Hydrogen Peroxide Activation over Ti^IV: What Have We Learned from Studies on Ti‐Containing Polyoxometalates?

Abstract

Keywords: Polyoxometalates / Titanium / Hydrogen peroxide / Epoxidation / OxidationThis short review paper summarizes recent findings in the field of H 2 O 2 activation over Ti IV centers on the basis of model studies of Ti-containing polyoxometalates (Ti-POM) that used both experimental and computational techniques. Alkene epoxidation with H 2 O 2 mediated by two different POMs, the Ti-monosubstituted Keggin-type POM, [PTi(OH)-W 11 O 39 ] 4-(1), and the Ti-disubstituted sandwich-type POM, [Ti 2 (OH) 2 As 2 W 19 O 67 (H 2 O)] 8-(2), which contain well-defined [a] Boreskov [H 2 O 2 ] = 0.4 m. [29,30] [e] Other products were epoxide and trans-diol overoxidation products, namely, 2-hydroxycyclohexanone (predominant), 5-hydroxycaprolactone, adipic aldehyde, and adipic acid.

Mechanism of Thioether Oxidation over Di‐ and Tetrameric Ti Centres: Kinetic and DFT Studies Based on Model Ti‐Containing Polyoxometalates

“…We also evaluated the alternative mechanism B (Figure 2), which starts from the hydrolysis of compound 1 D yielding the monomeric forms [(γ‐SiTi 2 W 10 O 38 H 2 )(OH) 2 ] 4− ( 1 M ). In general, Group IV‐containing POMs have a strong tendency to form intercluster linkages and to interconvert between dimeric and monomeric species 10a–d,g. 15 Moreover, the rate of thioether oxidation with H 2 O 2 catalysed by Ti‐substituted Keggin anions was related to the dissociation of the dimer to the monomer 10d.…”

“…Computational techniques have been successfully employed to get insight into the mechanisms of catalytic processes by POMs,11 including alkene epoxidation with H 2 O 2 catalysed by Ti‐substituted POMs 10b. 12 On the other hand, mechanistic studies (both experimental and theoretical) on POMs containing Ti dimers (small clusters) are scarce 10g. 13…”

Strengths, Weaknesses, Opportunities and Threats: Computational Studies of Mn- and Fe-Catalyzed Epoxidations

“…In the case of vanadium, this was first systematically studied by Vandichel et al [104], whereas the case of titanium was tackled earlier by Sever and Root [111]. Some theoretical studies on titanium-catalyzed epoxidations also focused on this metal's ability to efficiently catalyze the epoxidation of olefins when immobilized into a heterogeneous matrix, such as silicates [112] or polyoxometalates [113,114]. These theoretical studies follow the same trend observed in the case of manganese-and iron-catalyzed epoxidations, with a predominance of DFT, with B3LYP as the preferred density function.…”

Novel Gallium Polyoxometalate/Nano‐Gold Hybrid Material Supported on Nano‐sized Silica for Mild Cyclohexene Oxidation Using Molecular Oxygen

“…The difference in catalytic activity and product distribution between the two catalysts could be attributed to the different specific surface areas of these catalysts as seen in BET analysis where the specific surface area changes with Au content, and the chemical environment of the POM ions which could be altered after the deposition of Au species. Kholdeeva et al . suggested that the introduced species (Au) may act as the main active center for the activation of oxygen to form activated intermediates, which are suitable for the formation of oxidized products through a heterolytic oxidation mechanism.…”