Oxidation/elimination of heterocyclic sulfur compounds in a biphasic system with mesostructured FeOx/Ti-MCM-41 catalysts

A series of FeOx/Ti-MCM-41 mesostructured catalysts were synthesized and characterized by the X-ray diffraction (XRD), N₂ adsorption-desorption isotherms, transmission electron microscopy (TEM), and Mössbauer spectroscopy. The surface reducibility and acidity of the catalysts were measured by temperature-programmed reduction of hydrogen (TPR-H₂) and temperature-programmed desorption of ammonia (TPR-NH₃) methods. Incorporating Ti⁴⁺ in the MCM-41 framework and loading Fe₂O₃ in the surface of the catalysts would create the acid sites and enhance the surface oxygen reducibility. Most of Fe³⁺ ions were highly dispersed on the surface and some were inserted into the framework of the support with different structural environments. The amount of the Fe³⁺ ions locating around the structural defects increased as the Fe content increased. In a biphasic reaction system (n-hexadecane/acetonitrile), the oxidation/elimination of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) could correlate perfectly with the surface acidity and the amount of iron ions around the structural defects of the catalysts. The reactivity of sulfur compounds decreased with increasing of the steric hindrance, showing an order as: DBT > 4-MDBT > 4,6-DMDBT. In the presence of catalyst and oxidant, the polar/non-polar biphasic reaction system developed herein could effectively oxidize the sterically hindered sulfur compounds and simultaneously extracted the produced sulfones from oil phase into polar phase.