Pyrazolium ionic liquids with multiple active sites immobilized on mesoporous MCM-41 for chemical fixation of CO2 under mild conditions

Abstract

The conversion of carbon dioxide (CO₂) into valuable industrial materials effectively combats rising emissions and facilitates carbon recycling, which requires a top-tier catalyst. In this study, a series of dicationic pyrazolium ionic liquids were grafted onto pre-functionalized MCM-41, and the resulting C_nPz–APTES/MCM-41 (n = 3–6), which integrate hydrogen bond donors, Lewis basic tertiary amines (dual N⁺ sites), nucleophilic ionic sites, high surface areas, and mesoporous structures, were employed as catalysts in the cycloaddition of CO₂ and epoxides to yield cyclic carbonates. Among them, C₆Pz–APTES/MCM-41 exhibited excellent catalytic performance achieving 98.2% PC yield and 100% selectivity under mild reaction conditions (70 °C and 0.5 MPa CO₂ pressure) without a metal, co-catalyst, or solvent. The effect of reaction parameters on the production of cyclic carbonates was thoroughly investigated. Furthermore, C₆Pz–APTES/MCM-41 could be efficiently recovered and reused six times without loss of its catalytic activity. It also showed significant adaptability to different epoxides. Structure–activity studies indicated that PO activation was synergistically promoted by the presence of C3/C5 hydrogens from dicationic pyrazolium rings bound by alkyl linkers and dibromide ions in C_nPz–APTES/MCM-41. Finally, the reaction mechanism was further investigated using X-ray photoelectron spectroscopy (XPS), proton nuclear magnetic resonance (¹H NMR), and density functional theory (DFT) computations.