Titanium framework structures

New materials are also finding application in the area of catalysis reiated to the Chemicals industry. For example, microporous [10] materials which have titanium incorporated into the framework structure (e.g. so-calied TS-1) show selective oxidation behaviour with aqueous hydrogen peroxide as oxidizing agent (Figure 5). Two processes based on these new catalytic materials have now been developed and commercialized by ENl. These include the selective oxidation of phenol to catechol and hydroquinone and the ammoxidation of cyclohexanone to e-caproiactam. 

Wang Y, Tang X, Lin L, Huang W, Haochen Y, Gedanken A (2000) Sonochemical synthesis of mesoporous titanium oxide with wormhole-like framework structures. Adv Mater 12 1183-1186... 

Titanium silicalite-1 (TS-1), first synthesized in 1983, is well known for its outstanding ability to catalyze various oxidation and hydroxylation reactions. This catalytic activity is ascribed to the presence of Ti atoms in the zeolite. Knowledge of the effect of the Ti atoms on the framework structure and of the location of the Ti atoms in the zeolite would be useful in understanding the catalytic properties of TS-1. Although TS-1 has been characterized extensively, the location of the Ti atoms in the zeolite is still under discussion. The maximum amount of framework Ti has been reported to be 2.5 Ti atoms per... 

Y.Q. Wang, X.H. Tang, L.X. Yin, W.P. Huang, Y.R. Hacohen, and A. Gedanken, Sono-chemical Synthesis of Mesoporous Titanium Oxide with Wormhole-like Framework Structures. Adv. Mater, 2000, 12, 1183-1186. 

P. F. Henry, M. T. Weller, C. C. Wilson, Structural investigation of TS-1. Determination of the true nonrandom titanium framework substitution and silicon vacancy distribution from powder neutron diffraction studies using isotopes, /. Phys. Chem. B 105 (2001) 7452. 

The deeper oxidation of ethylbenzene over TS-2 can be explained with the slower diffusion of 1-phenylethanol and aeetophenone formed in the zeolite pores where they could undergo additional oxidation to aeetophenone or other products, respectively. Another possible reason could be some differences in the local geometry of the titanium sites due to the different framework structure of the two titanium silicalites. 

A family of titanium-containing molecular sieves with pentasil-type framework structures is represented by titanivun-silicalite-3 (TS-3) [74], characterized by a framework topology similar to that of TS-2 but with a different degree of stacking faults. In fact, as found by Perego et al. for the boron-substituted MFI/MEL molecular sieves, the frequency of stacking faults may be controlled by choosing the appropriate pair of tetraalkylammonium ions (e.g., TMAOH/TPAOH, TMAOH/TBAOH, TEAOH/TBAOH) [72]. 

One example of anodes alternative to graphite is lithium titanium oxide, Li4Ti50i2 (LTO). This material has a defective spinel-framework structure... 

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. 

Further support for the direct relationship of the 960 cm-1 band to the presence of 4-coordinated Ti atoms in the framework of TS-1 came from the photoluminescence investigations of Soult et al. (94). At 12 K, an emission band was observed at 490 nm, which was unequivocally attributed to titanium (Section II.A.4). This band showed a resolved vibrational structure of 966 24 cm-1, which clearly demonstrates that Ti is involved in the corresponding vibrational mode. 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

Following the discovery of TS-1 [125], a titanium-substituted MFl, the use of zeolitic materials for oxidation increased significantly. The presence of the Ti atom in the framework of a zeolite structure provides a site-isolated Ti center, a situation not possible with other Ti-containing materials while also allowing shape-selective oxidations. The combination of the two effects gives highly active and selective oxidation reactions [126]. 

The molecular structure of 124 has been elucidated by an X-ray structural analysis. The central structural motif of 124 is an unsymmetrically substituted six-membered TisOs ring. Two pentamethylcyclopentadienyl ligands are coordinated to one titanium atom, while the other two are free of Cp. They are both part of eight-membered TiSi304 ring systems within the silsesquioxane frameworks. This results in an unusual bis(spirocyclic) inorganic ring system in the molecular structure of 124. 

However, it became evident in the post-war period that, valuable as they were, these band-structure concepts could not be applied even qualitatively to key systems of industrial interest notably steels, nickel-base alloys, and other emerging materials such as titanium and uranium alloys. This led to a resurgence of interest in a more general thermodynamic approach both in Europe (Meijering 1948, Hillert 1953, Lumsden 1952, Andrews 1956, Svechnikov and Lesnik 1956, Meijering 1957) and in the USA (Kaufman and Cohen 1956, Weiss and Tauer 1956, Kaufman and Cohen 1958, Betterton 1958). Initially much of the work related only to relatively simple binary or ternary systems and calculations were performed largely by individuals, each with their own methodology, and there was no attempt to produce a co-ordinated framework. 

Transition metals and their complexes can be immobilized in the mesopores or incorporated in the structure to make silica-supported metal catalysts. For instance, titanium catalysts for selective oxidation can be formed by modifying the mesoporous structure with either Ti grafted on the surface (Tif MCM-41) or Ti substituted into the framework (Ti->MCM-41). The grafted version makes the better catalyst for the epoxidation of alkenes using peroxides, and has good resistance to leaching of the metal. 

The isomorphous substitution of Siiv by Ti,v was claimed by Taramasso, Perego, and Notari in 1983 for a new material with the composition xTi02(l - x)Si02 (0.0 x 0.04 M). This has the crystalline structure of silicalite-1 (or MF1) with Tilv in framework positions it was named titanium silicalite-1 or TS-1 (Taramasso el al., 1983). The occurrence of isomorphous substitution was deduced from the regular increase in unit-cell parameters with the degree of substitution and from the good agreement between the observed and calculated values of the Si—O and Ti—O distances. The same type of evidence had already been obtained by the same authors in the synthesis of crystalline microporous boron silicates, where the smaller B—O distance relative to Si—O causes a decrease in unit-cell parameters (Taramasso et al., 1980). 

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