Deep desulfurization of diesel by ionic liquid extraction coupled with catalytic oxidation using an Anderson-type catalyst [(C4H9)4N]4NiMo6O24H6 Original Research Article

A series of Anderson-type Q4NiMo6−xWxO24H6 (x = 0, 2, 4, 6) catalysts were synthesized and employed in ionic liquids (ILs) extraction coupled with catalytic oxidation desulfurization systems (ECODS) for removal of benzothiophene (BT), dibenzothiophene (DBT) and their derivates in a model diesel and an actual commercial diesel. The catalyst [(C4H9)4N]4NiMo6O24H6 in ECODS system, containing H2O2 and [Bmim]PF6, exhibited so high catalytic activity that DBT removal can reach 98% at 30 °C in 3 h. The catalytic activity of the catalysts depends on the polyoxometalate anion and cation. The DBT removal gradually increases with the increasing of the molybdenum ions in the Anderson-type polyoxometalates. Quaternary ammonium cations can effectively promote the oxidative desulfurization reaction, which was confirmed by the presence of the mixed-valence molybdenum ions in the Anderson-type polyoxometalate catalyst. ILs was used as an extractant, however it also promoted the oxidative desulfurization reaction greatly. The reaction rate was so sensitive to the oxidant dosage that the removal DBT first increased and then decreased with the increasing of O/S molar ratio. The change is probably due to the introduction of excess water in the system. Sulfur removal selectivity for S-compounds followed the order of DBT > 4-MDBT > 4,6-DMDBT > BT > 5-MBT. The reactivity of these S-compounds is sensitive to the electron density on sulfur atoms and the steric hindrance of the substituted groups of S-compounds. The sulfur level of an actual commercial diesel can be decreased from 700 to about 30 ppm after desulfurization reaction. The desulfurization system for the actual commercial diesel can be recycled ten times with an unnoticeable decrease in activity.