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 CO₂ separation during recyclability processes. Additionally, the performance of APILs toward CO₂ 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 CO₂ absorption capacity with the value of 0.74 mole fraction at 20 bar. Additionally, the regeneration of [TBA][C7] for CO₂ absorption was studied. Analysis of the measured CO₂ absorption data showed marginal reduction in the mole fraction of CO₂ absorbed between fresh and recycled [TBA][C7] thus proving the promising potential of APILs as good liquid absorbents for CO₂ removal.