Thiophene (T) is considered one of the most refractory organic sulfur compounds in oxidative desulfurization. This study used an ultrasound-assisted oxidative desulfurization process to optimize T oxidation on a bench scale. Four control factors, including durations of sonication, the amounts of transition metal catalyst (TMC), phase transfer agent (PTA), and hydrogen peroxide, were carefully examined. The best operation condition, evaluated by using the analysis of variance, was found at mass ratios of T:H2O 2:PTA:TMC at 1:1.5:0.005:0.01 with 20 min of sonication, where almost 73.5% of T, 89.9% of benzothiophene (BT), and 100% of dibenzothiophene (DBT) were oxidized to their corresponding sulfones. Moreover, the electron density on the sulfur atom of various compounds, including T, BT, and DBT, and their methyl-substituted derivatives and oxidation rate constants were also examined. The oxidative reactivity of sulfur compounds increased with the increasing electron density on sulfur atoms. The T that is commonly considered more difficult to the oxidized is attributed to the combined effect of low electron density of the sulfur atom and low boiling temperature under mild oxidation reaction. This information is essential for the deep desulfurization process under oxidative conditions.
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