Chromophores from hexeneuronic acids: identification of HexA-derived chromophores
Cellulose
Chromophores from hexeneuronic acids: identification of HexA-derived chromophores
0 K. Krainz Air Liquid GmbH , Sendnergasse 30, 2320 Schwechat , Austria
1 F. Plasser Faculty of Chemistry, Institute for Theoretical Chemistry, University of Vienna , Wa ̈hringerstr. 17, 1090 Vienna , Austria
2 T. Rosenau (&) A. Potthast N. S. Zwirchmayr H. Hettegger T. Hosoya 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 Turku , Finland
4 T. Dietz Evonik-Degussa , Rodenbacher Chaussee 4, 63457 Hanau-Wolfgang , Germany
Hexeneuronic acids (HexA) have long been known as triggers for discoloration processes in glucuronoxylan-containing cellulosic pulps. They are formed under the conditions of pulping from 4-Omethylglucuronic 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 4-8, which make up 90% of the HexAderived chromophores, are reported here for the first time. The compounds are ladder-type, mixed quinoidaromatic 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 a) HexA in pulp, b) the HexA model compound methyl 1-13C-4-deoxy-b-L-threo-hex-4enopyranosiduronic acid (1) or c) a mixture of the primary degradation intermediates of 1, namely 5-formyl-furancarboxylic acid (2) and 2-furancarboxylic acid (3). Isotopic labeling (13C) in combination with NMR spectroscopy and mass spectrometry served for structure elucidation, and final confirmation was provided by X-ray structure analysis. 13C-Isotopic labeling was also used to establish the formation mechanisms, showing all the compounds to be composed of condensed, but otherwise largely intact, 2-carbonylfuran and 2-carbonylfuran-5-carboxylic acid moieties. 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
-
Chromophores in cellulosic materials. Part XVI.
of ‘‘humins’’, which are formed as dark-colored
byproducts in depolymerization of pentosans and
hexosans in different biorefinery scenarios.
Introduction
The pulp and paper industry is a major economic
factor in many countries. Today, with biorefinery
topics becoming more and more prominent, paper
mills are in transition from classical pulp producers to
more diverse biorefineries. But still, the major product
of these industries is cellulose, with pulping and
bleaching being the two main operations in its
manufacture. A major part of chemical, energy and
technology costs is devoted to removing
chromophores from pulp to reach target brightness. The
fewer chromophores are left in pulp, the more difficult
it gets to remove the remaining ones. For
glucuronoxylan-containing pulps, the role of hexeneuronic acids
(HexA) has been recognized as critical with regard to
color formation (‘‘brightness reversion’’). The origin
of this moiety in pulp has been briefly repeated in the
previous part of this series
(Rosenau et al. 2017)
.
Studies by Clayton dating back to 1963
investigated the formation of
4-deoxy-b-L-threo-hex-4Scheme 1 Top: schematic
formation of HexA from
glucuronoxylan during
pulping; bottom: structure of
the HexA model compound
methyl
4-deoxy-b-L-threohex-4-enopyranosiduronic
acid (1) and its primary
degradation products
5-formylfuran-2-carboxylic
acid (2) and
furan-2carboxylic acid (3)
enopyranosiduronic acid (HexA) moieties from
hemicelluloses during alkaline treatment of wood at
temperatures up to 170 C. From 4-O-methyl
glucopyranosiduronic acid, HexA formation was
suggested to occur through b-elimination of methanol,
leaving a double bond that is always placed
regioselectively towards C-5, thus resulting in a conjugated
a,b-unsaturated acid structure (see Scheme 1)
(Clayton 1963)
. Further studies were able to confirm these
early findings
(Johansson and Samuelson 1977)
,
establishing definite knowledge of the structure of
HexA, when enzymatic hydrolysis was applied to kraft
pulps by
Teleman et al. (1995)
. The role of HexA in
decreasing brightness has been thoroughly
investigated, as were the bleaching conditions that permit its
minimization
(Teleman et al. 1995; Sjostrom 2006)
.
Acidic pulping conditions, e.g. at 130 C for 2–3 h
with 0.05 M H2SO4 (Johansson and Samuelson 1977),
or at 80 to 140 C and pH 3.0–3.5 for 2–5 h
(Chakar
et al. 2000; Clavijo et al. 2012)
, allow the removal of
possible HexA side groups, but also result in the
occurrence of furanoid primary degradation products,
e.g. 2-furancarboxylic acid and
5-formyl-2-furancarboxylic acid that lead to the formation (...truncated)