Effect of the addition of deep eutectic solvent to the anthracene separation

Abstract

Anthracene (AN) is an important fine chemical feedstock, mainly derived from the anthracene oil fraction of high-temperature coal tar. The traditional solvent crystallization method used to obtain anthracene involves large amounts of solvents. A method with a high separation capacity and a high selectivity needs to be developed urgently. To this end, the composition of crude AN oil and the solubility of its main components in xylene were determined, and phenanthrene in crude AN oil was extracted with xylene. Pure deep eutectic solvents (DESs) and DESs–organic solvent mixture were then applied to separate anthracene and carbazole, respectively. Based on the distribution coefficient, selectivity, and purity of the AN product, the influence of organic solvent type, the molar ratio of DESs to organic solvent, and DESs type on the separation efficiency were investigated. Finally, the separation mechanism was investigated with density functional theory calculations and UV–visible spectroscopy. The results show that the phenanthrene content decreased from 25.5% to 2.6% and most of other condensed aromatics in crude AN were removed after washing twice with xylene. DESs–organic solvent mixtures had better performance than pure DESs for the separation of anthracene and carbazole, and under optimal condition, the 97.5% purity of AN is acquired in a single separation step with a tetraethyl ammonium chloride–glycerol DESs–DMF mixture. The more complicated interactions between phenanthrene and xylene than between anthracene / carbazole and xylene favored the removal of phenanthrene. The hydrogen bond with the minimum bond energy (-17.6 kJ/mol) between DESs and carbazole is also stronger than that with the maximum bond energy (-16.8 kJ/mol) between DESs and anthracene. Adding DESs to organic solvent can enhance the interactions between carbazole and the solvent, but had no significantly influence on interactions between anthracene and the solvent, thereby facilitating the separation of anthracene and carbazole.