Oxidation of Thioanisole Using Niobium–Silica Catalysts: Theoretical and Experimental Studies

Catalysts containing supported metal oxides are very versatile materials used in liquid phase oxidation reactions. The present study aimed to determine the influence of niobium supported on a silica matrix (SiO 2 ) for the oxidation of thioanisole to methyl phenyl sulfoxide and methyl phenyl sulfone. Characterization of the synthesized Nb5SiO 2 and Nb10SiO 2 catalysts revealed that although they are not completely crystalline, these microporous materials displayed even niobium dispersion on the SiO 2 support whilst containing Lewis and Brönsted acidic sites on their surface. Nb5SiO 2 and Nb10SiO 2 catalysts showed higher activities compared to pure SiO 2 catalyst. Niobium-containing catalysts promoted the subsequent oxidation reaction that converted methyl phenyl sulfoxide to methyl phenyl sulfone. The formation of methyl phenyl sulfone was observed in the presence of niobium-containing catalysts even when using a deficiency of H 2 O 2 (0.44 equivalents), whereas the same result could not be achieved for the SiO 2 catalyst. The formation of methyl phenyl sulfone occurred in a significantly shorter time in the presence of Nb10SiO 2 , demonstrating the effectiveness of niobium in producing active oxygen species that promote the subsequent reactions. Under the optimized conditions, methyl phenyl sulfoxide could be obtained with 100% conversion and selectivity at room temperature (25 °C) in either acetonitrile or methanol as solvent. Finally, theoretical calculations showed, through energy values, that the niobium-containing catalysts have a lower relative surface energy compared to the pure silica catalyst. The results presented in this research show the potential of the synthesized materials to promote thioanisole catalytic oxidation reactions using hydrogen peroxide as an oxidant. Graphical Abstract