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Cleavage of the cellulose chain under dioxygen-alkaline conditions has been studied
with simple model compounds such as methyl-4- O -methyl-b-d-glucopyranoside
[200], methyl-b-d-glucopyranoside [201–203] and methyl-b-d-cellobioside
[204]. These compounds represent the inner cellulose units, and result in the formation
of glycolic acid, lactic acid, formic acid, acetic acid and carbon dioxide
[183] and methyl-b-d-glucoside, d-glucose, d-arabinose, d-arabinonic acid, d-erythronic
acid, and d-glyceric acid [204]. Additionally, carboxy-furanosides, methyl-2-
C -carboxy-b-d-pentafuranosides, have been identified as oxidation products of
both glycosides [200] and the corresponding methyl-3- C -carboxy-b-d-pentafuranoside
has also been formed from methyl-b-d-glucopyranoside. The formation of
these furanosidic acids is suggested via benzilic acid rearrangement of a diketo
intermediate [201].
It has been generally suggested that the oxidative peeling of a cellulose chain
proceeds via oxidation of the C2 or C3 hydroxyl group, followed by b-alkoxy-elim-
662 7Pulp Bleaching
ination at C4 [188]. In contrast, the b-elimination is more pronounced when the 4-
hydroxyl-group is substituted (as in cellulose), as is known from model-compound
studies [185,200]. As a result of b-elimination at C1, preceded by oxidation at C2
or C3, the formation of methyl-b-d-glucopyranoside from oxidation of methyl-b-dcellobioside
can be regarded [205]. The acids which clearly result from the oxidative
cleavage of the C1–C2, C2–C3, and C3–C4 linkages have been identified
among the oxidation products [183]. Furthermore, an attack of the C6 hydroxyl
group by a ROS seems very probable [205,206], because methyl-b-d-glucopyranoside
was more rapidly oxidized than methyl-b-d-xylopyranoside [183,206,207] and
methyl-6-deoxy-b-d-glucopyranoside [206]. Because the products formed from
methyl-4- O -methyl-b-d-glucopyranoside under alkaline hydrogen peroxide treatment
corresponded to those from alkaline dioxygen experiments with glycosides,
a common reactive species was inferred [206,208,209].
Cleavage of the xylan chain studied with methyl-b-d-xyloside as a model compound
[207] showed that the oxidation reaction products were similar to those of
methyl-b-d-glucopyranoside, methyl-4- O -methyl-b-d-glucopyranoside and methyla-
d-mannopyranoside suggesting the same mechanism. Although the oxidation
of methyl-b-d-xyloside was slower, the oxidative depolymerization of xylan was
more drastic compared with cellulose, but this may have been due to physical factors
[183,195] such as crystallinity [80,210].
The common reactive oxygen species [206,208,209] noted previously is thought
to be the hydroxyl radical [3,202–204,211]. A possible degradation mechanism for
carbohydrates proposed by Gierer [3] starts with an attack of a hydroxyl radical
(_OH) at the C2 position in the polysaccharide chain (Scheme 7.19), followed by
oxygenation of the resultant carbon-centered radical and elimination of superoxide
anion radical. This leads to the formation of a ketone in the polysaccharide
chain that allows cleavage of the glycosidic linkage by b-elimination (see Section
4.2.4.2, Carbohydrate reactions).
O
O
O
OH
OH
CH2OH
O
O
O
OH
OH
CH2OH
OH
-H2O -H+
O
O
O
O-
OH
CH2OH
pH > 10 O
O
O
O-
OH
CH2OH
O2
O
O
O
O
OH
CH2OH
-O2
-
H
H
H
H
H H
H H
H
H
H
O2
Fragmentation by β-elimination
("peeling")
Scheme 7.19 Mechanism for oxidative cleavage of carbohydrates
by hydroxyl radicals proposed by Gierer [3].
Guay et al. [204] have examined the proposed mechanism by using computational
methods, which revealed that the step involving elimination of superoxide
7.3 Oxygen Delignification 663
is energetically unfavorable. The highly reactive hydroxyl radical, which has been
generated by using hydrogen peroxide and UV light [Eq. (20)] [204], is capable of
reacting with most organic compounds, typically by hydrogen abstraction [139].
Hydroxyl radicals can react with both hydrogen peroxide and hydroperoxy anions
through Eq. (21) and Eq. (22), producing hydroperoxy radicals and superoxide
anions, respectively [212]. The reaction producing superoxide [Eq. (22)] is significantly
faster than the hydroperoxy radical formation [Eq. (21)] [213]. As shown in
Scheme 7.4, approximately half of the hydrogen peroxide is present as the conjugate
base at a pH of 11.8, and formation of superoxide anions should be more
important. At a lower pH, more hydroxyl radicals will be present to react with the
carbohydrates.
H2O2 _
h m 2_OH _20_
_OH _ H2O2→H2O _ HO_
2 _21_
_OH _ HO_2 →H2O__O_2 _22_
The experiments of Guay et al. with methyl-b-cellobioside have been conducted
with and without hydrogen peroxide at pH 10 and 12, and under oxygen pressure
(about 4 bar) at 90 °C [204]. Beside the predominant degradation products of
methyl-b-glucoside and d-glucose, d-arabinose, d-cellobionic acid, d-arabinonic
acid d-erythronic acid, d-glyceric acid, and glycolic acid, products that have also
been found by other groups [183,192,202,214–219], were identified. Moreover, no
degradation products were found in the control reactions, suggesting that dioxygen,
hydroxide ions, hydrogen peroxide, and hydroperoxy anions are not capable
of degrading carbohydrates without a radical initiator, such as lignin or metal ions
[204]. Due to the lower reactivity of methyl-b-cellobioside at higher pH (12) [202],
and the pH-dependence of the oxygen-species distribution [see Eqs. (21) and (22)],
the extent of the degradation decreased but the overall chemistry was unchanged
[204].
The mechanism of the formation of d-cellobioside is proposed to occur through
a two-step process (Scheme 7.20), starting with a hydroxyl ion attack at the anomeric
carbon displacing the methoxy radical. This radical can then abstract a hydrogen
from hydrogen peroxide or another hydrogen donor, forming a hydroperoxyl
radical and methanol (found experimentally).
The second degradative pathway (Scheme 7.21) is very similar to the first
(Scheme 7.20), except that the cleavage is between two pyranose rings, starting
with a hydroxyl attack at the anomeric carbon displacing d-glucose and methyl bglucoside
oxy radical at C4. The methyl b-glucoside radical then abstracts a hydrogen
from hydrogen peroxide, forming methyl b-d-glucoside [204].
664 7Pulp Bleaching
O
H
O
H
HO
H
H
H OH
OCH3
OH
O
H
HO
H
HO
H
H
H OH
OH
+H2O2
OH
O
H
O
H
HO
H
OH
OH
H
H
OH
O
H
HO
H
HO
H
H
OH
H
OH
+ CH3O.
HOO+.
O
H
O
H
HO
H
H
OH
H
OH
OH
O
H
HO
H
HO
H
H
OH
H
OH
CH3OH
Scheme 7.20 Proposed mechanism for cellobiose formation
(redrawn from Guay et al. [204]).
O
H
O
H
HO
H
H
H OH
OCH3
OH
O
H
HO
H
HO
H
H
H OH
OH
OH
OH
O
H
HO
H
HO
H
H
OH
H
OH
O
H
O
H
HO
H
H
OH
H
OCH3
OH
O
H
HO
H
HO
H
H
OH
H
OCH3
OH
+H2O2
OH
O
H
HO
H
HO
H
H
OH
H
OH
+
Scheme 7.21 Proposed mechanism for formation of methylb-
glucoside and D-glucose (redrawn from Guay et al. [204]).
Guay et al. [204] concluded that their experiments supported the view that hydroxyl
radicals are responsible for the degradation of carbohydrates during oxygen
delignification. Molecular oxygen, hydrogen peroxide, and hydroperoxy anions do
not appear to degrade carbohydrates directly. Previous studies also suggested that
superoxide anions do not degrade carbohydrates [3]. Guay et al. [204] reported that
no experimental evidence has been found to support the reaction mechanism
depicted in Scheme 7.19, though this may be due to different experimental conditions
being used in these studies and in previous research, which employed pulse
radiolysis to generate hydroxyl radicals. Evidence has been published suggesting
that cellulose degradation during pulse radiolysis arises from direct ionization of
the fibers rather than from hydroxyl radicals [216]. Moreover, the mechanism
(cleavage of the glycosidic linkage) shown in Scheme 7.21is supported by the
model-compound study with 1,4-anhydrocellobiotol and cellulose [211].
Details of the mechanisms regarding the involvement of superoxide elimination
[3,130] or no superoxide [204] are discussed – albeit controversially – in the literature,
there appears to be no doubt that the hydroxyl ion attacks the carbohydrates,
thereby starting the degradation reaction [4,220–226].
7.3 Oxygen Delignification 665
The hydroxyl radical (_OH) is one of the most reactive and short-lived of the
ROS, with a lifetime of about 1ns in biological systems [227]. Because of this,
methods used to detect _OH include electron spin resonance (ESR) [228] (using a
spin trap such as dimethylsulfoxide, DMSO), HPLC [229,230], rapid-flow ESR
[231], and fluorescence [232–237]. Two different methods can be used for the
detection of _OH. One is the direct reaction of a probe molecule with.OH. The
other method is to use a scavenger that creates a radical species with a longer lifetime.
The probe molecule then reacts with this radical species [229,234]. Superoxide
detection system have also been developed using ESR spin trapping [238], cytochrome
C [239,240], amperometric detection [241], or a chemiluminescence
assay [242,243], which may help to clarify whether the superoxide anion radical is
formed as a consequence of oxygen treatment. Moreover, a new chromatographic
method to determine hydroperoxides in cellulose [244], and a new colorimetric
method to determine hydroxyl radicals during the aging of cellulose [245] have
been published.
A compilation of important carbohydrate degradation products in dioxygenalkali
delignification processes of kraft and sulfite pulps (glycolic acid, 2,4-dihydroxibutyric
acid, 3,4-dihydroxibutyric acid, isosaccharinic acid, 2-deoxy-glyceric
acid, lactic acid, glyceric acid, formic acid, and acetic acid) according to Sjostrom
and Valttila [246] sums up this section.
7.3.2.6 Residual Lignin–Carbohydrate Complexes (RLCC)
It is well known that lignin and carbohydrates are linked in wood, and that new
linkages are formed during a kraft cook.
During oxygen delignification of pine pulp, the polysaccharides dissolve together
with lignin in the form of lignin–carbohydrate complexes (LCC) [247]. The
structures of these dissolved polysaccharides from pine and birch kraft pulps
treated under oxygen delignification conditions [247], when determined by using
methylation analysis [248], included 1,4-linked xylan, 1,3(,6)-linked and 1,4-linked
galactan, 1,5-linked arabinan, and notable amounts of a 1,3-linked glucan,
whereas the glucose-containing polysaccharide in the pine pulp effluent was 1,3-
linked glucan and not cellulose [247]. From the birch pulp mainly xylan, but also
traces of arabinan, 1,3-linked galactan and 1,4-linked glucan have been removed
[247].
Softwood kraft pulps with a kappa number between 50 and 20 and oxygendelignified
to a similar lignin content (kappa ~6) led to the isolation of LCCs using
a method based on selective enzymatic hydrolysis of the cellulose, and quantitative
fractionation of the LCC [63]. The large majority (85–90%) of the residual lignin
in the unbleached kraft pulp, and all of that in the oxygen-delignified pulps,
when isolated as LCC, was found as one of three types of complex, namely xylan–
lignin, glucomannan–lignin–xylan and glucan–lignin. Most of the lignin was
linked to xylan in high-kappa number pulps, but to glucomannan when the pulping
was extended to a low kappa number. Lawoko et al. [63] reported that, with
increasing degree of oxygen delignification, a similar trend in the delignification
666 7Pulp Bleaching
rates of LCC was observed; thus, the residual lignin was increasingly linked to glucomannan.
From this it was concluded that complex LCC network structures
appear to be degraded into simpler structures during delignification. Two excellent
schemes for the degradation of hemicellulose networks during pulping, and
possible differences in the accessibility of lignin under alkaline conditions between
a xylan–lignin complex and a glucomannan–lignin complex, were
described by Lawoko et al. [63]. Moreover, the chemical structure of the residual
lignin bound to xylan was different from that bound to glucomannan.
Enzymatically isolated residual lignin–carbohydrate complexes (RLCC) from
spruce and pine pulp (kappa number ca. 30) contained 4.9–9.4% carbohydrates,
with an enrichment of galactose and arabinose compared to the original pulp
samples. The main carbohydrate units present in the RLCC were 4-substituted
xylose, 4-, 3- and 3,6-substituted galactose, 4-substituted glucose, while 4- and 4,6-
substituted mannose were assigned to carbohydrate residues of xylan, 1,4- and
1,3/6-linked galactan, cellulose and glucomannan [65]. The comparison of RLCC
of surface material and the inner part of spruce kraft pulp fiber revealed that the
1,4-linked galactan was the major galactan in RLCC of fiber surface material of
spruce kraft pulp, and towards the inner part the proportion of 1,3/6-linked galactan
increased relative to 1,4-linked galactan [65]. It has been suggested that 1,3/6-
linked galactan structures may have a role in restricting lignin removal from the
secondary fiber wall. The RLCC of three different alkaline pine pulps studied by
Lawoko et al. [65] before and after oxygen delignification revealed small differences
in the carbohydrate structures of the unbleached pulps resulting from the
cooking method [conventional kraft pine pulp, a polysulfide/anthraquinone (AQ)
pine pulp and a soda/AQ pine pulp]. These authors found that all RLCC of oxygen-
delignified pulps had more nonreducing ends and less 1,3/ 6-linked galactan
than the corresponding RLCC of the unbleached pulps. Moreover, the oxygendelignified
soda/AQ pulp had a higher ratio of 1,4-galactan to 1,3/6- linked galactan
and shorter xylan residues than the RLCCs of oxygen-delignified conventional
kraft pine pulp and polysulfide/AQ pulps [65]. From the above results and the calculated
degree of polymerization, conclusions were drawn on the possible positions
of lignin–carbohydrate bonds (Fig. 7.27).
These authors concluded that xylan residues were partly bound to lignin via the
reducing end-groups, and that the RLCC contained either long galactan chains or
bonds linking galactans to lignin via the reducing ends [65]. Oxygen delignification
shortened the oligosaccharide chains present in RLCC and removed preferably
the 1,3/6- linked galactan compared to 1,4-linked galactan structures connected
to residual lignin. The RLCC of oxygen-delignified soda/AQ pulp differed
from those of the other two pulps after oxygen- delignification in that it had a
higher ratio of 1,4- to 1,3/6-linked galactan, and shorter xylan residues. However,
even this detailed analysis did not reveal any major differences in the soda/AQ
pulp that could explain its poor bleaching response. It is possible that factors other
than the chemical composition and interactions between lignin and carbohydrates
affect the bleachability of the pulps. These factors may be physical rather than
chemical [65].
7.3 Oxygen Delignification 667
gal-(1 4)-gal-(1 4)-gal-(1 4)-gal-(1 4)-gal-1
n
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