Oxidative desulfurization kinetics of refractory sulfur compounds in hydrotreated middle distillates

Oxidative desulfurization (ODS) has attracted considerable attention since it can remove the refractory sulfur compounds under relatively mild conditions. ODS kinetics of middle distillates, especially for the refractory sulfur compounds by using the oil-soluble oxidants, is crucial for the understanding of their ODS behavior, and benefits the design and development of a practical ODS process and catalytic system. However, relatively fewer studies in ODS kinetics of the individual sulfur compounds existing in real feedstock by using the oil-soluble oxidants have been reported in the literature. In the present study, oxidative kinetics of various sulfur compounds in a hydrotreated middle distillate using oil-soluble cumene hydroperoxide as an oxidant over a MoO3/Al2O3 catalyst at different temperatures was conducted in detail. All examined sulfur compounds follow the pseudo-first-order kinetics. The rate constants decrease in the order of 4-MDBT > 2,4,6-TMDBT > 3,4,6-TMDBT > 1,4,6-TMDBT > 4,6-DMDBT > 4-E,6-MDBT. Comparison of the molecular structures of the sulfur compounds with their rate constants indicates that both the number and size of the alkyl substituents at the 4- and 6-positions of dibenzothiophenes (DBTs) affect the rate constants significantly due to their steric hindrance. The alkyl substituents of alkyl DBTs at other positions may increase the rate constant by enhancing the electron density on the sulfur atom. The activation energies of oxidation of the refractory sulfur compounds increases in the order of 4-MDBT < 4,6-DMDBT < 4-E,6-MDBT, being 46.4, 48.9, and 49.3 kJ/mol, respectively.