Experimental Measurements of Amine-Functionalized Anion-Tethered Ionic Liquids with Carbon Dioxide

Abstract

Amine-functionalized anion-tethered ionic liquids (ILs), trihexyl(tetradecyl)phosphonium glycinate [P66614][Gly], alanate [P66614][Ala], sarcosinate [P66614][Sar], valinate [P66614][Val], leucinate [P66614][Leu], and isoleucinate [P66614][Ile], were synthesized and investigated as potential absorbents for CO2 capture from postcombustion flue gas. Their physical properties, including density, viscosity, glass transition temperature, and thermal decomposition temperature, were determined. The influence of changing the anion and, more specifically, the length of the alkyl chain is discussed. Furthermore, the CO2 absorption isotherms of [P66614][Gly], [P66614][Ile], [P66614][Sar], and [P66614][Ala] were measured using a volumetric method, and the results were modeled with two different Langmuir-type absorption models. All four ILs reached greater than 0.5 mol of CO2 per mole of IL at CO2 pressures of less than 1 bar. This indicates the predominance of the 1:1 mechanism, where the CO2 reacts with one IL to form a carbamic acid, over further reaction with another IL to make a carbamate (the 1:2 mechanism). The chemical absorption of CO2 dramatically increased the viscosity of the IL, but this can be mitigated to some extent by decreasing the number of hydrogens on the anion available for hydrogen-bonding.

Tuning the Carbon Dioxide Absorption in Amino Acid Ionic Liquids

“…To increase their CO 2 absorption capacities, ILs need to be modified in such a way that the chemical absorption of CO 2 becomes accessible. The introduction of amine groups in the cation [18] or anion [19][20][21] is one of the easiest and most straightforward methods among the different strategies developed for the enhancement of CO 2 capacity. [22][23][24][25][26][27][28] Amino acid ionic liquids (AAILs) are particular representatives of amine-functionalized ILs.…”

“…The existing equilibrium is sensitive to the external conditions and the molecular composition of the solution. For example, lower temperatures or higher CO 2 partial pressures [21,38,39] shift the equilibrium towards 1:1 capacity. [20,21,38,39] Similarly, the attachment of the amine group to the anion instead of to the cation provides higher capacities.…”

“…For example, lower temperatures or higher CO 2 partial pressures [21,38,39] shift the equilibrium towards 1:1 capacity. [20,21,38,39] Similarly, the attachment of the amine group to the anion instead of to the cation provides higher capacities. [18][19][20][21] Changing the location of the CO 2 absorption from the bulk to the surface of an IL with an amine-functionalized anion also increases the CO 2 capacity to 1:1 at a moderate CO 2 pressure.…”

“…[20,21,38,39] Similarly, the attachment of the amine group to the anion instead of to the cation provides higher capacities. [18][19][20][21] Changing the location of the CO 2 absorption from the bulk to the surface of an IL with an amine-functionalized anion also increases the CO 2 capacity to 1:1 at a moderate CO 2 pressure. [39] Finally, increasing the cation size is responsible for higher CO 2 absorption capacities for some AAILs.…”

Enhancement of CO2 capture performance of aqueous MEA by mixing with [NH2e-mim][BF4]

“…1 The regeneration temperature of the functional ILs was from 263 K to 343 K, 40 which was lower than that of the alcohol amine. Thus, the ILs needed less energy for the increase of the temperature.…”