4 years ago

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

Rate and Selectivity Control in Thioether and Alkene Oxidation with H2O2 over Phosphonate-Modified Niobium(V)–Silica Catalysts
Nicholas E. Thornburg, Justin M. Notestein
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. Tunable catalyst selectivity: Niobium(V)–silica catalysts are shown to be highly active and selective thioether oxidation catalysts for H2O2 compared to benchmark titanium(IV)–silica. Simple modification of the highly active Nb-SiO2 active sites with phenylphosphonic acid results in an unusually active and selective catalyst for sulfoxidation, but not for alkene epoxidation nor overoxidation to the sulfone.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/cctc.201700526

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