Historical Development and Overview of Catalysts

Shell subsequently developed a heterogeneous, silica-supported titania catalyst [11,12] which forms the basis of the commercial process for the epoxidation of propylene with ethylbenzene hydroperoxide. The co-product alcohol is dehydrated, in a separate step, to styrene. Ti(IV)Si02 was the first truly heterogeneous epoxidation catalyst useful for continuous operation in the liquid phase. 

TS-1 catalyzes the smooth epoxidation of relatively unreactive olefins, e. g. propylene and even allyl chloride, with 60 % aqueous H2O2 at 40-50 °C in methanol [20]. A serious shortcoming of TS-1 is, however, its restriction to substrates with kinetic diameters 5.5 A. For example, 1-hexene is readily epoxidized at 50 °C whereas cyclohexene is essentially unreactive. By the same token TS-1 does not catalyze epoxidations with the more bulky tert-butyl hydroperoxide (TBHP), 

The success of TS-1 sparked a flourish of activity, which still continues today, on the synthesis and application of other redox molecular sieves [21], This was driven by the expectation that TS-1 was the progenitor of a new class of novel catalytic materials. For example, titanium has been incorporated in the framework of wide variety of molecular sieves including mesoporous silicas, such as MCM-41, with pore diameters ranging from 20 to 50 A (see below). 

Finally, we note that caution should be exercised, because many of the solid catalysts reported in the literature have not been subjected to rigorous proof of heterogeneity under oxidizing conditions [31]. Both alkyl hydroperoxides and, especially, hydrogen peroxide can readily leach metal ions from solid catalysts. 

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