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

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: [Figure not available: see fulltext.]