Titanium-Oxo Species and Activity

If the tetra- and tripodal Ti structures and the titanium oxo species derived from these structures in the presence of ROOH (R = H, alkyl) are involved as active sites and reaction intermediates, the next step beyond their identification is to seek correlations between the structure and concentrations of these titanium oxo species and catalytic activity and selectivity. Clerici and Ingallina (204) were the first to propose the Ti(02H) group as the active site of alkene epoxidation by... 

A direct correlation between the concentration of the titanium oxo species and epoxidation activity was proposed by Lin and Frei (133). Loading TS-1/H202 with propene after evacuation, they observed by FTIR difference spectroscopy the loss of the bands characterizing propene (at 1646 cm-1) and TiOOH (at 837 and 3400 cm-1). Figure 48 is the infrared difference spectrum recorded immediately after loading the propene on TS-1/H202 Fig. 49 includes the spectra recorded 80 and 320 min later. 

While the mechanism for selective oxidation of an alkene using hydrogen peroxide, over these titanium silicates is not fully understood, it is believed that tetrahedral titanium sites atomically dispersed in a silica matrix are necessary. Furthermore, a titanium-oxo species has been proposed as a possible active species by several groups through EPR and UV-Vis spectroscopies, see Figure 4.27 ... 

Titanium dioxide, titania, is now extensively studied for its superior characteristics as a photo-active semiconductor. Of course, Ti02 films can be fabricated through alcoholic sol-gel method, the studies are also carried out on the use of aqueous solvents and the low temperature synthesis. Since the titanium ion, Ti +, is small (Shannon, 1976) and has a high valence of foiu, its charge density is very high. Consequently, water and titanium ions easily react to form titanium oxo species, which causes precipitation of hydrated Ti02. In order to avoid the formation of the precipitate, the solutions are usually prepared under highly acidic conditions. 

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. 

D. Srinivas, P. Manikandan, S. C. Laha, R. Kumar, and P Ratnasamy, Reactive oxo-titanium species in titanosilicate molecular sieves EPR investigations and structure—Activity correlations, J. Catal. 217, 160-171 (2003). 

The catalytic oxidation of hydrocarbons with peroxides, especially the epoxidation of olefins, in liquid phase by titanium catalysts is one of the most actively investigated reactions (60). The active species for this epoxidation reaction is usually assumed to be titanium peroxo moieties, derived from four-coordinate titanium and peroxides. However, the isolation of the active intermediate remains a challenge owing to the inherent instability of such species. We have been able to synthesize and stabilize the related cubic p-oxo-silicon-titanium complex (35) by reacting a bulky... 

As with most of the in situ generated catalysts the structure of the catalytically active species is not rigorously known but Maruoka and co-workers did observe a peak for the p-oxo titanium dimer 71 in ESI-MS. They also demonstrate a positive nonlinear effect in the enantioselectivity when the catalyst was generated from enriched BINOL as opposed to enantiopure BESfOL. This non linear effect reinforces the hypothesis that the active catalyst contains two molecules of BINOL. 

Katsuki and Saito reported that di-p-oxo titanium complexes of chiral salen ligands serve as efficient catalysts for asymmetric oxidation of a range of sulfides using H 2O2 or UHP as terminal oxidants [149]. Enantioselectivities as high as 94% were achieved [149]. A monomeric peroxo titanium species was proposed as the active oxidant based on MS and NMR studies [150]. 

Inspired by this finding, Katuski further developed a series of structurally tunable di- x-oxo titanium-salan complexes that can be easily prepared and possess amino protons to activate a putative peroxide species. The optimized catalyst (5 mol%) works well for the epoxidation of a variety of unfunctionalized olefins, giving the corresponding epoxides in moderate to good yields with 55-97% ee [262]. The ready availability of the salan ligand and aqueous H2O2 oxidant makes the process very practical for the production of optically active epoxides. 

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