Theoretical investigation on interactions between N-methylpyrrolidone-FeCl3 and components in model oil: The role of S-Fe coordination in thiophene removal

Abstract

Metal-based coordinated ionic liquids (ILs) have drawn wide attention due to their high sulfide selectivity and desulfurization rate on extractive desulfurization (EDS). Here, a systematical investigation on the interactions between N-methylpyrrolidone-FeCl3 (NMP-FeCl3) and thiophene (TH) in the presence of model oil consisting of 1-octene (OCT), toluene (TOL), cyclohexane (CHA) and cyclohexene (CHE) was carried out using density functional theory (DFT). Three stable structures of NMP-FeCl3-TH complexes at room temperature (298 K) were taken into consideration to compare different interactions between NMP-FeCl3 and TH. The interactions were analyzed using natural bond orbital, atoms in molecules theory, independent gradient model based on Hirshfeld partition, frontier molecular orbital theory and electron density difference. Results show that π-π stacking interactions, hydrogen bonds, CH···π interactions and S-Fe coordination exist in the complexes of NMP-FeCl3 and TH/OCT/TOL/CHA/CHE. The coordination of lone-pair electrons in S atom and empty orbital in Fe atom is the strongest interactions among all complexes, and thus is demonstrated as the predominant desulfurization mechanism of NMP-FeCl3 IL extracting TH from fuel oils.