Degradation of Carbohydrates

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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

H OH

R O

H OH

H O

O R

H O

O-

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

OCH3

CH2OH

+ O3

CH2OH

CH3

B A

CH2OH

CH3

O H

CH2OH

CH3

CH2OH

CH3

Ring closure

CH2OH

OCH3

CH2OH

Decarbonylation

Ring

closure

HOCH3 + O2

CH2OH

H2O

CH2OH

CO2

Ruff

degradation

CH2OH

HO C

COOH

OCH3

CH2OH

O O O-

CH2OH

CH3OOH + O2

+ H2O - H+

CH2OH

COOH

CH2OH

44 O

CH2OH

H2O

H+

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