Insight into the effect of physicochemical properties on CO2 absorption behavior of imidazole anion-functionalized ionic liquids

Abstract

Abstract 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), speed of sound (u) of the studied IFILs were measured, and molecular volume (Vm), standard entropy (S0), lattice energy (UPOT), and isentropic compressibility coefficient (κs) were calculated accordingly. CO2 absorption behavior of [P4442][Im] at T = 313.15-333.15 K and p = 0.2 and 1 bar were investigated as an example. The results show that with the increase of temperature, ρ, η, u, and UPOT decrease, while Vm, S0, and κs increase, due to a decrease in electrostatic interaction correspondingly. The orders of ρ, u, η, Vm, and S0 values are as follows: [P4442][Im] < [P4442][4-MeIm] < [P4442][4-BrIm], while UPOT and κs are in reverse order. Interestingly, CO2 capture capacity of IFILs is approximately linear with η or κs. Due to low η and high κs, 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.