Titania hydroperoxides

This simple picfure can only be partly correct, as it has been shown that both unsupported ultrafine titania nanoparticles [10] and silica-supported monodis-persed subnanometric titania particles [11] are active catalysts for olefin epoxidation with hydroperoxides. In these small titania particles, the titanium atoms possess coordination states between four and six. Quanfum chemical calculations derived similar activation energies for oxygen transfer on mononuclear four- and five-coordinafed fifanium sifes [12,13]. 

The importance of the acid-catalyzed side reactions are illustrated in Table 3 by the product distribution obtained using either TBHP or cumene hydroperoxide (CHP) as oxidant. The epoxidation with TBHP is faster and considerably more selective. When using CHP, about 20 mol% of the coproduct 2-phenyl-2-propanol was dehydrated to a-methylstyrene. It is likely that the simultaneously formed water increases the (Brpnsted) acidity of the aerogel and thus accelerates the ring opening and - to a smaller extent - the isomerization reactions. No oxidation products were formed in the absence of peroxide, as expected. Slow isomerization from p- to a-isophorone catalyzed by titania-silica was the only reaction observed. The data in Table 3 indicate that the simultaneous presence of peroxide and catalyst in the reaction mixture markedly accelerates the acid-catalyzed isomerization reaction. 

The epoxidation of two cycloalkenones, a- and P-isophorone, with alkyl hydroperoxides demonstrates that active and selective titania-silica aerogels can be prepared by the sol-gel method combined with extraction of the solvent with supercritical COg at low temperature. The key factors for obtaining high activity in the epoxidation of bulky cyclic olefins are the high Ti-distribution in the silica matrix, the mesoporous structure and high surface area. 

Figure 4 Effect of preparation method on the reactivity of titania/silica catalysts in the epoxidation of cyclooctene with tert-butyl hydroperoxide in tert-butanol...

Figure 5 shows effect of oxidant on the reactivity of the sol-gel titania/silica catalysts in the epoxidation of cyclooctene. The conversion in the epoxidation of cyclooctene with tert-butyl hydroperoxide sharply increases with reaction time. However, when the oxidant is hydrogen peroxide, the conversion is quite low... 

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. 

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 review of recent advances in transition-metal-catalysed oxidations by molecular oxygen has highlighted the scope and limitations, as well as the meehanisms of these reactions. " " An overview of the fundamental studies on a new method of synthesis of nicotinic acid by the gas-phase catalytic oxidation of -picoline by oxygen has been presented. The reactivity of vanadium species has been considered in order to discover the nature of the active catalyst. Kinetic equations for -picoline oxidation on vanadia-titania catalysts have been discussed. " The effect of quaternary ammonium salts or macrocyclic ethers on the autoxidation of ethylbenzene or the decomposition of the a-phenylethyl hydroperoxide intermediate catalysed by Ni(II) or Fe(III) acetylacetonates has been reviewed. ... 

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