Oxidative Desulfurization of Model Diesel Using Ionic Liquid 1-Octyl-3-methylimidazolium Hydrogen Sulfate: An Investigation of the Ultrasonic Irradiation Effect on Performance

A Brønsted acidic ionic liquid (IL), 1-octyl-3-methylimidazolium hydrogen sulfate ([Omim][HSO4]), was prepared and utilized as the extractant and catalyst to study the oxidative desulfurization of dibenzothiophene (DBT) in n-decane as the model oil. The effects of the alkyl chain length of the IL cation, temperature, H2O2/DBT molar ratio (O/S), IL/oil mass ratio, initial S-content, and sulfur species on the sulfur removal of the model oil were investigated. Complete removal of DBT was observed by [Omim][HSO4], O/S molar ratio of 5, and IL/oil mass ratio of 1:2 after 70 min at 25 °C. The order of observed oxidizing reactivity for different sulfur species was as follows: DBT > benzothiophene (BT) > thiophene (TH) > 4,6-dimethyldibenzothiophene (4,6-DMDBT). The IL could be reused six times without a significant decrease in the desulfurization activity. The kinetics of oxidative desulfurization for DBT by [Omim][HSO4] was found to be pseudo-first-order with an apparent rate constant of 0.0734 min–1 (at 298 K) and the apparent activation energy of 24.51 kJ/mol. The ultrasound-assisted oxidative desulfurization (UAOD) process was also applied and represented a high desulfurization performance for the model oil in a fast reaction. The effects of various parameters, including irradiation time, settling time, O/S molar ratio, and IL/model oil mass ratio, on the UAOD process were studied. The complete sulfur removal efficiency could be reached after 3 min of ultrasonic irradiation with an ultrasonic power of 30 W, ultrasonic frequency of 20 kHz, O/S molar ratio of 5, and IL/oil mass ratio of 1:2. It was observed that the application of ultrasonic irradiation allows the desulfurization process to be performed in a shorter time. The sulfur removal of real diesel was 77.2% in the ODS process, and 76.3% in the UAOD process under the optimal conditions.