Titanium, zeolites

Research Focus Epoxidation of propylene using palladium/titanium zeolite-1 as catalyst Originality This propene epoxidation method occurs in a neutral medium and is... 

TABLE 1. Reaction Scoping Using the Palladium/Titanium Zeolite Catalyst in the Epoxidation/Glycolization of Propylene... 

Arco have developed an integrated process for the production of industrially important epoxides via an adapted AO process (Figure 1.13).33 34 A sulfonic acid substituted alkylhydroanthraquinone alkylammonium salt is reacted with molecular oxygen to form the alkylanthraquinone and hydrogen peroxide. The hydrogen peroxide is then reacted with an alkene in the presence of a titanium zeolite catalyst (TS-1 see Chapter 4). The epoxide product is then separated, and the anthraquinone salt recycled to a hydrogenator for reaction with... 

Figure 3.34 Cleavage of a-methylstyrene with a hydrogen peroxide titanium zeolite system involving the production of 2-hydroxy-2-phenylpropan-l-ol.

A promising and cleaner route was opened by the discovery of titanium silica-lite-1 (TS-1) [1,2]. Its successful application in the hydroxylation of phenol started a surge of studies on related catalysts. Since then, and mostly in recent years, the preparation of several other zeolites, with different transition metals in their lattice and of different structure, has been claimed [3]. Few of them have been tested for the hydroxylation of benzene and substituted benzenes with hydrogen peroxide. Ongoing research on suppoi ted metals and metal oxides has continued simultaneously. As a result, knowledge in the field of aromatic hydroxylation has experienced major advances in recent years. For the sake of simplicity, the subject matter will be ordered according to four classes of catalyst medium-pore titanium zeolites, large-pore titanium zeolites, other transition metal-substituted molecular sieves, and supported metals and mixed oxides. 

Then, it would be of great interest to obtain large-pore titanium zeolites which would allow the diffusion of bulkier organic molecules to the internal Ti sites, thus broadening the possibilities of these materials. 

Selective oxidations with H2O2 titanium zeolite (TS-1) Enichem, 1983... 

Acid catalysts, transition metal redox catalysts, and titanium zeolites are all known to be effective for phenol hydroxylation. Acid catalysis proceeds by an ionic mechanism involving an intermediate hydroxonium ion (H3O2+) whereas some transition metal ions promote the formation of hydroxyl radicals to effect substitution. However the introduction of a second hydroxyl substituent onto the aromatic nucleus tends to activate the molecule towards further reaction and this leads to the formation of unwanted, tarry by-products. The commercial solution is to use very low mole ratios of hydrogen peroxide to phenol and to recycle the unreacted phenol, ie. operate at low conversion. Some typical commercial methods are given in Table 1. 

The discovery of the acidic titanium zeolite TS-1 has, in recent years, led to remarkable progress in the field of oxidations with H2O2 [15, 89 - 91]. For example, the direct hydroxylation of aromatic hydrocarbons with H2O2 in the presence of TS-1 is possible [90, 91]. The... 

Very recently, the use of titanium zeolites for oximation with ammonia has also been reported. 

Various substituted styrene oxides have been rearranged to their corresponding phenylacetaldehydes with good yields and selectivities using titanium zeolites in acetone or methanol at temperatures between 30 and 100°C [90]. 

Many oxidation reactions have been carried out using hydrogen peroxide and the titanosilicate, TS-1. However, this catalyst has relatively small pores and is therefore not an efficient catalyst for the oxidation of large molecules. This problem has been solved by the successful generation of a medium-pore titanium zeolite Beta-Ti [136]. Cyclododecane and cyclohexane are both oxidised selectively by H2O2 in the presence of the new titanium zeolite, favouring the ketone product. 

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