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Molybdate speciation in systems related to the bleaching of kraft pulp
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
2001 (English)In: Polyoxometalate Chemistry From Topology via Self-Assembly to Applications, Springer Netherlands , 2001, 161-73 p.Chapter in book (Other academic)
Abstract [en]

Peroxomolybdates have shown to be efficient selective agents in the degradation of lignin in non-chlorine based bleach processes of kraft pulp. Furthermore, the process can be improved when anions such as phosphate are present. To clarify the chemistry in aqueous solution, fundamental speciation studies of relevant systems have been made under conditions similar to those in the bleaching step.

In this article the equilibrium speciation in the system pH+ + qMoO42− ⇆ (H+)p(MoO42−)q in 0.300 M Na2(SO4) medium at 25 °C has been studied using potentiometric data in the range 2.5 <- pH <- 6.0, 1.25 <- Motot/mM <- 20. The speciation was found to consist of the monomers MoO42− (0,1), HMoO4− (1,1), H2MoO4(2,1) and the heptamers Mo7O246−(8,7), HMo7O245−(9,7), H2Mo7O244−(10,7) and H3Mo7O243−(11,7) (numbers in parentheses refer to the values of p and q in the general reaction above). The following formation constants and 3σ were obtained: log β1,1 = 3.40 ± 0.10, log β2,1 = 7.79 ± 0.06 (pKa = 4.39), log β8,7 = 52.43 ± 0.04, log β9,7 = 57.42 ± 0.03 (pKa = 4.99), log β10,7 = 61.24 ± 0.04 (pKa = 3.82) and log β11,7 = 63.90 ± 0.10 (pKa = 2.66). The pKa value for HSO4− was determined to 1.27 ± 0.01. The effects of different ionic media on this system are discussed.

Finally, this article presents some preliminary results of the ongoing speciation studies in the H+ - MoO42− - H2O2 and H+ - MoO42− - H2O2 - H2PO4− systems.

Place, publisher, year, edition, pages
Springer Netherlands , 2001. 161-73 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:umu:diva-8991DOI: 10.1007/0-306-47625-8_12ISBN: 978-0-7923-7011-6 (print)ISBN: 978-0-306-47625-9 (print)OAI: oai:DiVA.org:umu-8991DiVA: diva2:148662
Available from: 2008-02-28 Created: 2008-02-28 Last updated: 2012-06-28Bibliographically 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)
Opponent
Supervisors
Available from: 2003-06-13 Created: 2003-06-13 Last updated: 2012-06-28Bibliographically approved

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