Chromophores from hexeneuronic acids (HexA): synthesis of model compounds and primary degradation intermediates

Cellulose, Jul 2017

Hexeneuronic acid (HexA) is formed under pulping conditions from 4-O-methyl-glucuronic acid residues in xylans by methanol elimination. It is usually removed by an acidic washing treatment (A-stage) within the pulp bleaching sequence. Hexeneuronic acid has long been recognized as a source of color generation in pulps, but the chemical structure of the actual chromophoric compounds remained elusive. We report the synthesis of isotopically (13C) labeled HexA model units carrying a label at any of the six carbon atoms. Confirming pertinent literature accounts, it is shown that HexA forms three primary degradation intermediates, 2-furancarboxylic acid, 5-formyl-2-furancarboxylic acid, and formic acid, under mildly acidic conditions, and their formation mechanism is discussed. 2-Furancarboxylic acid is demonstrated to be deformylation product of 5-formyl-2-furancarboxylic acid. The three primary intermediates are colorless and do not represent chromophores themselves. Their mixture, upon thermal or acidic treatment, gives rise to the same chromophores that are also directly formed from HexA.

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Chromophores from hexeneuronic acids (HexA): synthesis of model compounds and primary degradation intermediates

Cellulose Chromophores from hexeneuronic acids (HexA): synthesis of model compounds and primary degradation intermediates 0 Y. Yoneda College of Agriculture, Shizuoka University , Ohya 836, Suruga-ku, Shizuoka 422-8529 , Japan 1 K. Krainz Air Liquid GmbH , Sendnergasse 30, 2320 Schwechat , Austria 2 T. Rosenau (&) A. Potthast N. S. Zwirchmayr T. Hosoya H. Hettegger M. Bacher Division of Chemistry of Renewable Resources, Department of Chemistry, BOKU University Vienna , Muthgasse 18, 1190 Vienna , Austria 3 T. Rosenau Johan Gadolin Process Chemistry Centre, A ̊ bo Akademi University , Porthansgatan 3, 20500 A ̊ bo/Turku , Finland 4 T. Dietz Evonik-Degussa , Rodenbacher Chaussee 4, 63457 Hanau-Wolfgang , Germany Hexeneuronic acid (HexA) is formed under pulping conditions from 4-O-methyl-glucuronic acid residues in xylans by methanol elimination. It is usually removed by an acidic washing treatment (Astage) within the pulp bleaching sequence. Hexeneuronic acid has long been recognized as a source of color generation in pulps, but the chemical structure of the actual chromophoric compounds remained elusive. We report the synthesis of isotopically (13C) labeled HexA model units carrying a label at any of the six carbon atoms. Confirming pertinent literature accounts, it is shown that HexA forms three primary degradation intermediates, 2-furancarboxylic acid, 5-formyl-2-furancarboxylic acid, and formic acid, under mildly acidic conditions, and their formation mechanism is discussed. 2-Furancarboxylic acid is demonstrated to be deformylation product of 5-formyl-2-furancarboxylic acid. The three primary intermediates are colorless and do not represent chromophores themselves. Their mixture, upon thermal or acidic treatment, gives rise to the same chromophores that are also directly formed from HexA. Cellulose; Pulp; Chromophores; Hexeneuronic acids; Bleaching; A-stage; Furan; Furancarboxylic acid; Ladder-type oligomers - Chromophores in cellulosic materials. Part XV. This work is dedicated to Dr. Hans-Ulrich Suess, formerly Degussa and Degussa-Evonik, who devoted his whole life to the understanding of chromophore formation in cellulosic pulps and bleaching phenomena. Introduction The CRI method (chromophore release and identification) opened the way to isolate and identify welldefined chromophoric structures from cellulosic matrices, despite their very low concentration (Rosenau et al. 2004) . This technique has been applied to different cellulosic materials, including cellulose I substrates (pulps, (Rosenau et al. 2007, 2008) bacterial cellulose, (Rosenau et al. 2014) cotton (Rosenau et al. 2011) , cellulose II substrates (regenerated celluloses such as rayon or Lyocell fibers), (Adorjan et al. 2005; Rosenau et al. 2005) and cellulose derivatives (cellulose 2.5-acetates, cellulose 3-acetates) (Rosenau et al. 2005) . In each case, different numbers of compounds (between 3 and 12 individual compounds) at different concentrations (between 4 and 42 ppm for the chromophore mixture) have been isolated. Making individual chromophoric structures known and rendering the compounds accessible to chemical, analytical, and bleaching studies can be seen as the main benefit of the method. With this basic knowledge at hand, industrial bleaching sequences can be optimized, discoloration treatments followed more easily, and destruction of chromophores optimized while at the same time keeping chemical usage and energy costs down, and accounts of this approach have been published. So far, the CRI method has been applied to ligninfree—or at least lignin-poor—cellulosic materials as higher content of lignin would overwhelm the separation and identification capability of the technique (Korntner et al. 2015) . The chromophores isolated so far thus resulted exclusively from cellulose, and their formation occurred either by oxidative ‘‘aging’’ (Potthast et al. 2005) with follow-up fragmentations/condensations or through side reactions of cellulose processing (Potthast et al. 2009), such as upon xanthogenation in rayon manufacture, dissolution in N-methylmorpholine-N-oxide in Lyocell production, or acetylation/deacetylation in cellulose acetate Scheme 1 Alkali catalyzed formation of HexA from glucuronoxylan by b-elemination of methanol during pulping. (Chakar et al. 2000) synthesis or acetic acid- based solvents (Potthast et al. 2002) . However, since the CRI approach is quite general, it can not only be used to address cellulosederived chromophores, but aromatic and quinoid chromophores in polysaccharide matrices in general. One of the most important of such non-cellulosederived chromophore sources is hexeneuronic acid (HexA), the term being used as shorthand for the 4-deoxy-b-L-threo-hex-4-enopyranosiduronic acid moiety. Within the series on chromophores in cellulosic materials, the present report and the subsequent parts will address the issue of chromophores from HexA, their formation mechanism, and their (...truncated)


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Thomas Rosenau, Antje Potthast, Nele Sophie Zwirchmayr, Takashi Hosoya, Hubert Hettegger, Markus Bacher, Karin Krainz, Yuko Yoneda, Thomas Dietz. Chromophores from hexeneuronic acids (HexA): synthesis of model compounds and primary degradation intermediates, Cellulose, 2017, pp. 3703-3723, Volume 24, Issue 9, DOI: 10.1007/s10570-017-1396-5