Tosylate-Based, Halogen-Free Ionic Liquids for Mild and Efficient Cycloaddition of CO ₂ to Propylene Oxide

The development of efficient and eco-friendly catalysts for the conversion of CO2 and propylene oxide (PO) to propylene carbonate (PC) is vital for carbon-neutral goals. To overcome the corrosion and environmental issues of halogenated ionic liquids (ILs), two novel halogen-free ILs were designed by pairing a p-toluenesulfonate anion with a carboxyl- or ether-functionalized 1-butyl-3-methylimidazolium cation. The carboxyl-functionalized IL, [HCPMIM][p-TSA], showed superior performance, achieving 98.60% PO conversion and 98.21% PC selectivity. Density functional theory (DFT) calculations confirmed its strong hydrogen-bond-donating ability, which led to the lowest energy barrier in the rate-determining step, consistent with the experimental data. This study provides a theoretical basis for designing efficient halogen-free catalysts and elucidates the reaction mechanisms.