Separation of 1,2-diols from ethylene glycol via biphasic-solvent extraction and thermodynamic insights

Impurities 1,2-propylene glycol (PG), 1,2-butylene glycol (BG), 2,3-butylene glycol (23BG), 1,2-pentanediol (PDO), and 1,2-hexanediol (HDO) were isolated from ethylene glycol (EG) via biphasic-solvent extraction, which involves immiscible solvents saline water (S1) and ethyl acetate (S2). After adding S1 to the diol-S2 system, the partition coefficients were almost unaltered, while the selectivity coefficients were doubled or tripled. The larger the carbon-atom number of diol was, the higher the removal rate was. The removal rates of PG, BG, 23BG, PDO, and HDO reached 68%, 95.3%, 90.1%, 100%, and 100%, respectively. The EG content was increased from 72.7 to 91.2 wt%, and EG recovery reached 80.1%. The phase diagrams showed that the S1-EG-S2, S1-PDO-S2, and S1-HDO-S2 systems exhibited Treybal's type-II behaviors, while the S1-PG-S2 and S1-BG-S2 systems showed type-I characteristics. In addition, the low-carbon-number diols were miscible with S1 and rejected S2, while the high-carbon-number diols exhibited opposite characteristics. The intense endotherm during the dissolution of EG in ester implied that the increase of enthalpy of system is an important reason for the high chemical potential of EG. The obvious Tyndall phenomena of C3–6 diols dispersed in saline water indicated that these diols formed into ordered micelles with low entropy, resulting in their high chemical potentials. The chemical potentials of diols in the biphasic solvents synergistically contributed to the separation selectivity.