Characterization by 51V solid state NMR of vanadium in dehydrated H4PVMo11O40 and Na1.5H2.5PVMo11O40 catalysts

Taouk, Bechara; Ghoussoub, Denise; Bennani, Abdelrhani; Crusson, E.; Rigole, Monique; Aboukaı̈s, Antoine; Decressain, R.; Fournier, Michel; Guelton, M.

doi:10.1051/jcp/1992890435

Cited by 21 publications

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“…For example, the rate of reduction by hydrogen is slower and less reversible for solid PMo 12 - x V x O 40 (3+ x )- than for solid PMo 12 O 40 3- although in solution the former systems are stronger oxidants than the latter . The thermal treatment of H 4 PMo 11 VO 40 formed PMo 12 O 40 3- and VO 2+ and the reduction of H 4 PMo 11 VO 40 with isobutyric acid formed PMo 12 O 40 4.09- , VO 2+ , PO 4 3- , and methacrylic acid. , These decomposition reactions are deeply related to the controversy about the effects of substitution by V for Mo on the catalytic activity. ,,− It is also unclear why the rate of reduction decreases with the content of alkali cation …”

Section: Effects Of Constituent Elements On the Redox Propertiesmentioning

confidence: 99%

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Heterogeneous Catalysis

1998

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Section: Effects Of Constituent Elements On the Redox Propertiesmentioning

confidence: 99%

“…47,48 These decomposition reactions are deeply related to the controversy about the effects of substitution by V for Mo on the catalytic activity. 47,48,[136][137][138][139][140] It is also unclear why the rate of reduction decreases with the content of alkali cation. 105…”

Section: Effects Of Constituent Elements On the Redox Propertiesmentioning

confidence: 99%

Heterogeneous Catalysis

1998

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“…62 Moreover, an increase in the replacement level of Mo by V atoms triggered changes in the infrared spectra, which was induced by a decrease in the symmetry of the heteropolyanion. 63,64 Particularly, the splitting of P-O a bond stretching band (ca. 1080 cm À1 ), as well as the presence of shoulders were more noticeable in the vanadium-trisubstituted phosphomolybdate anion.…”

Section: Resultsmentioning

confidence: 99%

“…A slight shi toward lower frequencies of main absorption bands was also noticed if compared to the infrared spectrum of the Na 3 PMo 12 O 40 salt 62. Moreover, an increase in the replacement level of Mo by V atoms triggered changes in the infrared spectra, which was induced by a decrease in the symmetry of the heteropolyanion 63,64. Particularly, the splitting of P-O a bond stretching band (ca.…”

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confidence: 81%

Vanadium-doped sodium phosphomolybdate salts as catalysts in the terpene alcohols oxidation with hydrogen peroxide

et al. 2021

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“…Oxo–vanadium bonding in the Keggin unit is strongly destabilized in the acid form. Removal of the “crystalline” water molecules from the secondary structure (see Scheme S1), without having an alternative cation present (e.g., Na + , K + ), initiates destruction of the primary structure by expulsion of vanadium − giving a lacunary (defective) Keggin. Expulsion of pentavalent vanadium nuclei from PVMo 11 is usually accompanied by rapid one-electron reduction to paramagnetic V 4+ .…”

Section: Introductionmentioning

confidence: 99%

Stable Pervanadyl Cation Encapsulated in Silica: Frivolous Vanadium in the H₄PVMo₁₁O₄₀ Keggin

Parker

Cavani

2015

J. Phys. Chem. C

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Encapsulation of H 4 PVMo 11 O 40 (1-vanado-11-molybdophosphoric acid) inside silica provided a single-site long-lived pervanadyl cation (VO 2 + ) for oxidation reactions. Protective silica residues allowed vanadium to sit in the secondary Keggin structure without being reduced, contrasting the classical situation for H 4 PVMo 11 O 40 supported on silica. The vanadium of this Keggin molecule is "frivolous", tending to exit the primary structure, with one-electron reduction, even during efflorescence under ambient conditions. Hydrated crystals circumvented this problem, allowing high resolution 51 V MAS NMR conditions and the first quadrupolar and chemical shift anisotropy interaction parameters for this intact Keggin. IntroductionPolyoxometalates (POM's) are nano-sized metal-oxygen cluster anions that self-assemble in aqueous solution. Heteropolyacids, or POM's in the acid form, are widely used to catalyze selective oxidation reactions under homogeneous and heterogeneous conditions. 1-7 Their acidbase and redox properties are finely altered by varying the chemical composition. The most stable POM's have the highly studied Keggin structure. 8 H 4 PVMo 11 O 40 ·xH 2 O is a more active oxidation catalyst than its fully molybdenum analogue (H 3 PMo 12 O 40 ), 4, 5, 9 yet its mode of action is less well understood. This work describes an interesting aspect of chemical behavior for PVMo 11 . POM formulas are abbreviated denoting only the central phosphorus and addenda metal atoms.One major obstacle in using POM's as heterogeneous catalysts is "containing" them in active form on a solid support with high surface area. Conventional wet impregnation on silica (i.e. POM-on-silica) does not provide a highly dispersed active phase with the desired "site isolation" effect. 10, 11 Far more stable and active catalysts are produced by sol-gel encapsulation methods 12 , where a "silica bottle" is built around a "POM ship". 13 Hydro-silanes, derived from silicon alkoxide precursors (e.g. TEOS, tetraethoxysilane), condense around the pre-formed POM to give "POM-in-silica". Preponderant examples exist for PW 12 -in-silica 5,14-19 , followed by PMo 12 20 , SiW 12 21, 22 and GeV 2 W 10 23 encapsulated inside silica. As we show here, little known PVMo 11 -in-silica 24,25, 26 has special properties. In addition to PVMo 11 -in-silica, more hydrophobic catalysts PVMo 11 -in-organosilica (Rsilica), with admixtures of MTOS (methyltriethoxysilane) or DDOS (dimethyldiethoxysilane) and TEOS, were also prepared and examined. The structures of these new catalysts, active in aerobic mercaptans oxidation 26 , are analyzed by multi-nuclear NMR spectroscopy. A technique deemed more effective than IR and Raman for discriminating the molecular species under consideration, especially under hydrated conditions. 27 However, an NMR approach ( 31 P, 51 V) has two shortcomings: 1) poor discrimination of species by 31 P chemical shift and 2) paramagnetic broadening affects from reduced (tetravalent) vanadium nuclei.Another, more intrinsic, obstacle exists fo...

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Characterization by ⁵¹V solid state NMR of vanadium in dehydrated H₄PVMo₁₁O₄₀ and Na_1.5H_2.5PVMo₁₁O₄₀ catalysts

Cited by 21 publications

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Heterogeneous Catalysis

Heterogeneous Catalysis

Vanadium-doped sodium phosphomolybdate salts as catalysts in the terpene alcohols oxidation with hydrogen peroxide

Stable Pervanadyl Cation Encapsulated in Silica: Frivolous Vanadium in the H₄PVMo₁₁O₄₀ Keggin

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Characterization by 51V solid state NMR of vanadium in dehydrated H4PVMo11O40 and Na1.5H2.5PVMo11O40 catalysts

Cited by 21 publications

References 0 publications

Heterogeneous Catalysis

Heterogeneous Catalysis

Vanadium-doped sodium phosphomolybdate salts as catalysts in the terpene alcohols oxidation with hydrogen peroxide

Stable Pervanadyl Cation Encapsulated in Silica: Frivolous Vanadium in the H4PVMo11O40 Keggin

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Characterization by ⁵¹V solid state NMR of vanadium in dehydrated H₄PVMo₁₁O₄₀ and Na_1.5H_2.5PVMo₁₁O₄₀ catalysts

Stable Pervanadyl Cation Encapsulated in Silica: Frivolous Vanadium in the H₄PVMo₁₁O₄₀ Keggin