Residual Lignin Structures

The principal pathway of oxidative degradation of phenolic units comprises the

abstraction of an electron from the phenolate anion by biradical oxygen, resulting

in the formation of the corresponding phenoxy and superoxide anion radicals.

The unstable cyclohexadienone hydroperoxide intermediates fragment to catechols

and various mono- and dicarboxylic acids, such as maleic and muconic

acids, by demethoxylation, ring-opening, and side-chain elimination reactions

[88]. The concentration of the catechols remain fairly constant throughout oxygen

delignification, implying that they are involved in the oxidation process as intermediates

[89]. Furthermore, the catechols also contribute to an improved solubility

of the oxidized lignin structures due to the increase in the hydrophilicity of the

lignin.

The overall outcome of these reactions is the degradation and elimination of

guaiacyl (softwood and hardwood) and syringyl phenolic (hardwood) units. The

degradation of the uncondensed phenolic units is more severe within the dissolved

lignins (51–67%) than in the residual lignins (3–46%) [90]. Condensed phenolic

units refer to those phenolic structures with C5 substituents other than

methoxyl. It is reported, that 56–58% of the total phenolic units present in the residual

softwood kraft lignin can be attributed to the condensed type [90,91]. The

proportion of condensed phenolic units in hardwood residual lignin is considerably

lower as compared to the softwood residual kraft lignin.

After applying oxygen delignification to a softwood kraft pulp, the content of

condensed phenolic units in the residual lignin is decreased only by 4–29%, while

the corresponding decrease within the dissolved lignin fraction is about 41–60%.

It is well established that 5,5′-biphenyl [92] and diphenylmethane structures [93]

are fairly resistant towards oxygen delignification. The latter structures were

found to accumulate during the whole oxygen delignification process. However,

some of the condensed phenolic structures seem to be susceptible to oxygen

delignification, as in general most reports demonstrate an overall decrease in the

total condensed phenolic units after oxygen delignification [90]. In a study by Lai

7.3 Oxygen Delignification 713

et al., it was indicated that b–5 linked phenylcoumaran-type structures are

degraded during the initial phase of oxygen delignification [94]. At very high temperatures

of about 140 °C the amount of condensed phenolic units decreases by

more than 50%, which confirms that the reactivity of softwood kraft lignin substantially

increases at temperatures above 110 °C.

The presence of carboxylic acid groups within the lignin structures is considered

to promote lignin solubilization during alkaline oxygen delignification. The

increase of the carboxylic content is particularly high in the residual softwood

kraft lignin, while the corresponding carboxylic content in the hardwood residual

lignin increases only moderately obviously due to the higher initial value [90].

Compared to the residual lignins, the dissolved lignins contain more carboxylic

acids. It is interesting to note that the rate of carboxylic group formation increases

drastically as the reaction temperature increases parallel to the degradation of the

uncondensed phenolic units. A significant increase in the carboxylic acid groups

is observed at temperatures greater than 120 °C [95]. After successive oxygen

delignification stages, the content of carboxylic acid groups in the residual lignin

decreases, which means that the lignin which is rich in carboxylic acid groups is

preferably removed in the subsequent oxygen delignification stages, leaving a residual

lignin containing fewer reactive groups [96]. The content of the residual lignin

(kappa number) also plays a decisive role in the overall reactivity of the lignin

structures. High-kappa number pulps are known to be easier to delignify than

low-kappa number pulps, clearly due to a lower proportion of condensed lignin

structures, particularly 5,5′-condensed lignin units and diphenylmethane structures

[93]. The extent of oxidation to carboxylic acid groups is found to be lower

than that in low-kappa lignins. It can be speculated that the lower amount of condensed

structures in these high-kappa number pulps requires less oxidation for

the removal. Nevertheless, it has been demonstrated in a study by Chirat and

Lachenal, and also later by Roost et al., that a significant fraction of the residual

lignin (>22–25% of the initial value) remains even after five oxygen treatments,

which suggests that the oxygen delignification levels off [97,98]. Elucidation of the

structures of residual lignin that is unsusceptible towards oxygen delignification

has certainly been the focus of recent research. Based on the experience that the

extent of lignin removal decreases with increasing hemicellulose content (particularly

xylan), it is assumed that the residual lignin is attached to carbohydrates by

lignin–carbohydrate complex (LCC) linkages [99–101]. In fact, xylan-linked lignin

is more resistant to oxidative reactions, while galactan-linked lignin is readily

degraded during oxygen delignification [96]. The removal of amorphous cellulose

during a first oxygen delignification stage causes an increase in cellulose crystallinity,

thus reducing the accessibility of residual lignin in the secondary wall.

Quite recently, the existence of p -hydroxyphenyl groups has been attributed to

the resistant lignin structures in residual kraft lignin [93,96,102]. Model experiments

indicated that these materials are less reactive than their guaiacol counterparts,

although they belong to the noncondensed phenolics. The content of p -hydroxyphenyl

groups in a residual softwood kraft lignin was shown to correlate

with delignification selectivity during oxygen delignification. The stability of p -

714 7Pulp Bleaching

hydroxybenzene during oxygen delignification is clearly confirmed by model compound

studies [93]. It can thus be concluded that the accumulation of 5,5′-biphenyl

structures occurring via phenoxy guaiacyl radical coupling reactions between

the liquor and the fiber and the p -hydroxyphenyl structures are amongst the major

factors impeding the efficiency of oxygen delignification.

The ratio between the light absorption coefficient and kappa number corrected

for the content of hexenuronic acids, k457 nm/kappa(**), is a measure of the specific

amount of chromophoric groups in the residual lignin (see Section 4.2.6, Influence

on bleachability). Quite surprisingly, the degree of oxygen delignification

was found to be somewhat higher for unbleached softwood kraft pulps with

increasing k457 nm/kappa(**) values, as shown in Fig. 7.46 [103].

0.6 0.9 1.2 1.5 1.8

k

457nm

/kappa, m2/kg

Degree of oxygen delignification, %

Fig. 7.46 Extent of oxygen delignification as a

function of the specific light absorption coefficient

[k457 nm/kappa(**)] (according to Gellerstedt

and Al-Dajani [103]). Unbleached Pinus sylvestris

kraft pulp, kappa numbers 15–19. Conditions

for oxygen delignification are given by Gustavvson

et al. [104]: 100 °C, 85 min, 12% consistency,

700 kPa pressure, 2.25% NaOH.

The slightly higher efficiency of oxygen delignification of the unbleached softwood

kraft pulps with the lower brightness was mainly attributed to the higher

content of reactive unsaturated aliphatic carbon atoms and phenolic hydroxyl

groups, as determined by a thorough analysis of the residual lignin structures

(two-dimensional NMR using the HSQC sequence) [103]. Prolonged cooking at a

low hydroxide ion concentration gives rise to the formation of these degraded lignin

structures due to preferred fragmentation reactions. However, the low hydroxide

ion concentration during residual delignification promotes the precipitation of

dissolved lignin, which then leads to a decrease in brightness and specific light

absorption coefficient. Despite their higher degree of delignification in the oxygen

stage, the pulps with a high unbleached k/kappa(**)-value require more bleaching

7.3 Oxygen Delignification 715

chemicals (expressed as OXE per kappa) for subsequent bleaching to full brightness

as compared to those pulps comprising a low unbleached k/kappa(**)-value

(see Fig. 7.47). The latter are produced with a high effective alkali charge and a

short cooking time.

0 300 600 900 1200 1500

0.6

0.8

1.0

1.2

1.4

1.6

1.8

k

457nm

/kappa, m2/kg

Consumed OXE / kappa

Fig. 7.47 k457 nm/kappa(**) versus quantity of oxidation equivalents

consumed (OXE) in an OQPQP-sequence to a brightness

of 87% ISO (according to Gellerstedt and Al-Dajani

[103]). The conditions for the QPQP-sequence are given by

Gustavvson et al. [104].

The better bleachability of pulps with low unbleached k/kappa(**)-values is associated

with a higher amount of b-O-4 structures [105]. It is generally agreed that

the bleachability of softwood kraft pulps in a TCF-sequence comprising oxygen

and peroxide as bleaching chemicals is positively correlated with the unbleached

pulp brightness, and thus with the amount of b-O-4-structures in the residual lignin

[106].

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