Temperature-responsive ionic liquid extraction and separation of the aromatic sulfur compounds

Extraction of aromatic sulfur compounds from model oil using a temperature-responsive ionic liquid (IL) N-butyl-N-methylpiperidinium tetrachloroferrate ([C4mpip]FeCl4) as extractant has been investigated. In the extractive desulfurization process, liquid-liquid equilibrium was obtained at 45 °C for 10 min. After the extraction system was cooled to room temperature, the IL returned to solid due to the reversible thermoregulated phase transition of the IL. Therefore, it was easy for separation of oil and solid [C4mpip]FeCl4. The extraction efficiency and Nernst partition coefficient (KN) could get to 45.5% and 2.92, respectively, after a single extraction of DBT. Some important factors, such as extraction temperature, the amount of IL, initial sulfur content, multiple extractions, reusability and the mutual solution were investigated. The extraction ability of different sulfur compounds followed the order DBT > BT > 4,6-DMDBT. Finally, the extraction mechanism was studied systematically with experimental and theoretical methods. Compared with the extraction abilities of another three ILs, [C8mpip]FeCl4, [C12mpip]FeCl4 and [C4Py]FeCl4, the size and aromatic structure of cation may be two positive factors but not the main extraction mechanism. The coordination interaction between Fe atom of IL and S atom of DBT was confirmed by IR spectra and theoretical computation.