Study of the electrochemical oxidation of 4,6-dimethyldibenzothiophene on a BDD electrode employing different techniques

Abstract

• Study of the oxidation steps of 4,6-DMDBT by different techniques using a BDD anode. • 4,6-DMDBT was removed after 60 min of electrolysis at E an = 1.50 V. • Selective oxidation of 4,6-DMDBT to sulfoxide or sulfone. • The conversion depended on concentration, applied potential, and water content. • A small amount of dimers was formed during the electrochemical oxidation. The electrochemical oxidation of 4,6-dimethyldibenzothiophene (4,6-DMDBT) at low concentrations on a BDD anode was investigated in a monophasic acetonitrile (93.5% v/v)–water (6.5% v/v, 0.01 M LiClO 4 ) solution. Two oxidation steps related to the sequential formation of sulfoxide and sulfone derivatives were identified. Kinetic parameters such as the electron transfer coefficient α, the number of electrons n α involved in the rate-determining step, the total number of electrons n , the reaction rate constant k 0 and the diffusion coefficient D of 4,6-DMDBT for the first transformation were determined by cyclic voltammetry, differential pulse voltammetry (DPV), square wave voltammetry and bulk electrolysis under potentiostatic conditions. The process was bielectronic with α = 0.57, n α = 1, k 0 = 7.46 × 10 −6 cm s −1 and D = 2.30 × 10 −6 cm 2 s −1 . DPV was the most sensitive electroanalytical technique. Using 27 mg L −1 of 4,6-DMTDB, DPV allowed determining a conversion of 91% to sulfoxide after 60 min of electrolysis in a BDD/BDD cell at an anodic potential of 1.50 V, with an apparent rate constant of 0.034 min −1 . The electrochemical characterization was corroborated via gas chromatography–mass spectrometry and ultra-high performance liquid chromatography coupled to electrospray ionization and quadrupole time-of-flight mass spectrometry, confirming the formation of the sulfoxide in the first step and the sulfone in the second one as main products, alongside a minor proportion of dimers.