Highly selective extraction of aromatics from aliphatics by using metal chloride-based ionic liquids

Abstract

The separation of aromatics from aliphatics is essential for achieving maximum exploitation of oil resources in the petrochemical industry. In this study, a series of metal chloride-based ionic liquids were prepared and their performances in the separation of 1,2,3,4-tetrahydronaphthalene (tetralin)/dodecane and tetralin/decalin systems were studied. Among these ionic liquids, 1-ethyl-3-methylimidazolium tetrachloroferrate ([EMIM][FeCl4]) with the highest selectivity was used as the extractant. Density functional theory calculations showed that [EMIM][FeCl4] interacted more strongly with tetralin than with dodecane and decalin. Energy decomposition analysis of [EMIM][FeCl4]–tetralin indicated that electrostatics and dispersion played essential roles, and induction cannot be neglected. The van der Waals forces was a main effect in [EMIM][FeCl4]–tetralin by independent gradient model analysis. The tetralin distribution coefficient and selectivity were 0.8 and 110, respectively, with 10% (mol) tetralin in the initial tetralin/dodecane system, and 0.67 and 19.5, respectively, with 10% (mol) tetralin in the initial tetralin/decalin system. The selectivity increased with decreasing alkyl chain length of the extractant. The influence of the extraction temperature, extractant dosage, and initial concentrations of the system components on the separation performance were studied. Recycling experiments showed that the regenerated [EMIM][FeCl4] could be used repeatedly.

1. Introduction

Diesel, which is a mixture that includes polycyclic aromatic hydrocarbons [1], is an important raw material in the chemical industry. However, polycyclic aromatic hydrocarbons in vehicle diesel decrease the cetane number [2]. They can also produce oxycarbide and particulate matter [3], which cause severe environmental pollution. Diesel is rich in paraffins and cycloalkanes, and these are ideal raw materials for ethylene cracking and catalytic reforming, respectively [4]. It is untoward that the aromatics were removed from non-aromatics in diesel via traditional distillation owing to their fairly approximate boiling points [5]. The separation of aromatics from non-aromatic compounds is therefore a knotty work for the petrochemical industry [6].

The conventional methods used to separate aromatics from aliphatics are liquid–liquid extraction [7], extractive distillation [8], and azeotropic distillation [9]. Liquid–liquid extraction with low energy consumption and mild operating conditions is most commonly adopted. Classical organic solvents such as sulfolane, N-methylpyrrolidone (NMP), N-formylmorpholine, dimethyl sulfoxide, and tetraethylene glycol are used in this process [10]. Nevertheless, these solvents usually appearing with low selectivity are often evaporable [11]. An extractant with high selectivity and fine recyclable feature is favored in industrial processes.

Ionic liquids (ILs), which are usually regarded as organic salts comprising cations and organic or inorganic anions [12], have been receiving increasing attention in the recent decades [13] due to their strong dissolving capacities, low vapor pressures, and high thermal and chemical stabilities [14–16]. They are used in many fields by tuning the chemical structures of the cation and anion to prepare targeted ILs with specified properties [17]; therefore, ILs have potential applications in liquid–liquid extraction. Li et al. [18] studied the use of NMP, sulfolane, and N-formylmorpholine for separating 1-methylnaphthalene from dodecane, which suggested that 1-methylnaphthalene had the highest solubility in NMP and that sulfolane had the highest selectivity for 1-methylnaphthalene. Furthermore, it was indicated that the extractant-possessing planar construction was inclined to have higher solubility for 1-methylnaphthalene because of the smaller steric hindrance and the group with more electronegativity had higher selectivity for 1-methylnaphthalene. Finally, it was also found that the van der Waals (VDW) force made a major contribution between the three solvents and 1-methylnaphthalene by reduced density gradient and atoms in molecules analyses. Ge et al. [19] used a deep eutectic solvent containing tetrabutylphosphonium bromide and levulinic acid to separate 1,2,3,4-tetrahydronaphthalene (tetralin) from dodecane, indicating that the tetrabutylphosphonium cation could interact with tetralin through CH–π interaction by spatial distribution function with molecular dynamic simulation. It was suggested that π–π interaction was found between C=O bond of levulinic acid and tetralin, and no hydrogen bonds were detected between deep eutectic solvent (DES) and tetralin. Li et al. [20] used [Bz-DBU][NTf2] to separate t...