Oxides of Ti and Si

The Ti02-Si02 catalyst can operate only with hydroperoxides, being rapidly deactivated by aqueous H202. Other titanium compounds such as titanium diacetylacetonate or tetrabutyl orthotitanate (TBOT) were found to be active catalysts for the epoxidation of olefins with extremely high selectivity (98%), but only with hydroperoxides (Sheldon et al.., 1973). 

As will be made clear in Section III, it is possible that finely dispersed Ti02 is present in titanium silicates. The observed catalytic activity would then be attributed to a combination of the two materials. It is therefore useful to review the chemistry of high-surface-area Ti02-Si02 and its catalytic behavior. 

From the difference in electronegativity between O (3.5) and Ti (1.32), it is inferred that the Ti—O bond has a marked ionic character, but the designation of these ions as Ti4+ and O2 seems exaggerated the notation Tilv will therefore be used, except in the case of reduction/oxidation reactions where Ti4+ and Ti3 + better clarify the transformation taking place. 

The composition of the mixed oxides can span the full range from Ti02 to Si02 and has a major influence on the properties of the materials. The procedures used to prepare the mixed oxides are also important in influencing structure and catalytic properties. The procedures can be summarized as follows  

Separate precipitation of titanium- and silicon-containing precursors followed by mixing of the precipitated products. 

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