Phase Behavior and Microscopic Mechanisms of Separation of n-Hexane and Methylcyclopentane Mixtures with Deep Eutectic Solvents

n-Hexane is widely used in industry, and methylcyclopentane is often generated as a byproduct during its production. The small boiling point difference (3.08 K) between these two compounds makes their separation challenging using traditional distillation methods. Extractive distillation, known for its high efficiency and energy savings, is effective in separating azeotropic and near-azeotropic mixtures. Deep Eutectic Solvents (DESs), a new class of green solvents, have shown promise in chemical separations. This study investigates the use of DESs as potential entrainers for the separation of n-hexane and methylcyclopentane in extractive distillation. The COSMO-SAC model was used to select two DESs: DES1 (tetrabutylammonium bromide (TBAB):decanoic acid = 1:2) and DES2 (TBAB:oleic acid = 1:3). Vapor-liquid equilibrium (VLE) data for the ternary system (n-hexane-methylcyclopentane-DES) were measured at 101.3 kPa. The results showed that both DESs effectively separated n-hexane and methylcyclopentane at a 25 mol % concentration, with DES1 demonstrating superior performance. The VLE data were fitted using the Non-Random Two-Liquid (NRTL) model, yielding satisfactory results. Quantum chemistry calculations further elucidated the molecular mechanisms behind the superior separation performance of DES1.