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In carbohydrates, the attack of hydroxyl radicals begins with a hydrogen abstraction,
followed by an oxygenation of the resultant carbon-centered radical, which
leads to the introduction of carbonyl groups (Scheme 7.34) [30]. It is generally
agreed that gylcosidic bond cleavage is caused by ozone itself and radical species
during ozonation in water [10], and that both are responsible for carbohydrate
decomposition during ozone bleaching [xx], depending on pH, the temperature,
and transition metal ions concentration [37]. Ni et al. [37] reported that from their
experiments it is unlikely that in-situ-generated hydroxyl radicals mainly cause
the carbohydrate degradation occurring during pulp ozonation. However, it was
shown that the presence of metal ions, such as copper and iron, or a rise in temperature
increases the formation of hydroxyl radicals during ozonation [yy]. They
found that the increase in the number of hydroxyl radicals at pH 10.5 is not significant
and concluded that a high pH molecular ozone is mainly responsible for cellulose
degradation. Ozonation of unbleached pulp leads to the formation of considerable
amounts of hydrogen peroxide leading to an additional degradation due
to the hydroxyl radicals generated by the peroxide formed during ozonation [yy].
794 7Pulp Bleaching
O
H
H
HO
H
H
H OH
OH
OH
O
R O
H
H
HO
H
H OH
OH
OH
O
R
O
H
H
HO
O
H
H O
OH
OH
O
R
O
H
H
HO
H
H O
OH
OH
O
O R
H
HO
H
H O
OH
OH
O
R
O-
H
OH O2
-
HOO-
tautomerisation
benzilic acid rearrangement
Scheme 7.34 Hydrogen abstraction from carbohydrates.
Three lignin–carbohydrate model systems were studied to determine the mechanism
of the effect of lignin on cellulose degradation during ozone bleaching. The
three model systems were: methyl a-d-glucopyranoside, dextran, and fully
bleached kraft pulps in the presence of various lignin model compounds [38]. The
results suggested that the lignin models can both promote and suppress degradation
of the carbohydrates during ozone treatment. The presence of the lignin
models can exert a protective effect by competing with the carbohydrates for the
ozone available in the system. On the other hand, the ozone–lignin reactions give
rise to the formation of hydroxyl radicals which promote carbohydrate degradation.
It is suggested that many more hydroxyl radicals are formed via the ozone–
lignin reaction route than via ozone self-decomposition. Among the lignin model
compounds studied, the phenolic lignin structures have a more pronounced effect
on carbohydrate degradation, though this can be explained by the easier formation
of hydroxyl radicals from phenolic rather than from nonphenolic lignin models
[38]. Margara et al. [14] observed a pronounced effect of free phenolic hydroxyl
groups on the accelerated degradation of cellulose. Moreover, in the presence of
lignin model compounds with etherified phenolic hydroxyl groups, there was a
marked retardation of cellobiose degradation [14].
In another study, methyl beta-d-glucopyranoside was subjected to ozone treatment
under different conditions. The results suggested that both direct attack by
ozone and the attack of secondary radical species were responsible for degradation
during the ozone treatment of aqueous solutions of carbohydrate model compounds.
When ozonation occurred in water, the experimental evidence suggested
that carbohydrate degradation was caused mainly by an attack of hydroxyl radicals
whilst, in the presence of methanol, tert -butyl alcohol or acetic acid, degradation
was due to direct ozone attack [39]. Moreover, radicals generated at the ozone–lignin
reaction front were thought to be mainly responsible for cellulose degradation.
The rate of direct cleavage of glycosidic bonds was also found to be similar to
that of carbonyl group formation, providing further support that cellulose degradation
is caused by radical attack. Cellulose degradation in the reacted region of
the pulp contributed only a few percent of the total cellulose degradation [17].
7.5 Ozone Delignification 795
Model compound studies using methyl 4- O -ethyl-b-d-glucopyranoside were
conducted to elucidate the role of radical species in the polysaccharide degradation
during ozone treatment [10]. It was concluded that both ozone itself and radical
species each participate in the glycosidic bond cleavage of carbohydrates during
ozonation in aqueous solutions. The free radical-mediated reaction may lead to
both direct cleavage and to the conversion of hydroxyl to carbonyl groups [10].
Ozonations of methylpyranosides (alpha- and beta-anomers of methyl-d-glucopyranoside,
methyl-d-mannopyranoside and methyl-d-xylopyranoside), as “model
compounds” for cellulose, were performed in unbuffered aqueous solution at
room temperature [40]. Methyl-alpha-d-xylopyranoside was found to degrade
more slowly than the other compounds, whereas the rate of degradation was highest
for methyl-d-mannopyranoside. In general, the degradation of alpha-anomers
was slower than that of the corresponding beta-anomers. Monosaccharides, lactones,
furanosides, and acidic compounds are formed during ozonation.
A lignin-carbohydrate complex (LCC), containing a D-xylose unit connected to
an aromatic part (three aromatic units and no free phenolic group) through a
beta-glycosidic bond, showed, that in dilute aqueous solution (6 mg LCC/100 mL
water) the initial reaction rate was extremely fast, and that the degradation of aromatic
structures in the lignin-mimicking portion of the LCC was complete during
the first 30 minutes of ozonation [40]. However, degradation of the carbohydrate
part occurred more slowly, suggesting that lignin might provide a degree of protection
against attack by ozone on cellulose in lignin-containing pulps. Ozonated
LCC samples showed further that C=O structures are produced during ozonation
[40]. A proposed reaction mechanism for the ozonation of methyl-b-d-glucopyranoside
(MbG) initiated by direct ionic attack by ozone at the glycosidic linkage is
described in Scheme 7.35 [40]. Electrophilic attack by ozone at the anomeric oxygen
of MbG (25) results in the formation of the tetroxide intermediate (38), which
decomposes to the carbonium ion (39), hydroperoxide and oxygen. On reaction
with water, (39) converts to glucose (40), which is further attacked by ozone and
gives gluconic acid (33) via radical oxidation. Arabinose (34) is produced via a
Ruff-type degradation involving hydroxyl radicals. Pan et al. [20] found that 33 was
the major product in the first stages of ozonation of MbG in buffered solution,
indicating that 33 and its lactone (32) could be obtained from sources other than
the initially formed glucose (40). These authors suggested that the b-glucoside is
also attacked by ozone at the C–H bond of the anomeric carbon by a 1,3-dipolar
addition mechanism (Scheme 7.35, path B). This results in the formation of a
hydrotrioxide hemiorthoester (26), which may undergo several routes of fragmentation.
Orbital-assisted fragmentation via the orthoester would produce methyl
gluconate (29). Fragmentation assisted by the intermolecular hydrogen-bonded
ring structure (28) would produce gluconic acid-d-lactone (32). For a successful
ozone attack at the anomeric C–H bond, each of the acetal groups must have an
electron pair orbital antiperiplanar to the C–H bond [40]. Arabinose (34) may be
attacked by ozone in a similar way to glucose, yielding arabinonic acid (35) and its
c-lactone (43), which was identified by Olkkonen et al. [40]. Methylgluconate (29)
was not identified, but the methylfuranosides (30) and (31) were found.
796 7Pulp Bleaching
O
OH
OCH3
OH
OH
CH2OH
+ O3
O
OH
O
OH
OH
CH2OH
CH3
O
O
O
H
B A
O
OH
O
OH
OH
CH2OH
CH3
O H
O2
O
OH
O
OH
OH
CH2OH
CH3
O
O
O
H
O2
OH
OH
O
OH
OH
CH2OH
CH3
O
Ring closure
CH
OH
OH
O
HO
CH2OH
OCH3
OH
OH
O
OCH3
CH2OH
Decarbonylation
Ring
closure
HOCH3 + O2
O
OH
OH
OH
CH2OH
O
H2O
C
OH
OH
OH
OH
CH2OH
O
OH
CO2
Ruff
degradation
C
OH
OH
O
OH
CH2OH
H
Ox
C
OH
OH
O
OH
CH2OH
OH
HO C
H2
COOH
C
H2
C
H
COOH
+
HO
OH
O
OH
OCH3
OH
OH
CH2OH
H
O O O-
+
O
OH
OH
OH
CH2OH
H
+
CH3OOH + O2
+ H2O - H+
O
OH
OH
OH
CH2OH
OH
C
OH
OH
OH
OH
CH2OH
H
O
Ox
O
OH
OH
OH
COOH
OH
C
OH
OH
OH
OH
COOH
H
O
CH2OH
OH
OH
O
OH
44 O
OH
OH
OH
OH
CH2OH
OH
OH
O
O
H2O
H+
Ox
O3
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