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

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

Valorisation of Underutilized Grass Fibre (Stem) as a Potential Material for Paper Production

“…The lignin substrate contains many chromophore groups, such as quinoids, catechols, aromatic ketones, stilbenes, conjugated carbonyls with phenolics, and metal complexes, and some additional chromophores within paper can be oxidised from leucochromophores and remnant carbohydrates [ 57 , 58 , 59 , 60 , 61 ]. Chromophores are coloured substances with a high degree of stability that reduce brightness in pulp and fibre [ 62 , 63 ]. The goal of mechanical pulping is to achieve a high yield with high strength properties.…”

Oven‐Drying Microfibrillated Cellulose Covered with a Low Molecular Weight Copolymer for Dispersion in a Polymer Matrix

“…Chromophores structures are responsible for imparting a characteristic color to a compound by the absorption of electromagnetic energy at a specific frequency and emission of a visible light frequency. [ 24 ]…”

“…Chromophores structures are responsible for imparting a characteristic color to a compound by the absorption of electromagnetic energy at a specific frequency and emission of a visible light frequency. [24] Bleaching agents aim to eliminate HexA from the cellulosic pulp. However, the process generates a high oxidized group content and high content of low molecular weight carbohydrate chains.…”

Lignin and Xylan as Interface Engineering Additives for Improved Environmental Durability of Sustainable Cellulose Nanopapers

“…The high intensity of UV radiation oxidized oxygen to ozone, which assisted in the sample oxidation process. The oxidation process creates radicals that can induce chain scission, thus reducing the components’ molecular weight and mechanical strength [ 47 ]. In addition, heat exposure has been reported to induce chain relaxation, promoting cellulose crystallization and the reformation of hydrogen bonds [ 29 ].…”