Thermophysical Properties and CO2 Absorption of Ammonium-Based Protic Ionic Liquids Containing Acetate and Butyrate Anions

Abstract

Ionic liquids, which are classified as new solvents, have been identified to be potential solvents in the application of CO2 capture. In this work, six ammonium-based protic ionic liquids, containing ethanolammonium [EtOHA], tributylammonium [TBA], bis(2-ethylhexyl)ammonium [BEHA] cations, and acetate [AC] and butyrate [BA] anions, were synthesized and characterized. The thermophysical properties of the ammonium-based protic ionic liquids were measured. Density, , and dynamic viscosity, , were determined at temperatures between 293.15 K and 363.15 K. The density and viscosity values were correlated using empirical correlations and the thermal coefficient expansion, p, and molecular volume, Vm, were estimated using density values. The thermal stability of the ammonium-based protic ionic liquids was investigated using thermogravimetric analyzer (TGA) at a heating rate of 10 C.min‒1. The CO2 absorption of the ammonium-based ionic liquids were measured up to 20 bar at 298.15 K. From the experimental results, [BEHA][BA] had the highest affinity towards CO2 with the mol fraction of CO2 absorbed approaching 0.5 at 20 bar. Generally, ionic liquids with butyrate anions have better CO2 absorption than that of acetate anions while [BEHA] ionic liquids have higher affinity towards CO2 followed by [TBA] and [EtOHA] ionic liquids.

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

“…As temperature increases, the volume of ionic liquids increases, and the density of the ionic liquids decreases accordingly. At higher temperatures, the intermolecular forces between the constituent ions weaken, and this increases the mobility of the ions which in turn increases the volume of these ions [ 37 , 52 , 53 ]. Further analysis also revealed that [EHA][C5] has the highest density values compared to the rest of the ammonium-based PILs.…”

“…Further analysis also revealed that [EHA][C5] has the highest density values compared to the rest of the ammonium-based PILs. The small size of [EHA] cation compared to [BEHA] cation affects local packing of the PIL structure and thus contributes to the increase in the density values of [EHA][C5] [ 37 , 54 ]. Comparable observations using PIL with ethylammonium cation were also found by several researchers, in which an increasing trending packing efficiency was proportional with the decreasing of molecular weight [ 51 , 55 ].…”

“…The calculated data is tabulated in Table 2 . Thermal expansion coefficients, α p for the ammonium-based PILs can be defined as [ 37 , 53 , 56 ]: α p = −1/ ρ . (δ ρ /δT) = −(A 2 )/(A 1 + A 2 T) …”

“…Despite promising results of CO 2 absorption by protic ionic liquids published in the literature [ 34 ], our current work is focusing on the utilization of much cheaper starting reagents, namely amine solutions, for the production of new ammonium-based protic ionic liquids. This work serves as a continuation from our previous work on CO 2 absorption utilizing ammonium-based protic ionic liquids (PILs) [ 37 ]. Previously, the CO 2 absorption of ammonium-based PILs utilizing bis (2-ethylhexyl) ammonium, tributylammonium and ethanolammonium cations coupled with acetate and butyrate anions have been reported.…”

"Comparison of Group Contribution Methods for Ionic Liquids"

“…Ethanolammonium butyrate 1.073 [17] 1.177 9.71% Bis(2-ethylhexyl) ammonium butyrate 0.870 [17] 0.935 7.48% 1.001 15.13% Tributylammonium acetate 0.912 [17] 0.928 1.71% 1.065 16.77%…”