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

Abstract

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 NbV, TiIV, and ZrIV 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.

Ionic Liquid Stabilized Niobium Oxoclusters Catalyzing Oxidation of Sulfides with Exceptional Activity

“…The linearity of the Arrhenius plots was examined to determine the activation energy, which was E a =36.1 kJ mol −1 . The activation energy under the present conditions was lower than or comparable with that of previous work on Nb‐catalyzed thioether oxidation …”

Epoxidation of Karanja (Millettia pinnata) Oil Methyl Esters in the Presence of Hydrogen Peroxide over a Simple Niobium-Containing Catalyst

“…It is worth highlighting that such a preparation approach is conceptually simpler and more affordable, in terms of economic resources, than other synthesis routes reported so far [31,32,50]. Indeed, the preparation steps required no operations under inert or controlled atmosphere, no costly and/or hazardous precursors and no organic solvents (cf.…”

Liquid-Phase Epoxidation of Light Olefins over W and Nb Nanocatalysts

“…The decrease of catalytic activity in epoxidation with Nb content agrees with the decrease of the concentration of Lewis acid sites of moderated strength. 105 Therefore, a mechanism involving Lewis acid sites of moderate strength during epoxidation of alkenes with H 2 O 2 is proposed 207,230 while strong Lewis acidity activates the H 2 O 2 decomposition reaction. 231,232 The low selectivity has been attributed to the presence of strong Brønsted acid sites, which enhance the reaction of ring epoxide opening by hydrolysis.…”