Efficient Extraction of Olefin from Liquid-Phase Olefin/Alkane Mixtures Using Novel Bimetallic Ionic Liquids

The separation of olefins and alkanes is beneficial for resource utilization of oil products, and liquid-liquid extraction is an important method of liquid-phase olefin/alkane separation. This study investigated the feasibility of bimetallic ionic liquids for the extraction of olefin from liquid-phase olefin/alkane mixtures using 1-hexene/<i>n</i>-hexane as a model hydrocarbon mixture. The bimetallic ionic liquids were first synthesized from different metal chlorides (ZnCl₂, SnCl₂, AlCl₃, FeCl₃) and CuCl with triethylamine hydrochloride. This newly prepared Fe-Cu bimetallic ionic liquid [Et₃NH]Cl-0.47FeCl₃-0.8CuCl had a good extraction performance, and the extraction selectivity could reach up to 8.41, which was about 4 times that of the organic solvent such as <i>N</i>-formylmorpholine and γ-butyrolactone used in the industry. The liquid-liquid equilibrium (LLE) of 1-hexene/<i>n</i>-hexane and Fe-Cu bimetallic ionic liquid system was investigated under different extraction temperatures, extraction times, and mass ratios of extractant to feed. Through 5 recycling experiments, the extraction selectivity decreased by less than 2%, and the characteristic peaks of the recovered bimetallic ionic liquids were unchanged by IR analysis, which proved that the bimetallic ionic liquid had excellent reversibility. Furthermore, the mechanism of 1-hexene extraction was investigated by quantum chemistry calculation and Raman spectroscopy, and there was π-complexation between Cu⁺ and 1-hexene. Then, the charge density of Cu⁺ in bimetallic ionic liquids was larger than that of cuprous-based ionic liquids. The results revealed that the addition of FeCl₃ could activate Cu⁺ and weaken the Cu-Cl bond, enhancing the ability of the bimetallic ionic liquid to extract 1-hexene. Therefore, [Et₃NH]Cl-0.47FeCl₃-0.8CuCl was a good potential extractant during the efficient extraction of 1-hexene.