Insight into the Effect of Physicochemical Properties on CO2 Absorption Behavior of Imidazole Anion-Functionalized Ionic Liquids

Three imidazole anion-functionalized ionic liquids (IFILs) with tributylethylphosphonium ([P₄₄₄₂]⁺) cation and imidazolate ([Im]⁻), 4-methylimidazolate ([4-MeIm]⁻), or 4-bromimidazolate ([4-BrIm]⁻) anions were prepared to study the effect of physicochemical properties on CO₂ absorption behavior. Density (ρ), viscosity (η), and speed of sound (u) of the studied IFILs were measured, and molecular volume (V_m), standard entropy (S⁰), lattice energy (U_POT), and isentropic compressibility coefficient (κ_s) were calculated accordingly. CO₂ absorption behavior of [P₄₄₄₂][Im] at T = 313.15–333.15 K and p = 0.2 and 1 bar was investigated as an example. The results show that with the increase of temperature, ρ, η, u, and U_POT decrease, while V_m, S⁰, and κ_s increase, due to a decrease in electrostatic interaction correspondingly. The orders of ρ, u, η, V_m, and S⁰ values are as follows: [P₄₄₄₂][Im] < [P₄₄₄₂][4-MeIm] < [P₄₄₄₂][4-BrIm], while U_POT and κ_s are in reverse order. Interestingly, CO₂ capture capacity of IFILs is approximately linear with η or κ_s. Due to low η and high κ_s, CO₂ absorption capacity of [P₄₄₄₂][Im] is almost independent of temperature and partial pressure, as high as 0.90 mol CO₂/mol IL at 333.15 K and 0.2 bar, indicating that [P₄₄₄₂][Im] has potential applications for CO₂ absorption at high temperature and low pressure.