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 molecular sieves, TS-1 and TiMCM-41, with aqueous H2O2 have been studied by EPR spectroscopy. Three paramagnetic Ti(IV)-superoxide radical species (A, B, and C, respectively) have been observed in TS-1 samples; mainly species B has been identified in TiMCM-41. The Ti(IV) ions in the superoxide are in framework positions. Species B is more abundant than species A and C. On interaction with aqueous H2O2 the geometry of titanium expands from tetra- to hexacoordination in species A and C and to pentacoordination in species B. The EPR spectra of the Ti(IV)-superoxide species are characterized by rhombic g tensors. The effects of solvents, substrate (allyl alcohol), and temperature 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 toward the oxidation of allylalcohol to the corresponding epoxide than species B. Variable-temperature EPR studies (77–190 K) have provided evidence, for the first time, of the superoxide–hydroperoxide/peroxide interconversions. At higher temperatures Ti(IV)-superoxide is the preferred geometry. At lower temperatures Ti(IV)-hydroperoxide/peroxide is formed. Species B did not 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 relative concentrations of the superoxide and hydroperoxide/peroxide under reaction conditions are crucial factors determining catalytic activity and, especially, selectivity.