Performance and Mechanism Study on Functionalized Phosphonium-Based Deep Eutectic Solvents for CO 2 Absorption

In this work, a series of functionalized phosphonium-based deep eutectic solvents (DESs) were prepared, and the solubility of CO 2 in DESs was determined at temperatures from 303.15 K to 333.15 K and pressures from 200 to 2500 kPa. The experimental results show that the addition of carboxyl, hydroxyl, or amino functional groups to the alkyl chain of phosphonium-based ionic liquid (IL) can improve the solubility of CO 2 in DESs. With the use of the nonrandom two-liquid (NRTL) model, the solubility data for the {CO 2 + DESs} system were correlated, and the average relative deviation (ARD%) between the calculated and experimental values was less than 5%. The maximum absorption of CO 2 was shown by 1-carboxyethyltributylphosphonium bromide–diethylene glycol ([P 4,4,4,2 COOH][Br]-DEG), whose mole fraction of CO 2 was 0.5335 at 303.15 K and 2500 kPa. At the same time, [P 4,4,4,2 COOH][Br]-DEG can still maintain high-CO 2 absorption performance after five cycles of absorption and desorption, indicating that the DES had good cycle stability. In addition, the interaction energy between CO 2 and four DESs was determined by density functional theory (DFT), and the chemical bond and weak interaction were revealed by interaction region indicator (IRI), to clarify the absorption mechanism.