Cl- induced reactive oxygen species synergy for deep oxidative desulfurization in deep eutectic solvents: Mechanism and process intensification via solid oxidant slow release

In high-viscosity, metal salt-based deep eutectic solvents (DESs), strong hydrogen-bonding interactions frequently hinder mass transfer, limit ROS formation, and reduce synergistic catalytic activity. To address these challenges, a low-viscosity DES incorporating Brønsted-Lewis acidic sites was synthesized from FeCl₃·6H₂O and propionic acid. In this configuration, solid PMS acts as a slow-release oxidant, thereby preserving the DES’s functional integrity and sustaining its catalytic efficiency. Characterization and oxidative desulfurization evaluation revealed alternating Fe ion redox states within the DES, along with a dual cyclic reaction pathway involving the regeneration of Cl⁻ and Cl radicals. The electron-transfer activity of Cl radicals not only generates multiple reactive oxygen species (·ClO and ·O 2 ⁻) but also modulates the nucleophilic and electrophilic reactivity of organosulfur compounds. Under mild conditions at 40 °C, the catalytic system achieves over 99.7% desulfurization of model oil containing up to 1500   mg   kg -1 sulfur within 20   minutes, while demonstrating excellent recyclability over seven consecutive cycles. DFT analyses indicate that radical oxidation serves as the dominant pathway for thiophene sulfone formation, with perpropionic acid playing an auxiliary role by extending the lifetime of reactive oxygen species and enhancing the oxidation process. Overall, the controlled release of solid PMS reduces impurity effects on the DES, while radicals and reactive oxygen species promote S-O bond formation and the removal of organosulfur compounds from the oil phase.