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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

Hosoya, Takashi, Zwirchmayr, Nele Sophie, Klinger, Karl Michael, Reiter, Heidemarie, Spitzbart, Martin, Dietz, Thomas, Eibinger, Klaus, Kreiner, Wolfgang, Mahler, Arnulf Kai, Winter, Heribert, Röder, Thomas, Potthast, Antje, Elder, Thomas, Rosenau, Thomas
Cellulose 2018 v.25 no.9 pp. 4941-4954
acetic acid, aromatic compounds, bleaching, carbon dioxide, cellulose, chlorine dioxide, glycolic acid, hydrogen, nuclear magnetic resonance spectroscopy, oxalic acid, pH, pulp
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.