Experimental Investigation on Thermophysical Properties of Ammonium-Based Protic Ionic Liquids and Their Potential Ability towards CO 2 Capture.

Ionic liquids, which are extensively known as low-melting-point salts, have received significant attention as the promising solvent for CO 2 capture. This work presents the synthesis, thermophysical properties and the CO 2 absorption of a series of ammonium cations coupled with carboxylate anions producing ammonium-based protic ionic liquids (PILs), namely 2-ethylhexylammonium pentanoate ([EHA][C5]), 2-ethylhexylammonium hexanoate ([EHA][C6]), 2-ethylhexylammonium heptanoate ([EHA][C7]), bis-(2-ethylhexyl)ammonium pentanoate ([BEHA][C5]), bis-(2-ethylhexyl)ammonium hexanoate ([BEHA][C6]) and bis-(2-ethylhexyl)ammonium heptanoate ([BEHA][C7]). The chemical structures of the PILs were confirmed by using Nuclear Magnetic Resonance (NMR) spectroscopy while the density ( ρ ) and the dynamic viscosity ( η ) of the PILs were determined and analyzed in a range from 293.15K up to 363.15K. The refractive index ( n D ) was also measured at T = (293.15 to 333.15) K. Thermal analyses conducted via a thermogravimetric analyzer (TGA) and differential scanning calorimeter (DSC) indicated that all PILs have the thermal decomposition temperature, T d of greater than 416K and the presence of glass transition, T g was detected in each PIL. The CO 2 absorption of the PILs was studied up to 29 bar at 298.15 K and the experimental results showed that [BEHA][C7] had the highest CO 2 absorption with 0.78 mol at 29 bar. The CO 2 absorption values increase in the order of [C5] < [C6] < [C7] anion regardless of the nature of the cation.