Introduction
The demand for energy has increased continuously in the last decades due to the world economic development and population growth. In this context, large quantities of greenhouse gases have been emitted, reflecting on global climate changes.1 Carbon dioxide (CO2) is one of the main gases emitted by anthropogenic activities such as burning of fossil fuels and other raw materials and may be associated with global warming and acidification of the oceans.2-4 Despite several attempts for sustainable development and agreements to reduce CO2 concentration, the emitted quantity of this polluting gas continues to increase.5,6 CO2 capture and storage emerges therefore as a way to control gas emission through various technologies. The captured CO2 may subsequently be used as material for manufacturing value-added products such as methanol, urea and formic acid, among others.7
Although technologies for CO2 capture have advanced, their development still poses a great challenge nowadays and several issues remain.8 Currently, chemical filtration using amine solvents, like monoethanolamine (MEA), is among the most employed processes to separate CO2. 9 However, it still shows some problems such as corrosion of equipment and high cost for regeneration.9 Some alternatives have emerged, aiming to improve the CO2 capture process. Microporous metal-organic frameworks (MOFs),10 porous solids,11 ionic liquids,12,13 and membranes14 are among the materials that have been tested for carbon capture. In all cases, regardless of the structure and form of utilization, some interaction between CO2 and the employed material is required.
Recent studies have focused on the interaction mechanism of different chemical absorbents with carbon dioxide using both Density Functional Theory (DFT) and wavefunction-based approaches.15,16Carneiro et al. studied the reaction between CO2 and amines using the CAM-B3LYP and ωB97X-D functionals and the MP2 method with the aug-cc-pVTZ basis sets.15 All energies were subsequently extrapolated to the complete basis set limit using Truhlar’s procedure.17 They have shown that the interaction energy and stabilization of the zwitterion intermediate show high correlation with the basicity of the amines.15Different kinds of basic species, such as amines9,16and benzyl-chalcogenides18 were also investigated by theoretical procedures, trying to quantify their capacity to react with CO2. Along the same lines, Varandas19and Sucarrat and Varandas20 studied the capture of a CO2 molecule by Li3N and Li3N3. Moreover, they investigated the interaction of 26 anions with CO2 in the gas phase and in three distinct solvents21 using the B3LYP-D3 and M06-2X functionals with the aug-cc-pVTZ basis set. From this study, it has been shown that alkoxylate and thiolate anions are promising candidates for the capture process due to their reaction reversibility, in spite of the fact that the solvent has some influence on the stability of the formed anion-CO2 adduct.
Although the above pioneering study21 has shed some light upon the thermodynamics of the reaction, some questions still remain, particularly on the effect of the solvent on the whole process. The reaction in gas phase is usually barrierless, while in a solvent of high dielectric constant it is less spontaneous and exothermic, involving the formation of a transition state. The thermodynamic results were rationalized in terms of the stabilization of the anions in the polar solvents that decreases their reactivity.21Furthermore, more recent studies suggest that the energies computed using the M06-2X/aug-cc-pVTZ method may underestimate the barrier energies, thus motivating the present work.22,23
In the present study, we compute the stationary points on the pathways for reaction of a set of oxygen, sulfur, phosphorous and nitrogen anions with CO2. Electronic energies, zero-point vibrational energies (ZPVE) and thermal corrections are also computed for all species, leading to enthalpies and Gibbs free energies at 298 K and 1 atm. The computational approach is the same as that used in the previous DFT study.21 The CBS extrapolation was used to enhance the results accuracy and confirm the trends reported by Sucarrat and Varandas in their DFT calculations for anions reacting with CO2.21