Redox zeolites

Whilst a whole host of redox zeolites have been prepared and employed with hydrogen peroxide for the oxidation of organic substrates, by far the most studied is a titanium-containing silicalite known as TS-1. For this reason, the bulk of this section will discuss this material in terms of its preparation, oxidation ability with aqueous hydrogen peroxide and the mechanistic pathways believed to be functioning during the oxidation of organic substrates. 

The discovery of TS-1 in 1983 at Enichem in Italy,12 opened up active research into the area of titano-silicalites. TS-1 is a ZSM-5 type molecular sieve with MFI topology. The discovery of TS-1 is interesting as it can be used in 

Structure type Framework structure Pore size (A) Metals incorporated 

TS-1 exhibits some quite remarkable activities and selectivities in the epoxidation of many substrates. For example, relatively unreactive olefins such as ethene and allyl chloride are epoxidized in high yield under mild conditions using methanol as the solvent.19 As a result of the shape selectivity effect, larger, more reactive olefins such as cyclohexene are epoxidized very slowly (Table 4.2). 

Whilst the discovery of TS-1 is a seminal milestone in heterogeneous catalysis, the smaller pore size of the zeolite framework restricts its use, even for some 

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Heterogeneous catalysts for liquid phase oxidations can be divided into three different categories (a) supported metals (e.g. Pd/C), (b) supported metal ions (e.g. ion exchange resins, metal ion exchanged zeolites) and (c) supported oxometal (oxidic) catalysts (e.g. Ti1v/SiOg, redox zeolites, redox pillared clays). This division of the various catalyst types will be used as a framework for the ensuing discussion. 

Redox zeolites as stable, selective oxidation catalysts... 

From a mechanistic viewpoint it is worth noting that the TS-1 catalyst contains the same chemical elements in roughly the same proportions as the Shell amorphous TiIV/Si02 catalyst referred to earlier. However, the former displays a much broader range of activities than the latter. A possible explanation may be that the TS-1 catalyst contains more (or more active) isolated titanyl centres than the amorphous Ti1v/Si02. Based on the quite remarkable results obtained with TS-1 we expect many more examples of redox zeolites, i.e. zeolites, alpos, etc. modified by isomorphous substitution with redox metal ions in the crystal lattice, as selective oxidation catalysts.66... 

Whilst the metal containing xerogels show reasonable catalytic activity with hydrogen peroxide, they are still inferior to the redox zeolites mentioned earlier. 

Another approach to minimizing the problem of metal peroxide dissolution was to incorporate a redox metal into a zeolite by replacing some or all of the aluminum in the framework. Of these redox zeolites, the most common catalysts for peroxide oxidations are the titanium silicalites, TS-1 and TS-2 (Chapter 10). 14,15,19-23 TS-1, the more generally used material, has a crystal structure analogous to ZSM-5 with two dimensional channels of 0.55-0.60 nm in... 

A number of other redox zeolites have peen prepared but they have not been as extensively studied as TS-l. O A titanium beta zeolite having a larger... 

A second approach to isolating redox metal ions in stable inorganic matrices, thereby creating oxidation catalysts with interesting activities and selectivities, is to incorporate them in a zeolite lattice framework. The first example of such a redox zeolite was the synthetic titanium(IV) zeolite, titanium siliealite (TS-1), developed by Enichem [30-32]. TS-1 was shown to catalyze a variety of synthetically useful oxidations with 30% H202, such as olefin epoxidation, oxidation of primary alcohols to aldehydes, aromatic hydroxylation, and ammoxi-mation of cyclohexanone to cyclohexanone oxime (see Fig. 9). 

Recent results in the field of catalytic oxidations with hydrogen peroxide are reviewed. Most effective catalysts fall into three categories metallorganic compounds, phase-transfer catalysts, redox zeolites. Metalloporphyrins and Pt-phosphine complexes are representative of first category. Mo and W polyoxome-talates and related systems, in association with phase transfer agents, belong to the second one. Titanium silicalite (TS-1) is the most studied redox zeolite. The oxidation of nitrogen and sulphur compounds and Fenton-like reactions are not reviewed. 

These drawbacks have been in part overcome by the discovery, in recent years, of new and effective catalysts which will be the subject of this review. Fentonlike reactions and the oxidation of sulphur and nitrogen compounds will not be considered. Emphasis will be put on selective catalysts, discovered over the last decade, and especially on phase transfer [9] and redox zeolite catalysts [10-11]. 

Although titanium silicalite was discovered over a decade ago, studies on other redox zeolites are just beginning. This is a field in which the most promising results are anticipated. The synthesis of large pore zeolites, containing Ti or other redox metal atoms, can open entirely new routes to catal)4ic oxidations and new products. 

Redox zeolites such as TS-1, and supported metal ion catalysts such as those that use ion-exchange resins as supports are among the other major oxidation catalysts. These were considered earlier in this chapter. 

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