Heteropolyanions as oxidation catalysts in a 2-phases system

Titanium-monosubstituted polyoxometalates: relation between homogeneous and heterogeneous Ti-single-site-based catalysis

“…Several research groups examined oxidation of cyclohexene (CyH) with H 2 O 2 over Ti-containing molecular sives [63,71,72,[145][146][147][148][149][150][151][152][153] and Ti-POMs [137,[154][155][156]. Mesoporous titanium-silicates typically produced a mixture of oxidation products (epoxide, trans-cyclohexane-1,2-diol, 2-cyclohexene-1-ol and 2-cyclohexene-1-one), which is consistent with a homolytic oxidation mechanism (scheme 3).…”

“…Some authors found good activity in epoxidation [154,156], while others found no activity at all [125,155]. The reasons for that have become clearer in the light of the recent findings [112,133,137,[157][158][159].…”

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

“…The structural stability of a POM under turnover conditions is a key issue that one should investigate carefully to be sure that the observed catalysis is due to a Ti IV site in the specific Ti‐POM environment and not due to W VI sites in low‐nuclearity peroxotungstates, primarily, the Venturello complex {PO 4 [W(O)(O 2 ) 2 ] 4 } 3– ,23 which can form as the result of POM destruction with an excess amount of H 2 O 2 and is known to be a highly active epoxidation catalyst. [14e,14f,23–25] Different reaction conditions and composition of counter cations (see below) were most likely responsible for the disagreement that can be found in the early results published on alkene epoxidation with H 2 O 2 in the presence of Keggin Ti‐POM 26–28. Whereas some groups found a rather good activity in the epoxidation of cyclohexene,26,28 others found none 27.…”

Liquid-Phase Selective Oxidation by Multimetallic Active Sites of Polyoxometalate-Based Molecular Catalysts

“…Although the lacunary POMs can activate H 2 O 2 to form peroxo species at vacant sites [47], the oxidation catalysis by lacunary POMs has scarcely been reported [73,[98][99][100] 4À and the epoxidation mechanism have been reported [104][105][106][107][108][109]. The inorganic-organic hybrid POMs have attracted much attention because the electron and steric properties of the POMs significantly affect the catalytic properties of the metal centers [110][111][112].…”