Titanium-silicalite

Titanium silicalite Titanium silicates Titanium-silicon alloy Titanium slag Titanium suboxides... 

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 quantification of the extra-framework titanium species in titanium silicalites of MFI structure, TS-1, was performed using either XANES at the Ti K-edge or XPS Ti (2p) photolines. In addition, two different framework sites, [Ti(OH)(OSi)3] and [Ti(OSi)4], were characterized in dehydrated samples using Diffuse Reflectance UV-visible, multiple scattering analysis of EXAFS, H and Si NMR spectroscopies. 

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. 

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. 

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. 

Recently, based on such an oxidative system, the synthesis of nitrone (12) an inhibitor of 5a-reductase has been carried out (Scheme 2.7) (50). Oxidation of amines with H2O2 can be catalyzed with peroxotungstophosphate (PCWP) (51), Se02 (52-54), and titanium silicalite molecular sieves TS-1 and TS-2 (55, 56). 

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. 

The titanium-silicalite composihon, TS-1, has achieved commercializahon in selective oxidation processes and iron-siUcalite in ethylbenzene synthesis. 

Oxidations of various organic substrates with aqueous hydrogen peroxide have been reported on titanium containing derivatives of silicalite-1, denoted as Titanium-Silicalite-1 or TS-1 [93-97]. Examples of reactions which are catalyzed by TS-1 with high H2O2 yields and product selectivities are listed in Table 6. The oxidations are generally carried out at atmospheric pressure and at temperatures ranging from 273 to 373 K. 

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. 

In the field of heterogeneous catalysis using H2O2 as oxygen source, examples of the use of titanium-silicalite (TS-1) or Ti-beta in the oxidation of selected alcohols, with formation of a Ti-peroxo species, have been reported... 

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