Active Sites and Reactive Intermediates in Titanium Silicate Molecular Sieves

This review is a summary and critical analysis of recent advances in the understanding of (a) the nature and coordination state of Ti ions and other functional groups (such as OH) on dehydrated titanium silicate molecular sieves, (b) the type and structure of the oxo intermediates generated by the interaction of these active sites with oxidant/reactant molecules during catalytic reactions, and (c) the factors that influence the reactivity and selectivity of these active sites and reaction intermediates. In the dehydrated state, most of the Ti4+ ions have the tetrapodal (Ti(OSi)4) or the tripodal (Ti(OSi)3OH) structure. On contact with H2O2, titanium oxo species, Ti(O2H) and Ti(O2−), respectively, are formed. On reaction with organic reactants, O–O bond cleavage in these titanium oxo species occurs in a hetero- or homolytic manner. Product selectivity is determined by the relative importance of these two modes of O–O cleavage. Factors such as the coordinative environment of titanium, substituents on the O–O bond (H or alkyl), temperature, solvent, nature of the organic reactant, etc. influence the mode of O–O cleavage. Correlations between the structure and catalytic activity of titanium sites and oxo-titanium intermediates are also described.