Tuning task-specific ionic liquids for the extractive desulfurization of liquid fuel

Extractive desulfurization of liquid fuel is a simple process that requires minimum energy input and can be operated via existing liquid-liquid extraction apparatuses. In particular, to achieve deep desulfurization, the conventional hydrodesulfurization (HDS) process has shown limitations in the removal of aromatic sulfur compounds. Recently, extractive desulfurization using a new type of nonvolatile solvent, ionic liquids (ILs), has yielded promising results. However, there is a lack of systematic evaluation of the effect of IL structure on desulfurization efficiency, and a lack of mechanistic understanding regarding how ILs lead to the partition of aromatic sulfur compounds from fuel to the IL phase. The present study examines a total of 71 ILs and two deep eutectic solvents (DESs) with combinations representing various cations and anions. We identify a number of ILs that yield high partition coefficients [up to 1.85 mg(S) kg (IL)^-1/mg(S) kg (oil)^-1] for the partition of aromatic sulfur compounds between ILs and n-octane or n-dodecane as surrogates for gasoline or diesel, respectively. We find that the high sulfur partition coefficient correlates with a high dipolarity/polarizability (π∗) or a low solvent polarizability (SP) of ILs carrying the same cation and different anions, but correlates with a low dipolarity/polarizability (π∗) for ILs carrying the same anion paired to cations bearing different alkyl chain lengths. We further demonstrate that a four-step extraction using ILs can achieve 99% dibenzothiophene (DBT) removal (i.e., an initial sulfur content of 500 ppm is reduced to <5 ppm following extraction).