Ultrasound-assisted extractive/oxidative desulfurization of oil using environmentally benign trihexyl tetradecyl phosphonium chloride

Abstract A phosphonium-based ionic liquid (PIL) trihexyl tetradecyl phosphonium chloride ([THTDP]Cl) was applied as an effective catalyst/extractant for the desulfurization of model oil (MO) as well as base oil. In the ultrasound-assisted extractive/oxidative desulfurization (UEODS) process, a liquid–liquid catalytic oxidative desulfurization system containing MO/base oil, hydrogen peroxide, and PIL was observed at 30 °C and maximum sulfur removal of 98.67% was obtained. After completion of the reaction, PIL was separated from MO. The PIL was then regenerated and recycled 5 times without a significant decrease in efficiency. The influence of UAODS of MO containing dibenzothiophene (DBT), benzothiophene (BT), thiophene (T), and 3-methyl thiophene (3-MT) was also observed, which shows fast oxidative desulfurization capability under ultrasound irradiation. The oxidation reactivity of sulfur compounds follows the order of DBT > BT > TH > 3-MT at the DBT optimized conditions. The quantity of PIL, H2O2/DBT mole ratio, oxidation temperature, and time all played significant roles in sulfur removal, which were also analyzed in detail to optimize the reaction conditions during UEODS. The density functional theory study was carried out to understand the exact mechanism responsible for the effective interaction between PIL and sulfur compounds. The desulfurization of base oil was also carried out to check the feasibility of the UEODS process and achieved 74.98% desulfurization efficiency. These results proved the efficacy of the UEODS process for industrial application.