A non-catalytic oxidative desulfurization of thiophene and benzothiophene by peroxyacetic acid oxidant

Oxidative desulfurization (ODS) is one of the most efficient methods in the removal of aromatic sulfur compounds from the combustion fuels. The present study explores the potential energy surface of thiophene (TH) and benzothiophene (BTH) oxidation reaction in the presence of peroxyacetic acid (PAA) as oxidizing agent using density functional theory (DFT). The structural and thermodynamical parameters of all the stationary points are obtained and characterized at B3LYP/6–311 +  + G(d,p) and ωB97X-D/6–311 + G(d,p) levels of theory. In order to study the influence of solvents (water and acetonitrile) in the considered ODS process; solvent effects are included via conductor like polarizable continuum model (CPCM). The natural bond orbital (NBO) analysis was carried out to understand the charge transfer mechanism during the oxidation process. Fukui function calculations are performed to determine the reactivity of the k^th atom site, for electrophilic \(({\text{f}}_{\text{k}}^{-})\) $({\text{f}}_{\text{k}}^{-})$ and nucleophilic \(({\text{f}}_{\text{k}}^{+})\) $({\text{f}}_{\text{k}}^{+})$ attacks. Rate constant calculations considering the quantum tunneling effects are also made. The results obtained suggest that the oxidation of benzothiophene into benzothiophene sulfoxide to be more favorable than thiophene oxidation. The presence of solvent is found to have a significant effect in reducing the barrier energy of the considered ODS process. Further this study elucidates the selective reactivity of peroxyacetic acid towards the compounds of high nucleophilicity, which makes it to be an effective oxidant in the uncatalyzed ODS in both gas and solvent phases when compared to hydrogen peroxide (H₂O₂) oxidant in the presence of catalyst that is available in the literature.