CO2 absorption and recycling performance of functionalized phosphonium amino acid ionic liquids as green absorbent

Functionalized phosphonium amino acid ionic liquids (PAAILs) have emerged as promising green absorbents offering a versatile alternative to conventional ionic liquids (ILs) and amine solutions for carbon dioxide (CO 2 ) capture. In this study, four PAAILs were synthesised using different amino acids: Glycine, L-Arginine, L-Lysine, and L-Histidine, and evaluated for CO 2 absorption performance at pressures up to 4.0 ± 0.2 bar and temperatures between 15 and 40 ℃. All synthesised PAAILs remained liquid at room temperature and demonstrated enhanced CO 2 absorption capacity compared with the precursor ionic liquid. Among the investigated absorbents, P[Arg] IL exhibited the highest CO 2 uptake and fastest absorption kinetics at 25 ℃. ATR-FTIR and 1 H/ 13 C NMR analyses confirmed carbamate formation during CO 2 absorption. Enhanced CO 2 absorption capacity was attributed to the lone pair (–NH 2 ) groups of amino acid anions of PAAILs, which act as absorption enhancers. Thermal stability, viscosity, and regeneration behaviour of the PAAILs were also investigated to assess their practical applicability. Furthermore, a multiple linear regression (MLR) model was developed to predict CO 2 absorption capacity based on physicochemical parameters, showing strong agreement with experimental data. This work provides molecular design insights into phosphonium amino acid ionic liquids and demonstrates their potential as efficient absorbents for low-temperature CO 2 capture applications.