Thermodynamic and Mechanistic Study of Indole Extraction from Simulated Wash Oil Using Quaternary Ammonium-Based Deep Eutectic Solvents

Abstract

In this work, three independent single-solute model systems─indole–toluene, naphthalene–toluene, and acenaphthene–toluene─were established to elucidate the component-specific phase-distribution behavior of representative species in simulated wash oil. These simplified systems were designed to isolate the distribution behavior of each component from the competitive effects present in real multicomponent wash oil. Three quaternary ammonium-based deep eutectic solvents, TEAC/EG, TPAC/EG, and TBAC/EG, denoted as DES1, DES2, and DES3, respectively, were prepared and used for the selective extraction of indole. The thermodynamic affinity, extraction performance, and interaction mechanism of the DESs were investigated using Hansen solubility parameters, Flory–Huggins interaction parameters, inverse gas chromatography, liquid–liquid equilibrium experiments, and quantum chemical calculations. The results showed that all three DESs promoted the transfer of indole into the DES-rich phase, among which DES2 exhibited the best overall performance, with a maximum indole/toluene selectivity of 33.95. Quantum chemical calculations showed that the DES2-indole interaction energy was −72.77 kJ·mol–1, which was more negative than those of DES2 with naphthalene, acenaphthene, and toluene. The preferential extraction of indole was mainly associated with a dominant weak closed-shell N–H···Cl hydrogen-bonding interaction, assisted by weak C–H···O and C–H···Cl interactions. After five cycles, the mass recoveries of the three DESs remained above 95%, and no obvious decrease in indole extraction performance was observed. This work provides experimental evidence and mechanistic insight into the extraction and separation of indole from simulated wash oil using quaternary ammonium-based DESs.