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Manfred Schwanninger
The residual lignin from unbleached or semibleached pulps, which could not be
removed by pulping, must be removed from pulp through oxidative lignin degradation
with bleaching reagents such as chlorine dioxide. The structure and reactivity
of the residual lignin have already been described (see Section 4.2.4, Lignin
structures and their reactivity; Composition of lignin, residual lignin after cooking
and bleaching).
Most knowledge regarding the proposed reaction pathways of chlorine dioxide
and lignin, and of compounds formed during bleaching, is derived from model
compound studies [1–8], molecular orbital calculations [9,10] and from bleached
residual lignin of kraft pulps [11–19]. However, whilst much information is available
on the reaction pathways and degradation products formed during chlorine
dioxide bleaching, for reasons of space only a minimal amount can be selected
and presented in this section.
In principle, a distinction can be made between: (a) oxidations by the oxidant
(chlorine dioxide), which affords a great variety of products and illustrates the
complexity of chlorine dioxide bleaching; and (b) the chlorination of aromatic
rings (this will be described later).
The initial steps of bleaching with chlorine dioxide are shown in Scheme 7.23.
Oxidations of aromatic substrates with chlorine dioxide are initiated by electrophilic
addition of the oxidant to the aromatic nuclei. This results in the generation
of charge-transfer (= p) complexes [1]. These complexes become protonated in
acidic media, thereby enhancing the formation of corresponding resonance-stabilized
cation radicals by the elimination of chlorous acid. In the case of phenolic
substrates (R1 = H), these cation radical intermediates readily lose a proton, affording
phenoxyl radicals in various mesomeric forms (Scheme 7.24; 8, 9, 10, and 11).
In the case of nonphenolic substrates (R1 = alkyl or aryl), the intermediary cation
radicals exist in various ortho- and para-oxonium ion forms (Scheme 7.25; 26 and
27) [1].
Creosol (Scheme 7.24), a simple phenolic compound, represents structural
types that were assumed to be present in residual lignin [1]. The methyl group
was thought to indicate side chains lacking a free or etherified hydroxyl group in
the a-position [1], although finding a methyl alpha position in lignin is highly unlikely.
The formation of the oxidations products (Scheme 7.242) begins from the
7.4 Chlorine Dioxide Bleaching 745
O
OCH3
R
+ ClO2
- HClO2
R1
O
OCH3
R
R1
ä +
ä −
CT - complex
ð - complex
+ H+
O
OCH3
R
R1
+
O
OCH3
R
R1
+ OH
OCH3
R
+
O
OCH3
R
R1
+
R1 = H R1 = alkyl or aryl
O
OCH3
R
phenolic structures non-phenolic structures
ClO2
Scheme 7.23 The initial steps of a general course of bleaching
with chlorine dioxide (from Ref. [1]).
phenoxyl radical in its ortho- and para-mesomeric forms (8 – 11) by coupling with
chlorine dioxide, yielding the chlorite esters of ortho- and para-quinols which
undergo various reactions. Hydrolysis of the chlorous ester intermediate (14)
accompanied by an oxidative demethoxylation gives the corresponding ortho-quinone
(15), and subsequent reduction leads to 4-methylcatechol (16). Heterolytic
fragmentation of the chlorous ester intermediate (14) leads to cleavage of the aromatic
ring with formation of 3-methylmuconic acid monomethyl ester (18), as
well as the cyclization product 4-carbomethoxymethyl-4-methylbutenolide (19).
The competing reactions, the hydrolysis and the heterolytic fragmentation of the
quinol chlorous ester (14), should provide quantitative correlation between the
products (15, 18 and 19), which is difficult to show since the quinone (15) readily
undergoes dimerization and polymerization in addition to the reduction yielding
16. Hydrolysis of the chlorous ester intermediate (12) gives a compound (13) to
which an oxirane structure is tentatively ascribed. In a competitive reaction, two
radicals of 11 may couple to give bis-creosol (17). Chlorine dioxide addition to the
para-form of the phenoxyl radical (10) followed by elimination of chlorous acid,
746 7Pulp Bleaching
results in the formation of a quinone methide intermediate (21). Nucleophilic
addition of water, chlorous acid or creosol to this intermediate gives rise to vanillyl
alcohol (23), vanillin (25) and the diarylmethane (22), respectively.
The formation of quinoid structures (e.g., 15) represents the main oxidative
change of creosol-type phenols subjected to chlorine dioxide. Under the conditions
usually employed, such structures are not stable and undergo reduction to
catechols, and possibly also further oxidation with chlorous acid [1]. These experimental
results are in good agreement with the thermodynamic results determined
by computational methods [9].
O
OCH3
CH3
O
H3CO
CH3
O
OCH3
CH3
O
OCH3
CH3
H
+ ClO2
O
H3CO
CH3
H
OClO
+ H2O - HClO2
OH
H3CO
CH3
O
O
OCH3
CH3
+ ClO2
- HClO2
- CH3OH
+ H2O
OClO
O
CH3
O
O
OCH3
CH3
O
- HClO
+ H2O
OH
CH3
OH
red
HO
O
O
H3C
COOCH3
Dimer and
polymers
+ ClO2
O
OCH3
H3C OClO
- HClO2
O
OCH3
CH2
OH
OCH3
CH2OH
+ H2O
2 x
Bis-creosol 17
OH
OCH3
H2C
+ HClO2
OClO
OH
OCH3
CHO
- HClO
OH
OCH3
OH
H3CO
CH3
+ creosol
Scheme 7.24 Reactions of chlorine dioxide with creosol (from Ref. [1]).
The methylated analogue of creosol, the 4-methylveratrole, represents a simple
model for nonphenolic structures in lignin, and reacts with chlorine dioxide at a
much lower rate (the rate constant, k, is about seven orders lower, which illustrates
the pronounced preference of chlorine dioxide for phenolic substrates [1])
than its phenolic counterpart. The reaction pathways starting from the two meth-
7.4 Chlorine Dioxide Bleaching 747
OCH3
OCH3
CH3
+ ClO2
+
OCH3
OCH3
CH3
+
OCH3
OCH3
CH3
+
H3CO
OCH3
CH3
+
H
+ ClO2 + ClO2
+ ClO2
OCH3
OCH3
CH3
+
H
OCl O
- HClO
- CH3OH
- H+
+ H2O
OCH3
O
CH3
O
H3CO
OCH3
CH3
+
O ClO
- HClO
- H+ H2O +
H3CO
OCH3
CH3
O
O
OCH3
OCH3
H3C
+
OCH3
OCH3
CH3
+
H
OCH3
OCH3
H3C
+
O ClO
- HClO2
- H+ H2O +
OCH3
OCH3
H3C O
- HClO
- CH3OH
+ ClO2
+ H2O
OCH3
O
H3C O
O
- HClO2
+ CH3OH
OCH3
OCH3
H3C
+
OCH3
- CH3OH
- H+ + H2O
O
OCH3
H3C OCH3
OCH3
OCH3
CH3
+
H
OCl O
+ H2O - HClO
H3CO
OCH3
CH3
+
H
O OH
- CH3OH
- H+ H2O +
H3CO
O
CH3
H
HO O
H3CO
O
CH3
Cl
Cl O
oxid. and
chlorination
reactions
HO
Scheme 7.25 Reactions of chlorine dioxide with creosol
methyl ether (4-methylveratrole) (adapted from Ref. [1]).
oxonium ions (26) are shown in Scheme 7.25. Electrophilic attack of chlorine dioxide
to the unsubstituted para-position (27 left) relative to the methoxyl group, followed
by elimination of hypochlorous acid and hydrolysis of the methyl aryl ether
group affords the para-quinone (29). Chlorine dioxide attack on a carbon bearing
748 7Pulp Bleaching
a methoxyl group yields compound 30. In the presence of water the intermediate
chlorous ester (30) undergoes heterolytic fragmentation and ring cleavage giving
rise to 3-methylmuconic acid dimethyl ester (31). Hydrolysis of the chlorous ester
intermediate 32, and rearrangement forms an oxirane (33) that is further oxidised
to oxirane (34). Alternatively, methanolysis of 32, in a methanol-containing solvent,
followed by hydrolysis of the methyl aryl ether bond, gives rise to the formation
of the para-quinol methyl ether (36). A chlorine dioxide attack on an unsubstituted
carbon atom ortho to a methoxyl group (27 right), followed by heterolytic
fragmentation, leads to ring opening with formation of an oxo carboxylic acid
ester (39) which, in a series of oxidation and chlorination reactions, is converted
into the dichloro-hydroxy-a-keto ester (40) [1].
4-Methyl-2,3′,4′-trimethoxydiphenyl ether (Scheme 7.26; 41) served as a model
for native diaryl ether structures (unaffected during pulping) or for diaryl ether
structures, arising by oxidative coupling during bleaching [2]. Electrophilic attack
takes place preferentially on the aromatic moiety of the diaryl ether which is substituted
by three activating ether groups, giving rise to the corresponding oxidation
products (44, 47, 50, and 51). The modes of formation of these cleavage products
are analogous to those of other nonphenolic models, such as those shown in
Scheme 7.25. Moreover, muconic acid esters (e.g., 31; Scheme 7.25), monochloromuconic
acid ester, a lactone (e.g., 19; Scheme 7.24) and quinolmethylether (e.g.,
36; Scheme 7.25) are formed [2].
Under weakly acidic or neutral conditions, veratrylglycerol-b-guaiacyl ether
(Scheme 7.27; 52), a nonphenolic residual lignin structure of the b-aryl ether type,
undergo (in part) oxidation by chlorine dioxide to form the corresponding a-keto
structure (58), and in part also oxidative cleavage of the Ca–Cb bond to yield the
corresponding aromatic aldehyde (e.g., veratrylaldehyde 56) [2]. Both reactions follow
the general course via the initial radical cation intermediate shown in Scheme
7.23. The Ca–Cb cleavage may be considered to involve homolytic fragmentation
of the radical cation. This is analogous to the reactions of nonphenolic b-aryl ether
structures with other reagents of the radical type, such as hydroxyl radicals during
oxygen bleaching (see Scheme 7.12; Oxygen delignification, path B).
Structures of the stilbene and vinyl ether type (see Oxygen delignification,
Table 7.9) were found to be more resistant to chlorine dioxide oxidation. Chlorine
dioxide reacts only with the double bonds between the aromatic moieties, however
extensive oxidative cleavage of the double bond did not occur and no ring-opening
products were detected [3]. The initial reaction steps involved are the formation of
intermediary cation radicals and their reaction with the oxidant, which are analogous
to those observed when chlorine dioxide reacts with the aromatic nuclei
(e.g., Scheme 7.23). The subsequent reactions of ring-conjugated structures with
ClO2 differ from those of aromatic nuclei. The attack at the unsaturated side
chains may be due to the high electron density at the b-carbon atom(s) in conjugated
systems and the deactivating effect on the reactivity of aromatic nuclei
exerted by the original conjugated double bonds [3].
The reaction between chlorine dioxide and different ligninmodel compounds (phenolic
and nonphenolic, with or without an alpha-hydroxyl group), when studied
7.4 Chlorine Dioxide Bleaching 749
+ ClO2 + ClO2 + ClO2
- HClO2
- H+ H2O +
+
- CH3OH
O
OCH3
CH3
OCH3
OCH3
O
OCH3
CH3
H3CO
OCH3
O
OCH3
CH3
OCH3
OCH3
O
OCH3
CH3
OCH3
OCH3
H
OClO
- H+
+
O
OCH3
CH3
OCH3
OCH3
OClO
- HClO
- CH3OH
+ H2O
O
OCH3
CH3
OCH3
O
O
OClO
+ H2O
- H+
O
OCH3
CH3
H3CO
OCH3
+
OClO
OH
- HClO
O
OCH3
CH3
H3CO
OCH3
O
O
OClO
O
OCH3
CH3
H3CO
OCH3
OH
OH
OH
OCH3
CH3
O
OCH3
O
Scheme 7.26 Reactions of chlorine dioxide with 4-methyl-
2,3′,4′-trimethoxy-diphenyl ether (from Ref. [2]).
750 7Pulp Bleaching
H3CO
OCH3
OCH3
- H
- H+
CHOH
HC O
CH2OH
+ ClO2, + H+
- HClO2
H3CO
OCH3
OCH3
CHOH
HC O
CH2OH
+
H3CO
OCH3
OCH3
CHOH
HC O
CH2OH
H3CO
OCH3
OCH3
HC
HC O
CH2OH
+
O H
+
- H+
OCH3
OCH3
CHO
+
H3CO
OCH3
OCH3
CO
HC O
CH2OH
Scheme 7.27 Homolytic fragmentation and oxidation in the
a-position of veratryl-b-guaiacyl ether (52) on treatment with
chlorine dioxide (from Ref. [2]).
under the effective elimination of hypochlorous acid by sulfamic acid during chlorine
dioxide treatment [4], showed that the reaction between ClO2 and lignin
model compounds is generally characterized by three independent parallel reactions:
(a) demethylation (demethoxylation [20]); (b) formation of 2-methoxy- p -quinone;
and (c) formation of muconic acid monomethyl ester and/or its derivatives
[15,18,19]. Nonphenolic lignin model compounds do react with ClO2 when ClO2 is
supplied in large excess [1,2,4]. These conditions are not representative of industrial
ClO2 application levels; therefore, the presence of free phenolic lignin is
thought to be a critical lignin component needed to increase oxidation efficiency
during pulp bleaching. However, recently Svenson et al. [zz] showed that the
7.4 Chlorine Dioxide Bleaching 751
removal of phenolic hydroxyl groups via pulp methylation did not adversely affect
the chlorine dioxide bleaching efficiency or the amount of chlorate formed during
exposure to chlorine dioxide. Due to the effective elimination of hypochlorous
acid, no chloroaromatic material could be detected [4], or can be substantially
reduced [21]. This supports the view that the reaction intermediate hypochlorous
acid (and chlorine) is solely responsible for the formation of chloro organic material
during chlorine dioxide bleaching [4,5]. Without elimination of the hypochlorous
acid formed during chlorine dioxide bleaching, several chlorination products
of all model compounds studies have been found [1–3]. Although alkaline decomposition
of chlorine dioxide has a complex behaviour and is not very well understood
[31], it should be mentioned that increasing pH (although decreasing reactivity)
leads to decreased formation of chlorinated organics [29].
Chlorine dioxide delignification (D0) preferentially oxidizes phenolic entities to
both quinonoid and muconic acid structures modifying kraft residual lignin [15],
indicating that quinones are significant reaction products formed in preference
over muconic methyl esters during D0 stage [4]. The amount of phenolic groups
of unbleached residual lignin is about 30–50% of all the phenylpropane units.
Kinetic studies with model compounds have demonstrated that ClO2 preferentially
oxidizes these phenolic groups orders of magnitude faster than nonphenolic
entities [4,22,23]. Further, differences in the reactivity of phenolic compounds are
observed and dependent on structural details [24]. Thus, it is generally accepted
that ClO2 will preferably react with these phenolic groups during D0 delignification
[4]. In general, these reaction products (quinonoid and muconic acid structures)
are generally resistant to further oxidation by ClO2. It is suspected that the
formation of these chromophores during bleaching, and their slow elimination
during later bleaching stages might explain why various bleaching sequences
encounter brightness ceilings, and why different bleach sequences are more efficient
at reaching higher brightness targets than others [15]. Alkali extraction (E
stage), in addition to removing solubilized lignin and saponifying muconic acid
methyl esters, reactivates the residual lignin towards ClO2 oxidation [13]. It
appears that the E stage behaves like a reductive bleaching stage, similar to that of
a Y stage (sodium hydrosulphite bleaching stages), which presumably results in
the aromatization of quinones to polyhydric phenols. Chlorine dioxide oxidation
of D0E and D0Y treated pulps generally afforded quinonoid structures onto the oxidized
lignin, like that of the D0 stage, which implies that polyphenols react with
ClO2 in a somewhat similar way as unbleached phenolic groups [13]. Clearly, the
formation of quinones during the ClO2 oxidation of phenolic and polyphenolic
moieties represents a significant reaction product. D0 stage quinones can be easily
re-activated towards ClO2 again upon alkali treatment [13].
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