Extractive and oxidative-extractive desulfurization of fuels with ionic liquids

Concerns about the pollution associated to the sulfur content of fuels have stimulated the search for new alternative technologies to the classical hydrodesulfurization process. The use of ionic liquids to remove sulfur compounds from hydrocarbons by solvent extraction is the object of ongoing research. Most of the publications in this front, however, present two serious drawbacks: they are not based on rigorous thermodynamic data, and they do not consider the effect of the ionic liquid on the different fuel constituents. In the present work, a comparative study of the extractive desulfurization and the oxidative-extractive desulfurization of fuels was carried out. In particular, the sulfur extraction ability of four ionic liquids (1-hexyl-2,4-dimethylpyridinium bis(trifluoromethylsulfonyl)imide ([C62,4mmpy][NTf2]), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]), 1-ethyl-3-methylimidazolium diethylphosphate ([C2mim][Et2PO4]), and 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc])) was tested in real and synthetic fuels (gasoline and diesel). The selection of the ionic liquids was based on previously determined liquid–liquid equilibrium data. In the case of the oxidative desulfurization, an optimization of the oxidation and extraction conditions was done, taking into account catalyst type, ratio of catalyst and oxidant, temperature, mixing and settling times, etc. On the basis of the results, [C2mim][OAc] is suggested for the desulfurization of gasoline in an extractive process, whereas [C2mim][NTf2] is preferred for the oxidative desulfurization of diesel.