion into fuel. As CO₂ is highly stable, it is quite challenging to activate or reduce CO₂. Recently, the ability of superalkalis to easily transfer an electron to CO₂ has been proposed in several studies. The superalkalis are species possessing lower ionization energy than alkali atoms. These are hypervalent species, having an excess of electrons. Owing to this, they possess strong reducing power and cause the linear structure of CO₂ to bend by transferring an electron to it. Herein, we present a comprehensive account of the single-electron reduction and activation of CO₂ by various kinds of superalkalis. This review also includes a novel strategy for the capture and storage of CO₂ by superalkali.

Introduction

Carbon dioxide (CO₂) is a colorless and odorless gas with the property of trapping greenhouse gases, which are produced due to human activities as well as natural processes. Recently, the sharp decline in CO₂ levels has been noticed at the expense of the COVID-19 pandemic, which has caused a severe health emergency in the world and is not sustainable. The fueling of CO₂ in our environment is mainly through the CO₂ emissions from power plants and other industrial facilities, primarily waste products, and the developed economies are the leading contributors. As mentioned, it traps greenhouse gases which generally cause a change in the behavior of climate since it is a major contributor to global warming. In order to reduce its contribution to global warming, it is necessary to convert CO₂ into value-added products. The best way to back-pedal climate change without using expensive methods is extricating CO₂ from the air and then converting it into a useful product like fuel. These important issues were addressed by numerous techniques, which can be employed to reduce and capture CO₂ by other molecules [1–7]. As CO₂ is an extremely stable molecule [8, 9], it is quite challenging to convert it into usable fuel. To convert CO₂ into fuel, it is needed to activate CO₂ by some means or chemically reduce it by catalysts. CO₂ can be reduced either electrochemically (electrical energy) or photoelectrochemically (incident light) into CO. Single-electron reduction of CO₂ to CO₂⁻ was experimentally not viable due to the large energy of reorganization between linear CO₂ and bent CO₂⁻ anion. Notably, the electron affinity of CO₂ is negative so that CO₂⁻ is metastable. CO₂⁻ anion is stable in the ²A₁ state [10] which can be treated as an activated CO₂ moiety with the weaker C-O bond. The potential energy surface of the CO₂⁻ anion suggests three vibronically coupled bound states [11].

It is difficult to extract an electron from carbon dioxide because of its high ionization energy (13 eV) [12]. However, it has been revealed that there is a possibility of oxidation of CO₂ using superhalogens [13], whose electron affinity overrides the halogen atoms [14]. The counterparts of superhalogens are superalkalis which bear lower ionization energy than alkali atoms [15]. Due to the stronger reducibility of superalkalis over alkali atoms, they might activate stable CO₂. To investigate this, a few studies have been performed recently. In this review, we will provide an overview of how superalkalis play a significant role in the activation or reduction of CO₂, which is the initial step to convert CO₂ into fuel. Exploration of chemical processes used in the reduction of CO₂ is of tremendous importance in various fields, like biological, environmental, and industrial processes [16]. Before we go further, let us first have a look at superalkalis.

What Are Superalkalis?

Alkali atoms possess the lowest ionization energy (IE), ranging from 5.39 to 3.89 eV, among all the elements in the periodic table. However, superalkalis are clusters whose ionization energies are even lower than this range. These clusters were originally introduced by Gutsev and Boldyrev in 1982 using sp-block elements [15]. They proposed species like Li₂F, Li₃O, and Li₄N as superalkalis. In the form of a superatom, these clusters impersonate the behavior of alkali atoms. There have been several studies on the design of various kinds of superalkalis [17–24]. For instance, the binuclear superalkalis including F₂Li_3, have been widely studied [17]. Hou et al. [18] described non-metallic binuclear cations such as F₂H₃⁺ and O₂H₅⁺. Zhao et al. [19] proposed some special superalkalis like N₄Mg₆Li, Al₃, Mn(B₃N₃H₆)₂, B₉C₃H₁₂, Al₁₂P, and C₅NH₆, which were designed by using different schemes like jellium rule, 18-electron rule, Wade–Mingos rule, and Huckle’s rule, respectively. Al₁₂P was reported to be an alkali–metal-like superatom [20, 21]. Recently, Sikorska and Gaston [22] reported the superalkali behavior of polynuclear N₄Mg₆M for M = Li, Na, and K. Srivastava [25] noticed that the IE of C₆Li₆, being lower than that of Li, makes it a closed-shell superalkali. The IE scale of these superalkalis is depicted in Figure 1.

FIGURE 1

FIGURE 1. Ionization energy scale of various superalkalis is considered here. The values are taken from Refs. 17–22, 25, and 35.

Superalkalis find applications in the design of supersalts [26–28], superbases [29–31], alkaloids [32–34], and so forth. Due to their low IE, superalkalis play an important role in chemical industries as reducing agents. Here, we provide an account of how superalkalis are exploited to activate the CO₂ molecule.

Activation of CO₂ by Superalkalis

CO₂ is known to be a highly stable molecule due to its very high IE [12], as mentioned earlier, and no positive electron affinity [36, 37]. However, the low IE of superalkalis enables them to transfer an electron to CO₂, reducing it to CO₂⁻ anion and thus activating it. In Figure 2, we show the structures of CO₂ and its anion, along with the charge distribution. One can see that the CO₂⁻ anion is bent, in which the bond length is increased as compared to neutral CO₂ due to the negative charge.

FIGURE 2

FIGURE 2. Structure and NPA charge distribution in (A) neutral CO₂ and (B) CO₂⁻anion, Ref. 19 with the permission of the Royal Society of Chemistry.

Thus, the activation of CO₂ requires the following conditions to be satisfied:

1) The negative charge on CO₂ moiety is close to unity.

2) The structure of the CO₂ moiety is bent.

3) The bond length of CO₂ moiety is increased.

It has been reported earlier [38] that CO₂ would assume a bent structure when an electron is transferred to it or due to its interaction with the electrons of the metal atom. One would expect that the stable geometry of the M-CO₂ complex depends upon the IE of the metal atom, M. This may lead us to infer that an atom with a smaller IE should be able to transfer an electron to CO₂ more easily than one with a large IE. Later, we will discuss the interaction of CO₂ with various superalkalis described earlier.

Interaction With Typical Superalkalis (FLi₂, OLi₃, and NLi₄)

Srivastava [35] studied the interaction of CO₂ with FLi₂, OLi₃, and NLi₄ superalkalis using the second-order Møller–Plesset perturbative (MP2) method [39] and the 6–311+G(d) basis set in the Gaussian 09 program [40]. Such interaction leads to the formation of complexes, as shown in Figure 3, and corresponding parameters can be found in Table 1. It is evident that the minimum energy of these complexes corresponds to the structure in which the interaction between CO₂, and superalkalis is mediated by both the O atoms of CO₂. The bond length of Li-O lies between 1.865 and 1.892 Å. The low-lying isomers of FLi₂-CO₂ and OLi₃-CO₂ are of higher energy in which CO₂ interacts via a single atom, whereas in the case of NLi₄-CO₂, there are no competing isomers obtained.

FIGURE 3

FIGURE 3. Equilibrium structures of CO₂ complexes with typical superalkalis, with bond lengths in Å from Ref. 35 with the permission of Wiley.

TABLE 1

TABLE 1. Relative energy, binding energy (BE), NPA charge (∆q), bond length (C-O), and bond angle (O-C-O) of the complexes of CO₂ with various superalkalis.

The binding energy (BE) of superalkali–CO₂ complexes is calculated and listed in Table 1. The BE of these complexes monotonically decreases with the increase in the size of superalkalis. This can be explained on the basis of a more delocalized electron cloud that is generally associated with the larger superalkalis. The natural population analysis (NPA) [45] charges (∆q) on CO₂ have also been listed. The most stable structure of superalkali–CO₂ complexes takes the values of ∆q as −0.90e for FLi₂, −0.88e for OLi₃, and −0.85e for NLi₄. In isomer (b) of FLi₂-CO₂, ∆q has a very small magnitude (−0.17e), which is consistent with an almost linear CO₂ moiety just as in a neutral CO₂ molecule. On the contrary, the ∆q in isomer (b) of OLi₃-CO₂ is, albeit smaller than that in its lowest energy structure (a), large enough to bend the CO₂ moiety. It should be noted that the size of the superalkalis is a more important factor than their IE in CO₂ activation. As per calculated binding energy and charge transfer, FLi₂ is more effective for CO₂ reduction. In these complexes, the CO₂ moiety is bent by 133° and the bond length C-O becomes 1.246 Å which is comparable to the bond lengths of 1.237 Å and bond angle 137° in the CO₂⁻ anion, obtained at the MP2/6-311+G(d) level. This study suggests simple and catalyst-free single-electron reduction of CO₂ by using typical superalkalis such as FLi₂, OLi₃, and NLi₄.

Interaction With Binuclear Superalkali (Li₃F₂)

Park and Meloni [41] reported the interaction of CO₂ and superalkali species Li₃F₂ using the CBS-OB3 composite model [46] through the Gaussian 09 program. They obtained three isomers, two planar (a) and (b), as well as one non-planar (c), of the Li₃F₂-CO₂ complex, as shown in Figure 4. There was no appreciable change in bond lengths between Li and F on interaction with CO₂. Despite some structural changes in the superalkalis, the structure of CO₂ changes from linear to bending. Therefore, it is clear that the strongly bound CO₂ is activated upon interaction with the superalkali. From Table 1, the BE of Li₃F₂-CO₂ isomers is found to be in the range of 1.06–1.63 eV (106–163 kJ/mol). The lowest BE was obtained for the isomer (c) in which one oxygen interacts with both the terminal Li atoms and the other oxygen with the central Li atom. The isomer (a) possesses greater BE and therefore stronger intermolecular interaction than isomer (b) because the electron density is more localized between the two terminal Li atoms and two oxygen atoms.

FIGURE 4

FIGURE 4. Equilibrium structures of CO₂ complex with binuclear superalkali with bond lengths from Ref. 41 with the permission of the Royal Society of Chemistry.

The BE of Li₃F₂-CO₂ complexes is comparable to or smaller than the BE of superalkali (FLi₂, OLi₃, NLi₄)-CO₂ complexes reported by Srivastava [35]. The charge transfer to CO₂ in Li₃F₂-CO₂ isomers ranges from −0.63e to −0.88e (see Table 1). Thus, the charge on the CO₂ moiety along with its structure suggests that CO₂ is reduced to a CO₂⁻ anion.

Interaction With Special Superalkalis [Al₃, Mn(B₃N₃H₆)₂, B₉C₃H₁₂, C₅NH₆]

Zhao et al. [19] presented the rational design of superalkalis and studied the activation of CO₂ by these special superalkalis using MP2/6-311+G(d) level in the Gaussian 09 program. They analyzed the interaction of CO₂ with special superalkalis like Al₃, Mn (B₃N₃H₆)₂, B₉C₃H₁₂, and C₅NH₆, which leads to the complexes shown in Figure 5. The distance between the CO₂ moiety and superalkali clusters has been calculated as 1.950 Å, 1.730 Å, 2.320 Å, and 1.040 Å for Al₃, Mn (B₃N₃H₆)₂, B₉C₃H₁₂, and C₅NH₆, respectively. In the case of Al₃, O-C bonds extend to 1.290 and 1.510 Å, about 4.4–22% longer than those in the CO₂⁻anion, whereas in B₉C₃H₁₂, the O-C bonds are extended to 1.290 and 1.280 Å, about 3.3–4.4% longer than those in CO₂⁻. The bond extension in Mn (B₃N₃H₆)₂ and C₅NH₆ is observed to be 1.250 Å which is slightly longer than that of 1.240 Å in CO₂⁻ and 1.35 Å which is about 9.3% longer than that in CO₂⁻, respectively. The bond angle of O-C-O in Al₃CO₂, Mn (B₃N₃H₆)₂CO₂, B₉C₃H₁₂CO₂, and C₅NH₆CO₂ is 126°, 133°, 131°, and 129° making the bond bend by 8°, 3, 4, and 7% more than the corresponding value in CO₂⁻. Thus, both the stretching of the O-C bonds and the bending of the O-C-O angle weaken the O-C bonds of CO₂, making it easy to activate.

FIGURE 5

FIGURE 5. Equilibrium structures of CO₂ complex with special superalkalis from Ref. 19 with the permission of the Royal Society of Chemistry.

The NPA charge has been listed in Table 1 to show how much charge is transferred to CO₂. This transfer of charge results in the bending of CO₂ and weakens the CO₂ bond, and therefore making it easier to break. The amount of charge transferred from Al₃, Mn (B₃N₃H₆)₂, B₉C₃H₁₂, and C₅NH₆ to CO₂ are −1.26e, −0.90e, −0.91e, and −0.63e, respectively. Note that this amount of charge transferred from Al₃, Mn (B₃N₃H₆)₂, B₉C₃H₁₂ are greater than that of noble gas (0.77e) [47] being very close to unity, whereas in the case of C₅NH₆, the amount of charge transferred is less. From this analysis, one may note that although the IE of Al₃ is not the lowest among these four superalkalis (see Figure 1), the charge transferred is the most and it is capable of bending the CO₂ molecule the most. This indicates that the quantitative nature of the activation of CO₂ depends on the electronic structure and size of the superalkalis, as seen in an earlier section.

Interaction With Non-Metallic Superalkalis (O₂H₅, N₂H₇)

Kumar et al. [42] explored the scope of non-metallic superalkalis in the activation of CO₂. They studied the interaction of CO₂ with non-metallic superalkalis such as O₂H₅ and N₂H_7, employing the MP2/6-311++G (d,p) level via the Gaussian 09 program. The equilibrium structures of O₂H₅-CO₂ and N₂H₇-CO₂ are shown in Figure 6, and related parameters are listed in Table 1. It was noticed that in O₂H₅-CO₂ complexes, O atoms of CO₂ interact with the H-atom of superalkali, unlike in N₂H₇-CO₂, in which the C-atom of CO₂ interacts with the H-atom. This may be due to the repulsion between excess electrons of N and O atoms.

FIGURE 6

FIGURE 6. Equilibrium structures of CO₂ complexes with non-metallic superalkalis from Ref. 42 with the permission of the Taylor & Francis.

The (b) isomers of O₂H₅-CO₂ and N₂H₇-CO₂ are 0.24 and 0.78 eV, higher in energy in which CO₂ interacts through the O-atom as well as the C-atom in the N₂H₇-CO₂ isomer. The relative stability of isomers can be explained on the basis of H-bond interactions. For instance, the bond lengths of O-H and C-H are 2.060 Å and 3.710 Å, respectively. The BE of these complexes provides relative strength through the interaction of CO₂ with non-metallic superalkalis. From Table 1, the BE suggests that O₂H₅-CO₂ isomers are stable, whereas N₂H₇-CO₂ is slightly destabilized due to the negative value of BE.

From Table 1, the value of NPA charges of CO₂ is calculated to be −0.75e for O₂H₅-CO₂ and −0.77e for N₂H₇-CO₂ lowest energy structures (a). Thus, the NPA charge values are very close to each other. In isomer (b) of O₂H₅-CO₂ and N₂H₇-CO₂, the CO₂ moiety is bent, similar to that in its lowest energy structure. Therefore, the activation and the consequential reduction of CO₂ can also be possible by non-metallic superalkalis such as O₂H₅, if not by N₂H₇.

Interaction With Polynuclear Species (Al₁₂P, N₄Mg₆M)

The compact (quasi) icosahedral Al₁₂X (X = Be, Al, C, and P) clusters have been employed to analyze the dissociation and absorption of small gas molecules [48–54]. Zhang et al. [43] studied the interaction of Al₁₂P superalkali with CO₂ using Minnesota density functional (M06-2X) [55] and 6-311+G(d) basis set in the Gaussian 09 program. They obtained three isomers of the Al₁₂P-CO₂ complex as shown in Figure 7. The lowest energy corresponds to the isomer (a) in which the interaction is mediated by both O atoms. The isomers (b) and (c) are found to have a high energy of 0.11 eV (2.64 kcal/mol) and 0.20 eV (4.52 kcal/mol), respectively. Obviously, the chemisorbed CO₂ molecule undergoes structural changes from linear to bending in each Al₁₂P-CO₂ isomer.

FIGURE 7

FIGURE 7. Equilibrium structures of CO₂ complex with Al₁₂P from Ref. 43 with the permission of Wiley.

They confined their analyses to the global minimum structure, that is, isomer (a). The bond distance of C-O in isomer (a) is 1.24 Å, which is found to be 7.6% larger than free CO₂ (1.15 Å), consequently, weakening the C-O bond. Moreover, the variation of the C-O-C angle from 180° to 130.4° in isomer (a) advocates the change in hybridization of carbon in CO₂ from sp to quasi-sp² after activation by Al₁₂P. Furthermore, the C-O bond is marginally larger (1.23 Å) as compared to isolated CO₂⁻, whereas bond bending is also larger, about 4.7%, than that in isolated CO₂⁻, which clearly supports the activation of CO₂ assimilated on the Al₁₂P cluster. The computed total NPA charge on the CO₂ subunit is −0.707e, which shows the transfer of almost one electron charge from Al₁₂P to CO₂ in the complex. Thus, CO₂ is successfully reduced to CO₂⁻ anion. The low IE of Al₁₂P superatom is the main source of CO₂ reduction as it facilitates the transfer of charge to CO_2, which ultimately results in the contraction of the O-C-O angle and the weakening of the C-O bond of the CO₂ moiety. The small activation barrier of 23 kcal/mol is calculated for the chemisorption of CO₂ on Al₁₂P to form the Al₁₂P-CO₂ complex (a), which further suggests the application of Al₁₂P as a potential catalyst for CO₂ conversion. It has been found that the Al₁₂P complex shows high adsorption intensities in the visible region and, hence, promotes photocatalysis or photothermal catalysis of CO₂ and its transformation by absorbing sunlight.

Recently, Sikorska and Gaston [22] explored new superalkali species, N₄Mg₆M (M = Li, Na, K) by performing the MP2/6-311+G(d) and single-point CCSD(T)/6-311+G (3df) calculations Ref. 56 in the Gaussian 09 program. They studied the catalytic behavior of N₄Mg₆Li, N₄Mg₆Na, and N₄Mg₆K for CO₂ activation. For the sake of brevity, we will discuss the interaction of CO₂ with N₄Mg₆Li superalkali. CO₂ interacts with N₄Mg₆M and the resultant N₄Mg₆M-CO₂ complexes are displayed in Figure 8 for M = Li. In all isomers, the interaction between CO₂ and N₄Mg₆M takes place via the C-N bond of 1.452–1.454 Å, which supports the evolution of a single bond between carbon and nitrogen.

FIGURE 8

FIGURE 8. Equilibrium structures of CO₂ complex with N₄Mg₆Li from Ref. 22 with the permission of the American Institute of Physics.

The isomers (b) and (c) are found to be 0.04 eV (0.97 kcal/mol) and 0.11 eV (2.48 kcal/mol) higher in energy than the lowest energy isomer (a) in the case of N₄Mg₆Li-CO₂. The binding energy of these isomers lies in the range of 1.46–1.57 eV (see Table 1). The C-O bond lengths, being in the range of 1.223–1.224 Å and 1.335–1.336 Å are indeed 8% larger than that of the CO₂ anion. Furthermore, the bending of the angle O-C-O, 123–126°, was 9% more than the angle of isolated CO₂⁻. The NPA charge transferred from N₄Mg₆M superalkalis to CO₂ varies from −0.799e to −0.806e. Ionization energy plays a major role in the activation process of CO_2, as the amount of transferred charge increases with the decrease in ionization energy because it is effortless to transfer charge from species with low ionization energy. Therefore, the extension of the bond distance of C-O along with the bending of angle O-C-O results in the weakening of C-O bonds of CO₂. Thus, the newly designed N₄Mg₆M superalkalis could be used as a catalyst for CO₂ activation.

Interaction With Li₃F₂ Superalkali Inside Buckminsterfullerene (C₆₀)

The interaction of CO₂ with the binuclear Li₃F₂ superalkali was reported by Park and Meloni as discussed in an earlier section. Recently, Meloni et al. [44] investigated the interaction of CO₂ with Li₃F₂ inside fullerene (C₆₀) by using B3LYP [57, 58] with the 6-31G (d) basis set in the Gaussian 09 program. They noticed two important features. When CO₂ is encapsulated within C₆₀, it gets destabilized as its binding energy is −147 kJ/mol. The Li₃F₂ inside C₆₀ assumes trigonal bipyramidal (D_3h) geometry with a binding energy of 119 kJ/mol, unlike the free Li₃F₂ linear cluster (see Figure 1). Thus, C₆₀ strongly interacts with Li₃F₂ and there is no reduction of C₆₀.

The unforeseen result has been noticed on insertion of CO₂ inside Li₃F₂(D₃h)@C₆₀ endofullerene, as displayed in Figure 9. On the inspection of the geometry inside the C₆₀, it was found that CO₂ has been activated by making a ∠OCO 132° and that the bond length of C-O has been increased to 1.20 Å. The activation of CO₂ has been attained by the transfer of F atom from Li₃F₂ to CO₂, due to the F-C interaction with the bond distance of 1.38 Å. Thus, the endo-reaction simulates a non-planar (trigonal pyramidal) FCO₂ interacting with the FLi₃-like species (also shown in Figure 9). There have been several studies [59, 60] in which the structures and interactions between species are greatly modified by encapsulation within C₆₀.

FIGURE 9

FIGURE 9. Equilibrium structures of [Li₃F₂-CO₂]@C₆₀ endofullerene and Li₃F₂-CO₂ inside C₆₀ from Ref. 44.

Interaction With C₆Li₆: Capture and Storage of CO₂

Thus, the strong reducing power enables superalkalis to reduce CO₂ as well as several other molecules [61, 62], So far, it has been found that the superalkalis are restricted to activate one CO₂ molecule per unit, that is, only one CO₂ molecule is reduced by a superalkali. This may limit the capture and storage of superalkalis for practical applications. Srivastava [63] reported that a single molecule of hexalithiobenzene (C₆Li₆) is not only capable of reducing but also capturing up to six CO₂ molecules sequentially using the ωB97xD functional [64] and 6-311+G(d) basis set in the Gaussian 09 program. The planar C₆Li₆ molecule has equal ring bond lengths of 1.418 Å, whose IE is reported to be lower than that of Li, thereby characterizing it as a superalkali molecule [25]. Note that planar star-like C₆Li₆ has been previously studied by several groups [65–67]. The sequential interaction of CO₂ molecules with C₆Li₆ results in the C₆Li₆-nCO₂ complexes as displayed in Figure 10.

FIGURE 10

FIGURE 10. Optimized structure of C₆Li₆-nCO₂ complexes (front and side views) for n = 1–6 from Ref. 63 with the permission of Wiley.

The interaction between CO₂ and C₆Li₆ is mediated by one C-C and two Li-O bonds in these complexes. The C₆Li₆ ring moiety in C₆Li₆-nCO₂is deviated from planarity due to the out-of-plane displacement of Li atoms because of the Coulomb-repulsion between neighboring Li-atoms for n = 1, 2, 4, and 5. However, both C₆Li₆-3CO₂ and C₆Li₆-6CO₂ possess the perfect planar ring moiety, having equal bond lengths of 1.42 Å and 1.40 Å, respectively. The CO₂ moiety in C₆Li₆-CO₂ has a bond length and an angle of 1.26 Å and 122.5°, respectively. With an increase in the number of CO₂, the average bond length and the average bond angle of CO₂ decrease and increase continuously up to 1.25 Å and 125.2°, respectively, for C₆Li₆-6CO₂.

The activation of CO₂ leads to an increase in the bond length and a decrease in the bond angle by bending. Table 2 lists the NBO charge (∆q) located at CO₂ moieties, adsorption (binding) energy (E_ad) per CO₂ molecule, and consecutive adsorption energy (ΔE_ad). It is clear that the Δq of C₆Li₆-nCO₂ becomes −0.83e, −0.81e, −0.79e, and −0.77e for n = 1–3, n = 4, n = 5, and n = 6, respectively. This suggests that C₆Li₆ can be employed in the activation of all CO₂ molecules and, consequently, their adsorption. One can note that the E_ad values are fairly large, ranging from 3.18 to 2.79 eV per CO₂, which predicts the stability of these C₆Li₆-nCO₂ complexes. It is to be noticed that the adsorption of molecules is not feasible with negative ΔE_ad values, whereas sequential adsorption becomes feasible with positive ΔE_ad values. Thus, the study suggests that C₆Li₆ is not only capable of activation but also effective in the sequential adsorption of six CO₂ molecules.

TABLE 2

TABLE 2. ωB97xD/6-311+G (d) calculated NBO charge on CO₂ (Δq), adsorption energy per CO₂ (E_ad), and consecutive adsorption energy (ΔE_ad) for C₆Li₆-nCO₂ complexes taken from Ref. 63.

Conclusion and Perspective

In summary, we have discussed the activation of CO₂ using various superalkalis. CO₂ is said to be activated when the charge is transferred to CO₂ from superalkali clusters, which ultimately results in the transformation of the linear structure of CO₂ to the bent structure of CO₂⁻. Based on quantum chemical methods, CO₂ is successfully reduced to CO₂⁻ by using typical superalkalis (FLi₂, OLi₃, and NLi₄), binuclear superalkali (Li₃F₂), special superalkalis (Al₃, Mn(B₃N₃H₆)₂, B₉C₃H₁₂, C₅NH₆), polynuclear superalkalis (Al₁₂P, N₄Mg₆M), and non-metallic superalkalis (O₂H₅ and N₂H₇). It was noticed that the amount of charge transfer depends on the electronic structure, size, and ionization energy of superalkalis. The activation of CO₂ by Li₃F₂ inside C₆₀ fullerene has also been discussed. It was also revealed that the C₆Li₆ molecule is not only capable of activating CO₂ but also capturing up to six CO₂ molecules. These results suggest that the superalkalis might be used as efficient catalysts for CO₂ activation. Thus, this activated CO₂ ion can be converted into fuel, such as methanol [68] via hydrogenation reaction.

Author Contributions

HS: literature survey, data collection, writing draft. AKS: conceptualization, supervision, editing, and finalizing the draft.

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.

Publisher’s Note

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