Performance and Mechanism Study on Functionalized Phosphonium-Based Deep Eutectic Solvents for CO2 Absorption

In this work, a series of functionalized phosphonium-based deep eutectic solvents (DESs) were prepared, and the solubility of CO2 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 CO2 in DESs. With the use of the nonrandom two-liquid (NRTL) model, the solubility data for the {CO2 + DESs} system were correlated, and the average relative deviation (ARD%) between the calculated and experimental values was less than 5%. The maximum absorption of CO2 was shown by 1-carboxyethyltributylphosphonium bromide-diethylene glycol ([P4,4,4,2COOH][Br]-DEG), whose mole fraction of CO2 was 0.5335 at 303.15 K and 2500 kPa. At the same time, [P4,4,4,2COOH][Br]-DEG can still maintain high-CO2 absorption performance after five cycles of absorption and desorption, indicating that the DES had good cycle stability. In addition, the interaction energy between CO2 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.

Automated summary

A series of functionalized phosphonium-based deep eutectic solvents (DESs) were prepared to enhance the solubility of CO2. The addition of carboxyl, hydroxyl, or amino functional groups to the alkyl chain of phosphonium-based ionic liquid (IL) was found to improve the solubility. The solubility data for the {CO2 + DESs} system were correlated using the nonrandom two-liquid (NRTL) model, with an average relative deviation (ARD%) less than 5%.