Protic vs aprotic ionic liquid for CO2 fixation: A simulation study

The cycloaddition of CO2 with epoxides catalyzed by ionic liquids (ILs) has been a widely ongoing studied hot topic over the years. Recent experimental research has shown that the protic ionic liquids (PILs) behave stronger hydrogen proton donating ability than aprotic ionic liquids (APILs), and can effectively catalyze the cycloaddition of CO2. Unfortunately, the mechanistic explanation remains primarily unraveled. Herein, a detailed simulation study on the cycloaddition reaction catalyzed by PIL ([HDBU][Mim]) in comparison with APIL ([MeDBU][Mim]) reaction catalysts was conducted, including the three-step route (ring-opening of PO (propylene oxide), insertion of CO2 and ring-closure of propylene carbonate (PC)) and two-step route (simultaneously ring-opening of PO and addition of CO2, and then ring-closure of PC). Based on the activation energy barrier of the rate-determining step, PIL preferentially activates PO as the optimal route for the reaction with the energy barrier of 23.2 kcal mol-1, while that of APIL is 31.2 kcal mol-1. The role of [HDBU]+ in the reaction was also explored and found that the direct formation of intermolecular hydrogen bond (H-bond) between [HDBU]+ and the reactants (PO + CO2) was unfavorable for the reaction, while the cooperation with the anion [Mim]- to assist indirectly was more conducive. To fully consider the reaction microenvironment of ILs, ONIOM calculation was used to study the solvent effect. At last, the above conclusions were further verified by the analysis of intermediates with charge, non-covalent interaction (NCI), and atoms in molecules (AIM) methods. The computational findings show that ILs studied in this work have dual functions of catalyst and solvent, enabling a microscopic understanding of the ILs catalyst for CO2 utilization as well as providing guidance for the rational design of more efficient ILs-based catalysts.