Desulfurization of fuels with ionic liquids by extraction and oxidative extraction processes

Abstract

The excessive emission of pollutants to the atmosphere has been a problem during the last few decades due to, among other reasons, the massive use of transports. In order to avoid this problem, many methods are being investigated as an alternative to the classical hydrodesulfurization technique, such as adsorption, biodesulfurization, extraction and oxidation. The extractive desulfurization is one of the most promising alternatives. Moreover, its enhancement with selective oxidation of sulfur compounds has been a trending field of research over the last years. Ionic liquids have turned into perfect solvents for many different tasks. Since they are composed by a cation and an anion, plenty combinations are possible, giving to these designer solvents different and defined properties. Among them, the negligible vapor pressure make these solvents easily recoverable while also avoiding atmospheric contamination. Amid other applications, ionic liquids are being proposed for the extraction of sulfur from fuels. In this work 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 acetate, [C2mim][OAc] and 1-ethyl-3-methylimidazolium diethylphosphate, [C2mim][DEP], were studied for the desulfurization of fuels. Liquid-liquid equilibrium data were obtained for the systems ionic liquid + thiophene + toluene, ionic liquid + thiophene + hexane and ionic liquid + pyridine + hexane at 298.15 K and atmospheric pressure. The obtained data were correlated using the UNIQUAC and NRTL models. The suitability of the ionic liquids as solvents was evaluated in terms of solute distribution ratio and selectivity. Then, the ionic liquids were tested in a three-stage extractive desulfurization using synthetic samples of fuels (gasoline and diesel oil). With the aim of enhancing the process, an oxidative desulfurization was also studied. Besides the traditional oxidative system for desulfurization (acetic acid and hydrogen peroxide), different ionanofluids, formed by the ionic liquid and metallic oxide nanoparticles (Cr2O3, Al2O3, TiO2), were tested as oxocatalytic system. In the case of H2O2-AcOH system, the research on the model fuels was carried out to optimize the operation conditions (ratio of oxidant and catalyst, temperature, extraction times and operation mode). Finally, the extractive and oxidative desulfurization methods were tested using the four ILs with real gasoline and diesel oil samples. Based on the results, [C2mim][OAc] is suggested for the desulfurization of gasoline in an extractive desulfurization process, whereas [C2mim][NTf2] is preferred for the oxidative desulfurization of diesel oil.