Vanadium silicate xerogels in hydrogen peroxide catalyzed oxidations

Vanadium silicate xerogels (V2O5-SiO2) were prepared by the sol-gel method by hydrolysis of vanadium and silicon alkoxides. The use of these xerogels as catalysts for oxidation of alkenes, alcohols and phenol was studied using 30% aqueous hydrogen peroxide as oxidant. It was found that the manner of xerogel preparation strongly influenced the catalytic activity of V2O5-SiO2. Alcohols were the preferred solvents for the reaction and did not leach vanadium oxide into solution. For alkenes, epoxidation was the dominant oxidation reaction the yield and selectivity depending on the nucleophilicity and oxidizability of the substrate. For alkenes of intermediate nucleophilicity allylic oxidation was significant. Secondary alcohols were oxidized in low to fair yields whereas primary alcohols were inert. Phenol was oxidized selectively to a 2:1 mixture of hydroquinone/catechol. UV-vis and electron spin resonance spectra of V2O5-SiO2 treated with hydrogen peroxide conclusively showed formation of vanadium-peroxo species with polarization of the O-O bond as seen by formation of vanadium(IV) electron spin resonance spectra leading to oxidation by both heterolytic and homolytic cleavage.