Rate and Selectivity Control in Thioether and Alkene Oxidation with H₂O₂ over Phosphonate-Modified Niobium(V)–Silica Catalysts

Supported metal oxide catalysts are versatile materials for liquid-phase oxidations, including alkene epoxidation and thioether sulfoxidation with H₂O₂. Periodic trends in H₂O₂ activation was recently demonstrated for alkene epoxidation, highlighting Nb-SiO₂ as a more active and selective catalyst than Ti-SiO₂. Three representative catalysts are studied consisting of Nb^V, Ti^IV, and Zr^IV on silica, each made through a molecular precursor approach that yields highly dispersed oxide sites, for thioanisole oxidation by H₂O₂. Initial rates trend Nb>Ti≫Zr, as for epoxidation, and Nb outperforms Ti for a number of other thioethers. In contrast, selectivity to sulfoxide vs. sulfone trends Ti>Nb≫Zr at all conversions. Modifying the Nb-SiO₂ catalyst with phenylphosphonic acid does not completely remove sulfoxidation reactivity, as it did for photooxidation and epoxidation, and results in an unusual material active for sulfoxidation but neither epoxidation nor overoxidation to the sulfone.