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 η and η 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.

QM/MM study of the stability of dimethyl ether in zeolites H-ZSM-5 and H-Y

“…In our previous work, we showed that an appropriate QM region includes atoms up to the fifth nearest neighbour (the third oxygen atom) from the central T-site. 30,31 For the QM calculations, the terminal oxygens at the edge of the QM region are saturated with ''link'' hydrogen atoms to ensure correct electronic structure for the terminal oxygens. The terminal ''link'' atoms do not inadvertently affect the electronic solution of the overall QM calculations as an additional bond-dipole correction is added at the bond bridging the QM and MM boundary, which ensures an accurate local electrostatic potential.…”

The activity of titanium silicalite-1 (TS-1): Some considerations on its origin

“…(II)). The reversible opening of a second Ti-OSi bond that would favour the inert Ti(η 2 O 2 ) peroxide, albeit feasible, is regarded to be restrained by the crystalline lattice [4]. It is possible that a certain amount of Ti is in the form of Ti(η 2 O 2 ), but theoretical studies and the acidity devel oped by TS 1 on chemisorption of hydrogen peroxide are against a major formation of it.…”

Quantum Mechanical/Molecular Mechanical (QM/MM) Approaches

“…By considering the site reactivity towards H2O2 in anhydrous and hydrous conditions, (To et al, 2008) confirmed the close competition between η 1 and η 2 complexes as candidates for the oxygen donating species in TS-1. Curiously, from electron paramagnetic resonance (EPR) studies, a much higher energy (by ca.…”

“…400-450 kJ mol -1 ) superoxide radical species had been reported to emerge as a result of interaction of aqueous H2O2 and non-aqueous urea H2O2 (Shetti et al, 2003). By calculating the related spectroscopic signature, (To et al, 2008) unambiguously identified the superoxide supported on tripodal Ti as the Table 4, where the calculated components of the g tensor are compared to experimental values, which are expected to be accurate to at least the third decimal place), which allowed them to argue in support of the tetrahedral site inversion mechanism as essential for promoting the catalytic activity of these systems. In the absence of definitive structural experimental characterisation, this combination of spectroscopic evidence and supporting computational results is key to unravelling the mechanisms of catalysis in atomistic and electronic detail (for more examples of such combinations see Chapters 9, 19, 21 of this volume).…”