Protic pyrazolium ionic liquids for efficient chemical fixation of CO2: design, synthesis, and catalysis

Abstract

The conversion of carbon dioxide into organic products under benign conditions is still challenging. Ionic liquids are regarded as efficient and “green” catalysts from a sustainability view point. However, little attention has been focused on pyrazolium ILs although they are structural isomers of imidazolium ILs. Even fewer studies have been performed on protic pyrazolium ILs. In this work, three new protic pyrazolium ILs, HTMPzBr, HMM₃PzBr, and HMM₅PzBr, have been synthesized to explore their catalytic activity for the coupling reaction of carbon dioxide and propylene oxide. Both theoretical calculations and experimental characterization have verified that HTMPzBr and HMM₅PzBr have similar catalytic activity, which is higher than that of HMM₃PzBr. The role of various weak interactions, especially hydrogen bonds, in the reaction is elucidated by detailed theoretical analysis. The sequence of catalytic activity predicted by the Double-IL theoretical model is totally consistent with the experimental result. It is reasonable to design ionic liquids from a molecular level if a suitable theoretical model is applied, and this would provide some guidance for further experimental study.