Experimental Investigation and Mechanism Analysis for the Separation of <i>sec</i>-Butanol and <i>sec</i>-Butyl Acetate System via Liquid–Liquid Extraction with Ionic Liquids

Efficient separation of sec-butanol (SBA) and sec-butyl acetate (SBAC) is important for the sustainability of methyl ethyl ketone (MEK) and its subsequent solvent and pharmaceutical chemical processes. However, SBA and SBAC form a binary azeotrope, which complicates the separation. In this work, three ionic liquids: 1-propyl-3-methylimidazolium dihydrogen phosphate ([PMIM][H2PO4]), 1-butyl-3-methylimidazolium dihydrogen phosphate ([BMIM][H2PO4]), and 1-butyl-3-methylimidazolium hydrosulfate ([BMIM][HSO4]) are screened and employed to separate SBAC and SBA azeotropic mixture using liquid–liquid extraction. Ternary liquid–liquid equilibrium (LLE) data at 303.15 K and atmospheric pressure are determined with distribution coefficients and selectivity evaluated, and data reliability is confirmed via Othmer-Tobias and Hand correlations. The experimental LLE data are correlated by the nonrandom two-liquids (NRTL) model, with the consistency of NRTL model parameters verified through the GMcal_TieLinesLL. The underlying separation mechanism is comprehensively elucidated through QC calculations, with ESP mapping, bond length and interaction energy, IGMH analysis, and QTAIM topological analysis, demonstrating that the interaction between ionic liquids (ILs) and SBA is stronger than the ILs and SBAC, among which hydrogen bond plays a dominant role. The feasibility of industrial-scale application is confirmed by a process flowsheet simulated and optimized with Aspen Plus.