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

Abstract

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.