Ti-O-Si bonding

These spectral differences are related to different Ti-O-Si bond angle of the Ti sites. Indeed, an angle opening will shift the bridging oxygen hybridization from sp3 to sp2 and... 

These structures should, in principle, show LMCT transitions at two different positions. Except for TS-1, data representing these angles for other titanosilicates are not available. Such data would be useful in determining the influence of the Ti-O-Si angle on the ease of hydrolysis of the Ti-O-Si bond, which is crucially important for the stability and, hence, utility of the material in catalytic applications. 

The increase in coordination around Tiiv could take place by the simple addition of H2O to Tilvor as a result of hydrolysis of Ti—O—Si bonds and formation of TiOH groups, followed by the addition of H20 to the TiOH groups. Ti02 exhibits little tendency to form surface hydroxyl groups, but could be forced to do so by the unusual condition in which titanium is present in titanium silicates. The steric hindrance of the crystalline lattice prevents Tilv from acquiring the same regular octahedral coordination that it assumes in many of its compounds. A shorthand notation of H20 on Tilv will be used to indicate the increased coordination ... 

The maximum content of titanium in Ti-beta zeolite appears to be higher than in the other materials. A value of x = 0.038 has been reported without formation of extra-framework Ti02. From the characterization of Ti-beta zeolite by XANES and EXAFS, it has been concluded that Tilv in the calcined material is tetrahedrally coordinated, isolated from other TiIV ions, and surrounded by OSi groups. In the presence of H20, Tilv increases its coordination and very likely undergoes hydrolysis of the Ti—O—Si bonds forming TiOH and SiOH groups (Blasco et al., 1993). 

In any case, they suggested, one of the Ti-O-Si bonds of the bridged Si-O-Ti-O-Si species reacts with AlMe3 and a Si-Me group is formed. 

Pbres of anodic alumina have been used for the growth of coaxial nanotubes of Ti02 sheathed SiO 92 Electrochemical deposition in the pores of a polymeric alumina membrane generates arrays of Ti02 nanotubes. R>r the coaxial nanotubes, the SiOj nanotubes are first grown inside the pores of the anodic alumina membrane The Ti02 nanotubes are then grown inside the SiOj nanotubes. The existence of Ti-O-Si bonds in the amorphous sheaths is believed to play a role in the formation of these composite nanotubes... 

In the case of titanium, a number of earlier [2] and more recent [3] studies revealed that Ti-O-Si bonds can be formed using alkoxysilanes [2, 3], chlorosilanes [4], or hydrosilanes [5]. In the latter case, Ti-O-Si bonds resulted from the reaction of surface Ti-OH groups with the reagent s Si-H groups accompanied by molecular hydrogen production. This type of reaction was also shown to be operative between hydrosilanes and Si-OH-containing surfaces [6]. 

Ti centers are likely to be tripodally held to Si-O as Ti-OH species in order to be sterically accessible [440]. This tripodal Ti site can be readily obtained by hydrolysis of a Ti-O-Si bond in a tetrapodal Ti site, and evidence for this occurrence has been presented by Sinclair et al. [441], Gold particles are found in the vicinity of these Ti centers due to its affinity for oxidized Ti [442], although not necessarily bonded directly to the Ti [118]. 

We have reported that properties of titania/silicas depend on their preparation methods and a complexing-agent assisted sol-gel method gives the most homogeneous titania/silicas [4]. In the sol-gel titania/silicas, Ti-O-Si bonds are more effectively formed and Si and Ti components are higher dispersed than those in conventional titania/silicas [4]. Therefore, it is expected that the sol-gel titania/silicas are also effective catalysts for oxidation reactions. 

Bellussi et al. demonstrated that hydrolysis of Ti-O-Si bond takes place with the formation of Ti-OH and Si-OH groups [8]. This means that the structure of active site, that is, isolated Ti atoms by long chains of -O-Si-O-Si-0-, which gives high selectivity for the formation of epoxides is destroyed by water. The decomposition of active site must occur in the initial stage. Therefore, the conversion in presence of hydrogen peroxide slightly increase with reaction time. 

---