Titanium Silicalite Catalysts

TS-l and titanium silicalite-2 (TS-2) are microporous solid materials made of Si02 and Ti02 that have silicalite structures (TS-1 has the ZSM-5 structure and TS-2, the ZSM-11 structure) modified by isomorphous substitution of Si(IV) with Ti(IV). TS-1 and TS-2, the former being most studied, show similar properties in catalysis of H202 oxidations. 

Several methods have been employed for the synthesis of TS-1.1 3 A typical method begins with the addition of tetraethylorthotitanate (TEOT) to tetra-ethylorthosilicate followed by the addition of a certain concentration of tetra-propylammonium hydroxide. The resulting mixture is stirred until clear, heated to remove alcohol, put into an autoclave, and heated to 175°C for 24 hours.4 Other methods use Ti(OBu)45,6 or TiF47 instead of TEOT. TS-2 contains a slightly larger cavity and is prepared in a similar manner.8,9 

Not only has the Ti precursor been investigated, but also the structure of the molecular sieve has been heavily investigated. Thus we now have an array of silica and silica-alumina molecular sieve supported Ti catalysts. These include Ti on amorphous Si02,10,11 Ti on a variety of Si02 mixed oxides,12 Ti-0 (titanium-beta),13 17 Ti-MCM-48,18 Ti-MCM-41,19 Ti-HMS,18 titanium- 

This popular and rapidly maturing held has generated several reviews,31-37 which contain many references. The types of reactions typical of the titanium silicalites and similar titanium-incorporated catalytic systems are summarized in the following paragraphs. 

By 1990, most of the catalytic reactions of TS-1 had been discovered. The wide scope of these reactions is shown in Fig. 6.1.35 Conversions include olefins and diolefins to epoxides,6,7 12 16 19 21 24 34 36 38 13 aromatic compounds to phenols,7,9 19 25 27 36 ketones to oximes,11 20 34 46 primary alcohols to aldehydes and then to acids, secondary alcohols to ketones,34-36 42 47-30 and alkanes to secondary and tertiary alcohols and ketones.6 34 43 31 52 

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The advantage of the above two methods are high yields of epoxides, and the titanium silicalite catalyst is not deactivated or poisoned by the contaminants in the crude oxidation mixture. Hence, the processes are commercially attractive. The in situ hydrogen peroxide generation based on the AO process from either the anthraquinone/anthrahydroquinone or ketone/alcohol redox couples has also been used for the following synthetic reactions ... 

G. F. Tliiele, E. Roland, Propylene epoxidation with hydrogen peroxide and titanium silicalite catalyst Activitv, deactivation, and regeneration of the catalyst, /. Mol. Catal. A Chem. 117(1997)351. 

Clerici and Bellussi" have shown that hexane in methanol can be selectively oxidized to 2-hexanol, 3-hexanol, 2-hexanone and 3-hexanone using a mixture of oxygen and hydrogen at 25 - 30 . The reactions were run in the presence of HCl for 20 - 24 hours. Several titanium silicalite catalysts containing Pd (0.01 mol ratio to Ti02) were prepared and used in these reactions. Presumably, hydrogen plays the role of an electron-donor activating the Ti catalyst. However, no explanations were offered. 

Hydroxylamine can be prepared from ammonia and hydrogen peroxide using a titanium silicalite catalyst in 83% yield.68 The by-product acetic acid could be recycled to acetic anhydride by pyrolyzing part of it to ketene. 

P-14 - Epoxidation of propylene in fixed bed reactor using supported titanium silicalite catalyst... 

Two molding methods were adopted to prepare the titanium silicalite catalyst used in a fixed bed reactor. One is the traditional extruding method in which strip TS-1 catalyst is obtained. The other is a new kind of molding method in which lamina TS-1 catalyst is made by spraying the powdery TS-1 on a small inert ball. The lamina TS-1 catalyst exhibits better performance in propylene epoxidation than the strip TS-1 catalyst. 

Solid catalysts may also be used for reactions implying oxidants differ by dioxygen. The most popular case is that of titanium-silicalite catalysts for oxidation with hydrogen peroxide. The active species in the presence of water has been characterised to be a side-on peroxo complex characterised by a Raman-detected 0-0 stretching at 618 cm Upon drying, this species converts into a hydroperoxo species characterised using IR by an 0-0 stretching at 837 cm and a broad OH band at 3400 cm ... 

Zr- and Ti-based POMs have been suggested as molecular models for the well-known heterogeneous titanium silicalite catalyst, TS-1, with major industrial appeal in the field of H2O2 activation and selective oxidation. In some cases, POMs... 

Tetrahedrally-coordinated cations are also encountered in the lattice of zeolites (Figure 3.8). The well-defined nature of the lattice, the variety of microchannel dimensions, and the many possibilities for isomorphous substitution make them ideal catalysts for the production of fine chemicals. A recent application is the oxidation of phenol to catechol by hydrogen peroxide using a titanium-silicalite catalyst with Ti ions in tetrahedral sites. 

A well-known example of the latter type is titanium silicalite-1 (TS-1), a redox molecular sieve catalyst [7]. 

In the 1980 s zeolites attracted a renewed attention. They were shown to be rather promising catalysts if, instead of O2, a chemically pre-modified oxygen entering the oxygen-containing molecules is used. The most known example is an excellent catalytic performance of titanium silicalites in the liquid phase oxidations with H2O2 [5]. A gas phase oxidation with nitrous oxide is another approach in this field being intensively developed in the last years [2],... 

The reaction is carried out using a titanium silicalite-1 (TS-1) zeolite catalyst [30, 122]. This type of catalyst is known to accelerate the selective oxidation of alcohols, epoxidation of alkenes and hydroxylation of aromatics. These reactions have importance for fine-chemical production. 

Many of these problems disappeared in 1983 when Taramasso, Perego, and Notari synthesized titanium silicalite-1 (TS-1),1 which greatly affected the use of zeolite catalysts for practical oxidation chemistry. This catalyst shows outstanding activity, selectivity, and stability below 100°C. 

Cyclohexanone is thus converted to cyclohexanone oxime, an intermediate in the manufacture of Nylon-6. The catalyst is titanium silicalite-2. Commercialized by Enichem who built a 12,000 ton/year plant in Porta Marghera in 1994. 

Indeed, several interesting procedures based on three families of active catalysts organometallic complexes, phase-transfer compounds and titanium silicalite (TS-1), and peroxides have been settled and used also in industrial processes in the last decades of the 20th century. The most impressive breakthrough in this field was achieved by Katsuki and Sharpless, who obtained the enantioselective oxidation of prochiral allylic alcohols with alkyl hydroperoxides catalyzed by titanium tetra-alkoxides in the presence of chiral nonracemic tartrates. In fact Sharpless was awarded the Nobel Prize in 2001. 

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