Hybrid QM/MM Investigations into the Structure and Properties of Oxygen-Donating Species in TS-1

Abstract

The active oxidizing species in the H2O2/TS-1 catalytic system is investigated using a hybrid quantum mechanical/molecular mechanical approach. In this computational technique, the site of interest is described with the density functional theory using the BB1K exchange and correlation functional, and the remainder of the system is treated with a valence force field. We have examined the formation of dioxygen adsorbate structures with η1 and η2 Ti-peroxo configurations, including radical species, which arise from the attack of hydrogen peroxide on a tetrahedral TiIV site along with effects of water co-adsorption. Our results show that hydrogen peroxide physisorbed on tetra- and tripodal Ti sites, as well as coadsorbed H2O2/H2O, is energetically favorable and involves minor structural modifications in the framework, as Ti atoms change their coordination from four to six. Furthermore, our calculations of various Ti-peroxo complexes formed from tripodal sites and H2O2 strongly suggest that Ti-η2(OOH), Ti-η1(OOH), and Ti-η1(O(H)OH) complexes will all form in the pores of TS-1 under reactions with H2O2 with a preferential coordination of six in the presence of water. In particular, both six-coordinate Ti-η1(OOH) and Ti-η2(OOH) compare favorably with EXAFS data obtained from the mesoporous materials with the isomorphous Ti active sites. These results indicate that water is not just a medium for transporting reactants and products at the catalytic sites, but has an active role in stabilizing the peroxo species present on the working catalyst. The Ti−O bond distances are also close to those in the five-coordinated complexes reported for TS-1 (also based on EXAFS data). Although, we find that both anhydrous and hydrated η2(O2·) type species are unlikely to be the predominant oxygen-donating species, we have demonstrated that our calculated g-tensors for the short-lived radical species are in line with EPR data, which supports our assertion of the tripodal Ti site as the main active site in TS-1 for partial oxidation catalysis.