Controlling dual-positively charged pyrazolium ionic liquids for efficient catalytic conversion of CO2 into carbonates under mild conditions

Abstract

To address the ongoing rise in carbon dioxide (CO₂) emissions, CO₂ utilization presents a promising approach due to its ability to convert CO₂ into valuable industrial products and enable carbon recycling. For this reason, a high-quality catalyst is required to ensure the effective activation and conversion of CO₂. In this study, a series of dicationic pyrazolium ionic liquids (DPzILs) were first synthesized via a one-step process and employed as catalysts in the cycloaddition reaction of CO₂ and epoxides, yielding cyclic carbonates. Among the synthesized DPzILs, [DMPz-6]I₂ exhibited outstanding catalytic performance on diluted CO₂ from simulated flue gas (60% CO₂ in N₂), achieving 94.1% PC yield and 100% selectivity under reaction conditions (100 °C and 10 bar CO₂ pressure) without metal, co-catalyst, or solvent. The study investigated the effects of DPzILs structures, catalyst dosage, CO₂ pressure, reaction temperature, and reaction time on the production of cyclic carbonates. Furthermore, [DMPz-6]I₂ could be efficiently recovered and reused seven times without significant degradation of catalytic activity. It demonstrated significant adaptability to various epoxides. Structure–activity studies indicated that PO activation is synergistically facilitated by the presence of C3/C5 hydrogen from dual-pyrazolium cation rings tethered by alkyl chain lengths and a paired halide anion (I⁻/Br⁻/Cl⁻) in DPzILs. Finally, the reaction mechanism was investigated using FT-IR, ¹H NMR, and DFT calculations.