Physicochemical Properties and CO2 Absorption of the Mixed System of Sodium Phenoxide and [Emim]Cl–EG Deep Eutectic Solvents

Abstract

This study aims to develop and characterize a novel mixed system composed of sodium phenoxide (NaOPh) and a deep eutectic solvent (DES) formed from 1-ethyl-3-methylimidazole chloride ([Emim]Cl) and ethylene glycol (EG), in order to address the phase separation issue commonly encountered in aqueous phenoxide solutions during CO₂ absorption—caused by the generation of phenol. The physicochemical properties, CO₂ absorption performance and absorption mechanism of the NaOPh/DES mixed system were investigated. The results indicate that the density, viscosity, and CO₂ absorption capacity of the studied NaOPh/DES mixed systems increase with NaOPh content. The NaOPh1.0/DES system exhibits the best absorption performance of these systems, with a CO₂ absorption capacity of 2.10 mol‧kg⁻¹ at 313.15 K and 0.1 MPa. DFT calculations reveal that the Na⁺ from NaOPh, the O–H group from EG, and the Cl⁻ from [Emim] Cl can form a stable four-membered ring. This weakens the electrostatic interaction between Na⁺ and [PhO]⁻, promoting the dissociation of the NaOPh and facilitating the chemical reaction between the “free” [PhO]⁻ and CO₂ to form carbonates. This mechanism is confirmed by FT-IR and ¹³C NMR spectra. Using [Emim]Cl–EG DES as a solvent to dissolve NaOPh leverages the reactivity of [PhO]⁻ with CO₂ for efficient absorption, while effectively avoiding phase separation. This expands the application of phenoxide-based systems in CO₂ capture.