CHEMISTRY OF PULPING AND BLEACHING
Responsible teacher: Tapani Vuorinen
PART I – CHEMICAL PULPING ................................................................................................................. 4
1
GENERAL ASPECTS OF PULPING ................................................................................................... 5
1.1
1.2
2
REACTIONS OF WOOD POLYSACCHARIDES IN PULPING ........................................................ 9
2.1
2.2
2.3
2.4
2.5
2.6
2.7
3
REACTIVE SITES .............................................................................................................................. 28
CLEAVAGE OF b-ARYL ETHER (b-O-4) LINKAGES IN KRAFT PULPING ................................................. 29
CARBON-CARBON BOND CLEAVAGE ................................................................................................. 38
CONDENSATION .............................................................................................................................. 38
SUMMARY – THE MOST IMPORTANT LIGNIN REACTIONS IN KRAFT PULPING ........................................ 38
LIGNIN-CARBOHYDRATE COMPLEXES (LCC) .......................................................................... 40
4.1
4.2
5
WOOD POLYSACCHARIDES AND REACTIVITY ...................................................................................... 9
REACTIVE SITES .............................................................................................................................. 11
HYDROXYL ION-CATALYSED PEELING OF REDUCING END GROUPS ..................................................... 12
ACID-ALKALI-CATALYSED HYDROLYSIS OF ESTERS .......................................................................... 20
ACID-ALKALI-CATALYSED HYDROLYSIS OF GLYCOSIDIC BONDS ........................................................ 22
REACTIONS OF URONIC ACID GROUPS ............................................................................................... 24
SUMMARY - CARBOHYDRATE REACTIONS ........................................................................................ 26
LIGNIN REACTIONS IN PULPING .................................................................................................. 28
3.1
3.2
3.3
3.4
3.5
4
ACID DELIGNIFICATION ..................................................................................................................... 6
ALKALINE DELIGNIFICATION –KRAFT PULPING ................................................................................... 7
NATIVE LIGNIN-CARBOHYDRATE COMPLEXES IN WOOD .................................................................... 40
LC COMPLEXES IN KRAFT PULP AND COOKING LIQUOR ..................................................................... 41
REACTIONS OF EXTRACTIVES IN KRAFT PULPING ................................................................ 43
5.1
5.2
5.3
EXTRACTIVES IN WOOD ................................................................................................................... 43
REACTIONS OF EXTRACTIVES IN PULPING ......................................................................................... 49
DERESINATION AND PITCH CONTROL ............................................................................................... 49
PART II – BLEACHING.............................................................................................................................. 52
6
CHROMOPHORES IN PULP – REMOVABLE GROUPS IN BLEACHING .................................. 53
7
GENERAL ASPECTS OF PULP BLEACHING................................................................................. 56
7.1
7.2
8
CLASSIFICATION OF BLEACHING CHEMICALS ....................................................................... 58
8.1
8.2
8.3
8.4
8.5
8.6
9
ELECTROPHILES .............................................................................................................................. 58
NUCLEOPHILES ............................................................................................................................... 60
RADICALS ...................................................................................................................................... 61
AUXILIARY CHEMICALS IN BLEACHING ............................................................................................ 62
DESIGN OF BLEACHING SEQUENCES ................................................................................................. 63
SUMMARY -MAIN BLEACHING CHEMICALS ....................................................................................... 63
SELECTIVE HYDROLYSIS (A) ........................................................................................................ 65
10
OXYGEN DELIGNIFICATION (O) ............................................................................................... 67
10.1
10.2
11
CHEMISTRY OF OXYGEN .................................................................................................................. 67
REACTIONS OF WOOD CONSTITUENTS .............................................................................................. 68
CHLORINE DIOXIDE BLEACHING (D)...................................................................................... 71
11.1
11.2
12
REACTIONS OF WOOD CONSTITUENTS .............................................................................................. 71
COMBINED A AND D STAGES ........................................................................................................... 74
HYDROGEN PEROXIDE (P) ......................................................................................................... 75
12.1
12.2
2
AIM ................................................................................................................................................ 56
MASS TRANSFER IN PULP BLEACHING ............................................................................................... 56
CHEMISTRY OF HYDROGEN PEROXIDE BLEACHING ............................................................................ 75
REACTIONS WITH WOOD COMPONENTS ............................................................................................ 76
13
PERACIDS / PERACETIC ACID (PAA) ....................................................................................... 78
13.1
13.2
REACTIONS OF WOOD COMPONENTS ................................................................................................ 78
FACTORS AFFECTING THE REACTIONS .............................................................................................. 80
14
OZONE BLEACHING (Z) .............................................................................................................. 82
15
DONNAN EFFECT AND CATALYTIC BLEACHING ................................................................ 86
16
SOME ENVIRONMENTAL ASPECTS OF KRAFT PULPING AND BLEACHING ................. 88
16.1
16.2
ENVIRONMENTAL IMPACTS OF PULPING AND BLEACHING EFFLUENTS AND AIR EMISSIONS .................. 88
BAT (BEST AVAILABLE TECHNIQUES) ............................................................................................. 88
3
PART I – Chemical pulping
Studying Part I
When approaching this material, it will be useful to first scan the table of contents and then
proceed to the chapters. It is advisable to study relevant chapters before the actual lecture.
By doing so, you will find the lecture more interesting and understandable. After the lecture,
re-read the chapter and your lecture notes to check your understanding.
It is essential for Finnish students that translations are available for all unfamiliar words and
concepts. The Glossary in Appendix A will help you with nomenclature and the structures of
chemical compounds. Please make sure that you understand the basic chemical reaction
mechanisms (addition, elimination, substitution, cleavage, ionisation, depolymerisation and
hydrolysis, etc) and what they mean.
Part I focuses on chemical pulping and in particular on alkaline kraft pulping. A few
examples of other chemical pulping methods are also given. Parts I and II contain notable
amount of kinetic equations and often students find these equations hard to understand. To
address this Appendix B, on reaction kinetics, has been provided which gives the necessary
prerequisites, with some examples and study tips.
Learning objectives are stated at the beginning of each of the chapters. The aim of these
learning objectives is to guide your learning and help you to concentrate on the most
important issues. Since this study material is relatively broad it is useful to highlight these
key issues. However, it is not sufficient to study just the learning objectives as you also need
the other information to support your understanding of the key issues.
An effective way of studying and learning is to pose/ask questions of yourself or to your
study colleagues and then seek explanations or solutions. At the end of each chapter a
section called Study problems contains questions to check understanding. In order to be able
to answer these questions you need both the key basic but also some deeper knowledge. A
few chapters also contain summaries and additional tips.
4
1 General Aspects of Pulping
Learning objectives
1. Understand the aim of chemical pulping.
2. Describe the mains steps of delignification and know how impregnation of wood
chips proceeds.
3. List the main methods and applications of chemical pulping.
4. Name and explain the main delignification stages in kraft cooking.
The main purpose of pulping is to liberate the fibres from the wood matrix. Before the actual
chemical pulping the wood is debarked and cut into chips. Wood chips are treated with
aqueous cooking solution at elevated temperature and pressure. Depending on the pulping
method the solution may be alkaline, neutral, or acidic.
The initial stage of any kind of chemical pulping method is an impregnation stage. During
this stage wood chips are impregnated with the cooking liquor. Impregnation proceeds via
liquid penetration into wood cavities, such as lumens, and the diffusion of dissolved pulping
chemicals. Diffusion controlled impregnation is a slow process in comparison with
penetration. The rate of penetration depends on the pressure gradient. Furthermore both
pore size distribution and capillary forces affect the penetration of cooking liquor into the
chips. The rate of diffusion is mainly dependent on the concentration of dissolved chemicals.
Diffusion is affected only by the total cross-sectional area of accessible pores (pores large
enough for a molecule of a given size to pass).
There are also other steps in pulp delignification during pulping. As mentioned before
during the first step wood chips are impregnated with cooking liquor by means of
penetration and diffusion. Step 2 involves the sorption of cooking chemicals on the surfaces
of wood chips. In the third step the chemical reactions between cooking chemicals and wood
components take place. In step 4 the reaction products are desorbed. Step 5 involves the
diffusion of reaction products onto the outer surfaces of wood chips. Similar kinds of
reactions happen also during the pulp bleaching although in this case wood fibres are
already separated from the each other. This is described more in detail in Chapter 7.
In order to liberate the fibres, lignin has to be removed from the middle lamella. Lignin is
also present in the secondary cell wall. As the cooking chemicals travel from the lumen
through the secondary and primary cell walls towards the middle lamella, both lignin and
polysaccharides are also attacked by the cooking chemicals. Consequently the yield in
chemical pulping is relatively low, being only 35-65 % of wood. The lignin in the middle
lamella is dissolved last. Although the main part of the lignin is removed during the
pulping, the wood chips maintain their shape. Only after the pulp and spent liquor are
blown out of the digestor do the chips come apart into individual fibres. Refining can also be
used to improve disintegration of fibres.
5
The alkaline kraft (i.e. sulphate) pulping process is the dominant wood pulping method.
This course deals mainly with chemistry of kraft pulping but some examples of other
methods are also provided.
1.1 Acid delignification
1.1.1 Sulphite pulping
It has been estimated that nowadays less than 20 % of chemical pulp is produced by the
sulphite process. The sulphite pulping method is based on the use of a bisulphite solution as
the delignifying agent. Active chemicals in the pulping solution are sulphur dioxide (SO 2),
hydrogen sulphite ions (HSO3-), and sulphite ions (SO32-). Calcium, sodium, magnesium or
ammonium is often used as the cation. The pH of the solution sets limits for the selection of
the cation. Calcium is suitable for pH < 2, magnesium for pH < 4 and both sodium and
ammonium are suitable for alkaline conditions. For an acid process the pH is 1-2. The
bisulphite process is carried out at pH 2-6. The pH of the neutral sulphite semichemical
process (NSSC) is 6-9 and for alkaline processes pH is 9-13. As can be observed the sulphite
pulping process includes a wide range of pH levels and cations.
Reactions of wood constituents
The main lignin reactions are sulphonation, hydrolysis and condensation reactions.
Sulphonation leads to generation of hydrophilic sulphonic acid groups. Hydrolysis breaks
aryl ether linkages between the phenyl propane units of lignin. Sulphonation and hydrolysis
are the reactions behind delignification and increase in the hydrophilicity of lignin.
Condensation reactions generate new stable carbon-carbon linkages and the solubilisation of
lignin is retarded.
The main reactions of cooking liquor and hemicelluloses include cleavage of acetyl groups,
hydrolysis of glycosidic bonds, oxidation of aldoses to aldonic acids, and dehydratation of
monosaccharides. Sulphite pulping causes also changes in the chemical structure of
extractives. The main reactions in this case are dehydration, sulphonation, hydrolysis of fats
and waxes and evaporation.
Process variables
Wood raw material (wood species, chip thickness and moisture content), cooking
temperature and pH are the main sulphite pulping process variables. In order to achieve a
sufficient degree of delignification the impregnation of wood chips must be sufficient. If the
moisture content is low, the cell pits are closed and impregnation is retarded and
insufficient. The same applies if the chip thickness is excessive. The rate of delignification is
temperature dependent. It is also well known that pH is dependent on temperature. Acid
sulphite pulping is carried out at lower temperatures (120-150 ºC) than bisulphite (150-160
ºC) or neutral sulphite (160-180 ºC) processes.
1.1.2 Organosolv pulping
The principle of acid-catalysed organosolv delignification is a steam hydrolysis in the
presence of organic solvents. The most commonly used solvents are methanol, ethanol, and
6
acetic and formic acids. Other suitable chemicals are phenols, amines, glycols, nitrobenzene,
dioxane, dimethyl sulphoxide, sulpholane, and liquid carbon dioxide. Solvent-assisted
alkaline organosolv processes also exists.
One application of organosolv pulping is the Milox process. The cooking chemicals are
formic acid and hydrogen peroxide. The method is applicable for production of straw pulp.
This kind of pulp is more easily bleached in than kraft pulp since the pulp contains minor
amounts of condensed lignin structures. In addition lignin contains reactive phenolic
hydroxyl groups. Due to acidic conditions the hemicellulose yield is low in the Milox
process. Another application of organosolv pulping is Alcell technology using ethanol-water
as cooking liquor.
1.2 Alkaline delignification –kraft pulping
1.2.1 Process description
The alkaline cooking liquor is composed of white liquor, water in chips, condensed steam
and also weak black liquor. The main components of white liquor are NaOH and Na 2S. In
addition to the main components minor amounts of Na 2CO3, Na2SO4, Na2S2O3, NaCl and
CaCO3 and accumulated salts are also present in the cooking liquor. In white liquor both
NaOH and Na2S are dissociated. The main reactions are shown below:
NaOH + H 2O « Na + + OH - + H 2O
Na2 S + H 2O « 2 Na + + S 2 - + H 2O
S 2- + H 2O « HS - + OH -
(1)
(2)
(3)
The kraft delignification process is usually carried out at elevated temperature (160º-180º) for
1-2 hours.
1.2.2 Delignification stages
Alkaline kraft pulping has three distinct stages: the initial (ID), bulk (BD) and residual (RD)
phases. These stages have different selectivity with respect to carbohydrate losses, as
illustrated in Figure 1.
During the initial delignification (ID) stage wood chips are impregnated with cooking
liquor. The initial stage is also called the heating-up period since during this stage the
temperature is elevated up to 140 ºC. The lignin content is not significantly reduced. The
reduction is only 15 % - 25 %. The selectivity is low and the carbohydrate losses are extensive
during the initial phase. Approximately 40 % of hemicelluloses are degraded during the
initial delignification.
In the bulk delignification (BD) stage cooking temperature exceeds 140 ºC. The increasing
temperature accelerates the reactivity and depolymerisation of lignin. The main part of
lignin is removed during this stage. After the BD stage about 90 % of the lignin has been
removed from chips.
7
Figure 1. Changes of polysaccharide and lignin contents during the initial (ID), bulk (BD) and
residual (RD) stages of kraft pulping (Smook, G.A., Handbook for Pulp and Paper Technologist, 2 nd
edition, Angus Wilde publication, Vancouver, Canada, 1992, 419p.)
The degradation and dissolution of hemicelluloses during the ID stage contributes to the
removal of lignin from the cell wall and compound middle lamella. As the porosity of cell
wall increases, the diffusion of lignin degradation products from the cell wall to the cooking
liquid phase becomes easier. In the BD stage the selectivity is fairly good and carbohydrate
losses are relatively low. At the end of the bulk delignification the selectivity starts to
decrease.
During the residual delignification (RD) the depolymerisation of cellulose increases notably
and delignification decreases. Approximately only 10 % of lignin is removed. However new
carbohydrate-lignin linkages are also formed. These structures are often stable in kraft
pulping conditions.
Literature
Alén R., Chapter 2. Basic Chemistry of wood delignification. Forest Products Chemistry. Ed.
Stenius, P., Publ. Fapet, Jyväskylä, Finland, 2000, pp. 59-104.
Gullichsen, J., Chapter 6. Fiber line operations. Chemical Pulping. Ed. Gullichsen, J. &
Fogelholm, C-J., Publ. Fapet, Jyväskylä, Finland, 2000, pp.
Smook, G.A., Handbook for Pulp and Paper Technologist, 2nd edition., Angus Wilde
publication, Vancouver, Canada, 1992, 419 p.
8
2 Reactions of wood polysaccharides in pulping
The selectivity of kraft pulping is relatively low. This means that unwanted side reactions
occur and polysaccharides are attacked during pulping. The reactivity of wood
polysaccharides varies depending on their accessibility and the type and amount of
monomer components and their substituents. Hemicelluloses are branched and have
remarkably lower molecular mass (i.e. degree of polymerisation) than cellulose.
Hemicelluloses have an amorphous structure and are not as stable to chemical attack as
cellulose, which contains both amorphous (i.e. non-crystalline) and crystalline regions. In
comparison to hemicelluloses, cellulose is relatively stable to acid and alkaline attack. A
large amount of hemicelluloses are dissolved and degraded mainly due to peeling reactions,
which take place even at low temperatures.
The main reactions are
1. peeling (i.e. endwise degradation) (see Ch. 2.3)
2. cleavage of glycosidic bonds (see Ch. 2.5)
3. also hydrolysis of esters occurs in kraft pulping (see Ch. 2.4)
After this chapter you should be able to describe the main reactions of polysaccharides and
their effects on pulp quality and properties. First, concentrate on the properties of wood
polysaccharides. It is important that you understand the differences between hemicelluloses
and cellulose. Make sure that you comprehend why hemicelluloses are more reactive than
cellulose. Then concentrate on the reactions of polysaccharides during pulping. Below is list
of key questions you should be able to answer after reading this chapter.
What does selectivity mean? Does the degradation of polysaccharides have a negative or
positive effect on the pulping process? Or maybe both? What are the main reactions
involved? Are some of the reactions more important than others? How do process
conditions, such as pH and temperature affect the rate of these reactions? Which reactions
happen in the initial, bulk and residual delignification stages?
Appendix A helps you with nomenclature and the structures of different chemical
compounds mentioned in this and other chapters.
2.1 Wood polysaccharides and reactivity
A. Cellulose
Cellulose consists of (1→4)-linked b-D-glucose units, which form a linear glucan chain
(Scheme 1, p. 11). The degree of polymerisation (DP) of native wood cellulose is
approximately 10 000. Cellulose chains form crystalline and non-crystalline (i.e. amorphous)
regions. The reactivity of crystalline regions is much lower than it is in non-crystalline
regions.
9
In the initial delignification stage cellulose is more resistant to primary peeling than are the
hemicelluloses. This is due to the high degree of polymerisation and crystallinity of the
cellulose structure. During the bulk delignification stage the temperature is above 140
degrees. The increasing temperature accelerates the cleavage of glycosidic bonds between
glucopyranose units of cellulose. Alkaline hydrolysis of glycosidic bonds regenerates new
reducing end groups leading to secondary peeling reactions.
B. Hemicelluloses
Hemicelluloses consist of a large variety of heteropolysaccharides. The main function of
hemicelluloses is to regulate the water content of the cell wall. Hemicelluloses are branched
and the degree of polymerisation is notably lower, approximately 150-200, than that of
cellulose. The main wood hemicelluloses are shown in Table 1.
Glucomannan chains are relatively short and have less substituents (i.e. they have less
branched structure) and therefore they are easily degraded and dissolved during pulping.
Xylan polymers are more favourably substituted (1®2 and 1®3 linkages) than glucomannans
and therefore they survive the cook to a greater extent. However, the degree of
polymerisation (DP) decreases. The main reactions of both glucomannans and xylans are
primary peeling reactions during the initial delignification stage. In addition, hexenuronic
acid groups are generated from the 4-O-methylglucuronic acid groups of xylan.
Table 1. The major hemicellulose components in wood. (Sjöström, E., Wood Chemistry.
Fundamentals and Applications. 2nd edition. Academic Press. 1993, 293 pp.)
10
Hemicellulose
Occurrence
Galactoglucomannan
Softwood
Amount
(% of wood)
5-8
(Galacto)glucomannan
Softwood
10-15
Arabinoglucuronxylan
Softwood
7-10
Arabinogalactan
(non-cell wall
polysaccharide in larch
heartwood)
Glucuronoxylan
Larch wood
5-35
Hardwood
15-30
Glucomannan
Hardwood
2-5
Units
b-D-Manp
b-D-Glcp
a-D-Galp
Acetyl
b-D-Manp
b-D-Glcp
a-D-Galp
Acetyl
b-D-Xylp
4-O-Me-a-D-GlcpA
a-L-Araf
b-D-Galp
a-L-Araf
b-L-Arap
b-D-Glcp
b-D-Xylp
4-O-Me-a-D-GlcpA
Acetyl
b-D-Manp
b-D-Glcp
Molar ratios Linkage
3
1
1
1
4
1
0,1
1
10
2
1,3
6
2/3
1/3
Little
10
1
7
1-2
1
1®4
1®4
1®6
1®4
1®4
1®6
1®4
1®2
1®3
1®3,6
1®6
1®3
1®6
1®4
1®2
1®4
1®4
2.2 Reactive sites
The main functional units of wood polysaccharides are glycosidic bonds, hydroxyl groups
and reducing end groups, as illustrated in Figure 2. Also ester structures are reactive. The
reactivity of these units varies among cellulose and hemicellulose components and
contributes to their differences in chemical and supramolecular structure.
Glycosidic bonds
HOH2C
HO
CellO
HOH2C
Hydroxyl groups (-OH)
O
O
O
HO
OH
HOH2C
OH
O
O
OH
HO
OH
Reducing end group
Figure 2. The major functional groups of wood polysaccharides. Here as an example reactive sites
of (1→4)-linked b-D-glucan, i.e. cellulose.
A. Reducing end groups
Reducing carbohydrates are polysaccharides that can be oxidised by a mild oxidising agent.
In other words the reducing carbohydrate is oxidised and the oxidising agent is reduced in
the course of the reaction. Reducing carbohydrates contain a reducing end group that is a
hemiacetal unit, Fig. 2. In solution, cyclic hemiacetal end group is in equilibrium with the
open-chain aldehyde form. Although the amount of open-chain forms is very small (<< 1 %),
it has a significant effect on the degradation of wood polysaccharides in pulping.
B. Glycosidic bonds
Glycosidic bonds connect monosaccharide units together to form a polysaccharide structure.
These bonds are acetal in nature and can be hydrolysed under acidic and alkaline conditions
and through oxidation. Acid hydrolysis proceeds in milder conditions whereas alkaline
hydrolysis requires more drastic conditions. The hydrolytic reaction leads to cleavage of
glycosidic bonds and weakens the mechanical properties of fibres.
C. Hydroxyl groups
The relative reactivity of hydroxyl groups may vary within a monosaccharide unit. For
example the reactivity of free hydroxyl groups of cellulose in C2, C3 and C6 positions
depends on the reaction conditions and steric hindrances.
The reactivity of hydroxyl groups in a heterogeneous system depends on the accessibility of
these functional groups. Hydroxyl groups may be in a region inaccessible to a reagent or the
groups might be hydrogen-bonded to other units. Naturally this lowers the reactivity.
11
D. Ester bonds
Certain hemicelluloses contain ester linkages. For example softwood galactoglucomannans
and hardwood xylan are partly acetylated. In other words these hemicelluloses contain
acetyl groups that form ester bonds with monosaccharide units, Fig. 3. Galacturonic acid
groups in the primary cell wall pectin exist partly as methyl esters.
HOOC
MeO
Hardwood xylan (methylglucuronoxylan)
HO
OH
O
O
HO
AcO
O
HO
4
O
O
O
OH
HO
3
O
5
2
3
2
AcO
O
OH 1
OH
O
1
5
4
O
O
OH
HO
OAc
O
Softwood galactoglucomannan
Ester bonds (
HOH2C
OH
O
C
CH3 )
O
HO
OH
O
HOH2C
HO
HO
O
OH
OH
HO
O
O
HOH2C
4 H2C 6
O
HO
5
2
3
3
O
OH
HO
O
1
5
4
HOH2C 6
2
OAC
O
HOH2C
1
O
O
OH
HO
OH
Figure 3. Softwood galactoglucomannans and hardwood xylans are partly acetylated (i.e. they
contain acetyl groups (Ac = CH3CO-)). These ester linkages are easily hydrolysed (see Chapter 2.4)
2.3 Hydroxyl ion-catalysed peeling of reducing end groups
The peeling of reducing end groups plays an important role in alkaline pulping. Already at
relatively low temperatures wood hemicelluloses and cellulose react in a way which leads to
peeling of reducing monosaccharide end units. Depolymerisation of polysaccharides occurs
so that one monosaccharide unit after another is cut off from the end of the polymer chain.
The reaction occurs in the reducing end of the chain, i.e. the end that contains a hemiacetal
unit (see 2.2, Figure 2). There are different pathways along which the peeling reaction can
proceed. The peeling of reducing end groups is known as primary peeling or end-wise
peeling. Sometimes the term end-wise depolymerisation is also used. Another kind of
peeling reaction also occurs during pulping known as secondary peeling and proceeds after
the hydrolysis of internal glycosidic bonds (see 2.5). The primary peeling reaction takes
places at the early stage of cooking, i.e. in the initial stage, whereas secondary peeling takes
place during bulk delignification. Peeling reactions are stopped by other reactions, which
lead to end group stabilisation. The primary peeling and stopping reactions are described in
more detail in the following examples:
Example A) With 1→4 bonded polysaccharides, such as glucomannans and cellulose (see
2.1), the peeling reaction begins with the isomerisation1 of a reducing aldehyde end unit. The
Isomerisation, rearrangement of atoms in a molecule without removing or adding any atoms to the original
molecule.
1
12
rearrangement of a reducing end unit to a keto intermediate is followed by β-alkoxy
elimination. In other words a glycosyl group (i.e. either glucose or mannose unit) which is at
β-position to the carbonyl group is easily eliminated. The reason is that glycosyl anion is a
good leaving group. Consecutive isomerisation and elimination reactions may lead to the
degradation of whole glucomannan chains, since glucomannans have a low degree of
polymerisation (DP<200), Scheme 1.
Rreducing end group
OH
OH
O
O
O
RO
PEELING
REACTION:
OH
HO
OH
HO
O
RO
O
H
Isomerisation of
reducing aldehyde
end unit
O
O
HO
OH O
HO
OH
OH
OH
OH
OH
OH
HO
OH
OH
O
RO
OH
HO
OH
O
RO
O
OH
O
HO
OH
HO
OH
Formation of keto
intermediate
O
OH
OH
b
O
RO
O O
O
RO
O
a
O
H
OH
O
OH
OH
HO
OH
HO
O
HO
HO
OH HO
O
RO
b
O
OH
a
Hydroxy acid
HO
OH
HO
OH
OH
RO
OH
O
OH
O
O
OH
OH
OH
New reducing
end group
HO
OH
OH
HO
b-alkoxy elimination and
formation of a new
reducing end group and
hydroxy acid
HO
Scheme 1. Endwise peeling reaction of glucomannan.
Primary peeling is less significant for cellulose, which consists of linear (1→4)-linked b-Dglucan chain. Usually the peeling stops when in average 65 sugar units have been peeled off
the cellulose chain. Why are only a few glucose monomers attacked and peeled off? The
explanation for this is that cellulose is a huge polymer and contains crystalline regions which
are very stable. The peeling reaction is stopped when these crystalline regions are reached.
Since the degree of polymerisation of native cellulose is approximately 10 000- 15 000, the
primary peeling reaction has a minor effect on the chain length and viscosity of cellulose.
Degradation products (i.e. diuloses, which are diketose compounds) are formed during the
peeling reaction. These chemical compounds are further degraded by isomerisation,
13
elimination and fragmentation forming a large number of different hydroxy carboxylic
acids. In addition acetic, formic acid and dicarboxylic acids are formed. See Table 2. The
neutralisation of these acids consumes the main part of the alkali in kraft cooking liquor
(70%-75% of charged alkali).
Table 2. Typical composition of the dry matter of Scots pine and silver birch kraft pulp liquors (%
of total dry matter) (Alén, 2000)
Component
Pine
Birch
Lignin
31
25
Aliphatic carboxylic acid
29
29
Formic acid
6
4
Acetic acid
4
8
Glycolic acid
2
2
Lactic acid
3
3
2-Hydroxybutanoic acid
1
5
3,4-Dideoxypentonic acid
2
1
3-Deoxypentonic acid
1
1
Xyloisosaccharinic acid
1
3
Glucoisosaccharinic acid
6
3
Others
3
3
Other organics
7
9
Extractives
4
3
Carbohydrates (hemicellulose-derived fragments)
2
5
Miscellaneous
1
1
Inorganics
33
33
Sodium bound to organics
11
11
Inorganic compounds
22
22
Example B) Typically wood contains only minor amounts of hemicellulose called
arabinogalactan (see Table 1, p.10). However, larch wood is an exception to this.
Arabinogalactans react readily with alkaline cooking liquor. Under alkaline cooking
condition peeling of larch arabinogalactan can proceed easily since the β-elimination takes
place at 1→3 bonded glycan2 (i.e. monosaccharide units) without isomerisation.
Example C) Softwoods contain arabinoglucuronoxylans. The backbone consists of (1→4)linked b-D-xylopyranose units which are partly substituted at C-2 by methylglucuronic acid
groups. The backbone also contains arabinofuranose units (see Table 1). During the course of
peeling arabinose substituents of softwood xylan (arabinoglucuronoxylan) may be
eliminated by b-alkoxy elimination without isomerisation. This reaction takes place if the
arabinose substituents are in the reducing end unit of xylan. As a result 3-deoxy aldosulose
(i.e. a compound containing both keto and aldehyde functions) becomes the end unit of
polymer chain. Due to rearrangements a final 3-deoxypentonic acid group is formed and this
can stabilise the xylan chain against further peeling. In other words the stopping reactions
lead to end group stabilisation, Scheme 2. Without such a competing stopping reaction a
whole molecule may be destroyed by peeling. However, the xylan content in softwoods is
relatively low which means that these reactions are less significant and have a minor
contribution to the total carbohydrate yield during kraft pulping. Glucuronoxylans are the
2
Glycan, a generic term for any monosachharide or assembly of monosaccharide.
14
main hemicellulose type in hardwoods. The behaviour of xylan during hardwood kraft
pulping is more important therefore. These reactions are described in example D.
-b-D-Xylp-1®4-b-D-Xylp-1®4-b-D-Xylp
HO
OH
HO
HO
OH
OH
O
O
OH
O
O
RO
RO
O
OH
O
HO
HO
OH
HO
OH
OH
ba
O
RO
O
OH
O
HO
OH
O
Formation of 3-deoxy
aldosulose
OH
OH
HO
O
OH
O
OH
RO
O
OH
OH
HO
O
O
O
RO
O
H
O
O
HO
b-alkoxy elimination (i.e.
cleavage of arabinose
side chain).
OH
OH
O
RO
OH
HO
Cleavage of a
reducing xylose end
unit
OH
OH
O
O
O
OH
O
OH
Araf
HO
O
OH
O
PEELING
REACTION:
O
O
O
OH
O
OH
HO
STOPPING
REACTION:
OH
Rearrangements in
the reducing end
O
O
group. Stabilisation of
OH
HO
xylan chain against
O
peeling.
O
Scheme 2. Peeling and stopping reactions of arabinoglucuronxylan (softwood). Xylp xylopyranose
unit, Araf arabinofuranose unit.
O
RO
O
OH
O
RO
O
Stopping reactions can proceed in different ways. For example 3-deoxyaldonic acid groups
can also be formed directly via β-hydroxy elimination. However the reaction is slow since
the hydroxyl ion is a worse leaving group than the glycosyl3 anion. This reaction may finally
stop the peeling of glucomannans.
It is possible, yet quite complicated, to describe the kinetics of the peeling reaction by using
Schemes 1 and 2. Rate of reaction paths is determined by enolisation (i.e. enol-keto
interconversion). Enolisation is followed by isomerisation or β-elimination. Both of these
reactions are extremely fast. Enolisation is an intramolecular reaction and therefore the total
rate of reaction depends on the degree of ionisation of the reducing end groups (Equation 4).
Glycosyl, a structure obtained by removing the hydroxyl group from the hemiacetal function of a
monosaccharide.
3
15
The ionisation constant for reducing sugars is approximately 30 M -1 at room temperature
(pKa 12.5). Total rate of peeling reaction is:
[
]
kK ion HO - × c
dc
= kx ion c =
dt
1 + K ion HO -
[
]
(4)
During the primary peeling reaction new reducing end groups are formed. Therefore the
amount of reducing end units is constant during the peeling (Scheme 2). A concurrent
stopping reaction can decrease the amount of reducing end groups (Scheme 3, Equation 5).
[RE ] = [RE ]0 e - k t
'
(5)
where [RE] is the amount of reducing end groups and k’ is a rate constant of the stopping
reaction. Formation of degradation products can now be described by Equation 6.
'
d [ A]
= k [RE ]0 e - k t
dt
(6)
where [A] is the concentration of degradation products and k is a rate constant of the peeling
reaction. Integration gives the following Equation:
[A] = [RE ]0 (1 - e -k t ) k'
'
k
(7)
The ration k/k’ describes how many monosaccharide units are peeled off from the
polysaccharide chain before the chain is stabilised. The magnitude of the ratio is 10 2.
Equation 7 is illustrated in Figure 4.
Figure 4. Formation of degradation products as a function of time (k/k’=100, k=1).
16
Both isomerisation and β-elimination are unimolecular reactions (i.e. involve one molecule)
which proceed via formation of an enol structure. Consequently the rate of the stopping
reaction is lower than rate of the peeling reaction indicating higher enthalpy for the stopping
reaction. In principle the degree of peeling is lower at higher temperatures. Due to high rate
of reaction the peeling of glucomannans occurs before the maximum temperature of cooking
is achieved. In other words reaction takes place in initial delignification.
Example D) Hardwood xylan (glucuronoxylan) is formed of (1→4)-linked b-D-xylose units.
In addition the xylan backbone contains methylglucuronic acid groups, acetyl groups and
sometimes also galacturonic acid and rhamnose groups in the vicinity of reducing end
groups. The first case shows an example where the main xylan backbone has galacturonic
acid and rhamnose substituents (Case 1, Scheme 3). Case 2 shows a situation where the xylan
backbone has methylglucuronic acid substituents (Case 2, Scheme 4).
Case 1 – The reducing end of a birch xylan chain contains galacturonic acid and rhamnose
units. In other words the unit next to the reducing xylose end unit is D-galacturonic acid
which is linked to an L-rhamnose unit through the C-2 position. The rhamnose unit is
connected through its C-3 position to the xylan chain:
-b-D-Xylp-1®3-a-L-Rhap-1®2-a-D-GalpA-1®4-b-D-Xylp (see also Scheme 3)
The reducing xylose end group of hardwood xylan is peeled off in a normal way (Scheme 3).
Isomerisation is not possible here since the galacturonic acid group is linked to the next
xylose unit through the C-2 position. In this case a direct β-hydroxy elimination is the main
reaction. The double bond formed during the reaction can shift to a 3,4-enolate which is
formed by the enolisation of a conjugated structure.
A glycosyl (i.e. rhamnose unit) substituent in β-position enables the elimination. The
releasing rhamnose unit is peeled off easily by a direct β-elimination. After that the peeling
reaction can proceed normally. It is useful to remember that both galacturonic acid and
rhamnose contents are very low in hardwood xylan.
17
Xylp
Rhamp
HO
HO
OH
O
RO
OH
O
RO
O
O
O
HO
HO
HO
O
HO
OH
OH
Cleavage of a
reducing xylose end
unit
O
O
Xylp
O
O
HO
OH
O
O
OH
OH
GalA
HO
O
HO
O
HO
PEELING
REACTION:
OH
OH
OH
HO
OH
O
RO
OH
RO
O
O
O
HO
OH
O
O
HO
O
O
H
O
HO
O
Enolisation of a
conjugated structure
and elimination
O
OH
OH
O
O
OH
HO
OH
OH
OH
O
RO
O
HO
HO
O
OH
OH
RO
OH
O
Cleavage of a reducing
rhamnose end unit and
formation of a new
reducing xylose end unit
Scheme 3. Case 1 – Peeling of hardwood xylan, when the chain contains galacturonic acid and
rhamnose groups. Xylp is xylopyranose, i.e. xylose unit, Rhamp is rhamnopyranose unit and GalA
is galacturonic acid.
Case 2 – A xylan chain is partially substituted with 4-O-methylglucuronic acid groups ( 1®2
linkages). A methylglucuronic acid substituent in a reducing xylose end group prevents its
isomerisation and thus the peeling of xylan at lower temperature (< 100°) and therefore the
xylan chain is not excessively attacked during the initial stage of delignification.
During bulk delignification the temperature is higher and methylglucuronic acid groups
offer only a partial protection against primary peeling. When there is a 2-O (4-O-methyl-αD-glucopryranosyluronic acid) substituent in the reducing end unit of xylan, conversion of
this carbon (C-2) to carbonyl group is impossible. Therefore the main reaction is a direct βhydroxy elimination (i.e. elimination of HO-3 group). 3,4-enolate can not be formed because
HO-4 group is not free. In this case the most probable reaction is a new β-hydroxy
elimination. The conjugated end group4 is likely to undergo a cyclisation. After that the
reaction can only continue via hydrolysis of the enol ether structure, Scheme 4.
4
Conjugated end group is an end group that contains conjugated double bonds.
18
-b-D-Xylp-1®4-b-D-Xylp-1®4-b-D-Xylp
HO
OH
HO
HO
OH
OH
O
HO
O
HO
OH
O
O
O
HO
O
HO
O
OH
O
O
OH
O
O
OH
GlcA
OH
O
O
HO
PEELING
REACTION:
O
O
OH
OMe
O
OMe
Cleavage of a
reducing xylose end
unit and formation of
a new reducing end
unit with GlcA-
OH
OH
HO
O
ba
O
HO
HO
OH
O
O
O
O
O
OH
O
O
HO
O
O
OMe
OH
O
HO
OMe
Conjugated end group
OH
HO
OH
HO
OH
O
OH
O
b-hydroxy elimination
(i.e. cleavage of OHgroup) takes place
and leads to
formation of a
conjugated end group
OH
HO
O
O
O
HO
O
O
OH
O
OH
O
O
HO
O
O
O
OH
O
OH
O
OMe
OH
O
OMe
Conjugated end group
undergoes
cyclisation.
OH
O
HO
O
O
O
OH
O
O
OH
O
OMe
Scheme 4. The course of the peeling reaction of hardwood xylan, when the chain has glucuronic
acid (GlcA) substituents.
The effects of primary peeling are drastic to hemicelluloses. Approximately 70-80 % of
glucomannans are dissolved during the kraft pulping. The main part is dissolved already
during the initial delignification. Compared with glucomannans, xylans are peeled off
slowly. Therefore the degree of peeling of xylan remains lower (40-50 %). In cooking solution
xylans exist as polymers whereas polymeric glucomannans are not present. Instead, they are
degraded mainly to monomeric hydroxyl carboxylic acids. It is obvious that at least in the
case of hardwood xylan the enol structures hinder the peeling. Nucleophiles are able to
attack a conjugated aldehyde group which is formed in the end of a xylan chain. Model
compound studies have shown that the reaction leads to the formation of stable lignincarbohydrate linkages. Formation of new stable LC-linkages is an unwanted reaction.
19
Stabilisation against alkaline primary peeling
The carbohydrate yield can be improved by using oxidative and reductive pre-treatments
which decrease the amount of reducing end groups in wood chips. Reductive pre-treatments
are based on the reduction of aldehyde groups to alcohols. Another possibility is to oxidise
aldehyde groups to carboxyl groups for example by carrying out the cooking process under
oxidative conditions (i.e. by adding polysulphide). Polysulphide is able to oxidise a part of
the enolates and therefore it increases the rate of the stopping reaction. (k’ in Equation 7,
page 16).
It is important to realise that the degradation of carbohydrates is essential in respect of
increasing the cell wall porosity and delignification. The aim of chemical pulping is to
liberate the fibres from the wood matrix and in order to do so lignin has to be removed from
the middle lamella. However, the main part of lignin is located in the secondary cell wall. As
the cooking chemicals travel from the lumen through the secondary and primary cell walls
towards middle lamella, both lignin and polysaccharides are attacked by the cooking
chemicals. The lignin in the middle lamella is dissolved last. In order for lignin to be
removed from the middle lamella and the secondary wall, the cell wall has to be sufficiently
porous so that the dissolved and degraded products can be removed from the cell wall.
Consequently it is practically impossible to obtain very high yield chemical pulp.
2.4 Acid-base-catalysed hydrolysis of esters
Esters are not very common linkages in polysaccharides or in wood. However, softwood
glucomannans and hardwood xylans are partly acetylated which means that they contain
ester bonds (Figure 3, page 12). Some of the linkages between lignin and hemicelluloses are
also esters. It is well known that esters are easily hydrolysed in alkaline and also in acidic
conditions.
Under alkaline pulping conditions the rate of hydrolysis is determined by the addition of
hydroxyl ion to an ester linkage, Scheme 5 right hand side reaction. What are the key factors
in this reaction? The rate of the ester bond cleavage is determined by the rate constant (kHO-)
and concentrations of ester ([E]) and hydroxyl ions ([OH-]). Therefore the rate equation is:
d [E ]
= - k HO - [E ] × HO dt
[
]
(8)
Equation 8 states that under alkaline conditions, such as in kraft pulping or alkaline
bleaching sequences, the cleavage of ester linkages increases with increasing pH. This is also
illustrated in Figure 5.
Hydrolysis of esters also occurs in acidic conditions, such as in acid sulphite pulping and
acidic bleaching sequences. The reaction path here is somewhat different from that in
alkaline conditions. In acid conditions hydrolysis of esters begins with the formation of a
carbonium ion, Scheme 5 left hand side reaction. The second step is the addition of water to
the carbonium ion, followed by the cleavage of the ester bond. Formation of a carbonium ion
determines the rate of reaction. Since an intermediate, which is formed in the reaction, is a
20
protonated ester structure, and the degree of protonation remains very low in all reaction
conditions, the rate of reaction can be written as:
[dE ] = -k
dt
H 3O +
[E ] × [H 3O + ]
(9)
The equation shows that the cleavage of ester bonds depends on the concentrations of ester
([E]) and hydronium ion [H3O+]. This is also illustrated in Figure 5. In other words the rate of
reaction increases with increasing acidity. The temperature dependent rate constant, k H3O+, is
specific for this reaction and is characterized by the entropy and enthalpy of activation
(energy of activation is often used instead of enthalpy) as explained earlier (see Appendix B).
There is also a third kind of reaction present. This so called water-catalysed reaction also
occurs during the hydrolysis of ester bonds. The reaction mechanism is a combination of
acid and alkali-catalysed reactions.
The overall rate of reaction in a large pH range is therefore (Equation 10, Scheme 5):
d [E ]
= - k H O + [E ] × H 3O + - k H 2O [E ] - k HO - [E ] × HO 3
dt
[
OH
OR'
]
OH
[
]
OR'
OR' k(HO-)
k(H3O+)
+
HOH3O+
R
OH
R
O
(10)
OR'
OH
OH
R
O
R
OH
R
O
R
O
Scheme 5. Acid-alkaline catalysed hydrolysis of esters. Alkaline catalysed reaction on the right
hand side and acid catalysed reaction on the left hand side.
Figure 5. Rate of ester hydrolysis as a function of pH.
In practice hydrolysis of ester bonds happens very fast even at room temperature, if pH > 8.
Consequently hemicelluloses are deacetylated (i.e. acetyl groups are cleaved off)
immediately when chips or pulp are put in contact with alkaline cooking or bleaching
solutions. Also, ester linkages between hemicelluloses and lignin are cleaved under these
conditions.
21
2.5 Acid-base-catalysed hydrolysis of glycosidic bonds
Glycosidic bonds connect monosaccharide units to each other. These linkages are cleaved off
easily in acidic conditions but they are more stable in alkaline conditions. Although at
elevated temperature (160-170 °C) the cleavage of glycosidic bonds also takes place during
alkaline pulping.
Acid-catalysed hydrolysis is a SN1 reaction (i.e. unimolecular nucleophilic substitution
reaction). The protonation of glycosidic oxygen (i.e. addition of a proton to oxygen )
improves the goodness of a leaving group. Therefore the glycosidic bond can be cleaved off
even without a nucleophilic attack to the anomeric carbon5 (Scheme 6). Because the degree of
protonation of glycosidic oxygen remains low even in highly acidic conditions, the rate of
reaction is proportional to the hydrogen ion concentration:
d [G ]
= - k H O + GH + = - k H O + K H 3O + × [G ]
3
3
dt
[
]
[
]
(11)
where [G] is the concentration of glycoside. In comparison to furanosides, pyranosides are
hydrolysed slowly. The difference in the rate of reaction between the compounds is due to
the stability of a carbonium ion. Pyranose carbonium ions exist in distorted conformation
which is a much more unfavourable form than the normal chair form, Scheme 6.
RO
O
H3O+,k
OR'
RO
O
+
OR'
H
k H3O+
RO
O +
+
H2O
RO
O
O
Scheme 6.
Acid hydrolysis of hemicelluloses is very intensive for example under the conditions in
which acid sulphite pulping is conducted (pH 1-2 and temperature 130-140 ºC). Hydrolysis
may continue until the dissolved hemicelluloses have degraded into monosaccharide units.
Although the main part of dissolved hemicelluloses is degraded into monosaccharide units,
there are also some polymer hemicelluloses left in the sulphite pulp. Acid hydrolysis of
cellulose is a slow reaction in sulphite pulping conditions and therefore the yield of cellulose
is not significantly affected. Configuration of polysaccharides has a considerable effect on
reactivity, especially in acidic media. Nonetheless the effect of configuration is difficult to
take that into account when studying the kinetics of the reactions.
Under alkaline conditions, the hydrolysis of glycosides is the SN2 reaction (i.e. bimolecular
nucleophilic substitution reaction). As mentioned earlier the depolymerisation of lignin is
also a unimolecular reaction. The unimolecular reaction is dominating due to higher
entropy. The internal nucleophile is usually an ionised hydroxyl group at C-2 position
(Scheme 7).
Anomeric carbon is a carbon atom of a carbohydrate that is adjacent to the ring oxygen atom and is part of an
acetal or hemiacetal.
5
22
HO2HC
CellO
K ion
O
HO
k
HO 2HC
OCell
CellO
OH
O
HO
CH2OH
OH
HO-
OCell
O
-CellO-
O
O
OCell
H2O
An ionised hydroxyl group
at C-2 position acts as an
internal nucleophile
HO2HC
CellO
HO
O
OH
OH
Scheme 7. Hydrolysis of glycosidic bonds under alkaline conditions.
Rate of reaction:
[
]
K HO - × [G ]
d [G ]
= - k HO - G - = - k HO - ion
dt
1 + K ion HO -
[ ]
[
(12)
]
where [G] is the concentration of the glycoside (total concentration of neutral and anionic
forms). In the kraft pulping bulk stage alkali-catalysed hydrolysis is significant in regard to
depolymerisation of cellulose and secondary peeling reactions. Yield losses in kraft pulping
correlate well with the decrease in the degree of polymerisation (viscosity) of cellulose.
Compared with pyranosides, furanosides are hydrolysed fast under alkaline conditions. An
example of this is the major decrease in the arabinose content of hardwood xylan during
kraft pulping.
The rate of hydrolysis of glycosidic bonds can be described in a general form that includes
reactions of acid-, alkali- and water-catalysed reactions:
[
]
K ion HO - × [G ]
d [G ]
+
= - k H O + K H 3O × [G ] - k H 2O [G ] - k HO 3
dt
1 + K ion HO -
[
]
Acid- catalysed
Water-catalysed
[
]
(13)
Alkali- catalysed
Equation 13 is illustrated in Figure 6. It is important to notice that the ratios of rate constant
are dependent on the character of the glycosidic bond. Nevertheless the graph shows that
hydrolysis is significant both in acidic and in highly alkaline conditions.
Because the alkali-catalysed degradation of cellulose is a very slow reaction, its enthalpy is
high (150 kJmol-1) in comparison to the enthalpy of lignin depolymerisation (120 kJmol -1)
during the bulk delignification stage.
23
Log (rate of reaction)
pH
Figure 6. Rate of hydrolysis of glycosidic bonds as a function of pH.
The carbohydrate material lost in peeling and chain cleavage is converted into various
hydroxy acids as mentioned earlier (Table 2, page 14). These acids consume notable amounts
of alkaline cooking liquors.
2.6 Reactions of uronic acid groups
Both hardwood and softwood xylans contain 4-O-methylglucuronic acid groups. Since
xylans are the main hemicellulose group in hardwood, the reactions of uronic acid groups
are important especially in hardwood kraft pulping. At the end of initial delignification 4-Omethylglucuronic acid groups start to demethoxylate (i.e. methoxy groups (-OCH3) are
cleaved off). The reaction is hydroxyl ion-catalysed, Scheme 8. Due to the demethoxylation
4-O-methylglucuronic acid groups are partly converted into hexenuronic acid groups.
During the bulk delignification demethoxylation continues. The hexenuronic acid groups are
subsequently cleaved off from the xylan chain. This reaction is also hydroxyl ion-catalysed.
HOH
MeO
COO-
HO-
O
O
HO
k2
COO-
k1
OXyl
OH
Methylglucuronic acid
MeGlcA
- MeO-
HO
Xylan and HexA
OXyl
OH
Hexenuronic acid
HexA
Scheme 8. Formation of hexenuronic acid groups of xylan.
These reactions occur when the two anions collide. Electric repulsion hinders the reaction.
Repulsion can be decreased and the rate of reaction increased by the addition of any auxliary
24
electrolyte (salt). Consequently the rate of reaction depends on the ionic strength (the
electrolyte concentration) of the solution. The effect of ionic strength can be described by the
Debye-Hückel equation:
æk ö
logçç ÷÷ = 2 Az A z B m 1 / 2
è k0 ø
(14)
where k0 is a rate constant at zero ionic strength, A is Debye-Hückel constant (0.51 M-1/2 in a
water solution, T= 25 ºC), zA and zB are the charge numbers of ions and µ is the ionic strength
(M). In practice the effect of ionic strength depends on the distance between the charge and
the reaction centre, etc. Hence the equation can be presented in a general form:
æk ö
logçç ÷÷ » m 1 / 2
è k0 ø
(15)
Figure 7. Effect of hydroxyl ion concentration and ionic strength on the formation rate of
hexenuronic acid groups.
Equation 16 describes the formation and degradation of HexA.
d [HexA]
= k1 HO - × [MeGlcA] - k 2 HO - × [HexA]
dt
[
]
[
]
(16)
where rate constants k1 and k2 are dependent on the ionic strength. MeGlcA stands for 4-Omethylglucuronic acid. Integration leads to Equation 17 which states that hexenuronic acid
content is dependent on the concentration of methylglucuronic acid side groups of the xylan
backbone, pH ([HO-]) and ionic strength of the solution.
(e - k1[HO ]t - e - k 2[HO ]t )
[HexA] = [MeGlcA]0 k1
k 2 - k1
(17)
25
In other words two main factors affect formation of HexA during kraft pulping:
1. Wood species (xylan content)
2. Cooking conditions (alkalinity, ion concentration)
The hexenuronic acid content of xylan during pulping is shown in Figure 8. The intensity of
cooking means the combined effect of temperature, ionic strength, hydroxyl ion
concentration and time. Hydroxyl ion concentration and ionic strength are dependent on the
cooking method. It has been observed that low hexenuronic acid content (softwood pulp)
can be obtained when hydroxyl ion concentration and ionic strength are high during the
bulk delignification phase. An example of this kind of cooking method is the SuperBatch
method.
Figure 8. The effect of cooking on the concentration of hexenuronic acid groups in hardwood
(continuous line) and softwood (dashed line) pulp. Fluctuations of cooking intensity (1.5-3,
arbitrary units) during hardwood pulping have only a minor effect on hexenuronic acid
concentration. The effect of the range of conditions (3-6) in softwood pulping is much larger.
The optimisation of cooking process is often done by analysing the kappa number of pulp.
Permanganate reacts with all unsaturated (i.e. doubly bonded) hydrocarbons. Therefore the
kappa number expresses the amount of lignin and hexenuronic acid groups in pulp.
Hexenuronic acid groups can be removed from the pulp by a mild acid hydrolysis (see Part II
Ch. 9). Consequently, it would be more important to analyse the real lignin content instead
of the kappa number.
2.7 Summary - Carbohydrate reactions
Table 3 summarises the main carbohydrate reactions during kraft pulping. In some cases
also acidic conditions are taken into consideration.
26
Table 3. Summary of carbohydrate reactions in kraft cooking. HC abbreviates hemicelluloses and
LC lignin-carbohydrate. * indicates that reaction also takes place under acidic conditions (e.g.
sulphite cooking). ID stands for initial delignification or heating-up period, BD is bulk
delignification.
Stage
Type of reaction
Target
Effect
ID
Primary peeling (elimination)
HC, cellulos e
Degradation of HC (and cellulose),
formation of aliphatic carboxylic
(T<140)
Hydrolysis of esters *
HC, LC-linkages
acids
Deacetylation of xylan and
glucomannan, cleavage of LC-linkages
BD
(T>140)
Hydrolysis of uronic acid groups
HC, xylan
Formation of methanol and
HC, xylan
hexenuronic acid groups
Readsorption of deacetylated xylan on
Hydrolysis of glycosidic
Cellulose, (HC)
fibres
Cleavage of linkages and
linkages *
Secondary peeling (elimination)
Cellulose, (HC)
depolymerisation of cellulose
Further degradation of cellulose
Reprecipitation of xylan
Study problems
1. What are the reactive sites of carbohydrates?
2. Name the main carbohydrate reactions taking place in kraft pulping. What is
their significance in relations to pulp quality?
3. What is meant by primary peeling, secondary peeling and the cleavage of ester
bonds? Give examples of these reactions.
4. How does the reaction rate of secondary peeling reaction depend on pH? Derive
a kinetic expression based the reaction.
5. What are hexenuronic acid groups (HexA) and where and how they are formed?
What are the effects of cooking conditions (such as pH and ion concentration) and
xylan content on formation of HexA?
Literature
Alén R., Chapter 2. Basic Chemistry of wood delignification. Forest Products Chemistry. Ed.
Stenius, P., Publ. Fapet, Jyväskylä, Finland, 2000, pp. 59-104.
Lai, Y.-Z., Chapter 10. Chemical Degradation. Wood and Cellulosic Chemistry, 2nd edition,
Eds. Hon, D. N.-S. & Shiraishi, N., Publ. Marcel Dekker, Inc. New York 2001, p.443-512.
Sjöström, E., Wood Chemistry. Fundamentals and Applications. 2nd edition. Academic Press.
1993, 293 pp.
27
3 Lignin reactions in pulping
Lignin is a highly branched three dimensional polymer that is located in the middle lamella
and between cellulose microfibrils in the cell wall. It is practically insoluble in water and it is
chemically bound to carbohydrates. These characteristics make lignin difficult to remove.
During pulping, lignin is depolymerised through cleavage of inter-unit linkages. This
increases the solubility of lignin and makes it possible to remove it from the wood chips.
Understanding the chemical reactions taking place during cooking is important in order to
analyse the effect of cooking parameters on delignification.
The aim of this chapter is to deepen your knowledge of lignin and give you a sufficient basis
in pulping and lignin chemistry to assist you in understanding the effect of pulping
conditions on the rate and type of reactions occurring during delignification. After studying
this chapter you should be able to:
· define the main lignin reactions in chemical pulping.
· evaluate the significance of these reactions on delignification. Which are the important,
main reactions and which are harmful or undesirable side-reactions?
· understand the effects of pH, sulphidity and temperature on the initial and bulk
delignification stages.
3.1 Reactive sites
Unlike in the case of cellulose and hemicelluloses, the chemical structure of lignin is not
defined precisely. The composition, distribution and structure of lignin vary depending on
the wood species, age of wood and morphological regions. There are several different
reactive sites in lignin but the most reactive sites are ether units in α- or b-positions. Reactive
sites in lignin are:
a) Hydrolysable ether linkages
i. b-aryl, α-aryl and α-alkyl ether linkages
b) Phenolic hydroxyl groups
c) Aliphatic hydroxyl groups
d) Uncondensed units
i. Units with C2, C3, C5 and C6 being unsubstituted or substituted only
by a methoxyl group.
e) Unsaturated units
i. Coniferyl alcohol or coniferyl aldehyde end groups and α-carbonyl
groups.
f) Ester groups
α-ester
28
g) Methoxyl groups (-OCH3)
Some of the other structures are illustrated in Figure 9.
b-O-4-linkage (b-aryl ether
linkage)
OMe
HO
OH
HO
HO
Aliphatic hydroxyl group
OH
O
OMe
MeO
HO
O
HO
OH
HO
OMe
HO
α-O-4 – linkage (α-aryl ether linkage)
OMe
O
HO
O
HO
Methoxy group (-OMe)
OMe
OMe
O
HO
O
OH
OH
OH
MeO
HO
O
O
OMe
Unsaturated unit
OH
HO
OH
O
HO
HO
OMe
Uncondensed unit
MeO
O
MeO
O
HO
O
OMe
OH
Phenolic hydroxyl group
OMe
OH
Figure 9. Some possible reactive sites of softwood lignin.
In addition to lignin-lignin linkages there are also reactive lignin-carbohydrate (LC) linkages.
These kinds of linkages are discussed more in detail in Chapter 4.
3.2 Cleavage of b -aryl ether (b -O-4) linkages in kraft pulping
Most of the linkages (40-60 %) in wood lignin are b-aryl ether linkages (b-O-4). These
linkages are cleaved during kraft pulping.
Ethers are usually relatively inert substances so why are b-O-4 linkages cleaved? The lone
pairs of oxygen are a source of reactivity and this increased polarity of the -C b-O- bond
29
makes the neighbouring b-carbon atom sensitive to nucleophilic attacks. In addition, the
neighbouring benzene ring is more stable due to the resonance structure.
The reactivity of b-O-4 depends on whether the structure is phenolic or etherified (i.e. nonphenolic), Figure 10.
HO
HO
R
g
b
a
Ö
Phenolic structures:
OMe
R is H
Etherified structures: R is C
OMe
OR
Figure 10. Phenolic and etherified (i.e. non-phenolic) lignin structures.
Under alkaline conditions the phenolic group ionises (Ar-OH ® Ar-O-) and increases the
reactivity of the structure. On the other hand, etherified structure is much less reactive site in
the alkaline pulping process.
a) Phenolic lignin structures – initial delignification
The reaction of a phenolic (i.e. nonetherified) b-O-4 structure begins with the formation of a
quinone methide (QM), Scheme 9. The quinone methide is formed if there is a hydroxyl
group (OH) or ether bond (OR) in the α-carbon of the lignin structure. Consequently the
formation of the quinone methide structure results in the elimination of a-OH group or
cleavage of a-OR linkage. If the substituent (R) is lignin, this reaction leads to
depolymerisation of lignin. The formation of quinone methide is followed by an attack of
nucleophilic hydrogen sulphide ion (HS-). As a result the β-O-4 linkage is cleaved and lignin
degrades.
30
RO
R
HO
R
HO
HSO
O
K1
- RO-
HS
OMe
OMe
OMe
OMe
O
O
Quinone methide
structure (QM)
R
HO
S
R
OMe
OMe
K3
OMe
OMe
OMe
OH
4
k4
R
O
O
K4
K2
HO
HS
S
OMe
1
HO
O
O
O
OMe
OMe
O
R
HO
OH
3
2
k3
OH
R
OH
S
R
S
OMe
OMe
O
O
OMe
O
OMe
O
Scheme 9. Cleavage of phenolic b-O-4 linkages during the initial delignification (i.e. heating up
period).
The equilibrium concentration and ionisation of a-SH group (a thiol or mercaptan) can be
described with several equations, Equations 18-21:
K1 =
K2 =
K3 =
K4 =
[1]
[QM ]× [HS - ]
[1]
[2] × [HO - ]
[3]
[2] × [HO - ]
[4]
[3]× [HO - ]
(18)
(19)
(20)
(21)
β-O-4 linkages degrade via ionisation of the sulphide in α-carbon. Therefore the reaction can
proceed only via structures 3 and 4 (Scheme 9). The equilibrium concentration of these
structures is obtained by combining equilibrium equations.
31
[
K 1 K 3 [QM ] × HS [3] =
K2
]
(22)
[4] = K1 K 3 K 4 [QM ] × [HS
-
]× [HO ]
-
(23)
K2
The rate of degradation is therefore dependent on the HS - and HO- concentrations
(sulphidity and pH)
Rate =
[
]
[
][
- k 3 K 1 K 3 [QM ] × HS - k 4 K 1 K 3 K 4 [QM ] × HS - × HO K2
K2
]
(24)
It is important that sulphidity is sufficient during the initial delignification. If the HS concentration is not high enough a competing reaction may take place. The reaction results
in the formation an enol ether structure via elimination of a proton or formaldehyde from
quinone methide, Scheme 10. The cleavage of a proton is a bimolecular elimination reaction
caused by a hydroxyl ion. The elimination of a formaldehyde group is an intramolecular
reaction and requires ionisation of a hydroxyl group in the γ-carbon. When considering
delignification, this competing reaction is unwanted because it does not result in bond
cleavage and lignin degradation.
R
OH
HO-
R
O
R
H
O
OMe
K5
O
k5
O
OMe
OMe
QM
O
OMe
O
KQM
OMe
OMe
5
O
R
HO
O
OMe
OMe
O
Scheme 10. A competing reaction – formation enol ether structures.
K5 =
[5]
[QM]× [HO- ]
(25)
As can be seen in Equation 26 the rate of reaction (i.e. formation of enol ether) depends on
the hydroxyl ion concentration and the degree of ionisation of the hydroxyl group in the γ- carbon,
but not on HS- concentration.
32
[
]
Rate = - k QM [QM ] × HO - -
[
]
k 5 K 5 HO - × [QM ]
1 + K 5 HO -
(
[
])
(26)
It can be concluded that to achieve a good degree of delignification in the initial phase of
kraft pulping the hydrogen sulphide ion concentration needs to be sufficient. Otherwise the
competing reactions take place causing formation of stable enol ether linkages which hinder
the degradation of lignin.
It has also been suggested that the hydroxyl ion concentration should be low at the
beginning of pulping. The HO- ion concentration affects both the wanted b-O-4 cleavage and
unwanted enol ether formation reactions. The effect of hydroxyl ion concentration on the
formation of enol ethers (Equations 24 and 26) is shown in Figure 11. The cleavage of β-O-4
(β-aryl ether) linkage occurs at about the same rate whether the reaction occurs via
structures 3 or 4 because the additional charge of the structure 4 is located relatively far
away from the reaction centre (k3 ≈ k4). The pKa values of organic sulphide, phenol and
aliphatic alcohol are approx. 7, 10-11 and 14-15. The pK1 and pK2 values of hydrogen
sulphide are approximately 7 and 15. In the pH range of 8-15 hydrogen sulphide mainly
exist as HS- ions.
Figure 11. Effect of hydroxyl ion concentration on the cleavage β-O-4 (β-aryl ether) linkages (solid
line) and the formation of enol ethers (dashed line). Reactions proceed via formation of a quinone
methide in a liquid containing sulphide. Ionisation constant for γ-hydroxyl group is assumed to be
0.3 M-1 (pKa= 14.5) and ionisation constant for phenol is 10-5 M-1 (pKa = 11). In practice the
mechanism by which enol ethers are formed has only a small effect on the pH dependency of the
reaction rate.
According to Figure 11 the formation of enol ethers decreases notably only when the pH
decreases to 11 or below. Therefore it can be said that the (possible) favourable effect of low
hydroxyl ion concentration on delignification does not depend on changes in the formation
33
of enol ethers. Consequently, other reasons must be found to explain the lower degree of
initial delignification in the absence of sufficient amount of HS - ions.
The total rate of reaction of the phenolic structures is controlled by their degree of ionisation.
In practice hydroxyl ion concentration does not affect the rate of reaction when pH is above
11.
b) Carbonyl structures – initial delignification
Native lignin consists of keto and aldehyde groups, such as α-carbonyls and coniferyl
aldehydes. In these structures the carbon atom has low electron density and therefore these
structures are potential reaction sites for nucleophiles. Nucleophiles attack the carbonyl
structures and form addition products. Addition of an HS- ion to a carbonyl structure is
shown in Scheme 11.
R
HO
R
HO
HS-
HS
O
O
K1
OMe
O
O
OMe
1
2
OMe
OMe
OR
OR
K2
R
HO
OH
R
S
k
HO
S
HO
O
OMe
O
OMe
OMe
3
OMe
OR
OR
Scheme 11. Addition of an HS- ion to a carbonyl structure causes b-O-4 cleavage and the
degradation of lignin.
The equilibrium concentrations of the reactions:
[2]
K1 =
[1] × [HS - ]
K2 =
[3]
[2]
(27)
(28)
The cleavage of the β-O-4 (β- aryl ether) linkage in structure 3 (Scheme 11) is similar to the
reaction of phenolic structures and it happens fast. The rate of reaction is practically
independent of hydroxyl ion concentration but depends on the HS - concentration as follows:
[
Rate = - k [3] = - kK 1 K 2 [1] × HS 34
]
(29)
c) Etherified (non-phenolic) lignin structures - bulk delignification
The cleavage of β-O-4 (beta aryl ether) linkages
In the main pulping stage, bulk delignification, the temperature is sufficiently high to cause
reactions in the non-phenolic structures of lignin. As mentioned earlier etherified (i.e. nonphenolic) lignin structures are much less reactive sites in the alkaline pulping process than
phenolic lignin structures. The reactions of non-phenolic structures are slow and therefore
those reactions control the rate of delignification in the bulk phase. The cleavage of β-O-4
(beta aryl ether) linkages of non-phenolic lignin structures takes place if hydroxyl groups in
α- or γ-carbon are ionised. The pKa of these OH groups is very high (»14) indicating that
ionisation requires very high HO- concentration (i.e. pH). Once this group is ionised it acts
as a nucleophile attacking β-carbon causing the cleavage of the aryl ether linkage, as shown
in Scheme 12.
R
HO
R
HO
O
HO
O
OMe
R
O
O
K ion
OH
OMe
k
OMe
O
OMe
OMe
OR
OR
OMe
OR
Scheme 12. Cleavage of non-phenolic lignin structure.
The rate of reaction (i.e. cleavage of aryl ether linkages) is dependent on the degree of
ionisation. The degree of ionisation describes the amount of ionised hydroxyl groups at the
α- or γ-carbon. Equation 30 presents the ionisation equilibrium.
K ion =
[RO ] ,
[ROH ][HO ]
-
(30)
-
where ROH is the structure containing a hydroxyl group. A relation between ionisation
constant (Kion) and acidity constant (Ka):
Kw =
Ka
,
K ion
(31)
where Kw is the ionic product for water. The degree of ionisation is:
xion =
[RO ] = K
[ROH ] + [RO ]
-
-
[HO ]
1 + K [HO ]
-
ion
-
(32)
ion
The rate of reaction is therefore:
35
[
]
dc
HO = - kxion c = - kcK ion
dt
1 + K ion HO -
[
(33)
]
where c is the concentration of the reactive lignin structure, t is time and k is a rate constant.
According to the transition state theory the rate constant can be presented accordingly:
k = (kT / h) × e DS / R × e - DH / RT
(34)
where k is Boltzmann constant, h is Planck’s constant, R is general gas constant, T is
temperature, ∆S is entropy, and ∆H is enthalpy. Entropy is defined as the level of disorder in
a system. The entropy of intramolecular reactions is large because reactive components are
constantly close to each other. Intermolecular reactions have smaller entropy since reactive
components are only randomly close to each other. β-aryl ether linkages are not cleaved
directly via intermolecular reactions (effect of hydroxyl ion) but the reaction is an
intramolecular reaction as seen in Scheme 12.
Enthalpy is the energy barrier of the reaction (energy of activation, E, is often used instead of
enthalpy). It is the minimum energy needed to form a complex during a collision between
reactants. The enthalpy depends on chemical properties of reactants. For example in
nucleophilic substitution the ∆H depends on the nucleophilicity of a nucleophile and
chemical character of the leaving group. Ionisation increases the nucleophilicity of oxygen
and sulphur nucleophiles. Therefore the cleavage of β-aryl ether linkages cannot proceed as
an intramolecular reaction without ionisation of the lignin structure.
Demethylation
Sulphur nucleophiles are much more reactive than oxygen nucleophiles. For example HS - is
more reactive than HO-. Consequently, the HS- ion is able to demethylate methoxy groups of
lignin, Scheme 13. This reaction produces methyl mercaptan (CH3SH). At the end of pulping
the degree of demethylation of lignin, in other words the decrease in the total amount of
methoxy groups in lignins, is approximately 10 %.
Methyl mercaptan can form dimethyl sulphide and dimethyl disulphide in further
nucleophilic and oxidative reactions. Both of these compounds are very volatile and
malodourous causing trace emission in atmosphere and giving the typical odour of the kraft
cooking process.
R
HO
HO
HO
O
R
HO
O
OMe
O
OR
Me
OMe
HS-
MeS-
O
OR
Scheme 13. Demethylation and formation of malodourous methyl mercaptan (MeSH, Me = CH 3)
36
In principle the HS- ion is also able to cause a nucleophilic substitution in the b-aryl ether
structure (Scheme 14). The leaving group is the same in both cases, i.e. phenoxyl ion.
However, the methyl group is sterically less hindered than β-carbon of lignin and therefore
demethylation is more probable than the substitution in b-carbon. Consequently, the
cleavage of β-aryl ether linkages by this reaction mechanism is not a remarkably fast reaction
at the temperatures used in kraft pulping.
HS-
R
HO
HO
R
HO
HO
O
SH
O
OMe
OMe
O
Me
OH
OR
OR
Scheme 14. Nucleophilic substitution.
Total rate of reaction
During bulk delignification the total rate of hydrolysis of lignin β-aryl ether linkages is the
sum of the reactions in Scheme 12 and 14. In this way we get the rate equation described by
Equation 35. The HS- concentration is not significant with regard to the cleavage of b-O-4
linkages.
[
]
é kK ion HO ù
dc
= -c × ê
+ k ' HS - ú
dt
ë (1 + K ion HO )
û
[
The main reaction
in Scheme 12
FAST
]
[
]
(35)
The side reaction in
Scheme 14
SLOW
Figure 12. The effect of hydroxyl ion concentration on the hydrolysis of β-aryl ether linkages in
solution containing sulphides. The ionisation constant of α/γ- hydroxyl is presumed to be 0.3 M -1
(pKa 14.5)
37
3.3 Carbon-carbon bond cleavage
In general the carbon-carbon bonds of lignin are stable in alkaline media. Only few bonds
may be cleaved. These kinds of bonds exist for example between b- and γ-carbons of
phenolic lignin units. In this case the bond cleavage results in the formation of formaldehyde
and enol ether (b-O-4 structures) or formaldehyde and chromoric stilbene or stilbene-like
structures (b-5 = phenyl coumaran, b-1 and b-b = pinoresinol 6).
3.4 Condensation
A variety of condensation reactions take place in kraft pulping, like for example conjugate
addition, formaldehyde addition, epoxide addition and aldol condensation reactions. It has
been suggested that the majority of condensation reactions occur at the unoccupied C-5
position of phenolic units. Although the significance of lignin condensation in kraft pulping
is not still fully understood, it has been observed that the condensation reactions lead to the
formation of stable carbon-carbon bonds. Consequently the lignin depolymerisation is
retarded by this reaction especially during the final stages of kraft pulping.
3.5 Summary – the most important lignin reactions in kraft pulping
Table 4 summarises the most important lignin reactions taking place in the kraft pulping
process. It is good to remember that non-phenolic lignin structures are stable during the
initial delignification stage. As the intensity of cooking increases, these structures also begin
to react. Furthermore, a-ether linkages (e.g. a-O-4) in all non-phenolic structures are stable
under kraft pulping conditions.
Table 4. Lignin reactions in alkaline kraft pulping. ID means the initial delignification stage and
BD the bulk delignification stage.
Stage
Type of reaction
ID (T<140°) Cleavage of phenolic a-O-4
Cleavage of phenolic b-O-4
Enol ether formation
Effect on
delignification
+
+
-
Nucleophile
[HO-]
[HO-], [HS-]
[HO-]
Non-phenolic lignin structures are not reactive
+
BD (T>140°) Cleavage of non-phenolic b-O-4
[HO-], [HS-]
Demethylation
[HS-]
Condensation
6
38
As a result of cooking the lignin structure alters. This lignin is called kraft or residual lignin.
It is characterised by following features relative to native lignin:
·
·
·
·
·
less b-O-4 linkages
more phenolic hydroxyl groups
lower molar mass
more chromophores
more condensed lignin structures
Study problems
1. What are the possible reactive sites of native lignin? Why is the cleavage of b-O-4
bonds significant for delignification in pulping?
2. Summarise the reactions taking place in the initial delignification and bulk
delignification stages.
3. Determine the effects of sulphidity and pH on the rate of reactions in the initial
delignification stage using kinetic expressions.
4. Determine the effects of sulphidity and pH on the rate of reactions in the bulk
delignification stage using kinetic expressions.
5. In kraft pulping b-O-linkages are cleaved off by different chemical reactions. List
the reactions and name the factors affecting the reaction rate for bond cleavage. In
other words define the concepts of inter- and intramolecular reaction and the
properties of leaving groups.
Literature
Alén R., Chapter 2. Basic Chemistry of wood delignification. Forest Products Chemistry. Ed.
Stenius, P., Publ. Fapet, Jyväskylä, Finland, 2000, pp. 59-104.
Lai, Y.-Z., Chapter 10. Chemical Degradation. Wood and Cellulosic Chemistry, 2 nd edition,
Eds. Hon, D. N.-S. & Shiraishi, N., Publ. Marcel Dekker, Inc. New York 2001, p.443-512.
Sjöström, E., Wood Chemistry. Fundamentals and Applications. 2 nd edition. Academic Press.
1993, 293 pp.
39
4 Lignin-carbohydrate complexes (LCC)
Learning objectives
1. Identify linkages between native lignin and carbohydrates in wood.
2. Understand the chemical character of lignin-carbohydrate linkages.
3. Describe how lignin-carbohydrate complexes change due to pulping.
4.1 Native lignin-carbohydrate complexes in wood
The wood cell wall lignification process takes place at the end of cell wall biosynthesis.
Lignification starts at the cell corners and in the area of the compound middle lamella, after
which the cell wall is lignified. The polymerisation of lignin monomers begins through
ferulic acid groups which are linked to hemicelluloses by ester bonds. Since the hydrolysis of
esters (see 2.4) is a fast reaction both in acidic and alkaline media, the connecting linkages
between hemicelluloses and ferulic acid groups of lignin are cleaved off easily during
delignification of wood chips. This is the common reaction path in all pulping methods
(Scheme 15).
O
OHemicellulose
O
OH
H2O
+ Hemicellulose
OMe
OLignin
OMe
OLignin
Scheme 15. The hydrolysis of ester bond between lignin and hemicellulose units during pulping.
During the wood lignification quinone methides are formed. Usually these structures react
with water or with the hydroxyl groups of lignin forming polymerised lignin structures. It is
possible that quinone methides are attacked by nucleophilic cell wall polysaccharides.
Generally it is considered to be possible that the main part of native lignin-carbohydrate
complexes is of this origin. In kraft pulping α-ether linkages are cleaved only in phenolic
structures. α- ether linkages in non-phenolic structures are stable and therefore these kinds
of LC complexes are also stable. In acid sulphite pulping α- ether linkages are labile also in
non-phenolic structures, Scheme 16.
40
OH
HemicelluloseO
OH
HO
OLignin
OLignin
H3O+
OMe
OLignin
+ Hemicellulose
OMe
OLignin
Scheme 16. The cleavage of a a-O-4 linkage between a non-phenolic lignin unit and hemicellulose
in acid sulphite pulping. a-O-4 linkages are stable under alkaline conditions.
4.2 LC complexes in kraft pulp and cooking liquor
Lignin-carbohydrate (LC) complexes have been proven to exist in kraft pulp. Residual lignin
can be isolated from unbleached kraft pulp by an enzymatic treatment. A typical method is
to use a mixture of cellulases and hemicellulases. After the enzymatic treatment there are
always traces of wood polysaccharides in the water insoluble lignin fraction. Based on the
composition of lignin fractions it is possible to determine the morphological location of
lignin in the cell wall. Primary wall lignin contains a relatively large amount of galactans
which originate from primary wall pectins. Then again the secondary cell wall lignin residue
contains traces of xylan, glucomannans and cellulose.
Kraft pulp can be dissolved in a mixture of lithium chloride (approx. 1%) and dimethyl
acetamide resulting in liberation of wood polymers. It is possible to fraction these polymers
further by using chromatographic methods. Lignin has been observed to be linked with
cellulose (the higher molecular mass fraction) and also with hemicelluloses (the lower
molecular fraction). If the enzymatic treatment is carried out before the actual fractioning,
more specific information on lignin-carbohydrate bonding is obtained.
The polysaccharides dissolved as polymers in kraft pulping consist mainly of xylan and also
of arabinans and galactans originating from pectin. It has been observed that lignin is not
only bonded to these polysaccharides but it also links different polysaccharides together.
The probable formation mechanisms for lignin-carbohydrate complexes in kraft pulping: The first
mechanism suggests that LC complexes are formed as a result of a nucleophilic attack of
hemicelluloses to the epoxy structures of lignin, Scheme 17. The epoxy structures are formed
during bulk delignification. The addition reaction is quite unselective and both celluloses
and hemicelluloses can be linked with lignin. In kraft pulping the competing nucleophiles
are hydroxyl (HO-) and hydrogen sulphide (HS-) ions.
41
OH
OH
OH
O
HO
HO
OLignin
OHemicellulose
-O -Hemicellulose
HO-
OMe
OLignin
OMe
OLignin
OMe
OLignin
Scheme 17. The formation of LC complexes. Mechanism 1.
According to another mechanism phenolic lignin structures are linked to the reducing end
units of polysaccharides. This is also a nucleophilic reaction. An addition to a normal end
group is improbable due to the high reactivity of the end group. On the other hand a
conjugated aldehyde structure which is formed via β-elimination in glucuronoxylan is
relatively stable. This reaction path is interesting since the reaction and process conditions in
the initial delignification stage have an effect on the total amount of residual lignin. In this
reaction a hydrogen sulphide ion and a phenolate ion are competing nucleophiles. Therefore
the hydrogen sulphide ion concentration in the initial delignification is likely to affect the
amount of LC complexes formed during the pulping process (Scheme 18). However this is
not yet experimentally proven.
OMe
O
S
Φ(O-)
HSHO
LCC
k1
HO
O
O
ÖR
K
Xylan
Scheme 18. Formation of LC complexes. Mechanism 2.
42
O
O
Xylan
ÖR
Xylan-OH
k2
5 Reactions of extractives in kraft pulping
Learning objectives
1. Classify extractives present in wood.
2. Name and describe the main reactions of extractives in pulping.
3. Explain the pros and cons of the presence of extractives in pulping and
papermaking. Give examples of methods in pitch control and deresination.
5.1 Extractives in wood
Extractives comprise a large variety of chemical compounds of both lipophilic and
hydrophilic types. These components are soluble in neutral organic solvents (lipophilic
components) and water (hydrophilic components). Extractives are considered to be nonstructural wood components whereas cellulose, hemicelluloses and lignin are considered to
be structural components. The function, composition, and occurrence of different extractives
differ notably depending on wood species and the location in the wood. Even the growth
zone and season affect the composition and quantity of extractives. Figure 13 illustrates the
location and types of major extractives in wood.
Heartwood
Phenolic compounds
Resin canals
Extractives in wood
Terpenoids
Phenolic compounds
Bark
Terpenoids
Waxes
Fats
Parenchyma cells
Waxes
Fatty acids etc.
Figure 13. Extractives in wood. Morphological location and major compounds.
As a rule it can be said that even various parts of the same tree (i.e. stem, branches, roots,
bark, etc.) differ with respect to the amount and composition of extractives, Figure 14. In the
case of pines it is known that the heartwood contains more extractives than the sapwood.
43
Figure 14. The content of major wood resin component groups in fresh black spruce and jack pine
at three different heights of the stems (Nuget 1977).
The main function of heartwood and resin canal extractives (phenolic substances and lower
terpenoids such as mono- and diterpenoids) is to protect wood from microbiological damage
or insect attacks. Diterpenoids often occur as resin acids. Resin canal extractives exist only in
softwood. Parenchyma resin (fats, fatty acids etc.) constitute the energy source of the wood
cells. Extractives in the cambium and growth zone participate in the biosynthesis and also
act as a food reserve.
5.1.1 Terpenes, terpenoids and sterols
The term terpenoid is often used as a general term for both terpenes and their derivates with
hydroxyl, carbonyl and carboxyl functions. Strictly speaking the term terpenoid describes a
terpene which is substituted with one or more oxygen functions. The basic building unit of
terpenes is an isoprene unit (C5H8). Terpenes and terpenoids can be divided into subgroups
according the number of isoprene units, Figure 15. Terpenes, terpenoids and sterols are as
such insoluble in water.
44
Hemiterpenoid
(1 isoprene unit)
Monoterpenoid
(2 isoprene units)
Sesquiterpenoid
(3 isoprene untis)
Diterpenoid
(4 isoprene units)
isoprene
limonene
Sesterterpenoid
(5 isoprene units)
-codinene
Triterpenoid
(6 isoprene units)
abietadiene
O
O
Terpenoids
H
O
HO
-amyrin
OH
obhiobolin
Tetraterpenoid
(8 isoprene units)
Polyterpenoid
( >8 isoprene units)
n
-carotene
rubber and gutta-percha
Figure 15. General classification of terpenoids.
One of the best ways to learn some of main terpenoid structures is to memorise number of
isoprene units in a compound. For example, monoterpenoids – 2 isoprene units,
diterpenoids – 4 isoprene units and so on. Try to make a mental note of the most common
structures shown in this material. For example is the structure monocyclic, bicyclic or
tricyclic? Try to make connections, like for example: α-pinene – softwood (pine) - bicyclic –
monoterpenoid, betulinol - birch (betula in Latin) – pentacyclic – triterpenoid, etc. It is not
necessary to memorise all the structures but only the most important terpenoids affecting
pulping and papermaking.
Monoterpenes and monoterpenoids occur mainly in softwood canal resin (oleoresin) and are
relatively volatile compounds. These compounds exist in acyclic, monocyclic (limonene),
bicyclic (α-pinene, b-pinene and ∆3-carene) and tricyclic structural forms, Figure 16.
limonene
α-pinene
b-pinene
∆3-carene
Figure 16. Some monoterpenes from softwood.
Diterpenes and diterpenoids can be divided into acyclic, bicyclic, tricyclic, tetracyclic and so
called macrocyclic structural types. Diterpenoids, which constitute a major part of softwood
oleoresin, are present either as diterpenes or as derivates with one or more oxygencontaining functional groups (hydroxyl, carbonyl or carboxyl group).
45
The resin acids are diterpenoids with a carboxyl group. The most common resin acids in
softwood are pimarane and abietane types of tricyclic terpenoids, Figure 17.
PIMARANE TYPE
15
12
11
13
14
9
8
7
COOH
Sandropimaric acid
COOH
Pimaric acid
COOH
Isopimaric acid
ABIETENE TYPE
COOH
COOH
Levopimaric acid
Palustric acid
COOH
Abietic acid
COOH
COOH
Neoabietic acid
Dehydroabietic acid
Figure 17. Resin acids (tricyclic acidic terpenes i.e. terpenoids) from softwoods. These are common
in canal resin (oleoresin).
The pimarane type of resin acids have vinyl and methyl groups at the C-13 position, whereas
abietane type acids have isopropyl or isopropenyl group at the C-13 position.
Triterpenoids and sterols are structurally and biogenetically closely related. Both
compounds originate from squalene but the biosynthesis proceeds along different pathways.
Some typical steroids and triterpenoids are illustrated in Figure 18. b-sitosterol is the most
common steroid in wood. Betulinol is a triterpenoid compound (i.e. triterpenyl alcohol)
found in the outer bark of birch. Serratenediol is present in bark of pines.
STEROIDS
29
28
21 20
18
3
HO
4
12
9
10
5
25
23
26
24
17
11
1 19
2
22
27
13
8 14
16
15
7
HO
6
Sitosterol
Sitostanol
TRITERPENOIDS
OH
CH2OH
HO
HO
Betulinol
Serratenediol
Figure 18. Some common steroids and triterpenoids.
46
So what is the main difference between steroids and triterpenoids? Actually there is no
simple definition that allows easy distinction between steroids and triterpenoids. Usually
sterols have a tetracyclic ring system. All wood sterols have OH at the C-3 position and a
side chain at C-17. Most triterpenoids, or specifically triterpenyl alcohols, have a pentacyclic
ring system with a hydroxyl group at C-3.
5.1.2 Fats, waxes and their components
The fats and waxes are the main constituents of the lipophilic material in parenchyma cells.
The fats are glycerol esters of fatty acids. In wood they occur mainly as triglycerides. In a
living tree free fatty acids are present only in heartwood. However during storage fatty acids
are partially liberated from triglycerides, Figure 19.
Figure 19. The composition and amount of lipophilic extractives in fresh and water-stored logs of
Norway spruce. (Ekman 1993).
Waxes are ester of higher fatty alcohols (C18-C24). Some of the characteristic parenchyma
wood resin constituents are listed and illustrated below. Fatty acids and alcohols, fats, waxes
and other fatty acid derivatives are insoluble in water.
47
Fatty acids:
Triglycerides:
COOH
palmitic acid C16
COOH
oleic acid C18
COOH
linoleic acid C18
COOH
linolenic acid C18
COOH
pinolenic acid C18
H2
CO O C
CO O CH
trilinolein
CO O CH2
Fatty acid esters:
O
R1
O
C
H2
n
CH3
waxes
n=19-23
O
R2
steryl esters
O sterol
O
R2
O
triterpenyl esters
triterpenyl alcohol
R1= fatty acid chain, R2 = chains of saturated and unsaturated
C14-C20 fatty acids
Figure 20. Characteristic parenchyma wood resin constituents. (Ekman 2000).
5.1.3 Phenolic substances
Many of the phenolic compounds are derived from the phenyl propanol structures. There
are many different phenolic substances present in wood and the main groups are stilbenes,
lignans, flavonoids and tannins, Figure 21. To some extent theses phenolic substances are
hydrophilic. The phenolic substances are usually soluble in water.
Tannins
Stilbene
O
HO
O
HO
HO
OH
OH
HO
COOH
HO
HO
HO
gallic acid
O
Pinosylvin
O
Flavonoids
ellagic acid
Lignans
MeO
HO
OH
H
MeO
H
H
O
HO
O
O
O
O
OH
OH
O
OH
chrysin
H
HO
O
H
pinoresinol
HO
OMe
OMe
OH
conidendrin
Figure 21. Some phenolic extractives.
48
OH
OH
O
taxifolin
5.1.4 Inorganic compounds
Wood also contains traces of inorganic substances, like metals. The main metal compounds
are calcium, potassium and magnesium. Some heavy metal compounds, like iron, cobalt and
manganese are also present. These compounds have a detrimental effect on bleaching.
5.2 Reactions of extractives in pulping
As mentioned earlier extractives comprise a large variety of chemical compounds and in
pulping the reactions of these compounds vary. Hydrophilic phenolic structures are more
easily dissolved than hydrophobic (i.e. lipophilic) structures. During the pre-steaming and
heating up periods low molar mass terpenes (i.e. monoterpens and sesquiterpens) are
distilled out of wood chips and this volatile fraction, turpentine, can be recovered from the
digester relief condensate.
The non-volatile water-insoluble wood extractives are often referred as wood resin. This
nonvolatile wood resin remains in the system. Wood resin consists mainly of fatty acids and
their glycerides (fats), diterpenoids (resin acids), triterpenoids (sterols and triterpenyl
alcohols) and their fatty acid esters.
During alkaline pulping the following saponification reactions take place with wood resins:
I)
Free (fatty and resin) acids → Sodium soaps (reaction rate k1)
II )
Fats (mainly triglycerides) → Sodium soaps and glycerol (reaction rate k2)
III )
CH2O2C-R
CH2OH
CHO2C-R + 3 NaOH
CHOH
CH2O2C-R
CH2OH
triglyceride
glycerol
+ 3 RCO2Na
sodium soap of a fatty acid
Steryl esters → Sodium soaps and sterols (reaction rate k3)
The reaction rates of these reactions are in the order: k1 >> k2 > k3. Alkali concentration and
temperature are important factors in these reactions.
After cooking, the soap is removed from the black liquor and as a result tall oil soap is
recovered. Soap separation is effective with softwood or mixed softwood-hardwood cooking
liquors.
The term saponifiables is used for fatty and resin acids which yield soaps under the alkaline
hydrolysis of wood extractives. The unsaponifiables are the neutral components of wood
resin (e.g. hydrocarbons, fatty alcohols, sterols, triterpenyl alcohols). These water insoluble
components are liberated after hydrolysis of neutral esters.
5.3 Deresination and pitch control
Wood pitch can be considered to be wood resin that causes unwanted deposits in pulping
and papermaking. The main effects of lipophilic resin components are shown in Table 5.
49
Table 5. Some effects of lipophilic resin components in pulping and papermaking. (Holmbom
1998).
Effect
Major responsible component groups
Process disturbances
Foaming
Resin and fatty acid soaps
Deposits in kraft mills
Ca soaps of fatty acids, steryl esters, hydrocarbons
Deposits from mechanical pulps
All resin groups, but especially triglycerides and fatty acids
Wet-end chemistry disturbance
Colloidal pitch droplets, mainly composed of triglycerides, fatty acids
Effluent toxicity
Resin acids, diterpene aldehydes and alcohols, sterols
Product quality impairment
Lower sheet strength
Triglycerides, fatty acids
Lower water absorbance
All hydrophobic components
Lower friction
Fatty acids, triglycerides
Taste and odour
Unsaturated fatty acids (after oxidation)
Allergic reactions
Oxidised resin acid products
Different wood species have different extractive compositions and content, therefore each
species’ wood resin also has a different physical behaviour. For example softwood neutral
pitch components are more easily washed out of pulp after cooking than the neutral pitch
components of hardwood pulp. The main reason for this is that hardwood contains more
sterols and other neutral substances than softwood, Figure 22. In addition the structures of
these components also differ.
Figure 22. The composition and content of lipophilic wood resin compounds in different wood
species (Ekman 2000).
Saponifiables act as surface active agents since they consist of a hydrophobic end (usually
one or more hydrocarbon chains) and a hydrophilic end (often ionized carboxylic acid
group). These surfactants contribute to the removal of neutral lipophilic extractives from
chips during kraft pulping and the subsequent washing of pulp. If the concentration of
surfactants is sufficient, surfactants can form large aggregates, which are called micelles.
Micelles have the capacity to dissolve water-insoluble compounds (i.e. neutral substances) in
their interior. Consequently the solubilisation and the formation of micelles affect the
physical behaviour of wood extractives in pulping and bleaching.
50
Mixtures of fatty acids and resin acids are known to form efficient mixed micelles which
enable solubilisation of neutral components. Since hardwood doesn’t contain resin acids, it is
necessary to add softwood black liquor in order to enhance hardwood pulp deresination.
Talc can be used to some extent in deresination. Talc particles have lipophilic and
hydrophilic parts and as a consequence it can agglomerate on pitch particle surfaces and
decrease their tackiness. The use of talc is not accepted in all pulp grades however.
Despite washing and other means of pulp deresination, traces of lipophilic resins are still
present in pulp. One of the aims of bleaching therefore is to remove these compounds and
improve the colour and cleanliness of pulp.
Literature
Pitch control, Wood Resin and Deresination. Eds. Back, E., L. & Allen, L., H., Publ. TAPPI
Press, Atlanta, 2000.
Chapters:
Ekman, R. & Holmbom, B., Chapter 2: The Chemistry of Wood Resin. p. 37-76.
Ström, G., Chapter 5: Physico-chemical Properties and Surfactant Behavior. p. 139149.
Back E.,L., Chapter 8: Deresination in Pulping and Washing. p. 205-230.
Alén R., Chapter 2. Basic Chemistry of wood delignification. Forest Products Chemistry. Ed.
Stenius, P., Publ. Fapet, Jyväskylä, Finland, 2000, pp. 59-104.
Ekman, R. & Hafizoglu, H., Changes in spruce wood extractives due to log storage in water,
Seventh International Symposium on Wood and Pulping Chemistry, China Technical
Association of the Paper Industry Beijing, China, 25-28 May 1993, vol. 3, pp 92-96
Holmbom, B., Chapter 5 Extractives. Analytical Methods in Wood Chemistry, Pulping and
Papermaking, Eds. Sjöström, E., Alén, R., Springer 1998, pp.125-148.
Nuget, H.M., Allen, L.H. & Bolker, H.I., Trans. Tech. Sect. CPPA 3(4):103 (1977)
Sjöström, E., Wood Chemistry. Fundamentals and Applications. 2 nd edition. Academic Press.
1993, 293 pp.
51
PART II – Bleaching
Studying Part II
When studying the chapters related to pulp bleaching, make sure you understand why and
when cooking has to be stopped and then bleaching carried out (how the lignin structure has
changed, why the colour of kraft pulp is so dark, and what happens to carbohydrates if
cooking is continued, etc.)
After studying Part II you should be able to explain the main chemical reactions taking place
during chemical pulp bleaching and the effect of these reactions on pulp quality and
brightness. An effective way of learning is to pose questions and seek to find explanations.
Here are some questions to start with:
·
·
·
What are the reaction mechanisms of different bleaching chemicals? What does it mean if
a chemical reacts via a radical, electrophilic or nucleophilic mechanism?
Why and how do reaction conditions (T, pH) affect bleachability (O-stage, D-stage etc.)?
Why is delignifying bleaching performed in a bleaching sequence? What influences the
order of bleaching stages in a sequence? What is obtained by organizing bleaching stages
in a certain order?
Instead of simply reading the text through, try to see the relevance of each issue on
bleaching. It must be remembered that ecological, environmental and economic issues are
vital in pulp manufacture in addition to pulp quality.
52
6 Chromophores in pulp – removable groups in bleaching
The natural colour of wood varies from yellowish to dark brown. The word chromophore
means a chemical group that absorbs visible light at a specific frequency and so imparts
colour to a molecule. Therefore, chomophores are coloured chemical compounds. Light
colours in wood derive from certain lignin structures while dark colours originate from
extractives. The typical components of wood extractives (fats, waxes, monoterpens, resin
acids and sterols) are colourless. On the other hand the heartwood of certain wood species
contains intensely coloured phenolic extractives (Figure 21, page 48). Phenolic structures are
water soluble and therefore they can be easily removed from pulp. Some of the phenolic
structures are highly reactive. The condensation of pine heartwood pinosylvin with lignin in
acidic conditions leads to the formation of compounds that are chemically stable.
Consequently the delignification of the pine heartwood is impossible in acidic conditions.
The colour of lignin originates from different oxidised structures (Scheme 19). O-quinones
have the strongest impact on the colour of lignin. The wavelength of the absorption
maximum is approximately 420 nm. Reduced forms of o-quinones, catechols, are colourless
but these compounds are able to form intensely coloured complexes with transition metals,
such as iron (III) ions (λmax=550 nm). Coniferyl aldehydes and α-carbonyl structures also
have an impact on the colour of lignin although their absorption maximums (λmax=340 nm
and λmax=300 nm) are in the UV- range of spectrum.
O
OH
O
OAr
OMe
OMe
OR
OR
Coniferyl aldehyde
alpha-Carbonyl
OH
HO
OAr
[H]
HO
HO
OAr
[O]
O
O
o-Quinone
OH
OH
OH
OH
Catechol
Fe
3+
OAr
O
O Fe
OH
Scheme 19.
Since the kraft pulping method is a strongly nucleophilic treatment, the native
chromophores of wood are destroyed. However the colour of kraft lignin is substantially
more intensive than the colour of native lignin or lignosulphonates from sulphite pulping.
The main part of the colour of kraft lignin derives from stilbenes, originated from phenolic
phenyl coumaran and pinoresinol structures (Scheme 20). Unlike the bisulphite ion,
hydrogen sulphide ion cannot hinder the formation of these stilbene structures. Stilbenes are
easily oxidised to stilbene quinones which have even more intensive colour than stilbenes.
53
OH
O
OMe
OMe
OMe
OH
O
[O]
[O]
OMe
OH
OMe
OMe
O
OMe
OH
OMe
O
Scheme 20
In comparison with wood lignin kraft lignin contains a large amount of phenolic structures.
However the amount of β-aryl ether linkages has decreased notably. The amount of enol
ethers is low if the kraft cooking has been successful, however lignin-carbohydrate
complexes are present in the pulp. Different condensed structures are usually accumulated
in residual lignin. The primary cell wall residual lignin content is approximately five times
higher than the residual lignin content in the secondary cell wall. Why is this so? The
chemical composition of primary and secondary cell wall lignin is different. In addition,
secondary cell wall lignin is first attacked by pulping chemicals and the middle lamella and
primary wall lignin is attacked last. The difference in lignin content between primary and
secondary cell walls is emphasised in Figure 23. Surface lignin means lignin, which is
actually on the fibres and which derives mainly from the compound middle lamella.
Figure 23. The development of surface and total lignin contents during the bleaching of kraft pine
pulp. Results (i.e. points in the figure) are gathered from different bleaching sequences.
Primary wall lignin has a high molecular weight, a strongly condensed structure (it has a lot
of carbon-carbon linkages between lignin monomers) and it is chemically bonded with
pectins and other cell wall polysaccharides. Secondary wall lignin has a lower molecular
54
weight and it is less condensed. This type of lignin is bonded with celluloses and the most
typical hemicelluloses of the cell wall.
Study problems
1. Define the word chromophore. Where in wood you can detect these kinds of
structures?
2. Why is the colour of kraft pulp so dark?
3. How does surface lignin (i.e. lignin found on the surface of kraft pulp fibres) differ
from secondary cell wall residual lignin?
4. Compare the development (i.e. decrease) in surface lignin content vs. the decrease in
total residual lignin during pulp bleaching. See Fig. 23.
55
7
General aspects of pulp bleaching
7.1 Aim
In comparison with bleaching agents the chemicals used in kraft pulping are relatively
inexpensive and contain less potentially water-polluting agents. Consequently,
delignification should be extended as far as possible in the cooking stage. However, due to
extensive carbohydrate losses and retarded delignification, pulping must be interrupted at
the point that corresponds to a residual lignin content of approximately 5 %.
As mentioned in the previous chapter, the structure of lignin is altered during kraft pulping.
The residual lignin contains notably less b-O-4-linkages, and the amount of condensed
carbon-carbon linkages is higher. There are also new linkages between carbohydrates and
lignin units. In addition the kraft pulp contains low amounts of lipophilic extractives and
new chromophoric structures which give rise to its characteristic colour.
Unbleached kraft pulp is brown and the ISO brightness value is approximately 20-30% ISO,
whereas brightness values of sulphite and fully bleached chemical pulps are 60-70 and 8892% ISO, respectively.
The aim of chemical pulp bleaching is to remove residual lignin, chromophoric structures,
and improve cleanliness of the pulp by removing extractives and other contaminants, such
as bark residues and inorganic residues.
7.2 Mass transfer in pulp bleaching
Mass transfer is an important factor in pulp bleaching. Figure 24 shows a schematic
illustration of the pulp delignification steps during bleaching.
Step 1 is the dissolution of a gaseous bleaching agent, such as ozone or oxygen into the
bleaching phase. The solubility of the gas governs the rate of reaction. Step 2 describes the
transport of reagent from the bulk phase onto the fibre surface. The rate of this step is
dependent on the mixing. Step 3 is the diffusion of reagent into the fibre cell wall where it is
able to react with lignin or other cell wall components. Step 4 is considered to be the
chemical reaction itself. Step 5 is the transport of the reaction product out of the cell wall. The
diffusion may proceed towards the lumen or the outer surface of the cell wall. The final step
(6) is the transport of reaction products into the bulk phase. The rate of this step depends on
mixing.
56
Figure 24. Steps of mass transfer in bleaching.
Study problems
1. Explain the aim of bleaching.
2. How does residual lignin differ from native lignin?
3. Compare the differences and similarities of mass transfer in pulping and
bleaching. See also Ch. 1.
57
8 Classification of bleaching chemicals
Learning objectives
1. Classify the most common bleaching chemicals as electrophiles, nucleophiles or
radically reacting species.
2. List the chemicals which are reactive towards hexenuronic acid, phenolic and
non-phenolic lignin structures and chromophores.
3. Know the purpose of auxiliary bleaching chemicals.
Bleaching chemicals react via electrophilic, nucleophilic, or radical reaction mechanisms.
Some auxiliary chemicals are also necessary in the bleaching process. These auxiliary
chemicals do not have a bleaching ability but they improve the function of bleaching
chemicals. Some of the bleaching chemicals can function both as nucleophiles and
electrophiles. It is also possible that all the different bleaching chemical types (nucleophiles,
electrophiles and radicals) can be present in a single bleaching stage.
8.1 Electrophiles
An electrophile is a compound that is willing to accept a pair of electrons. In practice
electrophiles can react with all unsaturated structures (i.e. structures that contain C=C
bonds). Thus, electrophilic bleaching agents are able to react with phenolic and non-phenolic
lignin structures. In addition electrophiles can react with hexenuronic acid groups and with
certain extractives, since they contain carbon-carbon double bonds.
The main electrophilic bleaching chemicals are chlorine, hypochlorous acid, ozone and
peracetic acid. Also persulphate and permolybdate are used to some extent. Every
electrophilic bleaching chemical, except ozone, consist an electrophilic part and a leaving
group (Table 6). The reactivity of an electrophile depends on two things:
1. the character of the electrophilic part and
2. the goodness of the leaving group.
If the electrophilic parts are the same, the character of the leaving group determines the
reactivity. The reactivity of electrophiles increases in the following order: O 3 >> Cl2 > HOCl >
HOOSO3-, HOOMoO3- > HOOAc.
58
Table 6. Electrophilic bleaching agents.
Compound
Charge distribution
Leaving group
Conjugate acid
Clδ+ - Clδ-
Cl-
HCl
H2OCl
HOCl
HOOMoO3HOOSO3-
Cl - OH
HOδ+ - ClδHOδ+ - δ-OMoO3HOδ+ - δ-OSO3-
H2O
ClMoO42SO42-
H3O+
HCl
HMoO4HSO4-
HOOAc
HOδ+ - δ-OAc
AcO-
AcOH
O3
+
Cl2
+
O-O-Oδ+
2δ+
If an electrophile attacks an unsubstituted carbon of an aromatic ring, a carbocation is
formed. The carbocation is stabilised by the cleavage of a proton. The net reaction is
therefore an aromatic substitution. As a result the aromatic ring structure is chlorinated or
hydroxylated (Scheme 21).
Lignin
Lignin
Lignin
+
Cl
H
Cl
OMe
Cl
OMe
OLignin
Cl
OMe
OLignin
OLignin
Scheme 21. Chlorination (or hydroxylation in the case of other electrophiles than Cl2) of lignin.
Electrophiles are able to displace a side chain of a phenyl propane unit that contains an αhydroxyl group (Scheme 22). In this case lignin is depolymerised.
OH
HO
HO
Cl
OLignin
Cl
OH
OH
Cl
O
OLignin
Cl
OLignin
+
OMe
OMe
OLignin
OMe
OLignin
OLignin
Scheme 22. The chlorination (or hydroxylation in the case of other electrophiles than Cl 2) and
depolymerisation of lignin.
The aromatisation of a carbocation is not possible if the electrophile attacks an aromatic
carbon atom with an oxygen substituent (cf. situation in Scheme 22). In this kind of situation
a nucleophile (e.g. water) is added to a carbocation and a quinone is finally formed. The
reaction leads to the depolymerisation and demethoxylation of lignin (Scheme 23).
Lignin
Lignin
Lignin
Lignin
H2O
OH
Cl
Cl
OMe
OLignin
Cl
OMe
OLignin
Cl
-MeOH
OMe -LigninOH
OLignin
O
O
Scheme 23. Depolymerisation and demethoxylation of lignin.
59
Electrophilic reactions of isolated carbon-carbon double bonds are always followed by a
nucleophilic addition reaction to a carbocation. In most cases the net reaction is a
chlorination and / or hydroxylation.
The reactions of ozone are different from the reactions described above. The reaction product
(ozonide) is a strong nucleophile. Ozone reacts with all unsaturated structures in a similar
kind of way. The net reaction is a cleavage of a carbon-carbon double bond and formation of
carbonyl groups in both sp2 carbon atoms (Scheme 24). Usually lignin is depolymerised via
secondary hydrolysis of esters.
Lignin
Lignin
Lignin
OMe
+
OMe
O
OLignin
+
OMe
O
O
O
Attack of O3.
Formation of ozonide.
O
O
LigninO
O
OLignin
O
O
O
The secondary hydrolysis
of ester bond (see 2.4).
Depolymerisation and
demethoxylation of lignin.
Lignin
Lignin
OMe
-MeOH
-LigninOH
O
OH
O
O
HO
LigninO
Scheme 24. The reactions of ozone and an unsaturated lignin structure. Depolymerisation and
demethoxylation of lignin.
8.2 Nucleophiles
Nucleophiles are compounds that are able to donate a pair of electrons during a chemical
reaction. The compound accepting the pair of electrons is an electrophile. Nucleophilic
bleaching chemicals are hypochlorous acid (HOCl), hydrogen peroxide (H2O2), and peracids.
The nucleophilic reaction with the electrophilic lignin structure is a two-step reaction:
·
·
Step 1: an addition of a nucleophile to an electrophilic carbon atom. Usually the
electrophile is some type of a carbonyl structure, such as o-quinone.
Step 2: a cleavage of a carbon-carbon bond which is a rate determining step (Scheme 25).
Lignin
XO-
H2O
OH
O X
O
O
Lignin
Lignin
Lignin
O
-X
OH
HO
OH
O
X
HO
O
O
OH
Scheme 25. Nucleophilic attack on carbonyl lignin structures. X is the leaving group (Cl -, SO42-,
MoO42-, AcO-, or HO-).
60
The nucleophilic atom in nucleophilic bleaching agents is oxygen. Therefore the rate of
reaction is greatly determined by the properties of the leaving group. Reactivity decreases in
a following order: ClO- > HOOSO3-, HOOMoO3- > HOOAc > HOO - (corresponding leaving
groups are Cl - > SO42-, MoO42- > AcO - > HO-). The cleavage of a carbon-carbon bond is an
acid-base-catalysed reaction and therefore reactivity is also dependent on pH of the
bleaching stage. Usually nucleophiles react with chromophoric lignin structures but in some
lignin structures nucleophiles can cause depolymerisation of lignin.
8.3 Radicals
Chlorine dioxide and oxygen are bleaching chemicals reacting via a radical mechanism.
Chlorine dioxide is notably more reactive than oxygen. Nowadays catalytic oxygen
bleaching is actively studied. Polyoxometalates, transition metal complexes or laccasemediator pairs can be used as catalysts. Usually these catalysts also react in a radical
mechanism (the donation of a single electron).
The rate of reaction is affected by the stability of the radicals which originate from lignin.
Phenoxy radicals are very stable due to resonance structures. Oxygen is less reactive and it
reacts mainly with phenolic lignin structures. These reactions occur only if phenols are
ionised (in alkaline media) (Scheme 26).
R
R
O
OR'
R
OH
OR'
.
R
R
O
OR'
- HOO-
R'O
O2 R
O
OO-
O
O
R
O2 - ,-O2
.
OH
O
OR'
R
O2
OR'
HO-
O
OR'
R
R
OO .
O2 ,-O2 O
OR
O2- + H2O
O
OR
HO2 + HO-
HO2 + O2HO2- + O2
Scheme 26. Radical reaction of oxygen and phenolic lignin structures.
61
Oxygen and/or the in situ formed superoxide oxidise phenoxy radicals further. As a result
organic peroxy radicals or peroxides are formed. The final product of this reaction chain is a
quinone. In the course of the reaction lignin is depolymerised. Phenoxy radicals are easily
bonded with each other (cf. biosynthesis of lignin) and therefore the pressure of oxygen
must be sufficiently high during the bleaching. Superoxide, which is formed during the
oxygen bleaching stage, is partially degraded to water and oxygen (dismutation reaction).
Chlorine dioxide reacts analogically with phenolic lignin structures (Scheme 27). Secondary
oxidation products, organic chlorites, degrade by the cleavage of hypochlorite or chlorite
and quinones are formed. As a result of this reaction lignin is depolymerised. In acidic
media chlorine dioxide degrades forming hypochlorous acid which is in balance with
chlorine and hypochlorite.
R
R
O
R
OR'
.
OH
O
OR'
ClO2
OR'
H
R
R
O
OCl
O
R
O
-ClO-
OR'
R
O
OR'
.
O
R
ClO 2
R
OR'
OH
OR'
HO-
ClO2
-ClO2-
O
O
-ClO2-
R
O
OClO
OR'
R
ClO 2
R
O
OR
O
O
OR
HO-, ClO3 -
ClO 2
OR'
Scheme 27
Reactions of chlorine dioxide with non-phenolic lignin structures lead to the formation of
resonance stabilised benzyl radicals. These structures react almost analogically with the
reaction path shown in Scheme 27. As a result lignin is depolymerised. However, the
reactivity of non-phenolic benzylic structures is low and therefore their contribution to the
oxidation of lignin is generally insignificant.
8.4 Auxiliary chemicals in bleaching
Table 7 shows the main auxiliary chemicals and their function in bleaching. Auxiliary
chemicals have their own stages before or between the principal bleaching stages.
62
Table 7 Auxiliary chemicals and their functions in bleaching.
Chemical
Acid (A)
Function
Removal of hexenuronic acid groups by acid hydrolysis (see Ch. 9)
and/or removal of transition metals
Dissolution of oxidised lignin residue after the acid stage
Removal of transition metals (Cu, Fe, Mn) which consume
peroxide bleaching agent
Degradation of surface lignin-carbohydrate complexes
Alkali (E)
Chelant (Q)
Xylanase (X)
8.5 Design of bleaching sequences
A multistage bleaching process is necessary since it is impossible to obtain sufficient
decolourisation (i.e. brightening) of pulp in a single bleaching stage. In earlier days for
example the minimisation of environmental impacts may have been the driving force behind
the design of bleaching sequences. Nowadays the driving force is often the minimisation of
total bleaching costs.
The use of oxygen has increased since it is the cheapest of all bleaching chemicals. The
hexenuronic acid content of oxygen delignified hardwood pulp is high. Therefore it is
necessary to remove HexA groups by an acid hydrolysis step. The surface lignin of
hardwood pulp, which is normally difficult to remove, can be achieved by ozone treatment.
Ozone is not compatible with softwood pulps and use of O 3 leads to deterioration of pulp
quality. Consequently in some cases xylanases can be used to promote the removal of
surface lignin in softwood pulps. Chlorine dioxide stage is often used for removal of nonphenolic lignin structures (the non-phenolic lignin reacts actually with the in situ formed Cl 2
or HOCl). The final bleaching stage is preferably a stage where chromophores can be
oxidised. The final bleaching stage can be for example a peroxide stage, chlorine dioxide
stage conducted in a high pH (final pH 4-5) (in the high pH both hypochlorite and
hypochlorous acid are present) or even a peracetic acid treatment.
8.6 Summary -main bleaching chemicals
Table 8. Classification of bleaching chemicals.
Reaction
Bleaching
mechanism
chemical
Electrophilic
Chlorine (D)
Hypochlorous acid (D)
Ozone, Z
Peracids, Paa
Nucleophilic
Hypochlorous acid (D)
Hydrogen peroxide, P
Peracids, Paa
Radical
Chlorine dioxide, D
Oxygen, O
Target
Unsaturated structures
(-C=C-) [lignin, HexA,
extractives]
Effects
Chlorination of lignin (D)
Oxidation of lignin and HexA
Electrophilic carbon atoms,
such as carbonyl structures Brightening effect, oxidation of lignin
(C=O) [lignin, extractives]
Phenolic and nonphenolic
structures [lignin]
Brightening effect, oxidation and
chlorination of lignin (D)
Oxidation of lignin (O)
63
In bleaching the main reaction mechanism is oxidation. It is important to remember that
depending on bleaching conditions different intermediate products are formed. For example
as chlorine dioxide oxidises lignin it is reduced to chlorite. Depending on pH chlorite (ClO2-)
is partly converted to chlorous acid (HClO2), etc.
Reactions of bleaching chemicals are often very complex, because of the formation of
intermediate products and existence of several different chemical compounds (residual
lignin, carbohydrates, extractives, inorganic compounds). The main aspects and reaction
paths of individual bleaching chemicals are described in following chapters.
Study problems
1. Explain the following concepts: radical, nucleophile and electrophile.
2. Explain why different kinds of chemicals are used in pulp bleaching.
3. What kind of chemical structures are attacked by electrophilic bleaching
chemicals? What are the effects on lignin, HexA and chromophores?
4. Which lignin structures are attacked by radical bleaching chemicals?
5. Can a bleaching chemical be both a nucleophile and electrophile at the same
time? If so, give an example of this kind of bleaching chemical.
6. Give examples of auxiliary bleaching chemicals and explain their function.
64
9 Selective hydrolysis of HexA (A)
Learning objectives
1. Understand the reasons for HexA removal.
2. Describe how HexA can be removed from pulp.
Hexenuronic acid groups (HexA) are formed during kraft pulping (see 2.6). In pulp
bleaching, especially in hardwood pulp bleaching, these groups react with chlorine dioxide
and all electrophilic bleaching chemicals, such as chlorine, hypochlorous acid, ozone, and
peracids. When these agents are used, HexA in the pulp causes increased consumption of
bleaching chemicals, lowers the brightness of pulp, reduces brightness stability, binds heavy
metal ions, and increases deposit formation in the bleaching equipment. Consequently the
removal of hexenuronic acid groups prior to D-, O-, Z-, or Paa -stages is essential for
economic, ecological and environmental reasons.
Bleaching chemicals react with the “ene” functionality of HexA (i.e. with the carbon-carbon
double bond of the hexenuronic acid ring structure). These hexenuronic acid groups can be
removed by selective hydrolysis. As a result of mild acid hydrolysis the acid groups of HexA
are converted to 2-furoic acid, formic acid, and 5-carboxy-2-furaldehyde, Scheme 28.
COOH
MeO
O
COOH
kraft pulping
O
HCO2H
+
O
hydrolysis
COOH
~ 90%
HO
OXylan
OH
HO
OXylan
OH
O
O
COOH
~ 10%
Scheme 28. Hexenuronic acid groups are formed during kraft pulping from 4-O-methylglucuronic
acid groups of xylan. HexA is converted to 2-furoic acid, formic acid, and 5-carboxy-2-furaldehyde
during acid hydrolysis (A-stage).
The resulting hydrolysis products consume permanganate as does HexA itself, unwashed
pulps therefore, have higher kappa number than washed kraft pulps after such hydrolysis.
However, the hydrolysis products are less reactive than lignin and HexA themselves and in
some cases this means that the hydrolysis stage can be connected with other bleaching
stages, such as a D0-stage (see 11.2), without the need for intermediate washing stages. The
removal of HexA by acid hydrolysis decreases the kappa number of kraft pulp. For example
it has been observed that the removal of HexA from birch kraft pulp is most selective on
average at pH 3-3.5 and at a temperature of 80-100 ºC.
The HexA content of hardwood pulps is higher (e.g. birch, eucalyptus) than it is in softwood
pulps (e.g. pine) since xylan is the main type of hardwood hemicelluloses. In the course of
65
alkaline pulping these hardwood xylan methylglucuronic acid groups are converted into
hexenuronic acid groups, which as a consequence means that the removal of HexA is often
necessary prior to hardwood pulp bleaching.
Study problems
1. Why is HexA content more important in hardwood than softwood pulps? NB compare
properties of hardwood and softwood hemicelluloses.
2. Why it is useful to remove HexA prior to bleaching? Consider economic, ecological,
environmental and pulp quality issues.
3. How is the removal of HexA carried out?
4. Why is the selective hydrolysis of HexA (i.e. A-stage) unnecessary prior to the
bleaching of sulphite pulps? (see Ch. 1.1 also)
Literature
Vuorinen, T., Fagerström, P., Buchert, J., Tenkanen, M. & Teleman, A., Selective Hydrolysis of
Hexenuronic Acid Groups and its Application in ECF and TCF Bleaching of Kraft Pulps. J.
Pulp Pap. Sci. 25(1999)5, pp. 155-162.
Vuorinen, T., Fagerström, P., Räsänen, E. & Vikkula A., Hydrolysis of Hexenuronic Acid Groups
Opens New Possibilities for Development of Bleaching Processes. 9th International
symposium on wood and pulping chemistry (ISWPC), Montreal, Que, Canada, 9-12 June
1997, pp M4-1-M4-4.
66
10 Oxygen delignification (O)
Learning objectives
1. Understand the basic chemistry of oxygen and the formation of reactive
intermediates.
2. Describe the selectivity of oxygen and its intermediates.
There are many similarities between alkaline oxygen delignification and ozone and
hydrogen peroxide bleaching. All of these chemicals have the same reaction intermediate
species. For example a large quantity of H2O2 is formed during oxygen and ozone
delignification.
Oxygen delignification is often applied as an intermediate stage between kraft pulping and
pulp bleaching. Molecular oxygen is not very reactive and therefore activation of the
substrate and higher temperatures are required. Delignification is usually carried out at
relatively high temperatures (90-110 ºC) for 60 minutes under pressure (inlet and outlet
pressures about 750 kPa and 500 kPa, respectively). Nowadays most of the systems operate
with medium-consistency (10-14%).
10.1 Chemistry of oxygen
Oxygen is a suitable oxidising agent for delignification. Under alkaline conditions oxygen
oxidises organic substances and it is reduced to water. During the stepwise reduction of
oxygen reactive intermediates are also formed. The intermediates formed during the process
are hydroperoxyl radicals (HOO.), hydrogen peroxide (HOOH), hydroxyl radicals (HO.),
and their anions (O2-., HOO-, O-.). In alkaline conditions the superoxide radical anion (O -2.) is
also present, Scheme 29.
O2
pKa
+ e-, H+
HOO.
4.8
H+ + O2-.
+ e-, H+
HOOH
11.8
H+ + -OOH
+ e-, H+
HOH + .OH
+ e-, H+
2 HOH
11.9
H+ + O-.
Scheme 29. The stepwise reduction of oxygen to water and formation of reactive intermediates.
Some of the intermediates have low selectivity and therefore is necessary to control the
formation of these compounds in order to avoid severe degradation of polysaccharides. The
reaction pattern is very complex due to the high number of reactive intermediates formed.
67
Generally the oxidation process by oxygen includes a multitude of radical chain reactions
involving a variety of organic species derived from lignin and polysaccharides (Scheme 30).
It is still unclear whether the chain reactions are relevant at all in oxygen delignification.
Initiation
RO- + O2
RH + O2
RO. + O2-.
R∙ + HO2.
Propagation
R. + O2
RO2. + RH
RO2
RO2H + R.
Termination
RO. + R.
ROR
Scheme 30.
10.2 Reactions of wood constituents
RESIDUAL LIGNIN
Phenolic lignin structures are oxidised during the oxygen delignification. Under alkaline
conditions, free phenolic hydroxyl groups of residual lignin are ionised. Oxygen is capable
of attacking these ionised sites. Alkaline conditions are required in order to achieve
sufficient rates of delignification. The primary reaction starts when an ionised phenolic
group loses a single electron to a suitable acceptor, such as molecular oxygen. Then the
phenolic group is converted into phenoxy radical which is a resonance hybrid structure. In
the lignin structure the odd electron formally exists at the phenolic oxygen, at the b-carbon
of the side chain, or at one of several carbon atoms in the aromatic ring. In subsequent steps,
the phenoxy radical is converted into an organic hydroperoxide. The hydroperoxide
intermediates can react further leading to formation of oxidised products. As a result the
solubility of residual lignin is enhanced. The oxidative reactions of phenolic lignin structures
during oxygen delignification are illustrated in Scheme 26 (see Ch. 8, page 58). In addition to
the reactions shown in Scheme 26 monomeric extractive-based residues and volatile
substances such as methanol are released from the pulp.
CARBOHYDRATES
Carbohydrates are attacked during oxygen delignification and therefore the delignification
process has to be stopped once approximately 50% of the lignin is removed. Two main
carbohydrate reactions are a) transition metal catalyzed random chain cleavage (i.e. cleavage
of glycosidic bonds) and b) the peeling of reducing end groups. Pulp always contains small
quantities of transition metals (e.g. Fe, Cu, and Mn) which cause catalytic decomposition of
peroxides (an intermediate compound in stepwise reduction of oxygen) generating hydroxyl
radicals (HO·) which are able to attack the polysaccharide chains. These reactions lead to
degradation of carbohydrates and a decrease in pulp viscosity. If the reaction is allowed to
proceed far enough then strength properties are also adversely affected.
The term selectivity can be described as the ratio of attack on lignin to attack on
carbohydrates. The selectivity of a process is affected by the choice of process conditions and
by the presence of pulp contaminants, such as transition metals. The selectivity of oxygen
delignification can be improved by an addition of an inhibitor, i.e. magnesium. It is believed
that MgSO4 is converted to Mg(OH)2 which absorbs the metal ions and inhibits peroxide
decomposition.
68
The first step in the cleavage of glycosidic bonds involves the oxidation of a hydroxyl group
at C2-position of a monomeric sugar unit into a carbonyl group, as illustrated in Scheme 31
A. The glycosidic bond at C4-position is alkali-labile. The formation of a carbonyl group at
C2-position facilitates the cleavage of the glycosidic bond by b-alkoxy elimination.
Simultaneously a new reducing end group is formed. Oxidation at C 3 and at C6 can also lead
to the same result. A competing reaction takes places when oxygen attacks an ionised keto
form, resulting in the formation of a 2,3-diketo structure. This structure may be converted
into a furanosidic acid group or an open-chain structure containing two carboxylic acid
groups. It is important to notice that the last reaction takes place without chain cleavage as
shown in Scheme 31 B.
CH 2OH
CH2OH
A)
O
O2 /HO-
O R
OH
CH 2OH
O
R
R O
O
HO-
O R
OH
O
O R
R O
O
OH
HO
O
- RO-
CH2OH
O
O
COOCOO-
CH2OH
O
R
CH 2OH
O
R
O
O
+
COO-
HO
CH2OH
O
O
O R
R
HO
O
O
B)
CH2OH
CH2OH
O
R
O
OH
O
R
CH2OH
O
HOR
-
O
HO-
O R
OH
O
R
O
O
OH
O
OH
R
O
O2 /HOCH2OH
CH2OH
O
O
COOCOO-
R
CH2OH
O
+
R
O
HO
O
O
COO-
R
R
O R
O
O
O
Scheme 31
Oxidative depolymerisation is not responsible for polysaccharide yield loss in oxygen
delignification. However, it generates new reducing end groups which are subjected to the
alkali-catalysed peeling reaction of polysaccharides. The reaction mechanism was described
earlier in Chapter 2.3.
There are also other reactions occurring during oxygen delignification. The oxidation of
reducing end groups to alkali-stable aldonic acid end groups (i.e. arabinonic and other
aldonic acid end groups) also occurs. These groups stabilise cellulose chains.
EXTRACTIVES
Unsaturated lipophilic wood extractives, (i.e. resin components), can react with oxygen. As a
result a large mixture of oxidised products is formed. It appears that oxygen is not able to
penetrate into lipophilic resin aggregates. Pulps can contain high amounts of highly
69
saturated components (i.e. linoleic acid and betulaprenols) after oxygen bleaching. However
alkaline conditions with efficient washing can result in very extensive deresination during
oxygen bleaching.
Literature
Alén R., Chapter 2. Basic Chemistry of wood delignification. Forest Products Chemistry. Ed.
Stenius, P., Publ. Fapet, Jyväskylä, Finland, 2000, pp. 59-104.
Holmbom, B., Chapter 9: Resin Reactions and Deresination in Bleaching. Pitch control, Wood
Resin and Deresination. Eds. Back, E., L. & Allen, L., H., Publ. TAPPI Press, Atlanta, 2000. p.
231-244.
McDonough, T.J., Section IV: The Technology of Chemical Pulp Bleaching, Chapter 1: Oxygen
Delignification. Pulp Bleaching – Principles and Practice. Eds. Dence, C., W. & Reeve, D., W.,
Publ. TAPPI Press, Atlanta, 1996, p. 213-240.
70
11 Chlorine dioxide bleaching (D)
Learning objectives
1. Understand the basic reaction mechanisms of chlorine dioxide bleaching (i.e. the
formation of chlorite, chlorous acid, chlorate, chlorine etc.).
2. The effect of pH on the oxidation and chlorination of lignin.
3. The formation and composition of AOX compounds in the D-stage.
11.1 Reactions of wood constituents
LIGNIN
As chlorine dioxide oxidises lignin it is reduced to chlorite. Depending on the pH, chlorite
(ClO2-) is partly converted into chlorous acid (HClO2) (see Equation 43 and 44, pK1~2).
HClO2 + H2O D ClO2- + H3O+
K1 =
(43)
[ClO ]× [H O ]
2
+
(44)
3
[HClO 2 ]
Although chlorine dioxide can react with non-phenolic lignin structures, it reacts much more
easily with phenolic lignin and ionised lignin structures, Scheme 26 (see Ch. 8, page 61).
Equation 45 describes the rate of decomposition of chlorine dioxide.
] [
]
Lign phenolic
d [ClO 2 ]
= - k1 Lign non- phenolic - (k 2 + k 3 K 2 / H 3O + ) ×
dt
1 + K 2 / H 3O +
[
]
[
[
]
(45)
where K2 is a dissociation constant for phenolic lignin (pK2 ~10). The relative rates of the
different routes follow the order k1 << k2 << k3.
Neither chlorite nor chlorous acid are reactive toward lignin (except for aldehyde groups
that are oxidized to carboxylic acid groups by chlorous acid). This can be observed when a
chlorine dioxide treatment is carried out in alkaline media, for example by adding a small
amount of chlorine dioxide to the oxygen delignified pulp (pH ~ 9) which is not completely
washed. Due to the formation of chlorite, and also the formation of chlorate, hypochlorite
and chloride, pH decreases to 5-6 (according to Equation 44 this is a fast reaction). Increasing
reaction time does not increase the lignin removal although the chemical consumption
measured by titration is only 40-50 %. The reason for this phenomenon is the formation of
chlorite.
In acidic conditions chlorite decomposes via the formation of chlorous acid. This reaction is
partly catalysed by chloride ions. The rate of degradation can be described by the Equation
46.
71
[
]
[
][ ]
d ClO22
= -k 4 [HClO2 ] - k5 [HClO2 ]× H 3O + × Cl dt
(46)
The rate determining reaction is followed by several other reactions which lead to the
formation of hypochlorous acid, chlorine dioxide, chlorate, chloride, etc. Hypochlorous acid
is in balance with hypochlorite (pK4 ~ 7.5) and chlorine. The balance depends on pH and the
chloride ion concentration.
Cl2 + 2H2O D HOCl + H3O+ + Cl-
(47)
[HOCl ]× [H 3O + ]× [Cl - ]
[Cl2 ]
(48)
HOCl + H2O D ClO- + H3O+
(49)
K3 =
K4 =
[ClO ]× [H O ]
-
+
(50)
3
[HOCl ]
Both electrophiles (HOCl, Cl2) and nucleophiles (ClO-) are formed during chlorine dioxide
bleaching stages. The final pH of D0 stage is typically 2-3 and ClO2 exists mainly as chlorine.
During other D stages the final pH is so high (4-5) that the proportion of chlorine is low.
The reactions of chlorine with lignin lead to chlorination, whereas hypochlorous acid
increases also the amount of (phenolic) hydroxyl groups in lignin. The kinetics of
chlorination and hydroxylation are presented in Equations 51-54 (chlorination through
H2O+Cl is excluded here for clarity).
d [LignCl ]
= k 6 [Lign ] × [Cl 2 ]
dt
(51)
d [LignOH ]
= k 7 [Lign ] × [HOCl ]
dt
(52)
d [LignCl ]
k [Cl ]
= 6 2
d [LignOH ] k 7 [HOCl ]
(53)
By combining Equations 48 and 53:
[
][ ]
d [LignCl ] k 6 H 3O + × Cl =
d [LignOH ]
k7 K 3
(54)
According to Equation 54 the degree of lignin chlorination is linearly dependent on the
hydrogen and chloride ion concentrations during the chlorine dioxide stage. By increasing
the chemical dosage the amount of chloride present in the chlorine dioxide stage increases
leading to higher degree of chlorination. On the other hand the circulation of bleaching
72
(washing) filtrates may increase the chloride concentration in the pulp and consequently the
AOX load may increase.
The most harmful compounds are polychlorinated aromatic lignin degradation products. In
comparison with chlorine bleaching the amount of polychlorinated compounds is low in
chlorine dioxide bleaching.
High final pH leads to better bleachability because the proportion of nucleophilic
hypochlorite increases as the pH increases (Equation 49). The degradation of chlorite sets the
limit for pH (Figure 25). If the final pH is > 5 part of the bleaching potential remains unused.
It is important to remember that despite the high oxidation state chlorate is not reactive
toward lignin.
HEXENURONIC ACID GROUPS
ClO2 is a very selective lignin oxidant and reactions with carbohydrates are limited. Chlorine
dioxide is indirectly able to react with the hexenuronic acid groups of xylan. The
degradation products of HexA in chlorine dioxide bleaching are chlorinated and
hydroxylated dicarboxylic acids (Figure 26). The formation of these compounds can be
explained with a mechanism according to which chlorine or hypochlorous acid first reacts
with electrophilic carbon-carbon double bonds. Chlorine dioxide itself does not oxidise
hexenuronic acid groups. In a normal chlorine dioxide stage both lignin and HexA contents
decrease in parallel.
OH
High pH
O
HO
OH
O
OH
O
O
OH
OH
O
HO
OH
Cl
OH
OH
HO
O
OH
HO
OH
O
Low pH
O
OH
O Cl
OH
Cl
OH
AOX
Figure 25. The major products of HexA (mmol/kg) in ClO 2 bleaching of kraft birch pulp.
Chlorinated products of HexA are formed during the D0 stage since the pH is low. The
decrease in AOX level of hardwood pulp can be obtained by a selective hydrolysis (A-stage).
The bulk of hexenuronic acid groups are usually removed during the A-stage.
EXTRACTIVES
Chlorine dioxide reacts with unsaturated lipophilic extractive (i.e. resin) components. ClO 2
favours oxidation reactions. The reaction products of resin are comprised of a mixture of
oxidised products containing hydroxyl, carboxyl or carbonyl groups. The reaction products
are more hydrophilic than the original compounds and therefore they are easier to wash out
of the pulp. In comparison to Cl2 bleaching, ClO2 bleaching results in the formation of
relatively small quantities of chlorinated products.
73
11.2 Combined A and D stages
As observed earlier, chlorine dioxide and other chlorine compounds originating from ClO 2
react with lignin rather than with the hydrolysis products of HexA (mainly furan carboxylic
acids). This provides interesting possibilities for combining A (acid hydrolysis, see Ch. 9) and
D bleaching stages. The most natural option is A/D 0 stage (without intermediate washing).
The required delay between the stages is only on a scale of seconds to minutes if the
temperature is high enough. Another option is a D/A/D stage (also without intermediate
washing). During the first chlorine dioxide stage, phenolic lignin structures are oxidised at
high pH. During the second stage, the A stage, hexenuronic acid groups are removed by
selective hydrolysis. During the final D stage, non-phenolic lignin structures are oxidised
(Figure 27). In many mills D/A prebleaching is applied because HexA is often enriched in
hardwood pulp in chlorine dioxide prebleaching. According to the latest results A-E OP-D-P is
the most economic and ecological alternative for eucalyptus pulp bleaching.
Figure 26. Principles of A/D and D/A/D pre-bleaching.
Study problems
1. ClO2 is a bleaching chemical that reacts via a radical mechanism and attacks phenolic
lignin structures. However, in ECF bleaching (including D-stages) it removes almost
all lignin and hexenuronic acid groups in kraft pulp. Explain why.
2. Explain how pH affects the formation of chlorinated organic compounds (AOX) in
the D-stage.
74
12 Hydrogen peroxide (P)
12.1 Chemistry of hydrogen peroxide bleaching
Hydrogen peroxide is decomposed in alkaline conditions or if transition metals are present. It
has also been observed that reducing sugars and oxygen lead to catalytic decomposition. The
main decomposition products are hydroperoxy (or perhydroxyl) anions (HOO -), hydroxyl
anions (HO·) and superoxide anions (O2-·). In alkaline conditions the equilibrium between
H2O2 and HOO- is:
H 2 O2 + OH - « HOO - + H 2 O
(36)
Base catalysed decomposition and radical formation:
H 2 O2 + HOO - ® ×OH + O2- × + H 2 O
H 2 O2 + HOO - ® OH - + O2 + H 2 O
(37)
(38)
Metal catalysed decomposition and radical formation:
H 2 O2 + M ® HO × + HO - + M +
(39)
H 2 O2 + HO× ® HOO × + H 2 O
(40)
+
+
HOO × + M ® O2 + H + M
HOO × + H 2 O ® O2- × + H 3 O +
(41)
(42)
Catalysis by transition metals is much greater than by a base and rate constants are in the
order of 1000 times higher for transition metal catalysis. In Figure 28 a proposed mechanism
for metal ion catalysed decomposition of hydrogen peroxide is illustrated. The effect of
reducing sugars is taken into account in this model.
Sugarred
Fe(III) (aq)O2.-
HO. + HO-
Fe(III) (aq)
O2
H2O2
Fe(II) (aq)
H2O2
HOO.
HO2- Sugarred-
Sugarox
O2.Figure 27. A proposed mechanism for metal ion (Fe3+) catalysed decomposition of hydrogen
peroxide (Vuorinen and Heikkilä, 2003).
Hydrogen peroxide decomposition is necessary in H2O2 bleaching but the rate of
decomposition must be controlled. If the decomposition is too fast and the concentrations of
HO· and O2-· become too high, the selectivity towards lignin is lost and cellulose degradation
75
occurs. Untreated pulp contains high levels of metals. Transition metals can catalyse H2O2
decomposition, therefore the metal content must be controlled carefully. The two main
methods to control the metal content in kraft pulp prior to H2O2 bleaching are:
1. Metal chelation at pH 4-7 followed by washing
2. Acid wash at pH 1.5-3.0 followed by replenishment of magnesium ions.
Strong chelating agents, such as EDTA (ethylenediaminetetraacetic acid) or DTPA
(diethylenetriaminepentaacetic acid), can displace the metals and make them soluble by
binding the metals inside their structures. After chelation the metals are more easily
removed by washing. Acid wash or mild acid hydrolysis can also remove heavy metals. In
this case it is possible to bleach without using nitrogen-containing complexing agents.
A chelation or acid wash will not remove all the transition metals capable of decomposing
H2O2. If Mg2+ or SiO32- is present in bleaching the released transition metals are complexed
and their effect limited. Consequently the addition of Mg 2+ and/or SiO32- when using
hydrogen peroxide improves the bleaching efficiency. Peroxide bleaching requires an
optimal level of alkaline earth metals (e.g. Ca and Mg) and the lowest obtainable level of
transition metals (e.g. Mn, Fe and Cu). Nevertheless, even the best possible transition metal
removal leaves a certain amount of residual transition metals which are able to catalyse
hydrogen peroxide decomposition needed to achieve sufficient delignification.
12.2 Reactions with wood components
LIGNIN
Residual lignin contains coloured quinone structures. These quinone structures contain
conjugated carbonyl groups. The hydroperoxy anion (HOO-) causes an elimination of
conjugated carbonyl structures, Scheme 32.
HOOO
O
O
O
O
HO-
O
OH
O
O
OH
Scheme 32. Susceptible sites in residual lignin for attack by hydroperoxy anions. The degradation
product is a muconic acid derivate.
The nucleophilic hydroperoxy anion (HOO-) is a mild oxidant. Consequently this type of
attack is insufficient to cause rapid delignification. On the other hand HO· and O 2-· radicals
are powerful oxidants and can cause degradation and dissolution of lignin. The hydroxyl
and superoxide radicals are nonselective and can also attack carbohydrates causing
depolymerisation.
CARBOHYDRATES
The reactions of carbohydrates in a peroxide bleaching stage are very similar to those in an
oxygen stage, as described in Chapter 10.2. Hydroxyl radicals can attack polysaccharide
chains. As a result the chains are depolymerised and pulp viscosity decreases. If the chain
76
reaction is allowed to proceed far enough, pulp strength properties start to deteriorate. As in
the case of oxygen delignification, the hexenuronic acid groups of xylan survive an alkaline
peroxide stage.
EXTRACTIVES
Peroxide is reactive toward conjugated double bonds. For example abietic acid type acids are
widely oxidised in alkaline peroxide bleaching. Also, phenolic substances, such as lignans
and stilbenes, are reactive toward peroxide. However, fatty acids, which contain isolated
double bonds, are less reactive. Peroxide does not penetrate resin aggregates and therefore
reactions occur mainly with dissolved components.
Literature
Holmbom, B., Chapter 9: Resin Reactions and Deresination in Bleaching. Pitch control, Wood
Resin and Deresination. Eds. Back, E., L. & Allen, L., H., Publ. TAPPI Press, Atlanta, 2000.
pp. 231-244.
Lapierre, L., Bouchard, J., Berry, R.M. & van Lierop, B., Chelatation Prior to Hydrogen Peroxide
Bleaching of Kraft Pulps: An Overview. J. Pulp Pap. Sci. 21(1995)8, pp. J268-J272.
Vuorinen, T. & Heikkilä, M., Mechanism of Transition Metal Ion Catalyzed decomposition of
hydrogen peroxide in pulp bleaching. 28th EUCEPA conference. Sustainable development
for the pulp and paper industry, Lisbon, Portugal, 2-4 Apr. 2003, pp 46-51.
77
13 Peracids / Peracetic acid (Paa)
The most common peracids used in bleaching are peracetic acid (Paa (CH3CO3H)), performic
acid (Pfa (HCO3H)) and Caro’s acid (Caa (H2SO5)). In addition, perpropionic acid
(CH3CH2CO3H), peroxonitric acid (HNO4) and peroxybenzoic acid are also applicable for
laboratory scale bleaching. Peracids can be synthesised from peroxides and acid as
illustrated in Scheme 33.
Acid
+ hydrogen peroxide
peracid
+
R C
O H
+ water
O
O
H2O2
R C
H
O O
δ-
+
H2O
δ+
Scheme 33. Equilibrium reaction for peroxyacid formation from an acid and hydrogen peroxide.
13.1 Reactions of wood components
LIGNIN
Peracids are able to react both electrophilically and nucleophilically. Electrophilic peracid
attacks phenolic and non-phenolic lignin structures alike, whereas the nucleophilic
secondary reaction focuses on carbonyl structures. The main reactions types of peracetic acid
(Paa) with lignin are illustrated in Scheme 34. These reaction types are: ring hydroxylation
(1), oxidative demethylation (2), oxidative ring cleavage (3), displacement of side chains (4),
cleavage of b-O-4 (b-aryl ether) linkages (5) and epoxidation (6). The results are based on
model experiments.
78
Scheme 34. Reactions of lignin with hydroxonium ions (Gierer, J., Chemistry of delignification.
Part 2. Reactions of lignin during bleaching. Wood Sci. Technol. 20(1986):1, 1-30).
As shown earlier, the structure of residual lignin changes during peracetic acid bleaching.
Residual lignin consists of higher amounts of phenolic hydroxyl groups. In addition the
amount of acid groups is increased which improves the hydrophilicity of lignin. Due to the
cleavage of side chains the molecular mass of residual lignin is decreased which further
improves the hydrophilicity.
CARBOHYDRATES
Peracid treatment is very selective and therefore large amounts of lignin can be removed
without extensive yield losses. Yet, the presence of transition metals may cause
decomposition of peracids and lead to the formation of harmful radicals.
These radicals may then attack carbohydrates and cause degradation of polysaccharides.
Insufficient bleaching pH may also lead to acid hydrolysis of carbohydrates and cause yield
losses.
79
Peracids react readily with the reducing end groups of carbohydrates. These kinds of
reactions may consume a large amount of peracids. Nevertheless in kraft pulp the amount of
reducing end groups is relatively low.
Peracids react easily with hexenuronic acid groups (cf. Ch. 9 Selective hydrolysis (A)). The
reaction starts with an electrophilic reaction which is followed by a nucleophilic reaction.
The first step is the reaction of peracid with hexenuronic acid groups, which proceeds
through the formation of an intermediate product (5-oxohexuronic acid). Peracid may
further react with the intermediate product. As a result formic acid and carbon dioxide are
formed (Scheme 36). Reactions consume considerable amounts of peracids (1 HexA
consumes 6 equivalents of AcOOH) and therefore it is advisable to remove hexenuronic acid
groups prior to the peracid (peracetic acid) bleaching stage. The reaction of peracid with
HexA is faster in neutral than acidic pH’s. Hexenuronic acid decomposition is notably faster
then removal of lignin, i.e. delignification.
CO2H
AcOOH
O
Hydrolysis
OH
O
OXylan
HO
OH
OH
O
5 AcOOH
O
OH
CO2H
OXylan
HO
O
HO
HO
OH
2 CO2 + 4 HCO2H
OH
Scheme 35. A reaction scheme for the oxidation of HexA by peracetic acid.
13.2 Factors affecting the reactions
Lignin reactions
According to reaction kinetics the rate of pulp bleaching and delignification is increased as
the temperature and peracetic acid charge increase. As discussed earlier the rate constant is
temperature dependent.
d [Lignin ]
a
b
= - k L × [Lignin] × [Paa ] × H 3O +
dt
[
]
c
(55)
where [Lignin] is the lignin content of the pulp, kappa units
[Paa] is the peracetic acid concentration (g/l)
[H3O+] is the hydronium ion concentration
t is time (min)
kL is the rate constant (delignification)
a, b and c are the reaction orders for lignin, peracetic acid and the hydronium ion
pH has a significant effect on lignin reactions as well. The pKa values of acids and peracids
are shown in Table 9. In water solutions peracids are easily ionised:
RCOOOH + H2O D RCOOO- + H3O+
80
Table 9. pKa values of some acids and peracids.
Acid
HCO2H
CH3CO2H
H2SO4
pKa
3.8
4.7
-3
Peracid
HCO3H
CH3CO3H
H2SO5
pKa
7.1
8.2
Depending on pH peracids exist either as electrophiles or nucleophiles. The acid form and
ionised form of peracids react differently in bleaching. The acid form (RCOOOH) is an
electrophile whereas the ionised form (RCOOO-) is a nucleophile.
Peracetic acid can be used as an alternative to ozone in TCF (totally chlorine free) bleaching.
Usually the bleaching is carried out at pH 4-5. Under acidic conditions, peracids exist mainly
in undissociated form.
Carbohydrates and hexenuronic acid decomposition
The most significant carbohydrate reaction during peracetic acid treatment is the
decomposition of hexenuronic acid groups. The kinetics of hexenuronic acid decomposition
is presented in Equation 56.
d [HexA]
d
e
f
g
= - k H 1 × HexCOO - × [Paa ] - k H 2 × [HexCOOH ] × [Paa ]
dt
[
]
(56)
where [HexA] is the total hexenuronic acid content in pulp, kappa units
[HexCOO-] is hexenuronate anion content, kappa units
[HexCOOH] is undissociated hexenuronic acid group content, kappa units
[Paa] is peracetic acid concentration (g/l)
kH1 is the rate constant of hexenuronate anion decomposition
kH2 is the rate constant of undissociated hexenuronic acid decomposition
d, e, f and g are the reaction orders for hexenuronate anion, peracetic acid,
hexenuronic acid and peracetic acid.
Literature
Gierer, J., Chemistry of delignification. Part 2. Reactions of lignin during bleaching. Wood
Sci. Technol. 20(1986)1, pp.1-30
Jääskeläinen, A-S., Kraft pulp bleaching with peroxyacetic acid and other peroxy compounds.
Dissertation. Helsinki University of Technology, Department of Forest Products Technology,
Espoo, 1999, 149 p.
81
14 Ozone bleaching (Z)
Ozone reacts extremely fast with unsaturated structures, such as lignin, HexA and
unsaturated extractive structures. Thus, the reactions in the cell wall are diffusion limited.
The progress of a reaction front in a fibre is shown in Figure 29. Images show a sharp
reaction front of O3-lignin, which moves inwards with time.
5m
Ozonation time: 20 s
5m
Ozonation time: 30 s
Figure 28. Propagation of the reaction front in the cell wall. Microscopy pictures of fibre crosssections at two degrees of reactivity. Hemlock kraft fibres exposed to ozone at 0.5 kPa pressure.
The degree of propagation depends on the kappa number of the pulp and the dosage of
ozone. Molecular diffusion is extremely fast over short distances (i.e. thickness of a cell wall)
and therefore ozone is used up in approximately 1 second. Then again, molecular diffusion
is slow if the distance increases (>1 mm) and is nonexistent if the distance is more than 1 cm.
The “Shrinking core” –model illustrates the progress of the reaction front (Figure 30).
Figure 29. Propagation of the reaction front during a high consistency ozonisation. The medium
consistency ozonisation can be presented in a similar fashion. In that case the cell wall is
surrounded by water.
82
In practice the amount of ozone added to the pulp is large enough to remove half of the
lignin (and HexA). The reactions in individual fibres may differ notably depending on the
individual properties of the fibres (i.e. lignin content, thickness of the cell wall). As Figure 31
illustrates, the degree of depolymerisation of celluloses depends on the degree of
delignification. Hydroxyl radicals are formed during the lignin reactions and these radicals
oxidise cellulose instantly. If the reaction front advances through the cell wall, the liquid
phase ozone starts to oxidise cellulose and the selectivity of the bleaching stage decreases.
In medium consistency ozone treatment the liquid phase exterior to the fibres is saturated
with ozone. Unsaturated compounds in the liquid phase are rapidly oxidised and consume
ozone. Therefore pulps must be properly washed prior to such ozone treatment.
Results, obtained using electron spectroscopy for chemical analysis (ESCA)-measurement,
indicate that ozone is a surface selective bleaching agent (Figure 31). A xylanase treatment
also improves the removal of surface lignin in later bleaching stages.
Figure 30. Development of surface and total lignin contents of oxygen delignified kraft pulp in
chlorine dioxide, peroxide and ozone bleaching stages.
RESIDUAL LIGNIN
The reactions of ozone differ from the reactions of other electrophiles since the primary
reaction product of ozone (i.e. ozonide) is a strong nucleophile. The reactions of unsaturated
structures with ozone can be described with the similar kind of mechanism. The net reaction
is a cleavage of a carbon-carbon double bond and a formation of a carbonyl group in both
sp2- hybridised carbon atoms.
Usually the depolymerisation of lignin happens via the secondary hydrolysis of esters.
Muconic acids are formed in the course of the reaction. The carbon-carbon double bonds of
these acids may be further oxidised and glyoxylic acid (COOHCHO) can be liberated from
the residual lignin. Glyoxylic acid can then be further oxidised forming oxalic acid
(COOHCOOH). These secondary lignin reactions are slower than the primary reactions and
therefore the filtrate from ozone treatment can be recirculated back to the ozone stage. The
recirculation of ozone treatment filtrate is less harmful than the recirculation of other
bleaching filtrates to the ozone stage.
83
Lignin
Lignin
Lignin
OMe
+
OMe
O
+
OLignin
OMe
O
O
O
O
O
LigninO
O
OLignin
O
O
Lignin
Lignin
O
LigninO
OMe
-MeOH
-LigninOH
O
OH
O
HO
O
Scheme 36.
HEXENURONIC ACID GROUPS
According to the main reaction path (Scheme 38) ozone is able to partly oxidise HexA
(approx. 50 %). The first oxidation stage is followed by a partial oxidation of an aldehyde to
a corresponding carboxylic acid. One of the oxidation products is an ester of oxalic acid.
Since the ozone stage is rapid and the reaction conditions are unfavourable to the hydrolysis
of ester (pH ~ 3), the oxidation products are carried over/drifted to the next washer.
Usually the ozone stage is followed by some kind of an alkaline treatment, such as a
peroxide bleaching stage. The oxidation products are released in alkaline media. Formation
of calcium oxalate is probable if calcium ions are present. Poorly soluble calcium oxalate
precipitates are found for example in washers. Released four carbon compounds (C 4) are
also reducing sugars and can therefore consume peroxides directly or indirectly by
catalysing their degradation.
COOH
COOH
O
O
OH
O
O
O
O
HO
OXylan
HO
OXylan
OH
OH
OH
+
(COOH)2
COOH
O
OH
O
OH
O
OH
O
O
HO
OXylan
+
(COOH)2
OH
OH
Scheme 37
HexA can also be oxidised in another way (Scheme 39). The reaction leads to
decarboxylation and the formation of a lactone. Since this lactone is also an ester, it is
released from the pulp only during the next alkaline stage. The product of hydrolysis, 4ketopenturonic acid, is a reducing sugar and therefore it catalyses the degradation of
peroxides and increases their consumption due to cleavage of carbon-carbon double bonds.
84
COOH
O
O
O
HO
OXylan
OH
O
O
HO
OH
O
HO
OXylan
O
OH
OH
Scheme 38
EXTRACTIVES
Ozone reacts with unsaturated resin components. In the case of hardwood birch pulp the
amount of unsaturated fatty acids, squalene, betulaprenols and sitosterol is reduced by
ozone bleaching.
Study problems
1. Describe the topology of delignification occurring in the cell wall during ozone
bleaching.
2. What are the arguments for and against ozone bleaching?
3. Explain the effect of diffusion in ozone bleaching.
4. What is oxalic acid? How it is formed during ozone bleaching?
5. How can you reduce the formation of oxalic acid and minimise the amount
calcium oxalate deposits on bleaching equipment? (see also Ch. 9)
Literature
Griffin, R., Ni, Y., and van Heiningen, A.R.P., The Development of Delignification and LigninCellulose Selectivity During Ozone Bleaching. J. Pulp Pap. Sci. 24(1998)4, pp.111-115
Holmbom, B., Chapter 9: Resin Reactions and Deresination in Bleaching. Pitch control, Wood
Resin and Deresination. Eds. Back, E., L. & Allen, L., H., Publ. TAPPI Press, Atlanta, 2000.
pp. 231-244.
85
15 Donnan effect and catalytic bleaching
There are ionisable groups within the cell wall, such as carboxylic acids (pK a~3), phenols
(pKa~10) and alcohols (pKa~14). Depending on pH the fibre wall is negatively charged. The
anionic fibres repel soluble anions and attracts cations. The Donnan theory describes the
interactions between fibres and ions. The Donnan effect may greatly influence the efficiency
of catalytic bleaching with polyoxymetalates. The phenomenon is described in detail by
Ruuttunen and Vuorinen.
Catalytic bleaching with polyoxymetalates (POMs). Polyoxometalates consist of a large
group of inorganic clusters. The POMs can be used as agents in novel and chlorine free
oxygen bleaching processes. It has been observed that POMs are more selective than oxygen
or hydrogen peroxide. Polyoxometalates differ notably from other bleaching agents since the
reacted molecules can be regenerated.
To explain why the Donnan effect may effect the POM bleaching, a short introduction to the
Donnan theory is shown below. The theory describes the equilibrium distribution of mobile
ionic species between two aqueous phases. The phenomenon was first studied by Donnan
and Harris in a system where ionic species were distributed unequally on each side of a
semi-permeable membrane because of their different sizes. The theory has been applied to
describe distributions of ions in suspension of cellulose fibres. In pulp suspensions, the
ionisable groups attached to fibres cause the unequal distribution of ions, Figure 32.
Figure 31.. (Towers and Scallan 1996)
The pulp suspension is comprised of two phases. These phases are called the fibre phase and
the external phase. The fibre phase is the small volume of liquid contained within the waterswollen fibre wall. The external phase is the large volume phase surrounding the fibre.
According to the Donnan theory, the distribution of different mobile anions and cations
between the fibre phase and the external phase is described by a single factor, the
distribution coefficient λ (Equation 57).
86
[H ] = [M ]
l=
[H ] [M ]
+
+
f
+
where
H
M
I
s
f
=
+
s
[M ]
[M ]
2+
f
s
f
2+
s
[I ]
=
[I ]
-
s
-
f
=
[I ]
[I ]
2-
s
2-
f
=3
[I ]
[I ]
3-
s
3-
,
(57)
f
is a hydrogen ion
is a cation
is an anion
is the external phase
is the fibre phase
Equation 57 can be written in a more general form:
[M ] = [I ]
=
[M ] [I ]
z+
l
z
z-
f
s
z+
z-
s
(58)
f
where
z
is the absolute of the valence of the ion.
N.B. For anions the concentration of the fibre phase is the nominator, and for cations the
nominator is the concentration of the external phase.
Since the Donnan distribution coefficient, λ, depicts a relationship between ion
concentrations, its value depends on the values of fibre and external phase volumes. These
volumes are affected by pH and ionic strength.
The concentration of certain ions in the fibre phase is strongly dependent on the Donnan
distribution coefficient, λ, and the valence of the ion. If an anion has a valence of -6,
following equation applies:
[I ] = [Il ]
6-
6-
s
f
6
(59)
When λ is large, even high external concentrations of a strongly negative anions can lead to
concentration of the same anion in the fibre. Since polyoxymetalates are anions having a
very high negative charge, the POM bleaching is strongly affected by the Donnan
phenomenon. During bleaching POM anions need to get into a contact with residual lignin
molecules in the fibre wall for the reaction to take place. POMs are repelled by fibres in pulp
suspensions at low ionic strength.
Due to the Donnan effect, pH is lower in the fibre phase than in the external phase. Naturally
this affects the rate of pH dependent reactions.
87
16 Some environmental aspects of kraft pulping and
bleaching
16.1 Environmental impacts of pulping and bleaching effluents and
air emissions
Pulping and bleaching effluents are chemically complex, consisting of several components.
Many components are capable of causing environmental impacts on receiving water courses,
such as organic loads (leading to oxygen depletion), increased nutrients (causing
eutrophication), suspended solids (smothering of bottom-living flora and fauna) and colour
change impacts. There are many process factors that influence the quality of process
effluents, for example:
1. wood raw material
2. pulping process chemistry
3. the degree of closure, spill control etc.
4. added process chemicals
5. the organic load entering the bleaching plant (kappa number of unbleached
pulp and black liquor carryover)
6. the type and efficiency of effluent treatment
7. operation variability and upsets etc.
Toxic substances. Fatty acids, resin acids and sterols are considered to be toxic substances. They
are present both in black liquor and in bleaching effluents. The amounts of fatty and resin
acids and sterols in effluents are affected more by the wood raw-material or pulping
variables than by the bleaching process. Sterols and steroids are observed to have
biochemical and physiological effects on fish, i.e. the effects on growth, liver function, and
white blood cell patterns. However due to modern pulping techniques the negative effects
are fewer than in earlier times. The amount of chlorinated compounds (chlorinated resin and
fatty acids, chlorophenolics etc.) has degreased notably since the 80’s through the
substitution of elemental chlorine bleaching by chlorine dioxide bleaching.
VOC - volatile organic compounds are present or produced during pulping and bleaching.
The main volatile organic compounds in bleaching are chloroform, methanol, acetaldehyde,
and methyl ethyl ketone. There are also air emission of gaseous bleaching chemicals, such as
chlorine and chorine dioxide as well as ozone emissions.
ECF vs. TCF. Nowadays the main bleaching method is ECF (Elemental Chlorine Free)
bleaching. Another method is totally chlorine free (TCF) bleaching. Results from both
laboratory and model ecosystem studies of the biological effects of effluent from mills with
modern process technology suggest that there are not clear differences between mills using
low-kappa ECF and TCF bleaching. In other words modern ECF and TCF technique are
equally good from the environmental point of view.
16.2 BAT (Best available techniques)
The best available techniques are the most effective techniques for achieving a high level of
protection of the environment as a whole (air, water, land and wastes), which have been
88
developed on a scale that allows them to be used under economically and technically viable
condition taking in to account the costs and advantages. They include both the technology
and the way the installation is designed, built, maintained, operated and decommissioned.
The best available technology for kraft pulp mills are considered to be:
·
·
·
·
·
·
·
·
·
·
·
Dry debarking of wood
Increased delignification before the bleach plant
o Extended or modified cooking and additional oxygen stages
Efficient brown stock washing
Elemental chlorine free (ECF) bleaching with low AOX or Totally chlorine free (TCF)
bleaching
Recycling of some process waters from the bleach plant
o Mainly alkaline process water
Effective spill monitoring, containment and recovery system
Stripping and reuse of the condensates from the evaporation plant
Sufficient capacity of the black liquor evaporation plant and the recovery boiler
o To cope with the additional liquor and dry solids load formed during process
disturbances and upsets
Collection and reuse of clean cooling waters
Sufficiently large buffer tanks for storage are necessary
o For prevention of unnecessary loading and occasional upsets in the external
effluent treatment process due to cooking and recovery liquors and dirty
condensates.
In addition to process-integrated measures, also primary treatment and biological
treatment belong to BAT for kraft pulp mills
Literature
Hynninen P., Environmental control. Publ. Fapet, Jyväskylä, Finland, 2000.
McKague, A.B. & Carlberg, G., Section VIII: Pulp Bleaching and The Environment, Pulp
Bleaching – Principles and Practice. Eds. Dence, C., W. & Reeve, D., W., Publ. TAPPI Press,
Atlanta, 1996, p. 751-847.
Sandström, O., Förlin, O., Grahn, O., Larsson, Å. & Lindesjö, E., Assesment of the
Environmental Impact of Swedish Pulp and Paper Mill Effluents at the Beginning of the
Next Century. 3rd International Conference on Environmental Fate and Effects of Pulp and
Paper Mill Effluents, Rotorua, Nov. 9-16, 1998, pp. 513-518.
89
90