Understanding the catalytic processes in heteroatom zeolite and silica materials

State-of-the-art quantum mechanical/molecular mechanical (QM/MM) calculations have been performed to study the local structure and properties of active sites in heteroatom substituted zeolites for partial oxidation reactions. In our initial calculations, we applied the cluster approach to investigate the accuracy of current density functional for use in QM/MM embedding. We demonstrated that the recently developed density functional, BB1K, provides an accurate description of the electronic structure of localised and paramagnetic defects in silica materials. We then applied the QM/MM technique to study the structural and chemical properties of titanium silicalite. In particular, we investigated three main themes: the nature of the active sites in titanium silicalite, the structural properties of the oxygen donating species formed from the active site and hydrogen peroxide, and the reactivity of the oxygen donating species in reactions of various alkenes. The work was further extended to include other group 4 and 14 elements incorporated into silicalite, both in their formal and reduced oxidation states. Particular attention has been paid to the elucidation of a new, stable model of the heteroatom in silicalite based on the hydrolysis and inversion of tetrahedral sites in the zeolitic framework. We found that the inversion mechanism can help to stabilise as well as increase the accessibility of the active sites to guest molecules in the zeolite pores. We further reported that the structure of two major reactive intermediates resulting from the decomposition of H2O 2 over the active Ti site compares very favourably with structural data, obtained by fitting of Ti EXAFS data. Furthermore, we showed that the partial oxidation of ethene by the peroxo complexes can proceed via three alternative mechanisms yielding epoxides or hydroxide products, with the former being the less stable species. In agreement with experiments, we concluded that hydroxide species formed from small alkenes (e.g. ethene, propene) are the favoured product in hydrous conditions, whereas hydroxide species formed from longer chain or cyclic alkenes, notably benzene, are the likely products in both dry and hydrous conditions