Green and low-toxicity deep eutectic solvents for the recovery of methanol and methyl acetate from wastewater: Phase behavior and mechanistic investigation

During the alcoholysis of polyvinyl acetate to produce polyvinyl alcohol, large volumes of wastewater containing methanol (MeOH) and methyl acetate (MA) are generated. Due to the azeotropic nature of the MeOH-MA mixture under atmospheric pressure, their separation via conventional distillation is highly challenging. Extractive distillation presents a promising alternative; however, traditional entrainers often suffer from high volatility, toxicity, and significant energy requirements. Deep eutectic solvents (DESs), characterized by low vapor pressure, biodegradability, and ease of regeneration, have emerged as green alternatives. This study aims to identify efficient DESs for separating the MeOH/MA azeotrope. Guided by conductor-like screening model for segment activity coefficient (COSMO-SAC) model predictions, three choline chloride (ChCl)-based DESs—DES1 (ChCl:urea), DES2 (ChCl:ethylene glycol), and DES3 (ChCl:glycerol)—were selected and evaluated through vapor-liquid equilibrium (VLE) experiments at 101.3 kPa. The addition of 20 mol% DESs significantly enhanced the relative volatility of MA with respect to MeOH. The VLE data were well correlated using the non-random two-liquid (NRTL) model. To elucidate the separation mechanism, molecular dynamics simulations were performed to identify stable DES-MeOH/MA cluster configurations. Electrostatic potential (ESP) analysis, electron density difference mapping, van der Waals surface penetration, Hirshfeld surface (HS) analysis, the independent gradient model based on Hirshfeld partition (IGMH), and electron localization function (ELF) analyses revealed that hydrogen bonding plays a dominant role in selectivity. Finally, a quantitative structure-activity relationship (QS-AR) based toxicity assessment confirmed that all three DESs exhibit substantially lower toxicity than conventional extractants. These findings underscore the potential of DESs as green entrainers for the efficient and sustainable separation of the MeOH/MA azeotropic system.