Hybrid Approach for Selective Sulfoxidation via Bioelectrochemically Derived Hydrogen Peroxide over a Niobium(V)-Silica Catalyst

In this work, we demonstrate a combined bioelectrochemical and inorganic catalytic system for resource recovery from wastewater. We designed a microbial peroxide producing cell (MPPC) for hydrogen peroxide (H₂O₂) production and used this bioelectrochemically derived H₂O₂ as a green oxidant for sulfoxidation, an industrial reaction used for chemical synthesis and oxidative desulfurization of transportation fuels. We operated an MPPC equipped with a gas diffusion electrode cathode for six months, achieving a peak current density above 1.4 mA cm^-2 with 60% average acetate removal and 61% average anodic Coulombic efficiency. We evaluated several cathode buffers under batch and continuous-flow conditions for solubility and pH compatibility with downstream catalytic systems. During 24-h batch tests, a phosphate-buffered MPPC achieved a maximum H₂O₂ concentration of 4.6 g L^-1 and a citric acid-phosphate-buffered MPPC obtained a moderate H₂O₂ concentration (3.1 g L^-1) at a low energy input (1.6 Wh g^-1 H₂O₂) and pH (10). The MPPC-derived H₂O₂ was used directly as an oxidant for the catalytic sulfoxidation of 4-hydroxythioanisole over a solid niobium(V)-silica catalyst. We achieved 82% conversion of 50 mM 4-hydroxythioanisole to 4-(methylsulfinyl)phenol with 99% selectivity with a 0.5 mol % catalyst loading in 100 min in aqueous media. Our results demonstrate a new and versatile approach for valorization of wastewater through continuous production of H₂O₂ and its subsequent use as a selective green oxidant in aqueous conditions for green chemistry applications.