Titanium peroxo species

A titanium complex (1) with a salen ligand is an efficient catalyst for the enan-tioselective epoxidation of alkenes with hydrogen peroxide as the terminal oxidant. The participation of a titanium-peroxo species, activated by hydrogen bonding, in the reaction, has been postulated.73... 

The pre-edge intensity very often increases only upon removal of any adsorbed water vapor [64]. The water vapor acts as a ligand, which changes the intensity of the pre-edge peak. This is crucial for the understanding of how the oxidation catalysis works, since TS-1 is mostly used in aqueous solutions with hydrogen peroxide. It has been suggested that EXAFS can show the presence of titanium peroxo species on TS-1 [65]. 

The ammoxidation of cyclohexanone to cyclohexanone oxime is catalyzed by TS-1 with 98.2% selectivity to cyclohexanone oxime at 99.9 % conversion [177]. Selective oxidation of the nitrogen of ammonia by hydrogen peroxide is a key step of this reaction. The mechanism is still vividly debated and three possible routes are shown in Scheme 20. Recent evidence [163] seems to support a route via intermediate formation of hydroxylamine [mechanism B]. The high selectivity on the other hand supports the postulate that the reaction proceeds via a concerted reaction step that involves the titanium peroxo species, ammonia and cyclohexanone (mechanism C) [177],... 

Vibrational frequencies of some titanium peroxo complexes and of solids containing peroxo and/or superoxo species are summarized in Table III. The three infrared vibrations of the triangular peroxo group in the Qv structure... 

The nature of the titanium-containing active site has been investigated with different techniques, including theoretical calculations. The formation of a hydroperoxidic species or of a bidentate side-on titanium peroxo structure was suggested by many authors . Alternatively, some DFT calculations indicated an undissociated molecule of H2O2 weakly interacting with Ti centers or an active Ti-O-O-Si peroxo moiety as a reactive site . Recently, Lin and Frei reported the first direct detection, obtained using in situ FT-infrared spectroscopy, of a Ti-OOH moiety as active species in the oxidation of small olefins like ethylene or propylene . 

In the field of heterogeneous catalysis using H2O2 as oxygen source, examples of the use of titanium-silicalite (TS-1) or Ti-beta in the oxidation of selected alcohols, with formation of a Ti-peroxo species, have been reported... 

Titanium(III) octaethylporphyrin (OEP) has been found to react with dioxygen at low temperatures to give a titanium(IV) /i-peroxo species. Characterization was based on a comparison with the reaction product from H202 oxidation of (OEP)TiO. This compound was shown to have a U-peroxo group by a single crystal X-ray study, O—O = 1.445 A, v0 o = 895 cm-1.13... 

The initial coordination of reactants has indeed been proposed to explain the selective oxidation of alkenes in the presence of saturated hydrocarbons. It was argued that, owing to the hydrophobic nature of titanium silicates, the concentration of both hydrocarbons inside the catalyst pores is relatively high and hence the alkenes must coordinate to TiIv. Consequently, the titanium peroxo complex will be formed almost exclusively on Tilv centers that already have an alkene in their coordination sphere, and will therefore oxidize this alkene rather than an alkane which may be present in the catalyst (Huybrechts et al., 1992). Objections to this proposal are based on the fact that the intrinsically higher reactivity of alkenes with respect to saturated hydrocarbons is sufficient to account for the selectivity observed (Clerici et al., 1992). But coordination around the titanium center of an alcohol molecule, particularly methanol, is nevertheless proposed to explain the formation of acidic species, as was previously discussed. In summary, coordination around Tiiv could play a more important role than it does in solution chemistry as a consequence of the hydrophobicity of the environment where the reactions take place. 

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... 

The essential point arising from the above discussion is that the saturation coverage of titanium dioxide with the surface-bonded peroxo species, photogenerated in alkaline or neutral solutions, ranges most likely from 4 to 5 peroxo groups per nm . The latter value is close to the total number of active OH groups present initially on each nm of fully hydroxylated (and unilluminated) anatase surface. 

The mechanism proposed involves adsorption of propene onto gold, and the reaction of the adsorbed species with oxygen species (hydroperoxo and peroxo species) formed at the interface between the gold particles and the titanium support, through the reductive activation of oxygen with hydrogen [36j]. Scheme 6.7 shows the reaction mechanism proposed in the literature [37b,g,h]. 

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