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
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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)