Reactive oxo-titanium species in titanosilicate molecular sieves: EPR investigations and structure–activity correlations

Abstract

The structure of the reactive oxo-titanium species (hydroperoxo-, peroxo-, and superoxo-titanium) in titanosilicate molecular sieves (TS-1, Ti-β, amorphous Ti–SiO2, and TiMCM-41), generated on interaction with aqueous H2O2 or urea–H2O2 adducts, was investigated by electron paramagnetic resonance (EPR) and diffuse reflectance UV–visible (DRUV–visible) spectroscopies and magnetic susceptibility measurements. Two types of superoxo-titanium species (A and B) were identified in TS-1 and amorphous Ti–SiO2. An additional oxo species, A′, was also detected over Ti-β. TiMCM-41 generated, mainly, the B-type species. DRUV–visible spectroscopy and magnetic susceptibility measurements suggest the coexistence of both the superoxo- and hydroperoxo-titanium species. The A-type oxo-titanium species originate from the framework Ti sites having a tetrapodal (SiO)4Ti structure located inside the pores, while the B-type species originate predominantly from tripodal (SiO)3Ti(OH) structures, also located in the framework, probably at the external surface. The local structural environment of the Ti4+ ions, solvent, and temperature influence the concentration of these oxo-titanium species. The concentration of the A-type species decreased in the order: TS-1>Ti-β>amorphous Ti–SiO2. The B type varied as follows: TiMCM-41>amorphous Ti–SiO2>Ti-β>TS-1. In the oxidation of styrene, the A-type species correlate with styrene oxide formation, while the presence of B-type species lead to nonselective products. Experimental evidence is given for the involvement of the hydroperoxo-titanium species in olefinic epoxidations (styrene to styrene oxide) and the superoxo-titanium in aromatic hydroxylations (phenol to hydroquinone and catechol).