Hydroperoxo

Equilibria between Peroxo and Hydroperoxo species in the TS-I/H2O2/H2O System In Situ UV-Vis DRS... 

Keywords EXAFS H2O2 Hydroperoxo complexes IR Raman Partial oxidations Peroxo complexes TitanosUicate TS-1 UV-Vis XANES... 

Unfortunately, there is no Ti-hydroperoxo compound of known structure to be used as a model. Conversely, the structure of several Ti-peroxo complexes are known by diffraction studies [111-113], all of them showing the side on r] geometry. None of these compounds is known to be active in partial oxidation reactions [114,115]. Similar considerations can be addressed... 

Scheme 2 Representation of equilibria between Ti04 framework species and H2O2/H2O solutions a formation of hydroperoxo species upon hydrolysis of a Ti - O - Si bridge b formation of hydroperoxo species toward reaction with a pre-existing defective Ti - OH species (see Sect. 3.8). Adapted from [49] with permission. Copyright (2004) by ACS...

Concerning peroxo complexes, it is worth noticing that they can be formed in TS-1 by evolution of both or rf- hydroperoxo complexes upon a further deprotonation in presence ofwater with formation of H30 /H20. Very recently Bonino et al. [49] have shown, by titration in aqueous medium with NaOH, that the acidity of the TS-1/H2O system is remarkably increased by addition of H2O2 (compare full squares with full circles in Fig. 8), a feature not observed for the Ti-free silicalite-1 system (open circles and squares in Fig. 8). 

This surprising results implies that water is not just a medium for transporting products on the catalytic sites but has an active role in determining the peroxo/hydroperoxo species present on the working catalyst. Scheme 3 summarizes the model hypothesized by Prestipino et al. [50] on the basis of the whole set of experiments reported by Bonino et al. [49] and by them selves [50], and here reviewed in Fig. 10. 

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... 

Figure 18.5 Plausible sequence of steps responsible for rapid and selective reduction of O2 to H2O by mixed-valence CcO. The square frames signify the catalytic site (Fig. 18.4c) imidazole ligation of Cub is omitted for clarity in some or aU intermediates, Cub may additionally be ligated by an exogenous ligand, such as H2O (in Cu ) or OH (in Cu ) such ligation is not established, and hence is omitted in all but compound Pm and the putative hydroperoxo intermediate. The dashed frames signify the noncatalytic redox cofactors. Typically used phenomenological names of the spectroscopically observed intermediates (compounds A, E, H, etc.) are also indicated.

An increase in the fraction of the four-electron reduction pathway at more reducing potentials (Fig. 18.10a, b) may be rationalized within at least two mechanisms. The first is based on the kinetic competition between the release of H2O2 from the ferric-hydroperoxo intermediate [Reaction (18.16) in Fig. 18.11] and its (reversible) reduction to a ferrous-hydroperoxo species, which undergoes rapid 0-0 bond heterolysis (18.13b). Because H2O2 and particularly HO2 are more basic ligands... 

Within the mechanism in Fig. 18.11, it seems implausible that simple Fe porphyrins can be effective ORR catalysts, since large overpotentials are required to access intermediates in which 0-0 bond heterolysis is facile. The only strategy discovered so far to facilitate this 0-0 bond heterolysis in the ferric-hydroperoxo intermediate is to control both the distal and the proximal environments of Fe porphyrins. In those cases, the overpotential of ORR reduction appears to be controlled by the potential of the (por)Fe / couple (see Section 18.6). 

The low limit on the rate constant fehetero of 0-0 bond heterolysis in the putative ferric-hydroperoxo intermediate by analyzing the turnover frequency of H2O2 reduction at potentials 0.6-0.4 V (vs. NHE at pH 7). 

Figure 18.20 A plausible ORR catalytic cycle by biomimetic catalysts 2 (Fig. 18.17). Cu is ligated by three imidazoles (omitted for clarity) and potentially an exogenous ligand, whose nature is not known. All intermediates other than ferric-peroxo and ferric-hydroperoxo were prepared independently.

The ligand 4,5-bis(di(2-pyridylmethyl)aminomethyl)imidazole has been utilized in the formation of Cu-Zn dinuclear complex [CuZnL(CH3CN)2](C104)3. The resonance Raman spectra of hydroperoxo intermediates were studied.117... 

The kinetics and the mechanism of superoxide reduction by SORs have been studied by several researchers. It was suggested that SORs react with superoxide via an inner-sphere mechanism, binding superoxide at ferrous center to form a ferric hydroperoxo intermediate [46,48 50]. The rate constant for this reaction is equal to 108 109 1 mol-1 s-1 [46,49], This... 

The rate of the Ir(III) catalyzed reaction was found to be first-order in [Ir] and [H2DTBC], but independent of 02 concentration in chloroform (56). The mechanism proposed for the reaction (Scheme 4) postulates that the protonation of the hydroperoxo a-oxygen by the hydroxy group of the bonded catechol in Int 1 leads to the formation of H202. The o-qui-none ligand of Int 2 is replaced by the partially coordinated catechol in the next step. In order to comply with the experimental rate law, the rate-determining step needs to be the reaction of the oxygen adduct (B) with catechol. 

Infrared absorption of an unstable hydroperoxo species had been observed at 230 K by Tozzola et al. (63). A peak at 886 cm-1, strongly overlapping the peak at 877 cm-1 attributed to physisorbed H202, was attributed to TiOOH (771 end-on coordination), although a band at 837 cm-1 was assigned to anionic triangular Ti(02) (side-on coordination). 

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... 

---