Si—O—Ti bond

Figure 1 shows the FT-1R spectra of samples dispersed in KBr. All the spectra display a strong band at 960 cm 1. This band has been assigned to Si-O-Ti bonds [14] or to Si-OH groups [15, 16]. It is usually taken as the evidence for isomorphous substitution of Si by Ti, but it cannot be used to determine quantitatively the content of titanium into the framework of mesoporous materials [17]. In addition, the broad pattern between 3700 and 3000 cm 1, originated from hydrogen-bonded surface OH groups as well as from adsorbed H20 [18], decreases dramatically in the silylated samples. 

A method was developed to increase the formation of Si—O—Ti bonds it involves the partial hydrolysis of TEOS followed by the addition of the... 

Amorphous Ti/SiCL oxides and crystalline Ti zeolites are two classes of well-studied solid Ti catalysts (11-14). In both classes, a Lewis-acidic Ti atom is anchored to the surrounding siliceous matrix by Si-O-Ti bonds. The oxidant of choice for Ti zeolites such as titanium silicalite 1 (TS-1) and 11-/1 is H2O2, whereas the amorphous, silica-based materials function optimally with organic peroxides such as /-butyl hydroperoxide (/-BuOOH) or ethyl benzene hydroperoxide. However, there are strictly no homogeneous analogues of these materials, and they therefore do not fit within the context of anchoring of homogeneous catalysts. 

The vanadium silicalites (with MFI and MEL stmcture) are active oxidation catalyst in gas and liquid phase reactions [180]. As for the titanium silicalites, only the ftamework associated vandium exhibits redox properties [181]. For example, in the hydroxylation of phenol, silicalite impregnated with vanadium compounds is catalytically inactive [182]. The catalytically active vanadium species is speculated to be located in non-tetrahedral positions, most probably chemically bound to the framework. Vanadium bound in that way is not extractable from the lattice [ 183]. A proposed stmcture of the vanadium site is schematically shown in Scheme 21. Note that the Si-O-V bonds are longer than the Si-O-Ti bonds and that V seems to be more exposed. The redox properties are affiliated with the changes in the oxidation state of vanadium between +IV and +V. Vanadium silicates with SiA ratios ranging from 40 to 160 have been reported and these high values suggest (in accordance with V MAS-NMR measurements) that the V sites are isolated in the lattice. 

Cbncezning the cause of the detitanaiton it can be noted that it involves the breakage of Si-O Ti bonds, and like the dissolution, it requires the breakage of Si-O-Si bonds. Thus, the basicity of the reaction medium must be involved in the extraction of Ti from the framework. In fact an appropriate treatment of the fresh catalyst with an aqueous solution of ammonia causes detitanation as well as the reaction environment (Fig. 2). 

Isolated highly dispersed titanium oxide species on silica as photocatal5dic active sites for non-oxidative direct methane coupling, were successfiilly prepared by sol-gel method, in which hydrolysis of TEOS to form Si(OH)4 would be completed, followed by condensation to form Si-O-Ti bonds. It was found that both hydrolysis and condensation time much affected the formation of the isolated highly dispersed species. The sample prepared by the present sol-gel method showed higher activity than those prepared by conventional impregnation method. 

In order to examine the effect of hydrolysis time for the formation of Si-O-Ti bonds, we prepared the TS(x,y) samples with various hydrolysis and condensation time. It is expected in the first step in scheme 1 that the hydrolysis of TEOS would mainly proceed as follows. 

The important reaction in the second step would be the condensation reaction to form Si-O-Ti bonds. 

On the other hand, the shorter the time for the second step, the smaller the opportunity of the Si-O-Ti bond formation, leading to the aggregation of titanium oxide species during the calcination procedure. Thus, the optimum time to obtain highly dispersed titanium oxide species for each step was determined to be 30 minutes. 

FIGURE 2.10 (a) Relative amounts of Si-O-Ti bonds as a function of the total content of titania in ST samples estimated from the spectra recorded in (1) reflectance and (2) transmittance (recorded at 293 K) modes and (b) relative amounts of silanols as a function of the total content of alumina in SA or titania in ST samples from the diffuse reflectance spectra (all diffuse reflectance spectra were recorded at 573 K). (Adapted from /. Colloid Interface ScL, 314, Gun ko, V.M., Blitz, J.R, Gude, K. et al.. Surface structure and properties of mixed fumed oxides, 119-130, 2007a, Copyright 2007, with permission from Elsevier.)... 

Although the 950 cm" bands are rather broad and overlap with other silica bands at higher wavenumber, after the referred calcination treatment th can be considered as a fingerprint for the formation of Si-O-Ti bonds [22]. It is apparent from Figure 3 that the XCIO sample presents the highest absorbance and the AS 10 the lowest one. The intensity decreases from the xerogels to the aerogels and die use of ultrasound in our preparations does not seem to be helpful to get better dispersion of Ti atoms into the silica network. 

In the particular case of Ti02-Si02 gels, another problem to consider is the reactivity of Si-O-Ti bonds toward water. TiOj-SiOj homogeneous materials become inhomogeneous after a few hours in contact with a water vapor pressure of 0.9 bar at 380 K [21]. After such... 

Crouzet L., Leclercq D., Mutin P.H., Vioux A. Organosilsesquioxane-titanium oxide hybrids by non-hydrolytic sol-gel processes, study of the rearrangement of Si-O-Ti bonds. Chem. Mater. 2003 15 1530-1534... 

The incorporation of other elements, such as A1 or P (Fig. 4-13), also induces significant changes at the structural level the Si-O-Ti bonds are replaced with Si-O-Al or Si-O-P upon A1 or P co-doping, respectively (Almeida et al., 1999). Thus, the peak near 940 cm , due to Si-O-Ti bonding sequences, shows a decrease in intensity, due to the decrease in the concentration of such bonds, when AI2O3 or P2O5 are added the Si-O-Al and Si-O-P vibrational peak is probably hidden under the main Si-O-Si stretch at 1070 cm . ... 

In a typical hydrolytic xerogel synthesis, the silicon and titanium alkoxide precursors are mixed with the solvent in the presence of an acid or base catalyst under vigorous stirring at ambient condition. Typically applied solvents are alcohols, such as ethanol or isopropanol, due to their ability to effectively homogenize the metal precursors. Since Si(OR)4 is by far less reactive toward hydrolysis and condensation reactions than Ti(OR)4, phase separation is expected, as already discussed [23,42]. As one possibility to reduce the difference in reactivity of the various alkoxides, Yoldas proposed to prehydrolyze the less reactive silicon alkoxide prior to the addition of the titanium alkoxide [26]. Keeping in mind that the water/alkoxide ratio is the key parameter, which is typically kept below 2 to increase the time for gelation, thus favoring Si—O—Ti bond formation, this route is pursued in many of the reported studies [27,45]. 

Several techniques can be used to characterize the presence of Si-O-Ti bonds in the dried gels Infrared or Raman spectroscopies (20), Si Magic Angle Spinning (MAS) NMR (27,22), X-ray absorption (23) and Photoelectron spectroscopy (24). 

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