Extraction Of Aromatic Hydrocarbons From Alkanes Using Ionic Liquid-based Deep Eutectic Solvents

Naphtha is an important raw material for ethylene cracking.To raise the ethylene yield and reduce the equipment coking,it is necessary to remove aromatic compounds.The efficient separation of aromatic/alkane mixtures in naphtha is of great significance for the efficient and comprehensive utilization of petroleum resources while promoting the development of the ethylene industry.Liquid-liquid extraction is one of the effective separation methods,the key process of which is to design highly efficient extractants.Deep eutectic solvents(DESs)are promising solvents for different separation processes due to their designability,simple synthesis and low volatility.In this paper,two simulated oil systems,toluene/n-heptane and toluene/cyclohexane,were investigated.Deep eutectic solvent with pyridine-based ionic liquid(Epy Cl)as the hydrogen bond acceptor(HBA)and levionic acid(LA)as the hydrogen bond donor(HBD)was synthesized.The effective separation of aromatics/alkanes is realized mainly according to the π-πinteraction between the extractant and aromatics/alkanes as well as the principle of "polar similarity and solubility".Meanwhile the different induced dipole moments generated by polar extractants on aromatics/alkanes resulting in the difference of polar dipole moments between aromatics/alkanes also played an essential role.The molar ratio of HBA and HBD was determined to be Epy Cl:LA(2:1).The effects of the initial concentration of toluene,operating temperature and ratio of extractant to feed were systematically investigated.When the initial toluene molar content was 10% and the operating temperature was 298 K,the selectivity of toluene was 69.57 for the toluene-n-heptane system,while the value was 24.41 for the toluene-cyclohexane system.And the extractants designed and synthesized had favourable selectivity,recycling performance and stability.To further improve the separation performance,the law and mechanism of DESs in the extraction process were revealed.From the perspective of π-πinteraction between C=O and benzene rings on aromatic hydrocarbons and combining with the law of "polar similarity solubility",a series of imidazole ionic liquid-based DESs were designed and synthesized with imidazole ionic liquids as HBAs and amide compounds as HBDs.The effects of the molar ratio of HBA to HBD,the length of alkyl side chains of HBA and the structure of HBD were studied.The optimal DES was determined to be Emim Cl: DMU with molar ratio of 2:1.The separation mechanism between DES and toluene/n-heptane/cyclohexane was investigated by quantum chemistry calculation.When the initial toluene molar content was 10% and the operating temperature was 298 K,the selectivity of toluene was up to113.95 for the toluene-n-heptane system.The value for the toluene-cyclohexane system maintained the same level with pyridine-based DES.The imidazole ionic liquid-based DES could be reused for more than 6times without loss of separation performance.The reliability of the experimental data was verified by the Othmer-Tobias equation and the correlation coefficient was close to 1.The liquid-liquid equilibrium data of studied systems could be correlated adequately using the NRTL thermodynamic model.The ionic liquid-based deep eutectic solvent was designed and synthesized in this paper,which showed excellent separation performance in aromatic hydrocarbon/alkane mixture.The extraction mechanism was preliminarily explored by quantum chemistry calculation.This would provide theoretical guidance for the design of high performance extractant for the efficient utilization of naphtha.