Dioxygen-mediated oxidation of hydroquinone with cobalt ions in a bicarbonate aqueous solution for the production of active radicals

Abstract

The catalytic activity and mechanism of Co²⁺ in HCO₃⁻ aqueous solution on the oxidative transformation of 1,4-hydroquinone (H₂Q) to 1,4-benzoquinone (BQ) with molecular oxygen as the oxidant is investigated. The Co(II)–HCO₃⁻ system is much more efficient in the oxidation of H₂Q than HCO₃⁻, Co(II)–OH⁻ or other metal ion–HCO₃⁻ systems due to the fact that the formed [Co(II)(HCO₃⁻)]⁺ complex can facilitate the generation of reactive oxygen species. In HCO₃⁻ solution, H₂Q can react with dissolved oxygen to form the semiquinone radical (SQ˙⁻). As a chain carrying radical, SQ˙⁻ continues to transfer an electron to molecular oxygen to yield the superoxide radical, followed by the generation of H₂O₂. The resulting H₂O₂ is further utilized as an oxidant by the Co(II)–HCO₃⁻ complex in situ with the formation of the ˙OH radical, which is highly active for H₂Q oxidation and even for dye degradation.