Synthesis and characterization of ammonium-based protic ionic liquids for carbon dioxide absorption

A series of ammonium-based protic ionic liquids (APILs) namely ethanolammonium pentanoate [ETOHA][C5], ethanolammonium heptanoate [ETOHA][C7], triethanolammonium pentanoate [TRIETOHA][C5], triethanolammonium heptanoate [TRIETOHA][C7], tributylammonium pentanoate [TBA][C5] and tributylammonium heptanoate [TBA][C7] was synthesized via proton transfer. Their structural confirmation and physiochemical properties namely thermal stability, phase transition, density, heat capacity (C(p)) and refractive index (RI) have been determined. Specifically, [TRIETOHA] APILs have crystallization peaks ranging from −31.67 to −1.00 °C, owing to their large density values. A comparison study revealed the low C(p) values of APILs in comparison to monoethanolamine (MEA) which could be advantageous for APILs to be used in CO(2) separation during recyclability processes. Additionally, the performance of APILs toward CO(2) absorption was investigated by using a pressure drop technique under a pressure range of 1–20 bar at 298.15 K. It was observed that [TBA][C7] recorded the highest CO(2) absorption capacity with the value of 0.74 mole fraction at 20 bar. Additionally, the regeneration of [TBA][C7] for CO(2) absorption was studied. Analysis of the measured CO(2) absorption data showed marginal reduction in the mole fraction of CO(2) absorbed between fresh and recycled [TBA][C7] thus proving the promising potential of APILs as good liquid absorbents for CO(2) removal.