jp065494m_si_001.pdf (198.7 kB)
On the Role of Ti(IV) as a Lewis Acid in the Chemistry of Titanium Zeolites: Formation, Structure, Reactivity, and Aging of Ti−Peroxo Oxidizing Intermediates. A First Principles Study
journal contribution
posted on 2006-11-02, 00:00 authored by Eleonora Spanó, Gloria Tabacchi, Aldo Gamba, Ettore FoisThe ethylene epoxidation cycle in a H2O2/H2O-loaded Ti zeolite has been simulated by a Car−Parrinello
approach. Results indicate a process where the zeolitic framework is the active oxygen mediator. The
dissociative chemisorption of H2O2 leads, via a transient Ti−hydroperoxo species, to H2O and a Ti−peroxo
zeolite intermediate. Transfer of active oxygen to ethylene follows, giving the epoxide and recovering the
catalyst. A thorough theoretical characterization indicates that the active oxidizing species is an asymmetric
η2-Ti−peroxo, absorbing in the visible range. The lability of the intermediate is found related to η2 ↔ η
interconversions of the Ti−peroxo structure. The interconversions, triggered by water molecules, could account
for the experimentally found reduced catalytic activity in aged TS-1 catalysts. The results provide a microscopic
picture of the reactivity and dehydration/aging processes of the catalyst fully consistent with experiments and
highlight the fundamental role of the Lewis acid character of Ti in the formation, reactivity, and degradation
of the active oxidizing species.