Oxidative desulfurization of dibenzothiophene over titanate nanotubes

Abstract

The catalytic activity of titanate nanotubes in the oxidation of dibenzothiophene (DBT) by using hydrogen peroxide was investigated. Titanate nanotubes were synthesized by alkaline hydrothermal treatment of anatase TiO2 followed by washing either with water to produce titanates with high sodium content (Na–TiNTs) or with aqueous HCl to obtain protonated titanates (H–TiNTs). The materials were characterized by transmission and scanning electron microscopies, X-ray diffraction, Raman spectroscopy, and their textural properties were determined by nitrogen adsorption/desorption isotherms. The catalytic activity of the titanate nanotubes was found to be dependent on the counter ion. Sodium titanate nanotubes were inactive in DBT oxidation, whereas the protonated titanates were very efficient in removal of DBT under mild conditions. These results were correlated with the production of reactive radicals as observed by electron paramagnetic resonance and with DBT adsorption capacity. The effects of H2O2/DBT molar ratio and temperature on catalytic properties of H–TiNT were studied in detail. Favorable reaction conditions were found to be low temperatures (25°C) and H2O2/DBT=4. Reactions at elevated temperatures and H2O2 concentrations lead to significant reduction in catalyst activity, associated with partial phase transformation and premature deactivation of the catalytic sites. Under mild reaction conditions, H–TiNTs could be recycled four times without significant decrease in activity and the turnover number (TON) can reach up to 30,000. N-compounds (quinoline) can be simultaneously removed in the same oxidation process. A mechanism was proposed to explain the oxidation process.