Zeolite Confined Ti(OH)4 Nanoparticles in Highly Active and Selective Oxidation Catalyst

Abstract

Some highly active, hydrophilic, MFI structured titanium silicate catalysts are characterized by an intense UV absorption centered in the 213–233 nm (43000–47000 cm−1) range. The assignment of this band to a specific molecular structure has remained unclear unlike the well published 233 nm UV bands of higher coordinated Ti-atoms at lattice defects or in variously agglomerated extra lattice Ti x O y (OH) z species. To decipher the structural origin of this intermediate UV band, we used Time-Dependent Density Functional Theory (TD-DFT) to compute the UV spectra of various model compositions and show in this paper that only one model, in which isolated tetrahedral Ti(OH)4 molecules reside in the zeolite channels without any covalent bond to the framework, fits the experimental UV spectrum. Hence these confined tetrahedral Ti(OH)4 nanoparticles may account for the enhanced activity and selectivity of some MFI type titanium silicate catalysts.