Combination of experimental and theoretical methods to explore the amino-functionalized pyrazolium ionic liquids: An efficient single-component catalyst for chemical fixation of CO2 under mild conditions

Abstract Numerous ionic liquids have been developed to catalyze the coupling reaction of carbon dioxide with epoxides without co-catalyst and organic solvent. Although the catalytic activity is satisfied with cyclic carbonate yield over 90%, the reaction condition is still not regarded as benign enough, especially for reaction temperature and carbon dioxide initial pressure. Four new amino-functionalized pyrazolium ionic liquids, AEMPzBr, APMPzBr, AEEPzBr, and APEPzBr, have been firstly synthesized in simple reaction steps with cheap raw materials. Moreover, they would catalyze the coupling reaction of carbon dioxide with propyleneoxide under the optimal reaction conditions, 110 °C, 1.5 MPa carbon dioxide initial pressure as well as 1.0 mol% catalyst amount for 4.0 h, with propylene carbonate yield over 94%. The reaction temperature is decreased by 10–20 °C as compared with other single-component ionic liquids. At the same time, the carbon dioxide initial pressure is not increased to keep 1.5 MPa, which is also superior to lots of other ionic liquids, such as imidazolium, pyrazolium, and guanidinium ionic liquids. The recyclability and suitability of amino-functionalized pyrazolium ionic liquids are also explored. Finally, the catalytic mechanism is investigated by the density functional theory. Moreover, the influence of weak interactions on the reaction is analyzed by atoms in molecules and non-covalent interactions. As compared with other reported ionic liquids, the reaction condition catalyzed by APEPzBr is more benign with comparable product yield. More importantly, there is good balance among various reaction conditions without sacrificing one to ensure others. Amino-functionalized pyrazolium ionic liquids would be a new kind of powerful catalysts for the carbon dioxide fixation.