Feasibility of phosphonium-based ionic liquids as solvents for extractive desulfurization of liquid fuels

• Solubility of DBT in phosphonium-based ILs was determined.
• Thiophene was found to be completely miscible with all tested ionic liquids.
• NRTL equation was used to model the systems.
• Deep fuel desulfurization was achieved at optimized conditions.

Binary solid–liquid equilibria for phosphonium based ionic liquids with dibenzothiophene (DBT) were measured within the temperature range of 303.15–353.15 K. The influence of temperature and ionic liquid structure on solubility of dibenzothiophene is presented. The solubility of DBT in all tested ionic liquids increased with temperature and as high as 52.93 wt% DBT in solution was obtained at 353.15 K. At this temperature, the solubility of DBT in ILs based on the trihexyl(tetradecyl)phosphonium cation depends on the anion of the IL. This dependency was found to be in the order; Cl− > Br− ≈ N(CN)2− > [NTf2]−> bis(2,4,4-trimethylpentyl)phosphinate. The experimental solubility data was satisfactorily correlated using NRTL thermodynamic model. Finally, more than 99% (DBT) and 96% (thiophene) sulfur removal were achieved after 4th and 5th extraction stages respectively using the triisobutyl(methyl) phosphonium tosylate ionic liquid under mild operating conditions.