Amino acid ionic liquids as efficient catalysts for CO2 capture and chemical conversion with epoxides under metal/halogen/cocatalyst/solvent-free conditions

Abstract

Amino acid ionic liquids (AAILs) are widely considered as biocompatible and biodegradable green absorbents, solvents and catalysts that have been utilized in sustainable chemistry. Several AAILs containing an ether-functionalized imidazole cation and amino acid anion, such as 1-methoxylbutyl-3-methylimidazolium glycine [MOBMIM][Gly], 1-methoxylbutyl-3-methylimidazolium lysine [MOBMIM][Lys], 1-methoxylbutyl-3-methylimidazolium arginine [MOBMIM][Arg] and 1-methoxylbutyl-3-methylimidazolium histidine [MOBMIM][His], were synthesized by the neutralization method and characterized by ¹H NMR, ¹³C NMR, FT-IR spectroscopy and elemental analysis in detail. Due to the CO₂ affinity of the ether-functionalized imidazole cation and amino acid anion, AAILs were applied in CO₂ capture as well as the coupling reaction of CO₂ with propylene oxide (PO). Specifically, [MOBMIM][Gly] with the ether-functionalized imidazole cation and glycine anion displayed the highest CO₂ capture capacity (2.36 mmol (mmol-IL)⁻¹) under the conditions of 50 °C and 0.5 MPa. Additionally, by the synergistic effects of the amino acid anion and ether group functionalized imidazole cation, all AAILs achieved 99% propylene carbonate (PC) selectivity and more than 80% PC yields (110 °C, 2 MPa and 12 h) towards CO₂ cycloaddition with PO. More importantly, the as-synthesized AAILs also showed good universality over various epoxides. The presented halogen-free, metal-free, bifunctional and stable AAILs provided a new insight into the application of AAILs in CO₂ capture and utilization.