FBSKP-Aa FBSKP Water

I (OH + P) 22 }18.0 } 0.4 41 } 24

II (OH + P + Q) 25 } 3 12.2 } 0.4 130 } 40

III (OH + P + Mg) 190 } 50 36.0 } 13.0 300 } 130

IV Grp 1(Mg + Q)

+ Grp 2 (OH + P)

240 } 150 83.0 } 11.0

1330 } 590

V Grp 1(Mg + OH)

+ Grp 2 (P + Q)

370 } 130 39.0 } 13.0

a. Acid-treated at pH = 1.5.

7.6 Hydrogen Peroxide Bleaching 855

DTMPA (stabilize H2O2 at high temperatures and alkali [bb]) the lignin undergo

increasing levels of oxidation and degradation with increasing temperature. The

highest degree of selectivity was observed at 90 °C, i.e. the highest amount of phenolic

hydroxyl groups degraded and the highest amount of lignin degraded as a

function of hydrogen peroxide consumed. The highest amount of lignin degradation,

over 80%, occurred at 110 °C. Analyses of the degraded lignins indicated that

both phenolic and nonphenolic lignin moieties were degraded [aa].

Residual Lignin

The sites of nucleophilic attacks in lignins are shown in Fig. 7.116. By elimination

of an a– (see Section 4.2.4, Chemistry of kraft pulping) or, in conjugated structures,

a c-substituent, a quinone-methide intermediate is formed from the arylalkane

unit (Fig. 7.116), which involves the loss of two electrons, and results in the

generation of centers of low electron density (d+) that constitute the sites of attack

by nucleophiles [22].

O

R2 OCH3

CH

ä+ ä+

ä+

arylalkane unit

R1 = OH, OAr or OAlk

arylpropene unit

quinone-methide intermediate

O

R2 OCH3

CH

HC

CH2

O

R2 OCH3

C

C

CH2

R

O

R3

á-carbonyl group containing

R = OAr, Ar or Alk

ä+

ä+ ä+

ä+

ä+

ä+

ä+

ä+

C C C C O

ä+ ä+

Fig. 7.116 Sites of nucleophilic (d+) attacks in lignin (adapted from Ref. [22]).

A nucleophilic attack starts with the addition of the hydroperoxy anion to carbonyl

and conjugated carbonyl structures (Scheme 7.37, 1) giving a hydroperoxide

(2) which forms an epoxide (4). After an additional nucleophilic attack the Ca–Cb

bond will be cleaved.

C

C

C

O

HOO-

HOO

C

C

C

O-

O

C

C

C

O-

HO

C C C

O O

- OH -

1 2 3 4

Scheme 7.37 Formation of hydroperoxide via a nucleophilic reaction.

The hydroperoxide anion adds rapidly to quinoid structures (Scheme 7.38). By

addition to an ortho-quinone (5) hydroperoxides (6, 9) are formed, leading to the

formation of dioxetane (7) or oxirane (10) intermediates followed by cleavage of

856 7Pulp Bleaching

O

R1 O

O

OO- R1

+ HOO -

- H+ O-

-O

R1 O

O-

O O

OR1 -

O

O-

O

R1 O

+ HOO-

O

O- R1

OOH

- HO - + HOO-

O

R1 O

O

O

OR1 -

O

O-

O

+ HOO -

+ OH -

degradation

products

O

R1 OCH3

O

+ HOO -

- H+

O

OCH3

R1

O-

OO-

-O

OCH3

R1

O-

O

O O

O

R1

O-

O-

O-

+ H2O

- CH3OH

+ HOO-

+ OH -

degradation

products

O

R1 OCH3

CH

+ HOO-

O-

R1 OCH3

HC O OH

- OH -

O

R1 OCH3

HC O

+ HOO-

- OH -

O

R1 OCH3

O

+ HOO -

+ OH -

degradation

products

O

R1 OCH3

CH

CH

[ O- ]

C

H O

+ HOO -

O

R1 OCH3

CH

CH

[ O- ]

C

O- H

HOO

- OH -

O

R1 OCH3

CH

CH

[ O- ]

C

H O

O

+ HOO-

O

R1 OCH3

C

[ O- ]

O H

HCOO-

+ HOO-

+ OH -

degradation

products

O

R1 OCH3

C

C

CH2

+

+ HOO-

O

OR

O

R1 OCH3

C

C

CH2

O-

OR

HOO

O

R1 OCH3

C

C

CH2

O

OR

O

- OH - + HOO -

O

R1 OCH3

C

O O-

HCOO-

+

+

ROH

H3CO

R1

R =

5 7 8

5 10 11

12 13 14 15

16 17 18 19

20 21 22 23

24 25 26 27

HC O

+

CO2

CO2

Scheme 7.38 Addition of hydroperoxide anions to quinoid

structures and to side-chain enone structures (adapted from

Refs. [12,23]).

7.6 Hydrogen Peroxide Bleaching 857

the ring giving dicarboxylic acids (8, 11) that can be further degraded. Adding the

hydroperoxide anion to a para-quinone with a methoxyl group (12) gives via a

hydroperoxide (13), a dioxetane (14), and the ring is cleaved after demethoxylation,

giving a dicarboxylic acid (15). The hydroperoxide (17) formed after hydroperoxide

anion addition to an arylalkane (quinone methide structure) (16) leads to an oxirane

(18). A further nucleophilic attack cleaves the bond between the Ca-atom and

the ring, thereby forming an aldehyde group and a para-quinone (19) which can

be further degraded (12 – 15).

Side chains with enone structures (20, 24) also afford hydroperoxides (21, 25)

and subsequently oxirane intermediates (22, 26), leading to cleavage of the Ca–Cb

bonds and producing an aldehyde (23) or carboxylic acid (27) at the aromatic ring

and carboxylic acids groups on the split-off residues.

Phenylpropanols and phenylpropanones (Scheme 7.39, 28) react with the

hydroperoxide anion to form a hydroperoxide (29) that is rearranged to an ester

(30) which can be cleaved to an aldehyde and a phenolate (31) in a Dakin-like reaction.

The latter can be oxidized to a para-quinone (32) and further degraded (see

Scheme 7.38, 12 – 15).

In a lignin model study, guaiacylglycerol-b-guaiacyl-ether was oxidized with

alkaline H2O2 in the presence of pulp in order to simulate technical bleaching conditions

[24]. The phenolic b-O-4 structure was found to react rather rapidly with

H2O2 and, from the mixture of products formed, it was concluded that the main

reaction was a side-chain displacement that proceeded via the so-called Dakin-like

mechanism. This was followed by secondary reactions that resulted in cleavage of

the molecule, accompanied by an extensive formation of carboxyl groups [24].

O-

OCH3

C

R

-OOH

O

O-

OCH3

C

R

O-

O

OH

O-

OCH3

C

R

O

O-

OCH3

+

O-

Ox

O

OCH3

O

further oxidation

giving aliphatic

degradation products

O

Ox - OH -

C

R

- O O

+ 2OH-, - H2O

Scheme 7.39 Dakin reaction at the Ca-keto group of a phenolic

unit (adapted from Ref. [23]).

A bleaching sequence involving oxygen bleaching (O), treatment with a chelating

agent EDTA (Q), and an alkaline H2O2 stage (P), showed that partial removal

of the residual fiber lignin was accompanied by extensive removal of chromophoric

groups. It appeared that the chemical structure of lignin remaining in the

fibers after the OQP sequence was mainly unaffected by the treatment. The oxidation

resulted mainly in an increase in the number of hydrophilic groups, but the

lignin remained phenolic to a certain extent and the aromatic structure was preserved

[25].

858 7Pulp Bleaching

A new mechanism for the heterogeneous alkaline peroxide brightening reactions

of mechanical pulps consists of four key kinetic steps: adsorption of H2O2

and hydroxide to the pulp fiber walls; a chromophore-removing chemical reaction

on the fiber wall; desorption of “light” organic products formed from the fiber

wall; and oxidation chain reduction of the cleaved organic substances. The most

important step here is the surface reaction, rather than reactions occurring in the

liquid phase. In general, removal of the cleaved organic substances from the fiber

wall is not anticipated to occur completely during the brightening reaction operation

stage [26].

As shown, the main reaction mode of HO2

– is nucleophilic addition to enone

and other carbonyl structures, removing chromophoric groups by the destruction

of conjugated systems. Through addition of the hydroperoxide anion, certain peroxide

(anion) structures may be formed which can subsequently react in a way

similar to that of the peroxide (anion) structures arising from the addition of

superoxide anion radicals to substrate radicals; this gives rise to the formation of

C–C cleavage products [27–30].

Due to the fact that the number of enone and other carbonyl structures in lignin

and residual lignin is usually low, the extent of degradation during bleaching with

pure H2O2 also remains low. Therefore, the main part of this bleaching step is

chromophore removal and lignin retention. Due to the fact that the number of

enone and other carbonyl structures in lignin and residual lignin is usually low,

and the extent of degradation during bleaching with pure hydrogen peroxide

remains low too. Therefore, the main part of this bleaching step is chromophore

removing and lignin retaining. However, this needs to be put into context with

two facts: a) most peroxide stages follow other bleaching steps where enone structures

are formed, and b) at high temperature extensive delignification can occur

[aa, bb].

The hydroxyl radical is considered to be responsible for the small degree of lignin

degradation observed during H2O2 bleaching. This can be interpreted as the

chemical reactions of the hydroxyl radicals during oxygen bleaching (see Section

7.3.2.4, Chemistry of oxygen delignification). The occurrence of hydroxyl radicals

may possibly have a distinct beneficial effect that may be ascribed to the cleavage

of cross-links in the rigid lignin matrix, which will in turn facilitate the penetration

of bleaching reagent(s) [31] and thereby improve the bleaching result. This

interpretation is in accordance with results from studies where metal ions were

removed carefully from either the pulp [32,33] or from wood shavings before kraft

cooking [34], or were complexed with chelants [25,35–38], and increased the

brightness gain [33].

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