Barker, Carolyn Marie; (2002) Catalytic processes in porous transition metal silicates. Doctoral thesis (Ph.D.), University College London.

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Abstract

Quantum mechanical cluster calculations on the mechanism of alkene epoxidations within peroxide saturated titanium molecular sieve catalysts are reported. This thesis addresses three major themes; the first is the nature of the titanium active sites, in dehydrated and hydrated titanium molecular sieves. Secondly the nature and formation mechanism of the oxygen-donating species for oxidation catalysis is presented. Finally, elucidation of the complete mechanistic catalytic cycle for the titanium molecular sieve mediated oxidation of alkenes to epoxides, in the presence of peroxide, is reported. A focal point of this work is the relationship between reaction energetics and the structure and electronic properties of varied ligands and alkenes. Highlights include the elucidation of a new, stable oxygen-donating species, (=SiO)3Ti-η1[O(H)OH](OH2), isolated for the first time by this work, which has a low activation barrier of formation (< 20 kJmol-1) and shows favourable reaction energetics for the epoxidation of alkenes. Furthermore, the first experimental evidence for the existence of two particular oxygen-donating structures in the catalyst, (SiO)3Ti-η2(OOR).H2O and (SiO)3Ti-η1(OOR)(OH2).H2O, which were predicted to exist by quantum mechanical calculations by this work, is reported. One of the oxidation agents is shown to provide an energetically favourable route to epoxide formation and the other is proposed as the oxygen-donor for diol formation, one of the major by-products in epoxidation catalysis.

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