A novel pyrazolium ionic liquid used for CO2 cycloaddition

Abstract

A novel pyrazole-based catalyst system featuring –CH as a hydrogen bonding donor was developed and utilized for the conversion of CO₂ into cyclic carbonates without the use of metals or co-catalysts. It is intriguing that the synthesized pyrazolium ionic liquid ([Bn₂DMPz]Br) could efficiently catalyze the coupling reaction of carbon dioxide (CO₂) and epoxides at 90 °C, under a pressure of 0.1 MPa CO₂, achieving a product yield of up to 99% within just 5 hours. The catalyst can be recycled and reused multiple times without any decrease in its catalytic efficiency. In addition, the pyrazolium ionic liquid was polymerized to get the first pyrazole-based poly(ionic liquid) (PIL). The polymer also exhibits high catalytic activity, excellent stability and easy recyclability. In comparison to other catalysts based on ionic liquids, our catalyst system demonstrates outstanding performance and recyclability at the same time. DFT calculations show that “double-IL” model may easily make the bromide anion serve as a nucleophile and cation [Bn₂DMPz]⁺ serves as a H-donor to stabilize the O⁻ of the epoxide and the carbonate anion with weak interaction. The design strategy based on pyrazole ionic liquid will inspire researchers to develop more catalysts used for CO₂ cycloaddition in the future.