Titanosilicate

That observed for TS-1 is not peculiar for TS-1 only and can be observed on other titanosilicates like Ti-MSA, a mesoporous amorphous material that has Ti(Vl) centers exposed on the surface of the pores [124,125]. In this case, easier experiments could be performed by Prestipino et al. [50] as the peroxo/hydroperoxo complexes can be formed by dosing f-butyl hydroperoxide (which does not enter the 10-membered rings of TS-1). The XANES spectrum of Ti-MSA in vacuum is typical for almost r -like Ti(IV) centers (the intensity of the Ai T2 pre-edge peak being only 0.69, as compared with 0.91 for TS-1). Upon dosing the t-butyl hydroperoxide in decane solution on Ti-MSA, a spectrum similar to that obtained on TS-1 contacted with anhydrous H2O2 is observed on both XANES and EXAFS regions [50]. When the... 

Summarizing, the in situ UV-Vis, XANES, and EXAFS studies of Bonino et al. [49] and of Prestipino et al. [50] on hydrated and anhydrous peroxo/hy-droperoxo complexes on crystalhne microporous and amorphous meso-porous titanosilicates have shown, for the first time, the equilibriiun between r] side-on and end-on complexes. The amount of water is the key factor in the equilibrium displacement. In this regard please note that, owing to the hydrophobic character of TS-1, substrates such as olefins are the dominant species in the channels. This fact assures a relatively local low concentration of water, which in turn guarantees a sufficient presence of the active end-on... 

Prakash, A. M., H. M. Sung-Suh et al. (1998). Electron spin resonance evidence for isomorphous substitution of titanium into titanosilicate TiMCM-41 mesoporous molecular sieve. J. Phys. Chem. B 102 857-864. 

Rate Nitrogen from air by PSA using carbon molecular sieve Nitrogen and methane using titanosilicate ETS-4... 

Figure 8.27 Heat capacity of some glass-forming liquids close to their glass transition temperatures ZnCl2 [45], GeSe2 [46], and a selected titanosilicate [47], aluminosilicate [48] and borosilicate [49] system.

The forty-eighth volume of Advances in Catalysis includes a description of a new and increasingly well understood class of catalysts (titanosilicates), a review of transmission electron microscopy and related methods applied to catalyst characterization, and summaries of the chemistry and processes of isobutane-alkene alkylation and partial oxidation and C02 reforming of methane to synthesis gas. 

XPS (78-80) and XANES (81 —84) data indicate that in the as-synthesized and calcined state all the Ti ions in titanosilicates are in the +4 oxidation state. 

A useful fingerprint of an active TS-1 catalyst is the particle size of the titanosilicate (<0.4 pm). Although the particle size influences the catalytic activity of all molecular sieves, it is especially so in the case of TS-1 and due care should be exercised in comparing samples varying in particle size (89,90). 

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. 

Diffuse reflectance UV-visible data of titanosilicate samples... 

Electron paramagnetic resonance (EPR) spectroscopy is yet another diagnostic tool for the detection of isomorphous substitution of Ti. Its sensitivity is very high, and investigations can be performed with samples even with very low contents of paramagnetic species. The spectra and g parameters are sensitive to the local structure and associated molecular distortions. Hence, it is an ideal tool to characterize Ti in titanosilicates. Ti in the + 4 oxidation state in titanosilicates is diamagnetic and hence EPR-silent. Upon contacting with CO or H2 at elevated... 

EPR spin Hamiltonian parameters (at 77 K) ofTi3+ in titanosilicate molecular sieves generated by... 

In addition to the Ti, hydroxyl groups constitute a second class of surface functional groups on dehydrated samples that can be of importance in catalytic reactions. The presence of a large number of Si-OH groups on the surfaces of all the titanosilicates is apparent from the intense absorption in the 3200-3800 cm-1 region of the infrared spectra. The experimental evidence of surface... 

In conclusion, dehydrated TS-1 (and presumably other titanosilicates) most likely does not have Brpnsted acid centers. The observed activity for acid-catalyzed reactions that yield undesired side products is, therefore, inferred to be created under reaction conditions in the presence of aqueous H2O2 (vide infra). 

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. 

Fig. 23. EPR spectra (at 210 K) of titanosilicates interacting with aqueous H202 the gzz region at higher gain (X 5) is shown. The peaks corresponding to Af, A, and B-type Ti-superoxo species are indicated [(from Srinivas et al. (52)].

There has been an attempt to estimate the relative concentrations of the two superoxo and hydroperoxo species (54) by deconvolution into two bands of the broad UV-visible band observed on reaction of titanosilicates with aqueous... 

EPR parameters (at 77 K) for the superoxo-Ti(IV) species generated on titanosilicates by contacting with aqueous H202 (HP), urea-H202 adduct (UHP) and (H2 + 02)... 

The Ti4+ distribution in TS-1 has also been studied by computational methods (34,62,160-163). The actual location of the Ti atoms in the framework of titanosilicates is difficult to determine experimentally because of the low Ti content (Section II), and information obtained from theoretical methods is, therefore, of considerable interest. In the orthorhombic MFI structure, substitution can take place at 12 crystallographically different tetrahedral (T) sites (T1-T12) (Fig. 1 and Section II.A.l.b). In the monoclinic MFI framework, the mirror symmetry is lost and 24 crystallographically different T sites can be distinguished (Fig. 31) (160). 

The catalytic activity of the titanosilicate molecular sieves, especially those of TS-1, TS-2, Ti-beta and Ti-MCM-41 has been investigated extensively... 

Influence of titanosilicate structure on epoxidation of pentenol with H202... 

Epoxidation of alkeneic reactants is faster on titanium-grafted silicates (such as A, B and C) than on the coprecipitated titanosilicates (such as D and E). This difference was attributed to the fact that on extra-framework titanium-grafted silicates, the catalytically active sites are virtually all exposed and accessible, whereas on the coprecipitated material some of them may be buried within the silicate walls and, thus, cannot adsorb reactant molecules. 

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