Abstract
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 ([CnMIM][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.