Mixed Oxides by the Sol-Gel Method

These mesoporous mixed titania-silica oxides are hydrophilic materials and are excellent catalysts for epoxidations of olefins, allylic alcohols and a,jff-unsaturated ketones with alkyl hydroperoxides in non-aqueous media [37]. Their performance can be improved even further by adding organic or inorganic bases to neutralize acid sites present on the surface [38,39], The latter cause side-reactions, especially with acid sensitive epoxides. Amine addition was particularly effective and led to the development of a mesoporous Ti-Si mixed oxide containing surface-tethered tertiary amino groups as an active, selective, and recyclable catalyst for the epoxidation of allylic alcohols [38]. 

A shortcoming of these materials is their hydrophilicity, which means they are ineffective catalysts for epoxidations with hydrogen peroxide strong adsorption of water on the hydrophilic surface severely limits access of hydrophobic substrates to the active site. Hence, much effort has been devoted to the synthesis of hydrophobic analogs by polycondensation of a titanium(IV) alkoxide with RSi(OEt)3 (R = alkyl, aryl) instead of Si(OEt)4 [23,24,40]. 

Both microporous [23] and mesoporous [24] hydrophobic Ti-Si mixed oxides have been synthesized but their activities as epoxidation catalysts with aqueous hydrogen peroxide are, as yet, disappointingly low compared with the corresponding reactions with TBHP in organic media or with TS-1 (Section 9.1.6). 

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