NMR studies of intermolecular interaction for the separation of carbazole and anthracene by deep eutectic solvents

Abstract

Carbazole (CAR) and anthracene (AN) are high value compounds derived from coal tar, but their separation is challenging due to their structural similarity. Promising results, using deep eutectic solvents (DESs) for the separation, have recently been obtained. However, choosing the optimal DESs for the separation of CAR and AN is difficult due to a lack of knowledge about the interaction of DESs with CAR and AN. Herein, the interactions between CAR and AN with seven DESs containing ethylene glycol (EG) as the hydrogen bond donor (HBD) were investigated by nuclear magnetic resonance (NMR). The interaction between CAR and tetrabutylammonium chloride/ ethylene glycol (TBAC/EG) was investigated using various NMR methods such as chemical shift titration, variable temperature NMR (VT-NMR), relaxation time measurements and diffusion-ordered spectroscopy (DOSY). From the chemical shift differences of the NH proton of CAR, a strong interaction between CAR and the DES consisting of TBAC and EG, was revealed. TBAC/EG was therefore proposed as a promising solvent for the separation of CAR and AN. In addition, VT-NMR showed that the hydrogen bonding interaction between CAR and TBAC/EG increased with decreasing temperature. The changes in relaxation time (T₁, T₂), molecular correlation time (τ_c) and the molecular self-diffusion coefficients (D) supported the existence of interactions between CAR and TBAC/EG, presumably dominated by hydrogen bonding between the NH and the chloride ion. This study introduces an efficient method for screening solvents to separating CAR and AN, and it can also be applied to other fields of separating complex mixtures.