Pyridine Ionic Liquid-Based Deep Eutectic Solvents Selectively Separating Toluene from Alkanes

Abstract

Naphtha is an important raw material for ethylene cracking, and the aromatics in it can affect the ethylene yield due to the easy coking during the cracking process. Therefore, it is important to separate the aromatics from naphtha to increase the alkane content and improve the ethylene cracking production. Liquid–liquid extraction separation of the aromatic/alkane mixture is one of the effective pathways, and the key is the design and synthesis of new efficient extractants. In this work, a novel pyridine ionic liquid-based deep eutectic solvent (DES) for the separation of aromatics from alkanes was proposed and successfully synthesized. 1-Ethylpyridinium chloride (EpyCl) was used as the hydrogen bond acceptor and levulinic acid (LA) as the hydrogen bond donor. Among them, the π–π interactions between pyridine ring, C═O, and aromatic ring can enhance the interactions with aromatic compounds. In contrast, the introduction of acidic groups can increase the polarity of DES, enhance the induced polarization of unsaturated bonds in aromatic rings by DES, and thus increase the non-bonding force with aromatics. Toluene/n-heptane and toluene/cyclohexane were used as simulated oil systems. The optimal ratio of EpyCl/LA is 2:1, which has the best extraction efficiency, stability, and reusability; the selectivity of toluene was up to 69.57 for the toluene-n-heptane system, while it was 24.41 for the toluene-cyclohexane system. The reliability of the liquid–liquid equilibrium data was verified by using the Othmer–Tobias correlation equations, and the basic system physical property data were obtained by fitting with the NRTL thermodynamic model. The interaction mechanism between DES and toluene/alkanes was revealed by AIM topology analysis, IGMH analysis, and symmetry-adapted perturbation theory energy decomposition. The interaction between DES and toluene/alkanes is a van der Waals force, dominated by dispersion forces.