Insight on asym-Pyrazolium Ionic Liquids for Chemical Fixation of CO2 and Propylene Epoxide into Propylene Carbonate without Organic Solvent and Metal

From the perspective of environmental protection and resource utilization, the fixation of carbon dioxide (CO2) is an interesting and meaningful topic. In this work, a series of asym-dialkylpyrazolium ionic liquids are synthesized by us to explore their catalytic activity for the cycloaddition of CO2 and propylene epoxide to produce propylene carbonate (PC). They could be easily synthesized by a simple reaction, which is the premise for the large-scale application. The effect of alkyl chain length in cation on catalytic performance is investigated. Although asym-pyrazolium ILs present less catalytic activity than sym-pyrazolium ILs, the product yield catalyzed by EPPzBr is as high as 85% with PC selectivity of 99%, which is better than most of non-functionalized ionic liquids. Moreover, the catalyst could be reused at least five times without significant loss of catalytic activity. To elucidate the structure-property relationship, the difference between asym-pyrazolium ILs and sym-pyrazolium ILs is discussed in detail by the Double-IL model associated with the noncovalent interactions and atoms in molecule analysis. The kind of single-component catalysts for the fixation of CO2 is further enriched.