Study on the Thermodynamic Properties of Two Quaternary Ammonium Salt-Based Deep Eutectic Solvents and Their Efficient Extraction Performance of Phenol from Model Coal Tar

Phenol serves as a crucial fundamental raw material in the organic chemical industry. The extraction of phenol from model coal tar using deep eutectic solvents (DESs) offers significant economic, social, and environmental advantages. In this work, we design two quaternary ammonium salt-based DESs ([2choline chloride (2ChCl): oxalic acid (OA)] and [choline chloride: 2malonic acid]). The density, surface tension, and electrical conductivity of these DESs were measured, and the thermodynamic parameters such as the molecular volume (V_m), molar volume (V), molar surface Gibbs energy (g_s), and molar electrical conductivity (Λ_m) of all DESs were calculated. Based on thermodynamic properties, the effects of quaternary ammonium salt-based DESs on the extraction efficiency of phenol under various extraction conditions were investigated. Under optimal extraction conditions, the phenol extraction efficiency of [2ChCl:OA] remained as high as 97.74% after three extraction cycles. Combined with infrared spectroscopy (IR) characterization, DFT calculation, and molecular dynamics simulation (MD), the main mechanism of the efficient and selective extraction of phenol by DESs was discussed. Combining thermodynamic properties and extraction mechanism analysis in this work can provide a potential assistant method for the comprehensive performance evaluation of DESs in further application.