Titanium atomic properties

Heterogeneous Catalysis. The main discovery of the 1980s was the use of titanium sihcaUte (TS-1) a synthetic zeoHte from the ZSM family containing no aluminum and where some titanium atoms replace siUcon atoms in the crystalline system (Ti/Si = 5%) (33). This zeoHte can be obtained by the hydrolysis of a siUcate and an alkyl titanate in the presence of quaternary ammonium hydroxide followed by heating to 170°C. Mainly studies have been devoted to the stmcture of TS-1 and its behavior toward H2O2 (34). The oxidation properties of the couple H2O2/TS-I have been extensively developed in... 

The naturally occurring dioxide exists in three crystal forms anatase, rutile and brookite. While rutile, the most common form, has an octahedral structure. Anatase and brookite have very distorted octahedra of oxygen atoms surrounding each titanium atom. In such distorted octahedral structures, two oxygen atoms are relatively closer to titanium than the other four oxygen atoms. Anatase is more stable than the rutile form by about 8 to 12 kJ/mol (Cotton, F.A., Wilkinson, G., Murillo, C.A and M Bochmann. 1999. Advanced Inorganic Chemistry, 6 ed, p. 697, New York John Wiley Sons) Other physical properties are density 4.23g/cm3 Mohs hardness 5.8 g/cm ( anatase and brookite) and 6.2 g/cm ( rutile) index of refraction 2.488 (anatase), 2.583 (brookite) and 2.609 (rutile) melts at 1,843°C insoluble in water and dilute acids soluble in concentrated acids. 

In the following, we discuss our results on the characterization and photocatalytic properties of titania-halogenometal complexes of Pt(IV) and Rh(III). Considering that the surface of titania contains about 3-6 OH groups per nm and imder-coordi-nated titanium atoms it may act both as a mono- and bidentate ligand (Fig. la-c) and as a coordination centre (Fig. Id-f). Research on the latter topic was recently summarized 14). 

For instance, the Til atom has a three-fold co-ordination by oxygen on the facet ridges, as opposed to bulk co-ordination. In the final model, a surface octahedral interstitial site of the O lattice was found to be occupied by a surface titanium atom, with 40% occupancy per (1x3) cell. Partial occupancies of 60% were also found for the 01 and Ti5 surface atoms. The resulting stoiehiometry for this surface structure is TiOi.es, which is equivalent to a 15.5% oxygen deficiency on the surface relative to the bulk. Hence, the refined model can be described as the formation of strongly distorted 110 micro facets on the surface with oxygen defects and a partial occupancy of an interstitial site. Relaxations are found down to 9 A below the topmost layer. The different coordinations found for Ti might explain part of the photo-catalytic properties of this surface. 

Heterogeneous catalysts which are active for the catalysis of the MPVO reactions include amorphous metal oxides and zeolites. Their activity is related to their surface basicity or Lewis acidity. Zeolites are only recently being developed as catalysts in the MPVO reactions. Their potential is related to the possibility of shape-selectivity as illustrated by an example showing absolute stereoselectivity as a result of restricted transition-state selectivity. In case of alkali or alkaline earth exchanged zeolites with a high aluminium content (X-type) the catalytic activity is most likely related to basic properties. For zeolite BEA (Si/Al=12), however, the dynamic character of those aluminium atoms which are only partially connected to the framework appear to play a role in the catalytic activity. Similarly, the Lewis acid character of the titanium atoms in aluminium free [Ti]-BEA explains its activity in the MPVO reactions. 

The synthesis of titanium silicalites TS-1 [10] and TS-2 [84-85], with MFI and MEL structure respectively, opened new opportunities in the oxidations with H O. TS-1 and TS-2, the former being the most studied, show similar properties in catalysis. Catalytic sites are isolated titanium atoms, incorporated into the zeolitic framework, in a channel system of about 0.55 nm average diameter [86-88], Different Ti-peroxo species, at lattice position, result from complex equilibria between TS-1, H O, and protic molecules [89-91],... 

Some experiments regarding the reactivity of complexes 451, 453, and 454 were performed with respect to their catalytic properties. It was found that 451 catalyzes the dimerization of isoprene. When 0.001 equiv of 451 was used, the chemical yield was 90% with a ratio of cyclic (456 + 457) to acyclic (458) dimers of 0.94 1 and 450 turnovers per titanium atom (Scheme 84). When 0.002 equiv of 451 was used, the chemical yield was 84% with a ratio of 0.83 1 and 840 turnovers per titanium atom. ... 

Ten years later in 1879, Swedish chemist Lars Nilson discovered the missing element with properties in between calcium (atomic weight of 40) and titanium (atomic weight of 48) and named it scandium (Sc). Scandium has an atomic weight of 45. 

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