Chromophores in cellulosics, XVIII. Degradation of the cellulosic key chromophore 5,8-dihydroxy-[1,4]-naphthoquinone under conditions of chlorine dioxide pulp bleaching: a combined experimental and theoretical study

Takashi Hosoya, Nele Sophie Zwirchmayr, Karl Michael Klinger, Heidemarie Reiter, Martin Spitzbart, Thomas Dietz, Klaus Eibinger, Wolfgang Kreiner, Arnulf Kai Mahler, Heribert Winter, Thomas Röder, Antje Potthast, Thomas Elder, Thomas Rosenau

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    Sammanfattning

    5,8-Dihydroxy-[1,4]-naphthoquinone (DHNQ) is one of the key chromophores occurring in all types of aged cellulosics. This study investigates the degradation of DHNQ by chlorine dioxide at moderately acidic (pH 3) conditions, corresponding to the conditions of industrial bleaching (“D stage”). The degradation involves three major pathways. As initial reaction, a hydrogen transfer from DHNQ to chlorine dioxide via a PCET mechanism occurs to form a radical DHNQ· and chlorous acid. DHNQ· is then attacked by water to give a pentahydroxynaphthalene radical PHN· that is stabilized by strong delocalization of the non-paired electron into its aromatic ring. PHN· immediately disproportionates to give the observable intermediate 1,2,4,5,8-pentahydroxynapththalene (I), which was comprehensively confirmed by NMR and MS (path A). In the presence of excess ClO2, I is immediately further oxidized into acetic acid, glycolic acid, oxalic acid and CO2 as the final, stable, and non-colored products (path C). In the absence of excess ClO2, elimination of water from I regenerates DHNQ (path B), so that at roughly equimolar DHNQ/ClO2 ratios ClO2 is fully consumed while a major part of DHNQ is recovered. To avoid such DHNQ “recycling” under ClO2 consumption—and to completely degrade DHNQ to colorless degradation products instead—ClO2 must be applied in at least fivefold molar excess relative to DHNQ.

    OriginalspråkOdefinierat/okänt
    Sidor (från-till)4941–4954
    TidskriftCellulose
    Volym25
    Utgåva9
    DOI
    StatusPublicerad - 2018
    MoE-publikationstypA1 Tidskriftsartikel-refererad

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