Mechanism of NMA as entrainer in separating acetic acid from water with extractive distillation

N-methylacetamide (NMA) was used as entrainer in separating acetic acid from water with extractive distillation, and the interaction mechanism in this process was studied by using the density functional theory. The optimization geometries and vibration frequencies of NMA-acetic acid and NMA-water complexes were calculated at the B3LYP/6-311G (d, p) level. After the basis set superposition error (BSSE) correction and the zero point vibration energy (ZPE) correction, the interaction energies for three types of NMA-acetic acid complexes and one type of NMA-water complex were -44.3, -35.5, -32.4 and -16.0 kJ·mol^-1 respectively, which showed that NMA had a great affinity for acetic acid than water. The boiling point of the NMA-acetic acid complex was higher than that of acetic acid so that acetic acid is easily preserved in the liquid phase, which is in agreement with the VLE experiment for NMA + acetic acid + water system that gave a relative volatility of 3.0-4.0. The results of natural bond orbital (NBO) population analysis revealed that there was charge-transfer in the complex formation, and acetic acid acted as electron acceptor during this transformation.