Multiple-site absorption of CO₂ in 2-hydroxypyridium ionic liquids based task-specific deep eutectic solvents

Global warming caused primarily by excessive emissions of CO₂ has attracted worldwide attention. Herein, three 2-hydroxypyridium ionic liquids (ILs) based task-specific deep eutectic solvents (DESs) were synthesized to absorb CO₂ and physical properties including density, viscosity, and melting points were measured to explore the effect on CO₂ absorption. The CO₂ absorption capacities of the ILs-based task-specific DESs were investigated at different pressures and temperatures, which showed that the maximum absorption capacity of the DES was up to 1.48 mol_CO2·mol_DES⁻¹ or 0.233 g_CO2·g_DES⁻¹ at the atmospheric pressure and 25 ℃. The plausible absorption mechanism was also proposed by a combination of 1:1 and 2:1 stoichiometric reactions of CO₂ and the IL-based task-specific DES via multiple-site absorption, which was confirmed by ¹³C and ¹H nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, quantum chemical calculation, and reaction equilibrium thermodynamic modeling. The thermodynamic properties, including absorption Gibbs free energy, absorption enthalpy, and absorption entropy were rationally deduced and explained. Furthermore, the excellent CO₂ absorption capacity and regenerability of multiple-site task-specific DES make it a new environmentally eco-friendly choice for highly efficient CO₂ absorption and subsequent CO₂ transformation.