Thiourea-Based Bifunctional Ionic Liquids as Highly Efficient Catalysts for the Cycloaddition of CO2 to Epoxides

Abstract A series of thiourea-based ionic liquids (TBILs) were developed as efficient catalysts for the synthesis of cyclic carbonates by the cycloaddition of CO 2 with epoxides under metal-, cocatalyst- and solvent-free conditions. Different substituted groups including electron donating and withdrawing groups were introduced onto the para-position of the aromatic ring in cation of TBILs. The effect of the catalyst structures on the reaction activity was systematically investigated. Among the ionic liquids, TBIL3 with substituted OCH 3 group showed the best performance with 99% propylene oxide conversion as well as 99% propylene carbonate selectivity under mild conditions (1.5 MPa, 130 °C and 3 h) without any co-solvent. It was demonstrated that two secondary amine groups of the thiourea unit could dually activate the epoxides and CO 2 and have a synergetic effect with the halide anion. Graphical Abstract