Oxidative desulfurization of benzothiophene derivatives with cis-dioxomolybdenum(VI) catalyst precursors, under extractive conditions

Abstract

The study of the activity of a series of heterogeneous cis-dioxomolybdenum(VI) based catalyst precursors for the hydrogen peroxide promoted oxidation of benzothiophene, 2- and 3-methylbenzothiophene, dibenzothiophene and 4,6-dimethyldibenzothiophene, has been carried out. The last, chosen as model substrates of the common sulfurated derivatives of fuels, have been oxidized in the presence of selected imidazolium based ionic liquids, under extractive oxidative desulfurization conditions. The reactivity of sulfur-containing compounds toward the oxidation to their corresponding sulfones has been evaluated, firstly, for each type of substrate alone, and then applying the optimized procedure to a system formed by all sulfurated substrates. Our aim was to mimic the oxidation of a model diesel oil, under either thermal or microwave assisted heating. Cis-dioxomolybdenum (VI) compounds behaved as very active and stable catalytic systems retaining, in one selected case, the original activity at least for three cycles. Imidazolium ionic liquids played a dual fundamental role not only as extractant and reaction medium but also as stabilizing system for both the oxidant and the catalytic agents. Under optimized microwave-assisted conditions, the quantitative elimination of sulfurated substrates from the model diesel oil, was obtained after only 1 h or 2 h, when [BMIM]OTf or [BMIM]BF4 and [OMIM]BF4, respectively, have been used as ionic liquids.