Degradation of cellulosic key chromophores by ozone: a mechanistic and kinetic study

David Budischowsky,Markus Bacher,Thomas Rosenau,Nele Sophie Zwirchmayr,Takashi Hosoya,Hubert Hettegger,Antje Potthast

doi:10.1007/s10570-021-03909-4

Abstract

Chromophores, colored substances of rather high stability that reduce brightness, are present in all kinds of cellulosic products, such as pulp, fibers, aged cellulosic material, and even in very low concentrations in highly bleached pulps. Thus, they are the prime targets of industrial pulp bleaching. In this study, the three cellulosic key chromophores 2,5-dihydroxy-1,4-benzoquinone (DHBQ, 1), 5,8-dihydroxy-1,4-naphthoquinone (DHNQ, 2), and 2,5-dihydroxyacetophenone (DHAP, 3) were bleached with ozone at pH 2 resembling industrial conditions (Z-stage). Bleaching kinetics were followed by UV/Vis spectrophotometry. The chemical structures of the degradation products were analyzed using NMR spectroscopy as well as GC/MS and confirmed by comparison with authentic reference compounds. The main, stable intermediates in the ozonation reactions have been elucidated by employing ozone not in excess but roughly stoichiometric amounts: mesoxalic acid (4) from DHBQ (1), rhodizonic acid (5) from DHNQ (2), and hydroxy-[1,4]-benzoquinone (8) from DHAP (3). As the final products at a large excess of ozone, a complex mixture of carboxylic acids (C1 to C4) was obtained, with the C4 acids being formed by subsequent condensation of smaller fragments (malonic acid and mesoxalic acid) rather than directly as ozonation products. At shorter reaction times and lower ozone excess, some aldehydes and ketones (C2 and C3) were contained in addition. The mixture of the degradation products was not completely stable but tended to undergo further changes, such as decarboxylation and condensation reactions. The reaction mechanisms of degradation by ozone, intermediate formation and re-condensation are described and discussed.Graphic abstract

Highlights

Chromophores in cellulosic products, such as pulp and paper, are responsible for yellowing and brightness revision
The reason for re-formation of chromophores from polysaccharides is the substantial number of processing steps during pulping and bleaching, which might damage cellulose oxidatively, and natural as well as artificial aging processes which cause the introduction of oxidized groups along the cellulose chain, followed by condensation reactions to chromophoric moieties
Other reactions than the Criegee pathway involve charge transfer mechanisms due to the strong electron-deficient character of ozone, formation of reactive oxygen species of which the hydroxyl radicals are most detrimental to cellulose during bleaching (Chirat and Lachenal 1994, 1997, Ragnar et al 1999a, b; Hoigne and Bader 1976, Pouyet et al 2014)

Summary

Introduction

Chromophores in cellulosic products, such as pulp and paper, are responsible for yellowing and brightness revision. In wood-derived cellulosic materials, chromophores mainly originate from residual lignin and hemicelluloses and their degradation products. In lignin-free or highly bleached cellulosics, such as bacterial celluloses, cotton, highly bleached pulps, historic or aged rag papers, they stem from polysaccharides and their degradation products. The reason for re-formation of chromophores from polysaccharides is the substantial number of processing steps during pulping and bleaching, which might damage cellulose oxidatively, and natural as well as artificial aging processes (thermal, chemical, or irradiation stress) which cause the introduction of oxidized groups along the cellulose chain, followed by condensation reactions to chromophoric moieties (see for instance: Potthast et al.2005; Yoneda et al 2008; Henniges et al 2013). These complex product mixtures are sometimes described as ‘‘Theander products’’ (Popoff and Theander 1976, 1978; Popoff et al 1976; Olsson et al 1977; Forsskahl et al 1976; Theander and Westerlund 1980)

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