Experimental Investigation on Thermophysical Properties of Ammonium-Based Protic Ionic Liquids and Their Potential Ability towards CO2 Capture

Abstract

Ionic liquids, which are extensively known as low-melting-point salts, have received significant attention as the promising solvent for CO2 capture. This work presents the synthesis, thermophysical properties and the CO2 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 (nD) 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, Td of greater than 416K and the presence of glass transition, Tg was detected in each PIL. The CO2 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 CO2 absorption with 0.78 mol at 29 bar. The CO2 absorption values increase in the order of [C5] < [C6] < [C7] anion regardless of the nature of the cation.

Reviewing and screening ionic liquids and deep eutectic solvents for effective CO2 capture

“…After the absorption, [MOBMIM][His] and [MOBMIM][Gly] stayed in the liquid phase, while [MOBMIM][Arg] and [MOBMIM][Gly] turned to gel. Recently, Zailani et al (2022) synthesized ammonium based protic ILs using 2-ethylhexylammonium and bis-(2-ethylhexyl) ammonium coupled with pentanoate, hexanoate, and heptanoate ([EHA][C 5 ], [EHA][C 6 ], [EHA][C 7 ], [BEHA][C 5 ], [BEHA][C 6 ], and [BEHA][C 7 ]). The absorption capacity (mol/kg) decreased in the order of [C 5 ] < [C 6 ] < [C 7 ], and [EHA][C 7 ] showed the highest absorption capacity (∼3.0 mol/kg at 29 bar and room temperature).…”

Alkylphosphonium carboxylate ionic liquids with tuned microscopic structures and properties

“…72 In general, an increase of approximately 90 1C or 150 1C in decomposition temperatures is observed for the phosphonium carboxylate ILs (carboxylate anions with linear alkyl chains) compared to their equivalent imidazolium or ammonium counterparts, respectively. 73 When the carboxylate anions are derived from amino acids, the corresponding phosphonium ILs show a lower thermal stability. 74 As expected, shorter alkyl side-chains in the phosphonium cations lead, in general, to higher decomposition temperatures.…”

Editorial for Special Issue: “Recent Advances in Green Solvents”

“…As a comprehensive study, Zailani et al [ 7 ] reported the results of their research on the synthesis of a series of ammonium cations coupled with carboxylate anions producing ammonium-based protic ionic liquids, and their densities, viscosities, refractive indices, thermal decomposition temperatures, glass temperatures, and CO 2 absorption was also reported.…”