Chromophores in lignin-free cellulosic materials belong to three compound classes. Chromophores in cellulosics, XII

Abstract

The chromophore release and identification method isolates well-defined chromophoric substances from different cellulosic matrices, such as highly bleached pulps, cotton linters, bacterial cellulose, viscose or lyocell fibers, and cellulose acetates. The chromophores are present only in extremely low (ppm to ppb) concentrations. The concept of primary and secondary chromophores is introduced, with primary chromophores arising only from the polysaccharides inherent to cellulosic materials. Secondary chromophores also include atoms from the chemicals used to process the cellulose. Most primary chromophores belong to one of three compound classes: hydroxy-[1,4]-benzoquinones, hydroxy-[1,4]-naphthoquinones, and hydroxyacetophenones. Among them, three individual compounds dominate: 2,5-dihydroxy-[1,4]-benzoquinone, 5,8-hydroxy-[1,4]-naphthoquinone, and 2,5-dihydroxyacetophenones, amounting to more than 80 % of the total isolated chromophores in most cases. In lignin-free cellulosics, these three compounds can thus be regarded as key chromophores. The prevalence of these molecules is due to both exceptionally strong resonance stabilization, as reflected in delocalized double bonds, and their ready reformation from carbohydrate degradation products by recondensation reactions. The findings that (a) most chromophores in lignin-free cellulosic materials belong to only three compound classes and that (b) three chromophore compounds make up the bulk of the chromophore mixtures are foundational for future bleaching research: Based on this knowledge, specific searches for optimized bleaching conditions can now concentrate on these compounds and still cover the vast majority of chromophores.

Chromophores from hexeneuronic acids: identification of HexA-derived chromophores

“…By applying the well-established technique of CRI (chromophore release and identification method, see also the previous parts of this series) (Rosenau et al 2004Korntner et al 2015), identification of the chromophores formed from HexA or from its furanoid primary degradation products was possible, and their structures-all of them being of the bis(furano)- [1,4]benzoquinone or bis(benzofurano)-[1,4]benzoquinone type-are hereby reported.…”

Chromophores from hexeneuronic acids: chemical behavior under peroxide bleaching conditions

“…In fact, it was the only intermediate compound identifiable when starting from either 1, 2 or 5. DHBQ is the most prominent one of the three key chromophores identified in diverse cellulosic materials (Rosenau et al 2004(Rosenau et al , 2007Korntner et al 2015). In the case of compounds 3 and 4 as the starting materials, 2,5-dihydroxyacetophenone (HAP, 7) was found in addition to DHBQ, but in significantly smaller amounts, approx.…”

“…The chromophores are mixed aromatic/quinoid and furanoid/benzoid ladder-type oligomers consisting of largely unchanged 2-furanoyl units (see Scheme 1). The five compounds described account for 90% of the chromophoric material isolated by the CRI procedure (Rosenau et al 2004(Rosenau et al , 2007Korntner et al 2015). The linkage pattern of these units and the formation mechanisms of the chromophores have been clarified by means of 13 C-isotopic labeling.…”

Cyclic peroxides as key intermediates in the degradation of cellulosic key chromophores by alkaline hydrogen peroxide: first direct proof by 17O NMR

“…Three compounds have been established as key chromophores in cellulosic matrices: 2,5-dihydroxy-1,4-benzoquinone (1), 5,8-dihydroxy-naphthoquinone (2), and 2,5-dihydroxyacetophenone (Korntner et al 2015). Because of their exceptionally strong resonance stabilization they are the main survivors of bleaching attempts, and due to their exceptional thermodynamic stability they are readily re-formed from low-molecular weight degradation products upon aging and brightness reversion.…”