Effect of the nature of cation of ionic liquid on the CO₂ conversion to cyclic carbonates-a theoretical study

Conversion of CO₂ into useful products is one of the favorable methods for alleviation of CO₂ and its further utilization. In this regard, ionic liquids (ILs) have been reported as efficient CO₂ conversion materials. One of the unique characteristics of ILs is their tunable physicochemical properties. The slight changes in the structure of ions (cation or anion) results in a number of cation/anion combinations which change the characteristics of ILs and overall, their functioning is also affected in this way. In the current study, we have considered a series of 1-alkyl-3-methyl-imidazolium chlorides ([C_nMIM][Cl]) (n = 1 to 6) for conversion of CO₂ to cyclic carbonates. These ILs are different from each other based on substituents attached to the imidazolium. Epoxide i.e., propylene oxide, is used as another reactant with CO₂ in the presence of ILs. The detailed mechanistic study shows that the lowest energy barriers are observed for CO₂ conversion process with [C₂MIM][Cl] and [C₃MIM][Cl]. The change in nature of cation affects the energy barriers by decreasing the barrier from [C₁MIM][Cl] to [C₂MIM][Cl] and [C₃MIM][Cl]. However, further increase in barriers is observed with [C₄MIM][Cl], [C₅MIM][Cl], and [C₆MIM][Cl] due to combined electronic and steric effects. The trend in the energy barrier is well understood through non-covalent interactions (NCI) analysis. Our study reveals that imidazolium with medium-sized alkyl chain is predicted to exhibit higher catalytic efficiency based on the computed activation energy barriers and free energy changes. Among the ILs studied, [C₂MIM][Cl] and [C₃MIM][Cl] show the most exergonic overall free energy change (-2.43 kcal/mol for step 1 with [C₂MIM][Cl] and - 12.82 kcal/mol for step 2 with [C₃MIM][Cl]), suggesting a modest thermodynamic preference for medium-chain ILs, though the small ΔG differences (~ 1-2 kcal/mol for step 1) are within the typical uncertainty of DFT. Our computational findings may serve as a preliminary guide for experimental design of imidazolium-based ILs for CO₂ cycloaddition reactions.