Titanosilicate surface structures

Although the identification of tetrahedrally coordinated, tetra- and tripodal Ti4+ ions on the surface of titanosilicates, as the likely active sites in reactions that require Lewis acidity, seems convincing, the structure and role of the sites active in catalytic oxidation, presumably oxo-titanium species, formed by the interaction of H202 (or H2 + 02) with these surface Ti ions, are not clear. In recent years, this problem has been investigated by FTIR (133), Raman (39,40), XANES (46-48), electronic (54-57), and EPR (51-54) spectroscopies. This is one of the areas in which major progress has been made since the reviews of Notari (33) and Vayssilov (34). Zecchina et al. (153) recently summarized some of the salient features of this progress. 

Significant progress has been achieved in the preceding few years in the study of titanosilicate molecular sieves, especially TS-1, TS-2, Ti-beta, and Ti-MCM-41. In the dehydrated, pristine state most of the Ti4+ ions on the surfaces of these materials are tetrahedrally coordinated, being present in either one of two structures a tetrapodal (Ti(OSi)4) or a tripodal (Ti(OSi)3OH) structure. The former predominates in TS-1, TS-2, and Ti-beta, and the latter is prominent in Ti-MCM-41. The Ti ions are coordinatively unsaturated and act as Lewis acid sites that coordinatively bind molecules such as H20, NH3, CH3CN, and H202. Upon interaction with H202 or H2 + 02, the Ti ions form titanium oxo species. Spectroscopic techniques have been used to identify side-bound hydroperoxo species such as Ti(02H) and superoxo structures such as Ti(02 ) on these catalysts. 

Several hundreds of synthetic zeolites (crystalline mieroporous aluminosilieates) and zeotypes (crystalline mieroporous ferrisilicates, gallosilicates, titanosilicates, isomorphously substituted aluminophosphates, etc.) have been sueeessfully synthesized in recent decades. All these mieroporous materials have tetrahedral coordination of their eentral atoms (Si, Al, P, Fe, etc.), which are interconnected with four oxygen bridges to form a three-dimensional crystal structure. These structures exhibit regular micropores with dimensions up to 1.0 nm and cavities, high surface areas, and adsorption capacities, and shape selectivity toward reactants, products, and transition states.. 

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