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Tuning Task-Specific Ionic Liquids for the Extractive Desulfurization of Liquid Fuel

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journal contribution
posted on 26.07.2016, 00:00 by Hua Zhao, Gary A. Baker, Durgesh V. Wagle, Sudhir Ravula, Qi Zhang
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).