Requirements for effective photocatalytic oxidative desulfurization of a thiophene-containing solution using TiO₂

Photocatalytic oxidative desulfurization (PODS) is an emerging sulfur removal technology that operates at mild temperatures and pressures without the need for the hydrogen gas that is required in hydrodesulfurization (HDS). This study investigated the desulfurization of a liquid containing 0.95% by weight sulfur content (aromatic thiophene) in n-octane to simulate the characteristics of fuel. An 8 W mercury lamp was used as the source of irradiation (UV-C) and the reaction was catalyzed by the photocatalyst TiO2. The optimal temperature, solution pH, catalyst loading, oxidizer dosage, solvent requirement and oil/solvent ratio were investigated for the PODS process. The optimal operating parameters for a 5500 ppm dibenzothiophene (DBT) (0.95 wt% sulfur content) solution were a temperature of 40 °C, pH of 4, 6 g/L TiO2(with respect to oil), 0.7% v/v H2O2and methanol as the solvent. Prior to this work, an aqueous solvent has always been thought to be an essential ingredient in the PODS process. This study found that no effective oxidation occurred when only water was used as the solvent and that the use of methanol significantly reduced the mass-transfer limitation in the batch system to result in a much higher overall sulfur removal. At the optimum conditions, overall desulfurizations of 91% and 64% were achieved after 3 h for DBT and benzothiophene (BT), respectively. Kinetic studies determined that the reaction rate was highly dependent upon the solution composition. The results also indicated that the PODS using TiO2is more selective to DBT than BT.