Reactive oxygen species in epoxidation reactions over titanosilicate molecular sieves

Abstract

In an attempt to rationalize the differences in the catalytic behavior of titanosilicate molecular sieves when using H2O2 (TS-1, TiMCM-41) versus H2+O2 (Pd–TS-1) as oxidant, the Ti(IV)–superoxo and hydroperoxo/peroxo species formed in situ during the oxidation reactions were investigated by electron paramagnetic resonance (EPR) and diffuse reflectance UV–visible spectroscopies. Two types of superoxo species, A and B, were identified in TS-1; only the latter species was detected in TiMCM-41. EPR has provided evidence, for the first time, for the in situ generation of similar oxo species in reactions using H2+O2 instead of H2O2. The Ti sites adjacent to Pd ions (in Pd–TS-1) behave differently magnetically than the other Ti ions, generating a greater variety of superoxo species (A′, A″, A, B′, B, and C) and corresponding reaction products. Pd enhances the reducibility of Ti and promotes formation of these oxo species at lower temperatures ( 323 K). The epoxide selectivity in the oxidation of allyl alcohol over Pd–TS-1 is higher when using (H2+O2) than when using H2O2. In all cases, type A species predominantly catalyze epoxidations while type B favor the hydroxylations. The selectivity for epoxidation (vis-à-vis allylic oxidation) over these catalysts can be enhanced by controlling the type of Ti–oxo species formed in situ during the oxidation process by a suitable combination of catalyst, oxidant, solvent, and temperature.