Reactive oxygen species in titanosilicates TS-1 and TiMCM-41: An in situ EPR spectroscopic study

The superoxide species generated by the interaction of titanosilicate molec ular sieves, TS-1 and TiMCM-41, with aqueous H2O2 have been studied by EPR spectroscopy. Three paramagnetic Ti(IV)-superoxide radical species (A, B, a nd C, respectively) have been observed in TS-1 samples; mainly species B ha s been identified in TiMCM-41. The Ti(IV) ions in the superoxide are in fra mework positions. Species B is more abundant than species A and C. On inter action with aqueous H2O2 the geometry of titanium expands from tetra- to he xacoordination in species A and C and to pentacoordination in species B. Th e EPR spectra of the Ti(IV)-superoxide species are characterized by rhombic g tensors. The effects of solvents, substrate (allyl alcohol), and tempera ture on the EPR spectra and distribution of species A-C have been examined. In situ EPR studies have revealed that species A and C are more reactive t oward the oxidation of allylalcohol to the corresponding epoxide than speci es B. Variable-temperature EPR studies (77-190 K) have provided evidence, f or the first time, of the superoxide-hydroperoxide/peroxide interconversion s. At higher temperatures Ti(IV)-superoxide is the preferred geometry. At l ower temperatures Ti(IV)-hydroperoxide/peroxide is formed. Species B did no t exhibit such an interconversion. It is concluded that, in addition to the framework Ti centers being able to expand their coordination geometry from 4 to 6 and the hydrophobic nature of the titanosilicate structure, the rel ative concentrations of the superoxide and hydroperoxide/peroxide under rea ction conditions are crucial factors determining catalytic activity and, es pecially, selectivity. (C) 2001 Academic Press.