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Equilibria and dynamics of some aqueous peroxomolybdate catalysts: a 17O NMR spectroscopic study
Umeå University, Faculty of Science and Technology, Chemistry.
Umeå University, Faculty of Science and Technology, Chemistry.
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2002 (English)In: Journal of the Chemical Society. Dalton Transactions, ISSN 1472-7773, 4451-6 p.Article in journal (Refereed) Published
Abstract [en]

The equilibrium speciation at 278 K and the dynamics in the systems pH++qMoO42–+rH2O2+sL (H+)p(MoO42–)q(H2O2)r Ls(L = SO42– or Cl–) in 0.3 M Na2(SO4) and 0.6 M Na(Cl) medium has been determined from 17O NMR integral and shift data in the range 0.7 pH 4.1 and at excess of peroxide ([H2O2]tot/[Mo]tot= 3). In 0.3 M Na2(SO4) medium species with the following compositions were found: MoX2–(1,1,2,0), MoX2(2,1,2,0), MoX2S2–(2,1,2,1), MoX2S–(3,1,2,1), Mo2X42–(2,2,4,0), Mo2X4–(3,2,4,0) and Mo2X62–(2,2,6,0), where (Mo) corresponds to MoO42–, (X) to O22– or HOO– and (S) to SO42–. In 0.6 M Na(Cl) medium the diperoxomolybdosulfate complex MoX2S– is replaced with a novel diperoxomolybdochloride complex MoX2Cl–(2,1,2,1), where (Cl) corresponds to Cl–. The numbers in parentheses refer to the values of p,q,r and s in the formula above. The stoichiometries of each complex are given by the abbreviated formula MoqXrSs(2q+ 2s–p)– and MoqXrCls(2q+s–p)–. The following formation constants with 3 were obtained with values for 0.6 M Na(Cl) in parenthesis: log 1,1,2,0= 11.61 (11.61); log 2,1,2,0= 13.70 ± 0.10, pKa= 2.09 (13.86 ± 0.10, pKa= 2.25); log 2,1,2,1= 14.57 ± 0.07 (13.87 ± 0.15 ); log 3,1,2,1= 15.27 ± 0.17, pKa= 0.70, log 2,2,4,0= 24.06 ± 0.05 (24.08 ± 0.04); log 3,2,4,0= 25.99 ± 0.18, pKa= 1.93 (26.23 ± 0.17, pKa= 2.15); log 2,2,6,0= 24.02 ± 0.15 (23.9 ± 0.3). The pKa value for HSO4– was determined to be 1.06 ± 0.11. At 278 K the MoX2 and Mo2X4 complexes are in chemical exchange and the rate of exchange increases upon protonation. At 312 K there is also measurable exchange between MoX2S and the MoX2 and Mo2X4 complexes. The dynamics in 0.6 M Na(Cl) medium seem to be very similar to those in the sulfate medium.

Place, publisher, year, edition, pages
2002. 4451-6 p.
Identifiers
URN: urn:nbn:se:umu:diva-9056DOI: doi:10.1039/b206230eOAI: oai:DiVA.org:umu-9056DiVA: diva2:148727
Available from: 2008-02-28 Created: 2008-02-28 Last updated: 2009-12-08Bibliographically approved
In thesis
1. Characterization of Aqueous Peroxomolybdates with Catalytic Applicability
Open this publication in new window or tab >>Characterization of Aqueous Peroxomolybdates with Catalytic Applicability
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Abstract

This thesis is a summary of five papers, containing equilibrium and structure studies of aqueous molybdate and peroxomolybdate species. Some of the peroxomolybdate species have also been studied in terms of their dynamic and catalytic properties. The primary objective was to characterize species with potential catalytic activity, with emphasis on thebleach process of kraft pulp. For this, potentiometry, EXAFS and 17O, 31P, 1H and 95 Mo NMR have been used.

The molybdate speciation in 0.300 M Na2(SO4) medium was found to differ from that in 0.600 M Na(Cl) medium, in that the uncharged monomeric molybdate species H2MoO4 was stronger in the sulphate medium, while highly charged species, such as Mo7O24 6-, became somewhat less pronounced.

Diperoxomolybdate species, (MoX2)n (X = peroxo ligand, n = 1-2), dominated the peroxomolybdate systems when sufficient peroxide was available. Both sulphate and chloride coordinated to molybdenum in the presence of hydrogen peroxide and these species were more inert than diperoxomolybdate species without coordinated medium anions. Chemical exchange rates increased upon protonation. A dimeric triperoxomolydate species was the only species found that contained more than two peroxo groups per molybdenum atom. At low concentrations of hydrogen peroxide, monoperoxoheptamolybdate species, Mo7X, were found.

Phosphate was found to coordinate relatively weakly to molybdate in the presence of peroxide. Species with four different nuclearities, i.e. (MoX2)nP (n = 1-4), were found. At excess of peroxide, no molybdophosphates were present. Chemical exchange rates were found to be substantially lower than in the peroxomolybdate system.

The aqueous monomeric diperoxomolybdate species retain the pentagonal bipyramidal seven-coordination found in the solid state, although with increased bond lengths. Sulphate seems to coordinate to molybdenum in a monodentate fashion by replacing an oxygen atom. Chloride probably coordinates by replacing an oxygen atom as well. For the dimeric diperoxomolybdate species, a single oxygen-bridge was proposed.

Conjugated carbon double bonds in the side chains of lignin model compounds were found to be hydroxylated or epoxidised by peroxomolybdate species. The addition of phosphate did not affect the type or yield of oxidation products noticeably. It was also shown that hydrogen peroxide, in the absence of molybdate, did not react to any noticeable extent with the lignin model compounds under these conditions.

Place, publisher, year, edition, pages
Umeå: Kemi, 2003. 73 p.
Keyword
Chemistry, Molybdate, peroxomolybdate, peroxomolybdophosphate, equilibrium, speciation, formation constants, potentiometry, EXAFS, dynamic NMR, Kemi
National Category
Chemical Sciences
Research subject
Analytical Chemistry
Identifiers
urn:nbn:se:umu:diva-45 (URN)91-7305-361-9 (ISBN)
Public defence
2002-11-18, KB3 A9, KBC-huset, Umeå, 09:00 (English)
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Available from: 2003-06-13 Created: 2003-06-13 Last updated: 2012-06-28Bibliographically approved

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Andersson, IngegärdPettersson, Lage

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