Catalytic thiophene oxidation by groups 4 and 5 framework-substituted zeolites with hydrogen peroxide: Mechanistic and spectroscopic evidence for the effects of metal Lewis acidity and solvent Lewis basicity

Group 4 (Ti and Zr) and 5 (Nb and Ta) atoms substituted into the ^*BEA zeolite framework (M-BEA) irreversibly activate hydrogen peroxide (H₂O₂) and form pools of metal-hydroperoxide (M-OOH) and peroxide (M-(η²-O₂)) intermediates active for the oxidation of 2,5-dimethylthiophene (C₆H₈S), a model reactant representative of organosulfur species in fossil reserves and chemical weapons. Sequential oxidation pathways convert C₆H₈S into 2,5-dimethylthiophene oxide (C₆H₈SO) and subsequently into 2,5-dimethylthiophene dioxide by oxidative dearomatization. Oxidation rates measured as functions of reactant concentrations together with in situ UV–vis spectra show that all M-BEA activate H₂O₂ to form pools of M-OOH and M-(η²-O₂), which then react with either C₆H₈S or H₂O₂ to form the sulfoxide or to decompose into H₂O and O₂, respectively. Turnover rates for C₆H₈S oxidation and H₂O₂ decomposition both increase exponentially with the electron affinity of the active site, which is quantitatively probed via the adsorption enthalpy for deuterated acetonitrile to active sites. C₆H₈S oxidation rates depend also on the nucleophilicity of the solvent used, and rates decrease in the order acetonitrile > p-dioxane ∼ acetone > ethanol ∼ methanol. In situ UV–vis spectra show that highly nucleophilic solvent molecules compete effectively for active sites, inhibit H₂O₂ activation and formation of reactive M-OOH and M-(η²-O₂) species, and give lower turnover rates. Consequently, this work shows that turnover rates for sulfoxidation are highest when highly electrophilic active sites (i.e., stronger Lewis acids) are paired with weakly nucleophilic solvents, which can guide the design of increasingly productive catalytic systems for sulfide oxidation.