Heterogeneous epoxidation on Ti catalysts

The finding of an active solid redox system resulted in a flourish of activity in the development and application of diverse redox molecular sieves containing titanium (IV) and other metal ions [378-380]. Like the earlier ion-exchanged zeolites, many of the resulting catalysts, however, also suffered from loss by leaching, even when the redox element was substituted in the framework [102]. Ti-substituted zeolites remain special because of then stability. 

Ti-MCM-41 [126]. The high activity of the aerogel was attributed to isolated Ti centers. 

Solvent effects differ with large size zeolites which have lesser hydrophobicity than TS-1, and these are discussed by Clerici [373]. Larger sized zeolites, such as Ti-beta, have been found to be more effective for bulkier olefins such as norbor-nene, limonene, and a-terpineol [381,399]. The subject has been reviewed by Baiker et al. [127]. 

Clerici cites considerable support for the mechanism involving the Ti (q -OOH) species [373]. In TS-1, numerous spectroscopic studies [412] show the adsorption of protic molecules on Ti sites, for example, as observed by XANES by the decrease in intensity of the tetrahedral Ti pre-edge peak on adsorption of water and ammonia [413]. Rate laws suggest that one molecule of alcohol solvent is adsorbed on the active species in the oxidation of alcohols [414] and the 

Although these mechanisms appear similar to that suggested for Mo catalysts (See next section), it must be remembered [373] that soluble Mo catalysts are strongly inhibited by protic compounds that compete with the oxidant for the active sites. On the other hand, these compoimds promote the activity of TS-1. 

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