Oxidative desulfurization of diesel with oxygen: Reaction pathways on supported metal and metal oxide catalysts

We examined oxidative desulfurization (ODS) of a model diesel under conditions where O-2 is the oxidant. To help understand the overall reaction pathways, we focused on all oxidation reactions in the system, including those of the alkylaromatic and nitrogen heterocycle components of the diesel. Three catalyst classes were studied in particular: supported Pd (on activated carbons or alumina), supported metal carbides and mesoporous titanium silicates. It is shown that regardless of catalyst the reaction rate for ODS is heavily influenced by the other oxidizable components present, especially the alkylaromatics. These compounds are oxidized to a greater extent than the sulfur heterocycles (by almost an order of magnitude on a molar basis), and temperatures must be kept less than 100 degrees C in order to minimize co-oxidation. Attaining a measurable rate at these temperatures requires high O-2 partial pressures and therefore higher pressure than typically used for peroxide-based oxidations. The products formed from the oxidation of some typical alkylaromatics and nitrogen heterocycles were determined. These products suggest the same reaction pathways are present regardless of oxidant, and over a wide range of catalysts. The results strongly suggest that a key step in the oxidation of sulfur heterocycles with O-2 is the generation of hydroperoxides from alkylaromatics and similar oxidizable hydrocarbons; it is these hydroperoxides which in turn oxidize the heterocycles. (C) 2013 Elsevier B.V. All rights reserved.