Hexeneuronic acids (HexA) have been known for long as triggers for discoloration processes in glucuronoxylan-containing cellulosic pulps, but the chemical structure of the actual chromophoric compounds have remained elusive. HexA is formed under the conditions of pulping from 4-O-methylglucuronic acid residues, and are removed in an “A stage” along the bleaching sequences, which mainly comprises acidic washing treatments. The chemical structures of HexA-derived chromophoric compounds, which make up for 90% of the HexA-derived chromophores, have been clarified by means of isotopic labeling (13C) in combination with NMR spectroscopy and mass spectrometry. The labeled HexA model units carried a 13C label at either of the six carbon atoms. Final structure confirmation was provided by X-ray structure analysis. The HexA-derived chromophores compounds are ladder-type, mixed quinoid-aromatic oligomers of the bis(furano)-[1,4]benzoquinone and bis(benzofurano)-[1,4]benzoquinone type. The same chromophoric compounds are generated independently of the starting material, which can be either HexA in pulp or also HexA model compounds. These results disprove the frequent assumption that HexA-derived or furfural-derived chromophores are linear furanoid polymers, and might have a direct bearing on structure elucidation studies of “humins”, which are formed as dark-colored byproducts in depolymerization of pentosans and hexosans in different biorefinery scenarios. The HexA-derived chromophores were subject to conditions of peroxide bleaching. They are degraded relatively fast to one major product, 2,5-dihydroxy-[1,4]-benzoquinone (DHBQ), and a minor component, 2,5-dihydroxyacetophenone (DHA). These two compounds, which have already been identified as two of the three key chromophores (besides 5,8-dihydroxy-[1,4]-naphthoquinone, DHNQ) in aged cellulosics, are potent chromophores themselves and are subsequently more slowly degraded to non-colored degradation products. The occurrence of DHBQ and DHA in the bleaching treatment of the HexA-derived chromophores establishes the link between HexA chemistry and the three general key chromophore classes of residual chromophores found in aged cellulosic materials.

Chomophores from hexeneuronic acids (HexA) – identification of structures, formation mechanisms, and destruction by peroxide bleaching

255th National Meeting (2018)

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