Study on separation of acetonitrile and methanol azeotropic system using tetraethylammonium chloride-based deep eutectic solvents

Large quantities of waste streams containing mixed acetonitrile (ACN) and methanol (MeOH) are generated during pharmaceutical and chemical production. The polarity of these two substances is extremely similar, resulting in the formation of an azeotrope at room temperature and atmospheric pressure. In this study, deep eutectic solvents (DESs) were selected as extractants for the separation of ACN-MeOH binary azeotrope. To obtain suitable DESs, the σ-profiles of ACN, MeOH, DESs were calculated using the COSMO-SAC model, and the results demonstrated that the DESs synthesized when tetraethylammonium chloride (TEAC) was chosen as the hydrogen bond acceptor (HBA) and acetamide as the hydrogen bond donor (HBD) could effectively break the azeotropy of ACN and MeOH. Therefore, four DESs were synthesized with molar ratios of TEAC: acetamide = 1: n (n = 1, 2, 3, 4) for subsequent ACN-MeOH-DESs ternary vapor–liquid phase equilibrium (VLE) experiments at 101.3 kPa to determine the optimal extractant. The measured VLE data revealed the higher separation of DES TEAC-2acetamide on the ACN-MeOH binary azeotrope compared with other DESs. Afterward, the NRTL model was adopted to calculate the binary interaction parameters between ACN/MeOH and DESs. The results of the fitting indicated agreement with the experimental data. Finally, the separation mechanism was further analyzed using the interaction energy analysis, improved independent gradient model (IGMH), and the theory of atoms in molecules (AIM). The results indicated that the strong interaction between DESs and MeOH is the primary factor in the ability of DESs to break the azeotrope of the system.