- Title
- Homogeneous-phase catalytic H2O2 oxidation of isobutyraldehyde using Keggin, Dawson and transition metal-substituted lacunary heteropolyanions
- Creator
- Hu, Ji; Burns, Robert C.
- Relation
- Journal of Molecular Catalysis A: Chemical Vol. 184, Issue 1-2, p. 451-464
- Publisher Link
- http://dx.doi.org/10.1016/S1381-1169(02)00032-8
- Publisher
- Elsevier Science BV
- Resource Type
- journal article
- Date
- 2002
- Description
- The relative effectiveness of a range of heteropolyoxometalate catalysts with Keggin, Dawson and transition metal-substituted lacunary structures has been compared using the homogeneous-phase oxidation of isobutyraldehyde to isobutyric acid with H₂O₂ in acetonitrile at 50 °C. For the Keggin ([XM₁₂O₄₀]ⁿ⁻, X=P(V), Co(III), Fe(III), Co(II), Zn(II), M=W(VI); X=P(V), M=Mo(VI)) and Dawson ([X₂Mo₁₈O₆₂]m⁻, X=P(V), S(VI)) structures, catalytic activity increased with decreasing anionic charge. In the case of the prototypical [PW₁₂O₄₀]³⁻ ion, kinetic studies indicated that there was association of the isobutyraldehyde and phosphopolyoxotungstate prior to oxidation of the former by H₂O₂. No fragmentation of the tungsten-containing species, such as [PW₁₂O₄₀]³⁻ and [CoW₁₂O₄₀]⁶⁻, occurred under the reaction conditions, as shown by ³¹P NMR and UV–VIS spectroscopy, respectively while both [PW₁₂O₄₀]³⁻ and [P₂Mo₁₈O₆₂]⁶⁻ generated small amounts of peroxo-species based on ³¹P NMR studies. The transition metal-substituted lacunary polyoxomolybdate systems of the type [PZ(II)(Br)Mo₁₁O₃₉]⁶⁻, where Z=Mn(II), Co(II), Ni(II), Cu(II) and Zn(II), are likely degraded to (PO₄[MoO(O₂)₂]₄)³⁻ and/or related species by H₂O₂, based on UV–VIS spectroscopic studies and on ³¹P NMR studies of diamagnetic [PZn(Br)Mo₁₁O₃₉]⁶⁻. The resulting products are only slightly more active catalysts than the Keggin anions (a maximum increase in activity by a factor of ~7). The initial rates of reaction followed the sequence Co(II)>Ni(II)>Zn(II)>Mn(III)>Cu(II), with Mn(II) undergoing oxidation by H₂O₂ under the reaction conditions. The order likely corresponds to a combination of the catalytic abilities of the transition metal-substituted lacunary anions in conjunction with their rates of decomposition by H₂O₂, and/or the catalytic abilities of the freed transition metals and peroxo-products themselves
- Subject
- homogeneous catalysis; heteropolyoxometalate; hydrogen peroxide; oxidation
- Identifier
- http://hdl.handle.net/1959.13/27146
- Identifier
- uon:1406
- Identifier
- ISSN:1381-1169
- Language
- eng
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