Titanium silicates

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

Titanium Silicates. A number of titanium siUcate minerals are known (160) examples are Hsted in Table 19. In most cases, it is convenient to classify these on the basis of the connectivity of the SiO building blocks, eg, isolated tetrahedra, chains, and rings, that are typical of siUcates in general. In some cases, the SiO units may be replaced, even if only to a limited extent by TiO. For example, up to 6% of the SiO in the garnet schorlomite can be replaced by TiO. In general, replacement of SiO by TiO bull ding blocks increases the refractive indices of these minerals. Ti has also replaced Si in the framework of various zeofltes. In addition, the catalytic activity of both titanium-substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) has received attention (161), eg, the selective oxidation of phenol, with hydrogen peroxide, to hydroquinone and catechol over TS-1 has been operated at the 10,000 t/yr scale in Italy (162). 

TS-1 (titanium silicate molecular sieve), H2O2, MeOH, reflux, 4-18 h, 60-64% yield. ... 

Miyamoto, M. Minami, Y Ukaji, Y Kinoshita, H. Inomata, K. Chem. Lett., 1994,1149. For an example using titanium silicate/H202, see Sasidharan, M. Suresh, S. Sudalai, A. Tetrahedron Lett., 1995, 36, 9071. 

Enichem made one of the most important steps forward in the development of general heterogeneous oxidation catalysts in the early 1990s with the commercialization of titanium silicate (TS-1) catalysts. TS-1 has a structure similar to ZSM-5 in which the aluminium has been replaced by titanium it is prepared by reaction of tetraethylorthosilicate and tetra-ethylorthotitanate in the presence of an organic base such as tetrapropy-lammonium hydroxide. This catalyst is especially useful for oxidation reactions using hydrogen peroxide (Scheme 4.11), from which the only byproduct is water, clean production of hydroquinone being one of the possibilities. 

After the submission of this contribution, a remarkable review authored by Ratnasamy, Srinivas and Knozinger has appeared on the investigation, by means of both experimental and theoretical methods, of the active sites and reactive intermediates in titanium silicate molecular sieves [126]. 

In earlier work, Bhaumik and Kumar (1995) have reported that the use of two liquid phases in the oxidation of hydrophobic organic substances with aqueous H2O2 using titanium silicate as the catalyst not only enhances the rate of oxidation but also improves selectivity for species like toluene, anisole, and benzyl alcohol. For a single liquid phase acetonitrile was u.sed a solvent. The solid-liquid system gives high ortho selectivity. Thus, in the case of anisole the ratios of o to p for. solid-liquid and solid-liquid-liquid system were 2.22 1 and 0.35 1, respectively. 

Active Sites and Reactive Intermediates in Titanium Silicate Molecular Sieves... 

Appendix C. Synthesis of Titanium Silicate Molecular Sieves. 143... 

TS-1 and TS-2 microporous titanium silicate molecular sieves with MFI... 

Ti-beta (Ti-(3) large-pore titanium silicate with BEA structure... 

ETS-4 and ETS-10 Engelhard Corporation titanium silicate molecular sieves... 

The main objective of this review is to summarize and critically analyze recent advances made in the characterization and catalytic properties of titanium silicate molecular sieves after the reviews of Notari (33) and Vayssilov (34) in 1996 and 1997, respectively. Of special interest are... 

Formation of HO radicals by decomposition of H2O2 on contact with titanium silicates increases with temperature. At 77 K, this decomposition is less probable. 

V.C. Epoxidation on Titanium Silicate Molecular Sieves V.C.l. General Features of Epoxidations... 

Titanium silicate molecular sieves not only catalyze the oxidation of C=C double bonds but can be successfully employed for the oxidative cleavage of carbon-nitrogen double bonds as well. Tosylhydrazones and imines are oxidized to their corresponding carbonyl compounds (243) (Scheme 19). Similarly, oximes can be cleaved to their corresponding carbonyl compounds (165). The conversion of cyclic dienes into hydroxyl ketones or lactones is a novel reaction reported by Kumar et al. (165) (Scheme 20). Thus, when cyclopentadienes, 1,3-cyclohexadiene, or furan is treated with aqueous H202 in acetone at reflux temperatures for 6 h in the presence of TS-1, the corresponding hydroxyl ketone or lactone is obtained in moderate to good yields (208). 

Acid catalysis by titanium silicate molecular sieves another area characterized by recent major progress. Whereas only two categories of acid-catalyzed reactions (the Beckmann rearrangement and MTBE synthesis) were included in the review by Notari in 1996 (33), the list has grown significantly since then. In view of the presence of weak Lewis acid sites on the surfaces of these catalysts, they can be used for reactions that require such weak acidity. 

Formation of pinacol over various titanium-silicates... 

The review of Notari (33) covers the synthesis methodologies of titanium silicate molecular sieves available up to 1996. The reviews of Corma (279) and subsequently of Biz and Occelli (280) describe the synthesis of mesoporous molecular sieves. An informative article on the preparation of TS-1 was reported recently by Perego et al. (68). In this section we list some of the recent developments in the synthesis of micro and mesoporous titanosilicate molecular sieves. 

Compositions of the synthesis gel and other physical characteristics of titanium silicate materials obtained in various synthesis methodologies are listed in Table Cl. 

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