- •Recovered Paper and Recycled Fibers
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •2006, Isbn 3-527-30997-7
- •Volume 1
- •Isbn: 3-527-30999-3
- •4.1 Introduction 109
- •4.2.5.1 Introduction 185
- •4.3.1 Introduction 392
- •5.1 Introduction 511
- •6.1 Introduction 561
- •6.2.1 Introduction 563
- •6.4.1 Introduction 579
- •Volume 2
- •7.3.1 Introduction 628
- •7.4.1 Introduction 734
- •7.5.1 Introduction 777
- •7.6.1 Introduction 849
- •7.10.1 Introduction 887
- •8.1 Introduction 933
- •1 Introduction 1071
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and
- •1 Introduction 1149
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •1 Introduction
- •150.000 Annual Fiber Flow[kt]
- •1 Introduction
- •1 Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •Void volume
- •Void volume fraction
- •Xylan and Fiber Morphology
- •Initial bulk residual
- •4.2.5.1 Introduction
- •In (Ai) Model concept Reference
- •Initial value
- •Validation and Application of the Kinetic Model
- •Inititial
- •Viscosity
- •Influence on Bleachability
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Impregnation
- •Introduction
- •International
- •Impregnation
- •Influence of Substituents on the Rate of Hydrolysis
- •140 116 Total so2
- •Xylonic
- •Viscosity Brightness
- •Xyl Man Glu Ara Furf hoAc XyLa
- •Initial NaOh charge [% of total charge]:
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Introduction
- •Isbn: 3-527-30999-3
- •Introduction
- •Xylosec
- •Xylan residues
- •Viscosity
- •Introduction
- •Viscosity
- •Viscosity
- •Introduction
- •Initiator Promoter Inhibitor
- •Viscosity
- •Viscosity
- •Viscosity
- •Introduction
- •Viscosity
- •Introduction
- •Intra-Stage Circulation and Circulation between Stages
- •Implications of Liquor Circulation
- •Vid Chalmers Tekniska
- •Introduction
- •It is a well-known fact that the mechanical properties of the viscose fibers
- •Increase in the low molecular-weight fraction [2]. The short-chain molecules represent
- •Isbn: 3-527-30999-3
- •In the cooking process or, alternatively, white liquor can be used for the cold
- •Is defined as the precipitate formed upon acidification of an aqueous alkaline solution
- •934 8 Pulp Purification
- •8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution 935
- •Is essentially governed by chemical degradation reactions involving endwise depolymerization
- •80 °C [12]. Caustic treatment: 5%consistency ,
- •30 Min reaction time, NaOh concentrations:
- •8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution
- •80 °C is mainly governed by chemical degradation reactions (e.G. Peeling reaction).
- •Investigated using solid-state cp-mas 13c-nmr spectroscopy (Fig. 8.4).
- •Indicates cleavage of the intramolecular hydrogen bond between o-3-h and o-5′,
- •8 Pulp Purification
- •Interaction between alkali and cellulose, a separate retention tower is not really
- •In the following section.
- •3% In the untreated pulp must be ensured in order to avoid a change in the supramolecular
- •8.3 Cold Caustic Extraction
- •Xylan content [%]
- •8 Pulp Purification
- •Is calculated as effective alkali (ea). Assuming total ea losses (including ea consumption
- •Xylan content [%]
- •8.3 Cold Caustic Extraction
- •120 °C (occasionally 140 °c). As mentioned previously, hce is carried out solely
- •Involved in alkaline cooks (kraft, soda), at less severe conditions and thus avoiding
- •8.4Hot Caustic Extraction 953
- •954 8 Pulp Purification
- •120 Kg NaOh odt–1, 90–240 min, 8.4 bar (abs)
- •8.4Hot Caustic Extraction 955
- •956 8 Pulp Purification
- •Into the purification reaction, either in the same (eo) or in a separate stage
- •960 8 Pulp Purification
- •8.4.1.5 Composition of Hot Caustic Extract
- •8.4Hot Caustic Extraction 961
- •Isbn: 3-527-30999-3
- •Xyloisosaccharinic acid
- •Inorganicsa
- •Inorganic compounds
- •Value (nhv), which better reflects the actual energy release, accounts for the fact
- •968 9 Recovery
- •It should be noted that the recycling of bleach (e.G., oxygen delignification) and
- •9.1 Characterization of Black Liquors 969
- •9.1.2.1 Viscosity
- •9.1.2.3 Surface Tension
- •9.1.2.5 Heat Capacity [8,11]
- •9.2 Chemical Recovery Processes
- •Is described by the empirical equation:
- •9 Recovery
- •Vent gases from all areas of the pulp mill. From an environmental perspective,
- •9.2.2.1 Introduction
- •In the sump at the bottom of the evaporator. The generated vapor escapes
- •Incineration, whereas sulphite ncg can be re-used for cooking acid preparation.
- •9 Recovery
- •Values related to high dry solids concentrations. The heat transfer rate is pro-
- •9.2 Chemical Recovery Processes
- •9.2.2.3 Multiple-Effect Evaporation
- •7% Over effects 4 and 5, but more than 30% over effect 1 alone.
- •9.2 Chemical Recovery Processes
- •Increasing the dry solids concentration brings a number of considerable advantages
- •9.2.2.4 Vapor Recompression
- •Is driven by electrical power. In general, vapor coming from the liquor
- •Vapor of more elevated temperature, thus considerably improving their performance.
- •9 Recovery
- •Is typically around 6 °c. The resulting driving temperature difference
- •Is low, and hence vapor recompression plants require comparatively large heating
- •Vapor recompression systems need steam from another source for start-up.
- •9 Recovery
- •Its temperature is continuously falling to about 180 °c. After the superheaters,
- •In the furnace walls, and only 10–20% in the boiler bank. As water turns into
- •9.2.3.1.2 Material Balance
- •Is required before the boiler ash is mixed. In addition, any chemical make-up
- •In this simplified model, all the potassium from the black liquor (18 kg t–1
- •Values for the chemicals in Eq. (11) can be inserted on a molar basis, equivalent
- •9.2 Chemical Recovery Processes
- •Input/output
- •9 Recovery
- •9.2.3.1.3 Energy Balance
- •In the black liquor, from water formed out of hydrogen in organic material, and
- •9.2 Chemical Recovery Processes
- •9.2.3.2 Causticizing and Lime Reburning
- •9.2.3.2.1 Overview
- •9.2.3.2.2 Chemistry
- •986 9 Recovery
- •Insoluble metal salts are kept low. Several types of filters with and without lime
- •Is, however, not considered a loss because some lime mud must be
- •988 9 Recovery
- •In slakers and causticizers needs special attention in order to avoid particle disintegration,
- •9.2 Chemical Recovery Processes 989
- •Ing disks into the center shaft, and flows to the filtrate separator. There, the white
- •9.2.3.2.4 Lime Cycle Processes and Equipment
- •It is either dried with flue gas in a separate, pneumatic lime mud dryer or is fed
- •990 9 Recovery
- •Its temperature falls gradually. Only about one-half of the chemical energy in the
- •9.2.3.3.2 Black Liquor Gasification
- •Inorganics leave the reactor as solids, and into high-temperature techniques,
- •In the bed. Green liquor is produced from surplus bed solids. The product gas
- •992 9 Recovery
- •Incremental capacity for handling black liquor solids. The encountered difficulties
- •10% Of today’s largest recovery boilers. When the process and material issues are
- •9.2 Chemical Recovery Processes 993
- •9.2.3.3.3 In-Situ Causticization
- •Is still in the conceptual phase, and builds on the formation of sodium titanates
- •9.2.3.3.4 Vision Bio-Refinery
- •Into primary and secondary recovery steps. This definition relates to the recovery
- •994 9 Recovery
- •Is largely different between sulfite cooking bases. While magnesium and
- •Introduction
- •In alkaline pulping the operation of the lime kiln represents an emission source.
- •Isbn: 3-527-30999-3
- •Is by the sophisticated management of these sources. This comprises their collection,
- •Ions, potassium, or transition metals) in the process requires the introduction
- •Industry”. Similarly guidelines for a potential kraft pulp mill in Tasmania [3]
- •Initially, the bleaching of chemical pulp was limited to treatment with hypochlorite
- •In a hollander, and effluent from the bleach plant was discharged without
- •In a heh treatment and permitted higher brightness at about 80% iso (using
- •Increasing pulp production resulted in increasing effluent volumes and loads.
- •10.2 A Glimpse of the Historical Development 999
- •It became obvious that the bleaching process was extremely difficult to operate in
- •In a c stage was detected as aox in the effluent (50 kg Cl2 t–1 pulp generated
- •1% Of the active chlorine is converted into halogenated compounds (50 kg active
- •In chlorination effluent [12] led to the relatively rapid development of alternative
- •1000 10 Environmental Aspects of Pulp Production
- •10.2 A Glimpse of the Historical Development
- •In 1990, only about 5% of the world’s bleached pulp was produced using ecf
- •64 Million tons of pulp [14]. The level of pulp still bleached with chlorine
- •10 000 Tons. These are typically old-fashioned, non-wood mills pending an
- •In developed countries, kraft pulp mills began to use biodegradation plants for
- •10 Environmental Aspects of Pulp Production
- •Indeed, all processes are undergoing continual development and further improvement.
- •Vary slightly different depending upon the type of combustion unit and the fuel
- •10.3Emissions to the Atmosphere
- •Volatile organic
- •In 2004 for a potential pulp mill in Tasmania using “accepted
- •10 Environmental Aspects of Pulp Production
- •Is woodyard effluent (rain water), which must be collected and treated biologically
- •10.4 Emissions to the Aquatic Environment
- •Is converted into carbon dioxide, while the other half is converted into biomass
- •Into alcohols and aldehydes; (c) conversion of these intermediates into acetic acid and
- •10 Environmental Aspects of Pulp Production
- •In North America, effluent color is a parameter which must be monitored.
- •It is not contaminated with other trace elements such as mercury, lead, or cadmium.
- •10.6 Outlook
- •Increase pollution by causing a higher demand for a chemical to achieve identical
- •In addition negatively affect fiber strength, which in turn triggers a higher
- •Introduction
- •2002, Paper-grade pulp accounts for almost 98% of the total wood pulp production
- •Important pulping method until the 1930s) continuously loses ground and finds
- •Importance in newsprint has been declining in recent years with the increasing
- •Isbn: 3-527-30999-3
- •Virtually all paper and paperboard grades in order to improve strength properties.
- •In fact, the word kraft is the Swedish and German word for strength. Unbleached
- •Importance is in the printing and writing grades. In these grades, softwood
- •In this chapter, the main emphasis is placed on a comprehensive discussion of
- •1010 11 Pulp Properties and Applications
- •Is particularly sensitive to alkaline cleavage. The decrease in uronic acid content
- •Xylan in the surface layers of kraft pulps as compared to sulfite pulps has been
- •80% Cellulose content the fiber strength greatly diminishes [14]. This may be due
- •Viscoelastic and capable of absorbing more energy under mechanical stress. The
- •11.2 Paper-Grade Pulp 1011
- •Various pulping treatments using black spruce with low fibril
- •In the viscoelastic regions. Fibers of high modulus and elasticity tend to peel their
- •1012 11 Pulp Properties and Applications
- •11.2 Paper-Grade Pulp
- •Viscosity mL g–1 793 635 833 802 1020 868 1123
- •Xylose % od pulp 7.3 6.9 18.4 25.5 4.1 2.7 12.2
- •11 Pulp Properties and Applications
- •Inorganic Compounds
- •11.2 Paper-Grade Pulp
- •Insight into many aspects of pulp origin and properties, including the type of
- •Indicate oxidative damage of carbohydrates).
- •In general, the r-values of paper pulps are typically at higher levels as predicted
- •Is true for sulfite pulps. Even though the r-values of sulfite pulps are generally
- •Is rather unstable in acid sulfite pulping, and this results in a low (hemicellulose)
- •11 Pulp Properties and Applications
- •Ing process, for example the kraft process, the cellulose:hemicellulose ratio is
- •Increases by up to 100%. In contrast to fiber strength, the sheet strength is highly
- •Identified as the major influencing parameter of sheet strength properties. It has
- •In contrast to dissolving pulp specification, the standard characterization of
- •Is observed for beech kraft pulp, which seems to correlate with the enhanced
- •11.2 Paper-Grade Pulp
- •11 Pulp Properties and Applications
- •Is significantly higher for the sulfite as compared to the kraft pulps, and indicates
- •11.2 Paper-Grade Pulp
- •Xylan [24].
- •11 Pulp Properties and Applications
- •11.2 Paper-Grade Pulp
- •11 Pulp Properties and Applications
- •Introduction
- •Various cellulose-derived products such as regenerated fibers or films (e.G.,
- •Viscose, Lyocell), cellulose esters (acetates, propionates, butyrates, nitrates) and
- •In pulping and bleaching operations are required in order to obtain a highquality
- •Important pioneer of cellulose chemistry and technology, by the statement that
- •11.3 Dissolving Grade Pulp
- •Involves the extensive characterization of the cellulose structure at three different
- •Is an important characteristic of dissolving pulps. Finally, the qualitative and
- •Inorganic compounds
- •11 Pulp Properties and Applications
- •11.3.2.1 Pulp Origin, Pulp Consumers
- •Include the recently evaluated Formacell procedure [7], as well as the prehydrolysis-
- •11.3 Dissolving Grade Pulp
- •Viscose
- •11 Pulp Properties and Applications
- •11.3.2.2 Chemical Properties
- •11.3.2.2.1 Chemical Composition
- •In the polymer. The available purification processes – particularly the hot and cold
- •11.3 Dissolving Grade Pulp
- •In the steeping lye inhibits cellulose degradation during ageing due to the
- •Is governed by a low content of noncellulosic impurities, particularly pentosans,
- •Increase in the xylan content in the respective viscose fibers clearly support the
- •11.3 Dissolving Grade Pulp
- •Instability. Diacetate color is measured by determining the yellowness coefficient
- •Xylan content [%]
- •11 Pulp Properties and Applications
- •Xylan content [%]
- •11.3 Dissolving Grade Pulp
- •11.3 Dissolving Grade Pulp
- •Is, however, not the only factor determining the optical properties of cellulosic
- •In the case of alkaline derivatization procedures (e.G., viscose, ethers). In industrial
- •11.3 Dissolving Grade Pulp
- •Viscose
- •Viscose
- •In order to bring out the effect of mwd on the strength properties of viscose
- •Imitating the regular production of rayon fibers. To obtain a representative view
- •11 Pulp Properties and Applications
- •Viscose Ether (hv) Viscose Acetate Acetate
- •Xylan % 3.6 3.1 1.5 0.9 0.2
- •1.3 Dtex regular viscose fibers in the conditioned
- •11.3 Dissolving Grade Pulp
- •Is more pronounced for sulfite than for phk pulps. Surprisingly, a clear correlation
- •Viscose fibers in the conditioned state related to the carbonyl
- •1038 11 Pulp Properties and Applications
- •In a comprehensive study, the effect of placing ozonation before (z-p) and after
- •Increased from 22.9 to 38.4 lmol g–1 in the case of a pz-sequence, whereas
- •22.3 To 24.2 lmol g–1. The courses of viscosity and carboxyl group contents were
- •Viscosity measurement additionally induces depolymerization due to strong
- •11 Pulp Properties and Applications
- •Increasing ozone charges. For more detailed
- •11.3 Dissolving Grade Pulp
- •Is more selective when ozonation represents the final stage according to an
- •11.3.2.3 Supramolecular Structure
- •1042 11 Pulp Properties and Applications
- •Is further altered by subsequent bleaching and purification processes. This
- •Involved in intra- and intermolecular hydrogen bonds. The softened state favors
- •11.3 Dissolving Grade Pulp
- •Interestingly, the resistance to mercerization, which refers to the concentration of
- •11 Pulp Properties and Applications
- •Illustrate that the difference in lye concentration between the two types of dissolving
- •Intensity (see Fig. 11.18: hw-phk high p-factor) clearly changes the supramolecular
- •11.3 Dissolving Grade Pulp
- •Viscose filterability, thus indicating an improved reactivity.
- •11 Pulp Properties and Applications
- •Impairs the accessibility of the acetylation agent. When subjecting a low-grade dissolving
- •Identification of the cell wall layers is possible by the preferred orientation of
- •Viscose pulp (low p-factor) (Fig. 11.21b, top). Apparently, the type of pulp – as well
- •11 Pulp Properties and Applications
- •150 °C for 2 h, more than 70% of a xylan, which was added to the cooking liquor
- •20% In the case of alkali concentrations up to 50 g l–1 [67]. Xylan redeposition has
- •11.3 Dissolving Grade Pulp
- •Xylan added linters cooked without xylan linters cooked with xylan
- •Viscosity
- •In the surface layer than in the inner fiber wall. This is in agreement with
- •11 Pulp Properties and Applications
- •Xylan content in peelings [wt%]
- •Xylan content located in the outermost layers of the beech phk fibers suggests
- •11.3.2.5 Fiber Morphology
- •11 Pulp Properties and Applications
- •50 And 90%. Moreover, bleachability of the screened pulps from which the wood
- •11.3.2.6 Pore Structure, Accessibility
- •11.3 Dissolving Grade Pulp
- •Volume (Vp), wrv and specific pore surface (Op) were seen between acid sulfite
- •11 Pulp Properties and Applications
- •Irreversible loss of fiber swelling occurs; indeed, Maloney and Paulapuro reported
- •In microcrystalline areas as the main reason for hornification [85]. The effect of
- •105 °C, thermal degradation proceeds in parallel with hornification, as shown in
- •Increased, particularly at temperatures above 105 °c. The increase in carbonyl
- •In pore volume is clearly illustrated in Fig. 11.28.
- •11.3 Dissolving Grade Pulp
- •Viscosity
- •11 Pulp Properties and Applications
- •Increase in the yellowness coefficient, haze, and the amount of undissolved particles.
- •11.3.2.7 Degradation of Dissolving Pulps
- •In mwd. A comprehensive description of all relevant cellulose degradation processes
- •Is reviewed in Ref. [4]. The different modes of cellulose degradation comprise
- •11.3 Dissolving Grade Pulp
- •50 °C, is illustrated graphically in Fig. 11.29.
- •11 Pulp Properties and Applications
- •In the crystalline regions.
- •11.3 Dissolving Grade Pulp
- •Important dissolving pulps, derived from hardwood, softwood and cotton linters
- •11.3 Dissolving Grade Pulp 1061
- •Xylan rel% ax/ec-pad 2.5 3.5 1.3 1.0 3.2 0.4
- •Viscosity mL g–1 scan-cm 15:99 500 450 820 730 1500 2000
- •1062 11 Pulp Properties and Applications
- •Isbn: 3-527-30999-3
- •Introduction
- •Isbn: 3-527-30999-3
- •1072 1 Introduction
- •Isbn: 3-527-30999-3
- •Inventor of stone groundwood. Right: the second version
- •1074 2 A Short History of Mechanical Pulping
- •In refining, the thinnings (diameter 7–10cm) can also be processed.
- •In mechanical pulping as it causes foam; the situation is especially
- •In mechanical pulping, those fibers that are responsible for strength properties
- •Isbn: 3-527-30999-3
- •In mechanical pulping, the wood should have a high moisture content, and the
- •In the paper and reduced paper quality. The higher the quality of the paper, the
- •1076 3 Raw Materials for Mechanical Pulp
- •1, Transversal resistance; 2, Longitudinal resistance; 3, Tanning limit.
- •3.2 Processing of Wood 1077
- •In the industrial situation in order to avoid problems of pollution and also
- •1078 3 Raw Materials for Mechanical Pulp
- •2, Grinder pit; 3, weir; 4, shower water pipe;
- •5, Wood magazine; 6, finger plate; 7, pulp stone
- •Isbn: 3-527-30999-3
- •4.1.2.1 Softening of the Fibers
- •1080 4 Mechanical Pulping Processes
- •235 °C, whereas according to Styan and Bramshall [4] the softening temperatures
- •Isolated lignin, the softening takes place at 80–90 °c, and additional water
- •4.1 Grinding Processes 1081
- •1082 4 Mechanical Pulping Processes
- •1, Cool wood; 2, strongly heated wood layer; 3, actual grinding
- •4.1.2.2 Defibration (Deliberation) of Single Fibers from the Fiber Compound
- •4 Mechanical Pulping Processes
- •Influence of Parameters on the Properties of Groundwood
- •In the mechanical defibration of wood by grinding, several process parameters
- •Improved by increasing both parameters – grinding pressure and pulp stone
- •In practice, the temperature of the pit pulp is used to control the grinding process,
- •In Fig. 4.8, while the grit material of the pulp stone estimates the microstructure
- •4 Mechanical Pulping Processes
- •4.1 Grinding Processes
- •Is of major importance for process control in grinding.
- •4 Mechanical Pulping Processes
- •4.1.4.2 Chain Grinders
- •Is fed continuously, as shown in Fig. 4.17.
- •Initial thickness of the
- •75 Mm thickness, is much thinner than that of a concrete pulp stone, much
- •4 Mechanical Pulping Processes
- •Include:
- •Increases; from the vapor–pressure relationship, the boiling temperature is seen
- •4 Mechanical Pulping Processes
- •In the pgw proves, and to prevent the colder seal waters from bleeding onto the
- •4.1 Grinding Processes
- •In pressure grinding, the grinder shower water temperature and flow are
- •70 °C, a hot loop is no longer used, and the grinding process is
- •4 Mechanical Pulping Processes
- •Very briefly at a high temperature and then refined at high
- •4.2 Refiner Processes
- •4 Mechanical Pulping Processes
- •Intensity caused by plate design and rotational speed.
- •4.2 Refiner Processes
- •1. Reduction of the chips sizes to units of matches.
- •2. Reduction of those “matches” to fibers.
- •3. Fibrillation of the deliberated fibers and fiber bundles.
- •1970S as result of the improved tmp technology. Because the key subprocess in
- •4 Mechanical Pulping Processes
- •Impregnation Preheating Cooking Yield
- •30%. Because of their anatomic structure, hardwoods are able to absorb more
- •Is at least 2 mWh t–1 o.D. Pulp for strongly fibrillated tmp and ctmp pulps from
- •4 Mechanical Pulping Processes
- •4.2 Refiner Processes
- •1500 R.P.M. (50 Hz) or 1800 r.P.M. (60 Hz); designed pressure 1.4 mPa
- •1500 R.P.M. (50 Hz) or 1800 r.P.M. (60 Hz); designed pressure 1.4 mPa;
- •4.2 Refiner Processes
- •4 Mechanical Pulping Processes
- •In hardwoods makes them more favorable than softwoods for this purpose. A
- •4.2 Refiner Processes
- •Isbn: 3-527-30999-3
- •1114 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.2Machines and Aggregates for Screening and Cleaning 1115
- •In refiner mechanical pulping, there is virtually no such coarse material in the
- •1116 5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.2Machines and Aggregates for Screening and Cleaning
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning
- •5.3 Reject Treatment and Heat Recovery
- •55% Iso and 65% iso. The intensity of the bark removal, the wood species,
- •Isbn: 3-527-30999-3
- •1124 6 Bleaching of Mechanical Pulp
- •Initially, the zinc hydroxide is filtered off and reprocessed to zinc dust. Then,
- •2000 Kg of technical-grade product is common. Typically, a small amount of a chelant
- •6.1 Bleaching with Dithionite 1125
- •Vary, but are normally ca. 10 kg t–1 or 1% on fiber. As the number of available
- •1126 6 Bleaching of Mechanical Pulp
- •6.2 Bleaching with Hydrogen Peroxide
- •70 °C, 2 h, amount of NaOh adjusted.
- •6.2 Bleaching with Hydrogen Peroxide
- •Is shown in Fig. 6.5, where silicate addition leads to a higher brightness and a
- •Volume (bulk). For most paper-grade applications, fiber volume should be low in
- •Valid and stiff fibers with a high volume are an advantage; however, this requires
- •1130 6 Bleaching of Mechanical Pulp
- •6.2 Bleaching with Hydrogen Peroxide
- •Very high brightness can be achieved with two-stage peroxide bleaching, although
- •In a first step. This excess must be activated with an addition of caustic soda. The
- •Volume of liquid to be recycled depends on the dilution and dewatering conditions
- •6 Bleaching of Mechanical Pulp
- •6 Bleaching of Mechanical Pulp
- •Is an essential requirement for bleaching effectiveness. Modern twin-wire presses
- •Is discharged to the effluent treatment plant. After the main bleaching stage, the
- •6.3 Technology of Mechanical Pulp Bleaching
- •1136 6 Bleaching of Mechanical Pulp
- •Isbn: 3-527-30999-3
- •7.3 Shows the fractional composition according to the McNett principle versus
- •1138 7 Latency and Properties of Mechanical Pulp
- •7.2 Properties of Mechanical Pulp 1139
- •Isbn: 3-527-30999-3
- •In 1950, about 50% of the global paper production was produced. This proportion
- •4.0% Worldwide; 4.2% for the cepi countries; and 4.8% for Germany.
- •1150 1 Introduction
- •1 Introduction
- •1 Introduction
- •Virgin fibers
- •74.4 % Mixed grades
- •Indonesia
- •Virgin fibers
- •Inhomogeneous sample Homogeneous sample
- •Variance of sampling Variance of measurement
- •1.Quartile
- •3.Quartile
- •Insoluble
- •Insoluble
- •Insoluble
- •Integral
- •In Newtonion liquid
- •Velocity
- •Increasing dp
- •2Α filter
- •0 Reaction time
- •Increasing interaction of probe and cellulose
- •Increasing hydrodynamic size
- •Vessel cell of beech
- •Initial elastic range
- •Internal flow
- •Intact structure
- •Viscosity 457
- •Isbn: 3-527-30999-3
- •1292 Index
- •Visbatch® pulp 354
- •Index 1293
- •1294 Index
- •Impregnation 153
- •Viscosity–extinction 433
- •Index 1295
- •1296 Index
- •Index 1297
- •Inhibitor 789
- •1298 Index
- •Index 1299
- •Impregnation liquor 290–293
- •1300 Index
- •Industries
- •Index 1301
- •1302 Index
- •Index 1303
- •Xylose 463
- •1304 Index
- •Index 1305
- •1306 Index
- •Index 1307
- •1308 Index
- •In conventional kraft cooking 232
- •Visbatch® pulp 358
- •Index 1309
- •In prehydrolysis-kraft process 351
- •Visbatch® cook 349–350
- •1310 Index
- •Index 1311
- •1312 Index
- •Viscosity 456
- •Index 1313
- •Viscosity 459
- •Interactions 327
- •1314 Index
- •Index 1315
- •Viscosity 459
- •1316 Index
- •Index 1317
- •Xylose 461
- •Index 1319
- •Visbatch® pulp 355
- •Impregnation 151–158
- •1320 Index
- •Index 1321
- •1322 Index
- •Xylan water prehydrolysis 333
- •Index 1323
- •1324 Index
- •Viscosity 459
- •Index 1325
- •Xylose 940
- •1326 Index
- •Index 1327
- •In selected kinetics model 228–229
- •4OMeGlcA 940
- •1328 Index
- •Index 1329
- •Intermediate molecule 164–165
- •1330 Index
- •Viscosity 456
- •Index 1331
- •1332 Index
- •Impregnation liquor 290–293
- •Index 1333
- •1334 Index
- •Index 1335
- •1336 Index
- •Impregnation 153
- •Index 1337
- •1338 Index
- •Viscose process 7
- •Index 1339
- •Volumetric reject ratio 590
- •1340 Index
- •Index 1341
- •1342 Index
- •Index 1343
- •1344 Index
- •Index 1345
- •Initiator 788
- •Xylose 463
- •1346 Index
- •Index 1347
- •Vessel 385
- •Index 1349
- •1350 Index
- •Xylan 834
- •1352 Index
Xylosec
Mannosec
CKa,d 31.6 71 32 11 23 50 900 0.3 2.06 1.19 1.5 1.8
PSAQa,d 33.6 72 32 12 23 45 100 0.3 2.72 1.52 1.2 3.07
Sie,f6 29.0 79 20 6 28 80 000 0.33 –0.82
CK/Oa,d 17.2 62 21 18 13 33 000 0.45 1.28 1.36 1.57 1.93
PSAQ/Ob,d 18.7 65 20 19 13 39 100 0.41 2.0 1.56 1.22 2.86
Sie,g 17.9 85 17 8 21 76 400 0.54 0.18
a. Conventional kraft pulp (CK) after oxygen bleaching.
b. Polysulfide/anthraquinone pulp (PSAQ) after oxygen bleaching.
c. Carbohydrates in mg 100 mg–1 cellulose.
d. proRL; 6f RL; 7g NaCl RL; d, f and g indicate different purification
procedures.
e. Two stage neutral sulfite pulp (Si).
structures, an increase in the proportion of carboxylic acids, a decrease in the
molar mass, and an increase in the hydrophilicity were observed [62]. The arabinose
content increased and the galactose, as well as the mannose, content
decreased during oxygen bleaching (Tab. 7.10).
Tamminen and Hortling [62] showed that the CK pulp was the most hydrophilic
before oxygen delignification, with the best bleachability. The sulfite pulp lignin
was structurally quite different from the alkaline pulp lignins, and seemed to dissolve
during oxygen delignification without any major oxidation reactions [62].
7.3.2.3 Oxygen (Dioxygen) and its Derivatives
The element oxygen (chemical symbol O) exists in air as a diatomic molecule, O2,
which strictly should be called dioxygen. Over 99.7% of the O2 in the atmosphere
is the isotope oxygen-16 (16O), but there are also traces of oxygen-17 (17O, about
0.04%) and oxygen-18 (18O, about 0.2%) [125].
The solubility of dioxygen and the physical transport of dissolved dioxygen gas
in an aqueous phase are important properties. The model of Broden and Simonson
was used to estimate the solubility of oxygen in equilibrium conditions, as a
function of oxygen pressure, temperature and hydroxide ion concentration [126].
7.3 Oxygen Delignification 641
7.3.2.3.1 Dioxygen: Electronic Structure
The diatomic oxygen molecule O2 has two unpaired electrons, each located in a
different p* antibonding orbital, having the same spin quantum number or, as is
often written, having parallel spins (Fig. 7.24); this is the most stable state – or
ground state – of dioxygen. If dioxygen, which can act as an oxidizing agent,
attempts to oxidize another atom or molecule by accepting a pair of electrons
from it, then both of these electrons must be of antiparallel spin so as to fit in to
the vacant spaces in the p* orbitals (Fig. 7.24). However, a pair of electrons in an
atomic or molecular orbital would not meet this criterion, as they would have
opposite spins in accordance with Pauli’s principle. This imposes a restriction on
electron transfer that tends to make O2 accept its electrons one at a time, and contributes
to explaining why O2 reacts sluggishly with many nonradicals [125].
According to the law that electrons reacting with each other must have antiparallel
spin, ground-state dioxygen (triplet-state) cannot react with atoms or molecules in
the singlet state due to the spin restriction [127].
Fig. 7.24 A simplified version of bonding in the diatomic oxygen
molecule (16 electrons) in the ground state and of the
excited state of dioxygen and his reduction products [125].
7.3.2.3.2 Principals of Dioxygen Activation
Due to the electron-configuration, dioxygen takes up one electron at a time – a
process termed one-electron reduction [125,127]. By stepwise addition of electrons
to the molecular orbital of ground state dioxygen, the reduction products of oxygen
are formed (Fig. 7.25 and Scheme 7.3). On the addition of one electron, superoxide
is formed. A second electron produces peroxide. Two more produces 2 separated
oxides since no bonds connect the atoms (the number of electrons in antibonding
and bonding orbitals are identical). Each of these species can react with
protons to produce species such as HO2
_, H2O2 (hydrogen peroxide) and H2O.
642 7Pulp Bleaching
7.3 Oxygen Delignification 643
0 1 2 3 4
0
100
200
300
400
500
H
2
O
+e-
Activation energy ΔG
0
(kJ)
Reduction equivalent
O2
+e-
O2
-
+2H+
H
2
O
2
+e-
+H+ OH + H2O
+H+
+e-
Fig. 7.25 Energetics of oxygen-reduction [127].
O2 O2 H2O2 H2O + 2 H2O pH 7
- HO
- 0.33 - 0.89 + 0.30 + 2.40
+ 0.59
+ 1.20
+ 1.349
+ 0.29
+ 0.281
+ 0.815
Scheme 7.3 Dioxygen redox potentials at pH 7 [127].
Under the conditions used in dioxygen delignification, with the pH in the range
between 10 and 13, the standard redox potentials of the reactive species are substantially
reduced (Scheme 7.4) due to the lower potential of the ionized form.
O2
+e-, H+
HOOH H2O+ 2 H2O
pKa= 4.8 11.6 11.9
O2
-
H++ H++HOO- O- H+ +
+e-, H+
+e-, H+ +e-, H+
E - 0.33 0.20 - 0.03 1.77 0 at pH 14
Dioxygen
Hydroperoxyl
radical
Hydrogen
peroxide
Hydroxyl
radical
Superoxide
anion radical
Hydroperoxy
anion
Oxyl anion
radical
Oxygen species
Anionic form
45.71
Hydroxide
ion
OH-
HOO HO Water
Scheme 7.4 Dioxygen reductions proceeding in four consecutive
one-electron steps (E0 standard reduction potential)
(according to [128]).
Therefore, in order to initiate a reaction, increases of the temperature and the ionization
of functional groups (ionized phenolic hydroxyl groups on lignin) are necessary
to facilitate the electron transfer to dioxygen and its related species.
7.3.2.3.3 The Reactions of Dioxygen and its Reduction Products
Under alkaline conditions the reaction of dioxygen with an activated lignin model
compound (particularly a phenolate) generates a superoxide anion radical
[3,129,130]. This is generally the rate-determining step of the oxidation requiring
elevated temperatures [131] or the presence of metal ions [132], whereas the superoxide
anion radical can undergo a metal-catalyzed dismutation [133–135] forming
hydroperoxy anion that can further undergo a metal-catalyzed disproportionation
reaction forming a hydroxyl radical. The following equations show some interconversion
reactions that oxygen species can undergo. Most of these are extremely
rapid, but others [Eqs. (4) and (5)] are very slow; indeed, for some reactions, metals
or protons are required for catalysis [136].
_O_2 _OH_→OH__O2 _1_
HOO__HO_→HO___O_2 _2_
HO_ _ HO_→H2O2 _3_
_O_2 __O_2 _H2O→HO_2 _O2_OH_ _4_
_O_2 _H2O2→OH__HO_ _ O2 _5_
HOO__H2O2→HO_ _ _O_2 _H2O _6_
_O_2 _OH_ _ H2O→HOO__OH_ _7_
O2__O_→_O_3 _ __O_ O2_HOO_ _8_
7.3.2.3.4 Autoxidation
The term “autoxidation” comprises the oxidation with dioxygen [137] and a multitude
of free radical reactions catalyzed by transients in the system such as hydroxyl
radicals (autocatalysis) and superoxide anion radicals [1,7,138]. This chain process
with various phases of autoxidation starting with the initiation of the reaction
of dioxygen [131], being the least reactive, with the activated substrate (particularly
a phenolate) requires an elevated temperature [137] and/or the presence of heavy
metals [132], acting as redox catalysts [3], forming a superoxide anion radical and
a substrate radical [Eq. (9)]. As noted, oxygen bleaching must be conducted in an
alkaline environment (pH > 10) and a temperature of about 80–100 °C and beyond
to ensure reasonable rates. At higher pH (alkali charge) and temperature (about
120 °C), hydroperoxides decay homolytically to hydroxyl radicals. Whilst activation
644 7Pulp Bleaching
of the substrate and elevated temperatures are needed to initiate the reaction [Eq.
(9)], dioxygen, on the other hand, reacts very rapidly with any substrate radical to
the corresponding peroxyl radical [Eq. (10), propagation]. The recombination and
termination respectively is accomplished by coupling of two radicals and does not
require activation by ionization, as is needed for electron transfer.
Eq. (9): Initiation [4]
R__O2→R_ _ _O_2
RH _ HO_→R_ _ H2O
RH _ _O_2 →R_ _ HOO_
RH _ O2→R_ _ HOO_
Eq. (10): Propagation [4]
R_ _ O2→ROO_
ROO_ _ RΗ→ROOH _ R_
ROO_→R_ _ _O_2
R_ _ RΗ→RH _ R_
Eq. (11): Recombination – Termination
R_ _ HO_→ROH
R_ _ _O_2 →ROO_
R_ _ R_→R__R
ROO_ _ ROO_→ROOR _ O2
ROO_ _ R_→ROOR
The initiation step above occurs mostly at C atoms which can produce the most
stable free radicals (allylic, benzylic position, and 3 > 2 >> 1c arbons).Hence, unsaturated
fatty acids are extra-reactive at themethyleneCthat separates the double bonds.
7.3.2.3.5 Singlet O2 – Excited State
Singlet dioxygen, with a lifetime of about 0.06 s, can be generated from triplet
dioxygen by photoexcitation [127,139]. Alternatively, it can be made from triplet
oxygen through collision with an excited molecule (photosensitizer), which relaxes
to the ground state after a radiationless transfer of energy to triplet oxygen to form
reactive singlet oxygen [Eq. (12)] [125,140,141]. Furthermore, singlet oxygen is
generated by the reaction between a hydroxyl radical and a hydroperyl radical [Eq.
(13)], and between HOCl and peroxide [Eq. (14)] [142], the oxidation of the superoxide
anion radicals with heavy metals [Eq. (15)] or ozone [Eq. (16)], and in consequence
of the decay of polyoxides.
7.3 Oxygen Delignification 645
O2
hm
Photosensitizer _____
O2
_1Dg_ _12_
HO_ _ HOO_→O2
1Dg _ __H2O _13_
HOOH _ OCl_→Cl__H2O _ O2
1Dg _ _ _14_
_O_2 _Fe3_→Fe2_ _O2
1Dg _ _ _15_
O3 _ _O_2 →O_3 _O2
_1Dg_ _16_
Alkenes (double-bonds) react with oxygen to form hydroperoxides, potentially
through an epoxide intermediate, and dienes reacts with oxygen in a Diels–Alderlike
reaction to form endoperoxides.
7.3.2.3.6 Superoxide Anion Radical
This is generated during the initiation step of the autoxidation [Eq. (9)], and undergoes
several interconversion reactions with other dioxygen-derived species
[Eqs. (1), (4), (5), and (7)]. In the presence of metal ions, the superoxide anion radical
can be oxidized to dioxygen [Eq. (17)] or reduced to the hydroperoxy anion
[Eq. (18)] [125]. The reduction of Fe3+ by the superoxide anion can accelerate the
Fenton reaction, giving a superoxide-assisted Fenton reaction [Eq. (19)] [125].
_O_2 _Fe3_→Fe2_ _O21_5 _ 108M_1s_1 _17_
_O_2 _Fe2_ _H_→Fe3_ _HO_2 1 _ 107M_1s_1 _18_
Fe2_H2O2→Fe3___OH _ OH_
Fe3__ _O_2 →Fe2__O2
H2O2__O_2
Fe
catalyst __ _OH _ OH__O2
_19_
Due to its low oxidation potential, the superoxide anion radical is highly selective.
It is a very strong Bronsted base capable of accepting a hydrogen from weak
acidic structures, and preferentially reacting with dihydroxy structures through
deprotonation followed by dehydration.
7.3.2.3.7 Hydrogen Peroxide
In contrast to dioxygen, which contains multiple bonds between the O atoms,
hydrogen peroxide has only one bond, which can be easily broken. Remember,
bonds can be broken in a heterolytic manner (both electrons in a bond go to one
of the atoms), or in a homolytic fashion, in which one electron goes to each atom.
646 7Pulp Bleaching
During dioxygen delignification, hydrogen peroxide [143] and the hydroperoxide
anion respectively evolve in situ from:
_ nucleophilic substitution reactions
O
OOH
O
OH
+ OH -
+ H2O2
and
_ disproportionation reactions 2_O_2 _H2O _
H_ HOO__HO__O2
2_O_2 _2H_ HOOH _ O2
Some properties and reactions of hydrogen peroxide include the following:
_ Acid/Base: H2O2
pKa1
_H_ _ HO_2
pKa2
_H__ O2_ 2
(pKa1 = 11.8; pKa2 > 16–18 [127] or 30 [128]; see Scheme 7.4)
_ Reaction with Fe2+: The Fenton reaction: HOOH + Fe2+ →Fe3+ + HO· + HO–. In
this reaction, a homolytic cleavage of the O–O bond occurs, generating OH– and
the hydroxyl radical (OH·), which will react with any molecule it encounters.
The hydroxyl radical may be formed via an oxoiron(IV) intermediate [144]. The
peroxide can also be effective as an oxidant, and in a transition metal-induced
cleavage of the H–OO bond the hydroperoxyl radical (HOO_) is formed:
HOOH + Fe3+ →Fe2+HOO· + H+
_ Thermal or photochemical homolytic cleavage of hydrogen peroxide:
HOO_ _ HOOH Energy _ H2 _ _HO _ _H_
7.3.2.3.8 Hydroxyl Free Radical
As mentioned earlier, this species is extremely reactive [136]. It will react with any
molecule it encounters, and does so immediately. It can abstract a H atom, leaving
another free radical. The anionic form, the oxyl anion radical (see Scheme 7.4),
displays properties that are distinctly different from those of the hydroxyl radical.
In contrast to the latter, the oxyl radical reacts predominantly by hydrogen abstraction
and is therefore probably less selective than the hydroxyl radical [145].
Note: The terms hydroxyl free radical and hydroxyl radical are used synonymously.
Care must be taken using the term hydroxyl ion, which is the synonym
for the hydroxide ion (OH–).
7.3.2.3.9 Electrophilic–Nucleophilic Reactions
As noted, delignification during bleaching is initiated by electrophilic reactions,
which may be followed by nucleophilic processes [6–9]. The reactive oxygen species
(ROS) are listed in Tab. 7.11, according to their electrophilic–nucleophilic
character. Under the conditions of oxygen- alkali bleaching, the hydroperoxyl radical
is deprotonated to produce the superoxide anion radical. About half of the hydroxyl
radical is present as its base the oxyl anion radical (Tab. 7.11; see also
Scheme 7.4), and about half of the hydroperoxy anion is present as hydrogen peroxide
(Scheme 7.4).
7.3 Oxygen Delignification 647
Tab. 7.11 Reactive oxygen species (ROS) listed according to
their electrophilic – nucleophilic character.
Electrophiles
Triplet dioxygen 3O2
Hydroperoxyl radical HOO_
____
pKa_4_8
_O_2 Superoxide anion radical
Hydroxyl radical HOO_
____
pKa_11_9
_O_2 Oxyl anion radical
Nucleophiles
Hydroperoxy anion HOO–
Singlet dioxygen 1O2
The sites of electrophilic and nucleophilic attacks in lignins are shown in
Fig. 7.26. The p-system of the aromatic ring can be overlapped by the lone electron
pairs on the oxygen atom in para-hydroxy and the para-alkoxy groups, creating
centers of high electron density (Fig. 7.26), that can be attacked by electrophiles.
High electron density (d-) also appears at the Cb atom of aliphatic double bonds
O-
R2 OCH3
HC R1
arylalkane unit
R1 = OH, OAr or OAlk
arylpropene unit
O
R2 OCH3
CH2
CH
δ- δ-
δ-
H2C R1
δ-
O
R2 OCH3
C
C
H2C R1
R δ-
O-
R3
- δ-
α-carbonyl group containing
R = OAr, Ar or Alk
O
R2 OCH3
CH
δδ
δ
arylalkane unit
R1 = OH, OAr or OAlk
arylpropene unit
quinone-methide intermediate
O
R2 OCH3
CH
HC
CH2
O
R2 OCH3
C
C
CH2
R
O
R3
α-carbonyl group containing
R = OAr, Ar or Alk
δ
δδ
δ
δ
δ
δ
δ
C C C C O
δδ
C
C O-
C
C O
-
δ-
δ-
ELECTROPHILIC
NUCLEOPHILIC
O
R2 OCH3
CH R1
δ- - δ-
δ-
-
O-
R2 OCH3
CH
CH
H2C R1
Cβ
Cα
Fig. 7.26 Sites of electrophilic (d-) and nucleophilic (d+)
attacks in lignin (adapted from Ref. [2]).
648 7Pulp Bleaching
conjugated to the aromatic ring. By elimination of an a– (see Section 4.2.4, Chemistry
of kraft pulping, Scheme 3) or, in conjugated structures, a c-substituent, a
quinone-methide intermediate is formed from the arylalkane unit (Fig. 7.26),
which involves the loss of two electrons, resulting in the generation of centers of
low electron density (d+) that constitute the sites of attack by nucleophiles [2].
7.3.2.4 A Principal Reaction Schema for Oxygen Delignification
Over 30 years of research into the oxidation of lignin and lignin model compounds
with dioxygen has now elapsed, and has provided insights into the reactions
involved in the degradation, and their mechanisms. Based on the reaction
products formed from the degradation of lignin and lignin model compounds
with dioxygen and with ROS generated during bleaching, a number of mechanisms
have been proposed. Several excellent reviews have been produced on the
mechanisms involved in lignin degradation [1,2,4,6,7,72,101,122,138,146,147] and
the reactive species present in these reactions [3,9,90,129,130], including their
selectivity. The latter remains of interest [148,149], especially in connection with
protective systems and additives [94,150–156]. In addition, an excellent book on
oxygen delignification chemistry was published a few years ago [157]. It is impossible
to cover all of these mechanisms in detail within this chapter; thus, a general
summary with selected mechanisms will be provided.
Oxygen delignification is actually based on the competitive reactions of oxygen
or ROS within pulp lignin and carbohydrates [94]. Lignin removal under alkalioxygen
conditions is accompanied by a kinetically less favorable oxidation of carbohydrates,
whereas the oxidation of the carbohydrates becomes a more favorable
process when the kappa number decreases [94]. The reaction of phenolic compounds
with oxygen produces ROS, namely the hydroxyl radical (_OH), which
can degrade nonphenolic (model) compounds.
As shown previously (see Scheme 7.1), the initial step in oxygen-alkali bleaching
is the formation of the phenoxyl radical as a consequence of an electrophilic
attack by oxygen (Scheme 7.5A). Moreover, the hydroxyl radical formed during
oxygen treatment [Eqs. (5), (6), and (19)] is also capable of generating a phenoxyl
radical (Scheme 7.5B) being reduced to the hydroxide anion (OH–).
A principal reaction schema for oxygen delignification [3,6,7,138] starts with the
generation of hydroperoxides, which are key intermediates in the oxidation of lignins
and carbohydrates. They can be formed either by electrophilic or nucleophilic
reactions:
_ Formation of hydroperoxides [8,138]
_ Fragmentation of hydroperoxides (homolytic – forming radicals;
or heterolytic – forming hydrogen peroxide, singlet oxygen)
[8,138]
_ Involvement of the radicals in the bleaching process [3,9,129,130]
7.3 Oxygen Delignification 649
OH
OCH3
CH
CH
CH2OH
δ- δ-
δ-
δ-
O-
OCH3
CH
CH
CH2OH
+ OH-, - H2O
O
OCH3
CH
CH
CH2OH
O2 O2
-
1 2a 3
A
O-
OCH3
CH
CH
CH2OH
O
OCH3
CH
CH
CH2OH
OH
2b 3
OH
OCH3
CH
CH
CH2OH
1
OH-
B
+ OH-, - H2O
+ HO
Scheme 7.5 Formation of the phenoxyl radical by oxygen (A)
and the hydroxyl radical (B).
O
R1 OCH3
C
C R
O
R1 OCH3
C
C R
O
R1 OCH3
C
C R
O
R1 OCH3
C
C R
O
R1 OCH3
C
C R
O -O
O
R1 OCH3
C
C R
O
OCH3
R1
C
C R
O O-
4 5a 6 7 8 9
O
R1 OCH3
C
C R
O -O
O-
R1 OCH3
C
C R
O
O
O
R1 OCH3
C
C R
OH
O
4 5b 10 11 12 13
14 15 16 17 18 19
-O
OCH3
R1
C
C R
O
O
O
OCH3
R1
C
C R
O O-
HO
OCH3
R1
C
C R
O
O
+ H2O, - OH-
O
R1 OCH3
C
C R
O
R1 OCH3
C
O C R -O
O
R1 OCH3
C
O C R -O
O-
R1 OCH3
C
O C R
O
OH
R1 OCH3
C
O C R
O
+ O2
-
+ O2
-
+ O2
-
+ H2O, - OH-
+ H2O, - OH-
R = H, OAr, Ar or Alk
Scheme 7.6 Formation of hydroperoxide intermediates in
alkaline media followed by an intramolecular nucleophilic
attack of the hydroperoxide anions (adapted from Ref. [6]).
650 7Pulp Bleaching
C
C
C
C
O
R
H
C
C
C
C
O
R
+ OH - (-)
C
C
C
C
O
R
+ O2, - HOO
+ HOO , - HOOH
+ HO , - HOH
R = H, OH, organic moiety
+ HOO
+ O2
C
C
C
C
O
R
OOH
20 21 22 23
O-
O-
+ O2, + H+
OOH
O
O-
O
O
+ HOO-
24 25 26
Scheme 7.7 Formation of hydroperoxides in the autoxidation
of enolisateable and enediol structures, and the formation of
the hydroperoxy anion (adapted from Refs. [4,6]).
The abstraction of an electron from phenolate anions by oxygen (or the hydroxyl
radical) (Scheme 7.5) yields phenoxyl radicals (Scheme 7.6, 4 and 14) and the
mesomeric cyclohexadienonyl radicals (5a and 5b) or “quinone methide” radicals
(15). The superoxide anion radicals then form hydroperoxide intermediates (6 and
10) with the mesomeric cyclohexadienonyl radicals or the b-radical (15). A nucleophilic
attack by the peroxide anions on the carbonyl carbon (11) or a vinylogous
carbon of the cyclohexadienone- (7) or quinone methide (17) moieties yields the
corresponding dioxetane intermediates (8, 12 and 18). Intermediate 8 finally form
an oxirane structure (9). The rearrangement of 12 results in an opening of the peroxide
ring and heterolytic cleavage of the carbon–carbon bond, giving a “muconic
acid” ester (13), and 18 is fragmented by scission of the Ca–Cb bond of the former
conjugated double-bond forming the corresponding aldehydes (19) and/or a
ketone, depending on the nature of R.
The hydroperoxy intermediates formed during the autoxidation of phenolic
(Schemes 7.6 and 7.8) and enolic (Scheme 7.7) structures in lignin and carbohydrates
can be displaced by the hydroxide ion via a SN2 reaction (28–29), or the
bond can be cleaved heterolytically giving the hydroperoxy anion, which is
described elsewhere. Homolytic decomposition of hydroperoxy intermediates produces
phenoxy (31) and hydroperoxy (Scheme 7.8) radicals. The latter can be
reduced to the hydroperoxy anion.
The hydroxyl radical reacts with the main components of wood, and attacks
preferentially electron-rich aromatic and olefinic moieties in lignin. It also reacts
with aliphatic side chains in lignin and carbohydrates, but at a lower rate. Depending
on the pH, the hydroxyl radical is converted to its conjugate base, the oxyl
anion radical (see Scheme 7.4). The oxyl anion radical does not react with electron-
rich structures, but rather with aliphatic side chains in lignin and carbohydrates.
The first step in all reactions of the hydroxyl radical with aromatic substrates
(Scheme 7.9, 32) is a rapid addition to the p-electron system of the aromatic
ring forming a short-lived charge-transfer adduct (33) that decays under
alkaline conditions to give isomeric hydroxycyclohexadienyl radicals (34 and 37).
7.3 Oxygen Delignification 651
O-
R1 OCH3
O
R1 OCH3
+ O2, + H+
OOH
O
R1 OCH3
OH
+ OH -
+
- HOO
O
R1 OCH3
- HOO
+ e-
HOO-
27 28 29 30
31
HOO-
Scheme 7.8 Formation of hydroperoxides in the autoxidation
of phenolic structures, and the formation of the hydroperoxy
anion (from Ref. [6]).
OR
OCH3
R
OH
33
OR
OCH3
R
32
OH
OR
OCH3
R1
34
H
HO
OR
OCH3
R1
37
or OH
Scheme 7.9 Formation of hydroxycyclohexadienyl radicals
(adapted from Ref. [6]).
The hydroxycyclohexadienyl radical can be oxidized by addition of oxygen
(Scheme 7.10) followed by alkali-promoted elimination of the superoxide anion
radical forming a cation radical (35, 38 and 42) and elimination of a proton (rearomatization)
leading to hydroxylation (Scheme 7.10, path A, 36) or, in combination
with elimination of methanol and cleavage of an alkyl-aryl ether bond, to dealkoxylation
(path B) with formation of ortho-quinonoid structures (39). From conjugated
structures (Scheme 7.10, path C), “quinone methide” intermediates (41,
42 and 43) are formed, giving glycolic structures (44) by adding hydroxide ions
that undergo oxidative cleavage of the glycolic C–C bond (44) [6].
The hydroxyl radical adducts (Scheme 7.11, 45) can undergo disproportionation
reactions from which the same oxidation products (46 and 47) arise, together with
the corresponding reduction products (48 and 49) [6].
652 7Pulp Bleaching
OR
OCH3
R1
OH
33
OR
OCH3
R1
34
OR
OCH3
R1
OH
33
R = H or alkyl
H
HO
+ O2
- O2
-
OR
OCH3
R1
35
H
HO
+
- H+
OR
OCH3
R1
36
HO
OR
OCH3
R1
37
+ O2
- O2
-
OH
OR
OCH3
R1
38
OH
+
- CH3OH, - H+
( - ROH )
O
O
R1
39
O-
OCH3
C
OH
40
C
O-
OCH3
C
41
C OH
O-
OCH3
C
42
C OH
+ O2
- O2
-
+
O
OCH3
C
43
C OH
+ OH -
O-
OCH3
C
44
C
OH
OH cleavage
A
B
C
Scheme 7.10 Reactions of the hydroxyl radical adducts of aromatic
and ring-conjugated structures (adapted from Ref. [6]).
O-
OCH3
R1
OH
45
O-
OCH3
R1
HO
46
O
O
R1
47
O-
OCH3
R1
48
O-
OH
R1
49
O-
OCH3
R1
OH
45
Disproportionation
+
Scheme 7.11 Disproportionation of hydroxyl radical adducts
(adapted from Ref. [6]).
Another reaction mode of the hydroxycyclohexadienyl radical (Scheme 7.12, 51
and 56) is the elimination of the hydroxyl radical as hydroxide anion (Scheme
7.12, paths A and B). This results in the formation of cation radicals (52 and 57)
followed by the generation of side-chain oxidation products and products of homolytic
Ca–Cb bond cleavage (58). The elimination of a proton leads to a re-aromatization
(59).
7.3 Oxygen Delignification 653
OR
OCH3
CH
OH
50
- OH -
OR
OCH3
HC
OH
55
CR1
OH
A
B
R = H or alkyl
OR
OCH3
CH
51
OH
OR
OCH3
CH
52
+ - H+
OR
OCH3
CH
53
OR
OCH3
C
54
R = alkyl; R1 = aryl or aroxyl
further oxidation
OR
OCH3
HC
56
CR1
OH
HO
H
OR
OCH3
HC
57
CR1
OH
- OH -
+
OR
OCH3
HC
58
CR1
OH
+
OR
OCH3
HC
59
O
- H+
Scheme 7.12 Reactions of the hydroxyl radical adducts of
aromatic and side chain structures (adapted from Ref. [6]).
Elimination of the hydroxyl radical as hydroxide anion results in the formation
of a cation radical (62 and 63), followed by a phenolic coupling (64) (Scheme 7.13)
and elimination of two protons to form a diphenyl (5–5) structure (58). The formation
of diphenyl structures is an undesirable reaction, because the 5–5 bond is
very stable and can hardly be cleaved.
O-
OH
60
H3CO
O-
61
H3CO
H
OH
O-
62
H3CO
- OH -
+
O
63
H3CO
2 x
O
64
H3CO
O
OCH3
H
H
O-
65
H3CO
O-
OCH3
- H+
Scheme 7.13 Phenolic coupling of the hydroxyl radical
adducts of aromatic structures (adapted from Ref. [6])
Singlet oxygen that can be generated during oxygen bleaching in different ways
[Eqs. (13–16)] has been of growing research interest for the past few years
[140,142,158–180]. Both, lignin model compounds and pulp have been investigated.
However, in most of the studies photosensitizers, such as rose bengal
654 7Pulp Bleaching
[159,162,164–167,174,179], methylene blue [140,141,160,163,169,175,177] or titandioxide
(TiO2) [139,169,175,177] have been used to generate singlet oxygen using
light from the visible range to the UV, the latter also used for direct irradiation of,
for example, an a-carbonyl group-containing lignin. Alternatively, singlet oxygen
was produced from sodium hypochlorite (NaOCl) and hydrogen peroxide [142]
according to Eq. (14). Some of these studies were performed in organic solvents
[162,167,174,176] and others in aqueous alkaline solution [142,168,177,179,181],
with the latter category being of main interest for this chapter. The photo- and radiation
chemical-induced degradation of lignin model compounds have been
summarized in a very good review [171], including other ROS, and the photochemical
oxidation of lignin models in the presence of singlet oxygen has been
studied by using ab initio calculations [178].
As mentioned, singlet oxygen has a pronounced electrophilic character, and hence
reacts well with electron-rich groups such as olefinic or aromatic derivatives. These
electron-rich groups tend to form an intermediate exciplex as a result of charge transfer
reactions between the electron-rich substrate and the singlet oxygen. This exciplex
is able to later form dioxetanes, hydroperoxides, or endoperoxides.
Photosensitized degradation studies of a-carbonyl group-containing lignin
model compounds (Scheme 7.14) show that a hydrogen atom transfer from the
phenolic OH group (66) to 1O2 might occur, leading to a phenoxyl radical (67) and
subsequently to quinonoid species (path A). However, formation of an endoperoxide
(68) leading ultimately to p-quinones (70) is also possible (path B).
H OH
HO H
H OR
H OH
R1
OH
HO H
H OR
H OH
R1
H OH
O2/OH-
O OH
HO H
H OR
H OH
R1
O
O HO
HO H
H OR
H OH
R1
OH
O
OH
H
+
OH-
I
II
I
1. -HOO-
2. -HOH
O
HO H
H OR
H OH
R1
O
OH
HO H
H OR
H OH
R1
BAR
H O HO O
HO O
HO H
H OR
H OH
R1
II
1 2 3 5
8
7
4
R1 = -H for xylan
BAR = Benzilic Acid Rearrangement R = Polysaccharide chain
H
HO
HO H
H OR
H OH
R1
HO O
6
+
R1 = -CH2OH for cellulose and glucomannan
- HCOOH
- ROH
+ OH-
HO O
HO
H
H OH
R1
9
+ OH-
Degradation
products
1 D-Glucose
2 1-Hydroperoxy-ketose
3 2-Hydroperoxy-aldose
4 D-arabino-hexosulose
5 Gluconic acid
6 Mannonic acid
8 Arabinonic acid
9 3-Deoxy-D-glycero-2-keto-pentonic acid
1 D-Xylose
2 1-Hydroperoxy-ketose
3 2-Hydroperoxy-aldose
4 D-threo-pentosulose
5 Xylonic acid
6 Lyxonic acid
8 Threonic acid
9 3-Deoxy-2-keto-tetronic acid
Scheme 7.14 Photodegradation of a-carbonyl group-containing
lignin model compounds (from Ref. [171]).
Moreover, a-carbonyl-containing b-O-4 lignin model compounds intensively
used in singlet oxygen degradation studies have been degraded to products deriving
from b-C–O bond cleavage. The main reactions were conversion of phenolic
aromatic units into carboxylic acids and cleavage of the b-O-4 ether bonds, leading
to a depolymerization of the lignin framework into smaller fragments [177]. Cleavage
of the b-O-4 aryl ether bond has been found for phenolic as well as nonpheno-
7.3 Oxygen Delignification 655
656 7Pulp Bleaching
lic derivatives [162]. Photochemical oxidation of the phenolic b-O-4 aryl ether gave
the same type of product, which confirmed that, in this case, the presence of the
carbonyl group is not indispensable for the cleavage reaction to occur [162]. When
the phenoxy portion of the molecule [1-(4-hydroxy-3-methoxyphenyl)-2-(2,6-
dimethoxyphenoxy)-3-hydroxy- 1-propanol] shows a lower reactivity towards singlet
oxygen, the oxidation of the phenol moiety to hydroquinone can occur. The
photochemical behavior of this model compound can be rationalized from a reaction
of singlet oxygen with the phenoxy part of the molecule [162].
Due to the unknown real contribution of singlet oxygen to lignin degradation
during oxygen bleaching, and the fact that in processes interconversions between
reactive species occur, this section of the text will be minimized.
One example of a rose bengal photosensitized degradation of loblolly pine
(Pinus taeda) kraft pulp, the final product of which contained 4% by mass of residual
lignin with the remainder being carbohydrates, is presented [179]. In this
study, the reactivity of singlet oxygen with kraft softwood substrates with respect
to the chemistry of lignin and cellulose has been investigated. The results revealed
that, despite the relatively high selectivity of singlet oxygen for lignin aromatic
units, degradation of the cellulose nevertheless occurred after approximately 50%
removal of the lignin. A decrease was observed in the number of aliphatic hydroxyls
(17%), condensed phenolics (4%), and guaiacyl phenolics (7%), and an
increase in carboxylic acids (54%). This result is typical of what is observed in the
reactions of ground-state oxygen with pulp or lignin, and suggests that despite the
initial electrophilic reactions of singlet oxygen with lignin, it is likely that ensuing
oxidations follow some of the typical reactions associated with ground-state oxygen
reactions, such as ring additions by hydroperoxide and oxygen followed by
ring openings to the muconic esters and acids. However, unlike ground-state oxygen
reactions, the levels of condensed phenolics (e.g., conjugated lignin monomers
at the C5 positions of the benzene moieties) were reduced during the singlet
oxygen reactions. Thismay be a consequence of the high electrophilic reactivity of singlet
oxygen, and was tested by subjecting substrates enriched in condensed phenolics
to singlet oxygen reactions [179]. The most salient difference between this systemand
a typical ground-state oxygen delignification system is the absence of condensed
phenolic units in the lignin. Subsequently, it was discovered that both the condensed
and noncondensed (guaiacyl) units react well with singlet oxygen [179].
This finding is important since 5-condensed phenolic subunits (5–5 and diphenylmethane;
DPM) in lignin are quite resistant. Their relative robustness does
not, however, appear to be the main rationale for the inactivity of lignin towards
oxygen delignification, but serves to suggest that the nature and reactivity of the
free phenolics deserve increasing scrutiny [182].
Residual lignins isolated from unbleached and oxygen-bleached eucalyptus
kraft pulps by acid hydrolysis and dissolved lignins in the kraft cooked and oxygen-
bleached liquors were studied, and the results compared with the corresponding
residual lignins. The data showed that etherified syringyl structures were
quite resistant towards degradation in the oxygen bleaching, causing little depolymerization
in residual lignin and a small increase in carboxylic acid content, but
producing appreciable amounts of saturated aliphatic methylene groups [105].
7.3 Oxygen Delignification 657
7.3.2.5 Carbohydrate Reactions in Dioxygen-Alkali Delignification Processes
The reactions of wood polysaccharides during dioxygen-alkali treatment can be
classified according to Malinen [183] into the following main categories:
_ Stabilization of the reducing end-groups.
_ Peeling reactions starting from the reducing end-groups.
_ Peeling reactions starting from stabilized end-groups.
_ Cleavage of the polysaccharide chain.
Reaction steps involving dioxygen are drawn with thicker lines (bold) and the
numbers given in italic.
7.3.2.5.1 Stabilization of the Reducing End-Groups
The rapid stabilization of the reducing end-groups of polysaccharides by transformation
to aldonic acid end-residues has been considered to be one great advantage
of the dioxygen-alkali delignification of wood or pulp [184–186]. Under the conditions
of dioxygen-alkali treatment, oxidation of the glucose unit (1) may proceed
via a 1-hydroperoxy-ketose (2 [187]) and a 2-hydroperoxy-aldose (3) (Scheme 7.15).
The hydroperoxy-group can easily be replaced by a hydroxide anion followed by
dehydration (path I) resulting in a a, b– dicarbonyl (glucosone = d-arabino-hexosulose,
4), which converts into gluconic acid (5) and mannonic acid (6) via benzilic
acid rearrangement (BAR) (see Section 4.2.4.2, Carbohydrate reactions). Glucosone
(d-arabino-hexosulose) end-groups have been suggested to be intermediates
in the formation of aldonic end-residues [188,189], and Theander [185] stated that
the fact that mannonic acid and gluconic acid end-residues are obtained on cellulose
treatment with dioxygen in basic solution is the best support for the view that
glucosone is really an intermediate. Alternatively, the hydroperoxy-intermediates
are split to formic acid (7) and arabinonic acid (8) (path II), the latter being converted
to 3-deoxy-d-glycero–2-keto-pentonic acid (9) and further degraded.
H OH
HO H
H OR
H OH
R1
OH
HO H
H OR
H OH
R1
H OH
O2/OH-
O OH
HO H
H OR
H OH
R1
O
O HO
HO H
H OR
H OH
R1
OH
O
OH
H
+
OH-
I
II
I
1. -HOO-
2. -HOH
O
HO H
H OR
H OH
R1
O
OH
HO H
H OR
H OH
R1
BAR
H O HO O
HO O
HO H
H OR
H OH
R1
II
1 2 3 5
8
7
4
R1 = -H for xylan
BAR = Benzilic Acid Rearrangement R = Polysaccharide chain
H
HO
HO H
H OR
H OH
R1
HO O
6
+
R1 = -CH2OH for cellulose and glucomannan
- HCOOH
- ROH
+ OH -
HO O
HO
H
H OH
R1
9
+ OH -
Degradation
products
1 D-Glucose
2 1-Hydroperoxy-ketose
3 2-Hydroperoxy-aldose
4 D-arabino-hexosulose
5 Gluconic acid
6 Mannonic acid
8 Arabinonic acid
9 3-Deoxy-D-glycero-2-keto-pentonic acid
1 D-Xylose
2 1-Hydroperoxy-ketose
3 2-Hydroperoxy-aldose
4 D-threo-pentosulose
5 Xylonic acid
6 Lyxonic acid
8 Threonic acid
9 3-Deoxy-2-keto-tetronic acid
Scheme 7.15 Stabilization of reducing end-residues through
formation of aldonic acids (5) and mannonic acid (6)
(adapted from Malinen [183] and Theander [185]).
In the absence of dioxygen, large amounts of 3-deoxy-pentonic acids are formed
and under oxidative conditions arabinonic, erythronic and mannonic acids are the
major reaction products [190]. A relative composition of aldonic acid residues
from various treatments is shown in Tab. 7.12.
Tab. 7.12 Relative composition (mol. %) of aldonic acid residues
from various treatment (from Ref. [185]).
From d-glucosone From cellulose
Acid NaOH/air
0.04 M, 100 °C
4 h [189]
NaOH/O2
0.04 M, 95 °C
1bar , 5 min [187]
NaOH/N2
0.04 M, 95 °C
1bar , 5 min [187]
NaOH/air
18%, 25 °C
200 h [191]
NaOH/O2
0.5%, 100 °C
5 bar, 2 h [184]
Mannonic 11 18 47 15 27
Gluconic 2 5 5 2 3
Arabinonic 58 37 26 58 50
Ribonic 4 6 0 2 2
Erythronic 25 35 22 23 18
Two different pathways can form erythronic acid (11) (Scheme 7.16). The first
entails rearrangement of the glucosone to d-erythro–2,3-hexodiulose (10), followed
by an oxidative cleavage and loss of glycolic acid (12) [183]. In the second pathway,
erythronic acid (11) results from alkaline and oxidative degradation of the glucosone
(4) through arabinose (13) and arabinosone (14) as intermediates. In the
absence of dioxygen arabinose (13, Scheme 7.16) and arabinonic acid (8, Scheme
7.15), it may be formed by hydroxide ion attack at C1 and C2 respectively [185].
Minor amounts of 3-deoxy-pentonic acids (17, 18) are formed from an arabinose
intermediate (13), and the main pathway starts with a direct b-hydroxy-elimination
in the glucosone (4) followed by loss of the elements of carbon monoxide
from the intermediate 4-deoxy-d-glycero–2,3-hexodiulose (15) [187].
The yield of 3-deoxy-pentonic acids is lower in the presence of dioxygen [185],
and the formation of arabinonic and erythronic acid is particularly important.
Theander [185] stated that an attack of dioxygen to the glucosone (4, Scheme 7.17)
should give a hydroperoxide (20), which should further yield arabinonic acid (8)
and carbon dioxide. A similar attack at C3 could, via formation of a hydroperoxide
(21), result in the formation of an erythronic acid end-group (11) plus glyoxylic
acid (22).
About the same proportions of aldonic acids were produced from glucosone
and glucose treated with dioxygen and alkali [183], and cellobiose [190] and cellotriose
[192] yielded glucosyl- and cellobiosyl-arabinonic acids as the main products.
However, the presence of the substituted erythronic and mannonic acids was
also significant, especially at higher alkali concentrations. Malinen and Sjostrom
658 7Pulp Bleaching
R1 = -H for xylan
R = Polysaccharide chain
R1 = -CH2OH for cellulose and glucomannan
O
HO H
H OR
H OH
R1
O
4
H
+ OH-
CH2OH
O
O
H OR
H OH
R1
10
+
O2/OH-
COOH
H OR
H OH
R1
11
CH2OH
COOH
12
7
- HCOOH
+ OHH
O
H
H OR
H OH
R1
13
HO
O2/OH- O2/OH-
H O
O
H OR
H OH
R1
14
COOH
H OR
H OH
R1
11
H O
H
H OH
R1
OH + OH-
- ROH-
H O
H
H OH
R1
16
O
H
O
OH
H
H OH
R1
H O
+ OH-
- ROH
O
O
H
H OH
R1
O
15
H
H
HO O
H
H OH
R1
17
HO
H
H
HO O
H
H OH
R1
18
H
H
OH
HO O
H
H OH
R1
19
+ + H
10 D-erythro-2,3-hexodiulose
11 Erythronic acid
12 Glycolic acid
13 Arabinose
14 Arabinosone
15 4-Deoxy-D-glycero-2,3-hexodiulose
16 3-Deoxy-D-glycero-pentosulose
17 3-Deoxy-D-threo-pentonic acid
18 3-Deoxy-D-erythro-pentonic acid
19 3,4-Dihydroxybutyric acid
10 D-glycero-2,3-pentodiulose
11 Glyceric acid
13 Threose
14 Threosone
15 4-Deoxy-2,3-pentodiulose
16 3-Deoxy-Tetrosulose
17 2,4-Dihydroxybutyric acid
18 2,4-Dihydroxybutyric acid
19 2-Deoxy-glyceric acid
Scheme 7.16 Degradation pathways of the glucosone and
xylosone end-groups (adapted from Malinen [183] and
Theander [185]).
R = Cellulose chain
R1 = -CH2OH
O
HO H
H OR
H OH
R1
O
4
H
+
O2/OHCOOH
H OR
H OH
R1
11
C
COOH
22
O
HO H
H OR
H OH
R1
O
20
H
O
HO O2H
H OR
H OH
R1
O
21
H
O2H
H
- CO2
HO O
HO H
H OR
H OH
R1
8
H O
O2/OHO2/
OH-
Scheme 7.17 Degradation pathways of the glucosone endgroups
to the formation of arabinonic acid (8) and erythronic
acid (11) (adapted from Theander [185]).
7.3 Oxygen Delignification 659
[192] reported that, when hydrocellulose was subjected to dioxygen-alkali treatment,
erythronic acid was the dominating end-group, and that the reaction conditions
actually have a marked effect on the composition of the aldonic acid endgroups.
Extensive studies on the formation of aldonic acid groups on cellulose [192],
mannan [193], xylan [194] and the corresponding oligosaccharides under various
conditions revealed that arabinonic acid was highly predominant after oxidation
of 4-b-linked mannobiose, mannotriose, and mannotetraose. The stabilization
(and also peeling) reactions of glucomannan and cellulose proceed in a similar
way (Schemes 7.15 and 7.16) [193]. In contrast, mannose end-groups – which
react more slowly than glucose end-groups – are converted to the same reactive
“fructose intermediates” as glucose, and the same aldonic acid end-groups in
about the same proportions have been found from manno-oligosaccharides and
mannan as from cello-oligosaccharides and hydrocellulose [193]. The monosaccharides
glucose, mannose, and xylose degrade much faster under dioxygen pressure
than the reducing end-groups of the corresponding oligosaccharides, the degradation
rates of which are almost the same in dioxygen and nitrogen atmospheres
[193].
The formation of aldonic acid end-groups after dioxygen-alkali treatment of
birch xylan studied by Kolmodin and Samuelson [195] showed that xylonic (5,
Scheme 7.15), lyxonic (6), threonic (8) and glyceric (11, Scheme 7.16) acids were
formed as the major terminal acid residues, and xylosone 2,4-dihydroxy-butyric
acid (17, 18) was also extensively formed in non-oxidative treatments [194]. Lyxonic
and xylonic groups are expected from a benzilic-type rearrangement (BAR) of
pentosulose end-unit (Scheme 7.15), whereas oxidative or hydrolytic cleavage
leads to threonic acid (8). Glyceric acid (11) is probably formed via cleavage of dglycero–
2,3-pentodiulose (10) end-units formed by isomerization of pentosulose
units (4), and from alkaline and oxidative degradation of the xylosone (4) through
threose (13) and threosone (14) as intermediates.
7.3.2.5.2 Peeling Reactions Starting from the Reducing End-Groups
The peeling removes the terminal anhydro-sugar unit, generating a new reducing
end-group until a competitive stopping reaction sets in forming a stable saccharinic
acid end-group (see Section 4.2.4.2, Carbohydrate reactions).
In studying the oxidative alkaline peeling reaction of cellulose by using cello-oligosaccharides
and hydrocellulose, Malinen and Sjostrom ([190, 192]) found in
addition to the “normal” alkaline peeling products [isosaccharinic acid (27,
Scheme 7.18) and lactic acid (32)], large amounts of 3,4-dihydroxybutyric acid
(28), glycolic acid (33), 3-deoxy-pentonic acid (17, 18, Scheme 7.16), formic acid
(34) and glyceric acid (35). The formation of the two isomeric glucoisosaccharinic
acids (e.g., 27) by alkaline treatment of cellulose is much depressed in the presence
of dioxygen [185], and the 4-deoxy-d-glycero–2,3-hexodiulose (26) is instead
fragmented to 3,4-dihydroxybutyric acid (28) and glycolic acid (33). These are
formed via oxidative cleavage of 4-deoxy-d-glycero–2,3-hexodiulose (26), which can
660 7Pulp Bleaching
H OH
HO H
H OR
H OH
R1
CH2OH
O
HO H
H OR
H OH
R1
H O
23
R1 = -H for xylan
R = Polysaccharide chain
R1 = -CH2OH for cellulose / glucomannan
23 Glucose end-group
24 Mannose end-group
25 Fructose end-group
26 4-Deoxy-D-glycero-2,3-hexodiulose
27 Isosaccharinic acid
28 3,4-Dihydroxybutyric acid
29 Glycolic acid
30 Dihydroxyacetone and glyceraldehyd
31 Methyl glyoxyl
32 Lactic acid
33 Glycolic acid
34 Formic acid
35 Glyceric acid
23 Xylose end-group
25 Xylulose end-group
26 4-Deoxy-2,3-pentodiulose
27 Xyloisosaccharinic acid
28 2-Deoxy-glyceric acid
29 Glycolic acid
30 Dihydroxyacetone and glycolaldehyd
31 Methyl glyoxyl
32 Lactic acid
33 Glycolic acid
34 Formic acid
35 Glyceric acid
25
- ROH
+ OH-
CH2OH
O
OH
H
H OH
R1
26
CH2OH
O
O
H
H OH
R1
COOH
HO
H
H OH
R1
27
+ OH-
+ O2/OHCH2OH
H
COOH
H
H OH
R1
28
H
+
COOH
CH2OH
29
HO H
HO H
H OH
H OH
CH2OH
H O
24
OH
30
H
H O
R1
CH2OH
O
CH2OH
OH
CH2
COOH
OH
CH2OH
COOH
CH2OH
33
HCOOH
34
+ OH-
O2/OH-
O2/OHH
O
31
+
O
CH3
H O
+ OH-
H
32
COOH
OH
CH2OH
H
35
Scheme 7.18 Peeling reactions of polysaccharides during
alkaline and oxidative alkaline conditions (redrawn from
Ref. [183]).
also rearrange to isosaccharinic acids (27) or cleave to yield glyceraldehyde (30)
[183]. Glyceraldehyde is further converted to lactic (32), glycolic (33) and glyceric
(35) acids.
Malinen and Sjostrom [192] reported that the extent of the peeling reaction for
cello-oligosaccharides was very low and that stabilization proceeded quickly. However,
the stabilization of hydrocellulose – that is, the formation of aldonic acid
end-groups – was less extensive, and peeling resulted in a loss of 10–50 sugar
units, depending on the reaction.
The peeling reactions of xylan and glucomannan that take place under alkaline
conditions have been described in detail (see Section 4.2.4.2, Carbohydrate reactions).
In the presence of dioxygen, the peeling of xylan is more extensive than in
alkali alone, and greater than that of cellulose and glucomannan. However, in the
absence of dioxygen the degradation rate is lower for xylan than for cellulose and
glucomannan [192,193,195]. 2,4-Dihydroxy-butyric acid (17, 18, Scheme 7.16), 2-
deoxy-glyceric acid (28, Scheme 7.18), glycolic acid (33), glyceric acid (35), xyloisosaccharinic
acid (27), lactic acid (32) and formic acid (34) are the main peeling
products of xylan, which are analogous to the peeling products of cellulose.
The xylan chains are partly substituted with 4-O-methyl-glucuronic acid units at
C2 [196], which prevent migration of the carbonyl group to the b-position relative
to the glycosidic bond constraining b-elimination (see Section 4.2.4.2, Carbohydrate
reactions; specific reactions of xylan). Model studies with aldobiuronic acid
7.3 Oxygen Delignification 661
[194,197] revealed that, under alkaline conditions at 80 °C, the degradation rate
was rapid but much slower than that of xylobiose. Under dioxygen alkali conditions,
aldobiuronic acid degraded almost as fast as xylobiose, suggesting that the
substituent at C2 has a low retarding effect on the peeling reaction. The arabinose
substituent at C3 position of softwood xylan is easily cleaved by b-elimination
through the peeling process, and the chain is partly stabilized to xylometasaccharinic
acid end-groups [198].
7.3.2.5.3 Peeling Reactions Starting from Stabilized End-Groups
The formation of aldonic acid end-groups serves as a possible means of stabilizing
the reducing end of the polysaccharide chain. In the presence of dioxygen, arabinonic
acid end-groups (8, Scheme 7.15) are formed that are relatively stable under
typical oxygen bleaching conditions, but degrade rather rapidly above 120 °C
under both oxygen and nitrogen atmospheres (Scheme 7.15). The formed erythronic
acid (11) and gluconic acid (5) end-groups are essentially stable up to 150 °C
[192,199]. Glucitol end-groups, which are more stable against dioxygen-alkali
treatment than the reducing end-groups, are relatively rapidly oxidized at higher
temperatures to arabinose, and are cleaved further by b-elimination [183,199].
Mannitol end-groups are oxidized through the same arabinose intermediates as
the glucitol end-groups. The model-compound methyl-a-d-mannopyranoside was
oxidized more rapidly than methyl-a-d-glucopyranoside giving similar oxidation
products, whereas the yield of furanosidic carboxylic acid was greater for methyla-
d-mannopyranoside. This suggests that the oxidative attack is favored by the cisposition
of the C2 and C3 hydroxyl groups [183]. Furthermore, the threonic acid
end-groups that have been formed during oxidative stabilization of the reducing
end-groups of xylan, show a similar degradation rate to that of arabinonic acid
end-residues.
7.3.2.5.4 Cleavage of the Polysaccharide Chain
Cleavage of the cellulose chain under dioxygen-alkaline conditions has been studied
with simple model compounds such as methyl-4-O-methyl-b-d-glucopyranoside
[200], methyl-b-d-glucopyranoside [201–203] and methyl-b-d-cellobioside
[204]. These compounds represent the inner cellulose units, and result in the formation
of glycolic acid, lactic acid, formic acid, acetic acid and carbon dioxide
[183] and methyl-b-d-glucoside, d-glucose, d-arabinose, d-arabinonic acid, d-erythronic
acid, and d-glyceric acid [204]. Additionally, carboxy-furanosides, methyl-2-
C-carboxy-b-d-pentafuranosides, have been identified as oxidation products of
both glycosides [200] and the corresponding methyl-3-C-carboxy-b-d-pentafuranoside
has also been formed from methyl-b-d-glucopyranoside. The formation of
these furanosidic acids is suggested via benzilic acid rearrangement of a diketo
intermediate [201].
It has been generally suggested that the oxidative peeling of a cellulose chain
proceeds via oxidation of the C2 or C3 hydroxyl group, followed by b-alkoxy-elim-
662 7Pulp Bleaching
ination at C4 [188]. In contrast, the b-elimination is more pronounced when the 4-
hydroxyl-group is substituted (as in cellulose), as is known from model-compound
studies [185,200]. As a result of b-elimination at C1, preceded by oxidation at C2
or C3, the formation of methyl-b-d-glucopyranoside from oxidation of methyl-b-dcellobioside
can be regarded [205]. The acids which clearly result from the oxidative
cleavage of the C1–C2, C2–C3, and C3–C4 linkages have been identified
among the oxidation products [183]. Furthermore, an attack of the C6 hydroxyl
group by a ROS seems very probable [205,206], because methyl-b-d-glucopyranoside
was more rapidly oxidized than methyl-b-d-xylopyranoside [183,206,207] and
methyl-6-deoxy-b-d-glucopyranoside [206]. Because the products formed from
methyl-4-O-methyl-b-d-glucopyranoside under alkaline hydrogen peroxide treatment
corresponded to those from alkaline dioxygen experiments with glycosides,
a common reactive species was inferred [206,208,209].
Cleavage of the xylan chain studied with methyl-b-d-xyloside as a model compound
[207] showed that the oxidation reaction products were similar to those of
methyl-b-d-glucopyranoside, methyl-4-O-methyl-b-d-glucopyranoside and methyla-
d-mannopyranoside suggesting the same mechanism. Although the oxidation
of methyl-b-d-xyloside was slower, the oxidative depolymerization of xylan was
more drastic compared with cellulose, but this may have been due to physical factors
[183,195] such as crystallinity [80,210].
The common reactive oxygen species [206,208,209] noted previously is thought
to be the hydroxyl radical [3,202–204,211]. A possible degradation mechanism for
carbohydrates proposed by Gierer [3] starts with an attack of a hydroxyl radical
(_OH) at the C2 position in the polysaccharide chain (Scheme 7.19), followed by
oxygenation of the resultant carbon-centered radical and elimination of superoxide
anion radical. This leads to the formation of a ketone in the polysaccharide
chain that allows cleavage of the glycosidic linkage by b-elimination (see Section
4.2.4.2, Carbohydrate reactions).
O
O
O
OH
OH
CH2OH
O
O
O
OH
OH
CH2OH
.OH
-H2O -H+
O
O
O
O-
OH
CH2OH
pH > 10 O
O
O
O-
OH
CH2OH
O2
O
O
O
O
OH
CH2OH
-O2
-
H
H
H
H
H H
H H
H
H
H
O2
Fragmentation by β-elimination
("peeling")
Scheme 7.19 Mechanism for oxidative cleavage of carbohydrates
by hydroxyl radicals proposed by Gierer [3].
Guay et al. [204] have examined the proposed mechanism by using computational
methods, which revealed that the step involving elimination of superoxide
7.3 Oxygen Delignification 663
is energetically unfavorable. The highly reactive hydroxyl radical, which has been
generated by using hydrogen peroxide and UV light [Eq. (20)] [204], is capable of
reacting with most organic compounds, typically by hydrogen abstraction [139].
Hydroxyl radicals can react with both hydrogen peroxide and hydroperoxy anions
through Eq. (21) and Eq. (22), producing hydroperoxy radicals and superoxide
anions, respectively [212]. The reaction producing superoxide [Eq. (22)] is significantly
faster than the hydroperoxy radical formation [Eq. (21)] [213]. As shown in
Scheme 7.4, approximately half of the hydrogen peroxide is present as the conjugate
base at a pH of 11.8, and formation of superoxide anions should be more
important. At a lower pH, more hydroxyl radicals will be present to react with the
carbohydrates.
H2O2 _
hm 2_OH _20_
_OH _ H2O2→H2O _ HO_
2 _21_
_OH _ HO_2 →H2O__O_2 _22_
The experiments of Guay et al. with methyl-b-cellobioside have been conducted
with and without hydrogen peroxide at pH 10 and 12, and under oxygen pressure
(about 4 bar) at 90 °C [204]. Beside the predominant degradation products of
methyl-b-glucoside and d-glucose, d-arabinose, d-cellobionic acid, d-arabinonic
acid d-erythronic acid, d-glyceric acid, and glycolic acid, products that have also
been found by other groups [183,192,202,214–219], were identified. Moreover, no
degradation products were found in the control reactions, suggesting that dioxygen,
hydroxide ions, hydrogen peroxide, and hydroperoxy anions are not capable
of degrading carbohydrates without a radical initiator, such as lignin or metal ions
[204]. Due to the lower reactivity of methyl-b-cellobioside at higher pH (12) [202],
and the pH-dependence of the oxygen-species distribution [see Eqs. (21) and (22)],
the extent of the degradation decreased but the overall chemistry was unchanged
[204].
The mechanism of the formation of d-cellobioside is proposed to occur through
a two-step process (Scheme 7.20), starting with a hydroxyl ion attack at the anomeric
carbon displacing the methoxy radical. This radical can then abstract a hydrogen
from hydrogen peroxide or another hydrogen donor, forming a hydroperoxyl
radical and methanol (found experimentally).
The second degradative pathway (Scheme 7.21) is very similar to the first
(Scheme 7.20), except that the cleavage is between two pyranose rings, starting
with a hydroxyl attack at the anomeric carbon displacing d-glucose and methyl bglucoside
oxy radical at C4. The methyl b-glucoside radical then abstracts a hydrogen
from hydrogen peroxide, forming methyl b-d-glucoside [204].
664 7Pulp Bleaching
O
H
O
H
HO
H
H
H OH
OCH3
OH
O
H
HO
H
HO
H
H
H OH
OH
+H2O2
.OH
O
H
O
H
HO
H
OH
OH
H
H
OH
O
H
HO
H
HO
H
H
OH
H
OH
+ CH3O.
HOO+ .
O
H
O
H
HO
H
H
OH
H
OH
OH
O
H
HO
H
HO
H
H
OH
H
OH
CH3OH
Scheme 7.20 Proposed mechanism for cellobiose formation
(redrawn from Guay et al. [204]).
O
H
O
H
HO
H
H
H OH
OCH3
OH
O
H
HO
H
HO
H
H
H OH
OH
.OH
OH
O
H
HO
H
HO
H
H
OH
H
OH
O
H
O
H
HO
H
H
OH
H
OCH3
OH
O
H
HO
H
HO
H
H
OH
H
OCH3
OH
+H2O2
OH
O
H
HO
H
HO
H
H
OH
H
OH
+
Scheme 7.21 Proposed mechanism for formation of methylb-
glucoside and D-glucose (redrawn from Guay et al. [204]).
Guay et al. [204] concluded that their experiments supported the view that hydroxyl
radicals are responsible for the degradation of carbohydrates during oxygen
delignification. Molecular oxygen, hydrogen peroxide, and hydroperoxy anions do
not appear to degrade carbohydrates directly. Previous studies also suggested that
superoxide anions do not degrade carbohydrates [3]. Guay et al. [204] reported that
no experimental evidence has been found to support the reaction mechanism
depicted in Scheme 7.19, though this may be due to different experimental conditions
being used in these studies and in previous research, which employed pulse
radiolysis to generate hydroxyl radicals. Evidence has been published suggesting
that cellulose degradation during pulse radiolysis arises from direct ionization of
the fibers rather than from hydroxyl radicals [216]. Moreover, the mechanism
(cleavage of the glycosidic linkage) shown in Scheme 7.21is supported by the
model-compound study with 1,4-anhydrocellobiotol and cellulose [211].
Details of the mechanisms regarding the involvement of superoxide elimination
[3,130] or no superoxide [204] are discussed – albeit controversially – in the literature,
there appears to be no doubt that the hydroxyl ion attacks the carbohydrates,
thereby starting the degradation reaction [4,220–226].
7.3 Oxygen Delignification 665
The hydroxyl radical (_OH) is one of the most reactive and short-lived of the
ROS, with a lifetime of about 1ns in biological systems [227]. Because of this,
methods used to detect _OH include electron spin resonance (ESR) [228] (using a
spin trap such as dimethylsulfoxide, DMSO), HPLC [229,230], rapid-flow ESR
[231], and fluorescence [232–237]. Two different methods can be used for the
detection of _OH. One is the direct reaction of a probe molecule with .OH. The
other method is to use a scavenger that creates a radical species with a longer lifetime.
The probe molecule then reacts with this radical species [229,234]. Superoxide
detection system have also been developed using ESR spin trapping [238], cytochrome
C [239,240], amperometric detection [241], or a chemiluminescence
assay [242,243], which may help to clarify whether the superoxide anion radical is
formed as a consequence of oxygen treatment. Moreover, a new chromatographic
method to determine hydroperoxides in cellulose [244], and a new colorimetric
method to determine hydroxyl radicals during the aging of cellulose [245] have
been published.
A compilation of important carbohydrate degradation products in dioxygenalkali
delignification processes of kraft and sulfite pulps (glycolic acid, 2,4-dihydroxibutyric
acid, 3,4-dihydroxibutyric acid, isosaccharinic acid, 2-deoxy-glyceric
acid, lactic acid, glyceric acid, formic acid, and acetic acid) according to Sjostrom
and Valttila [246] sums up this section.
7.3.2.6 Residual Lignin–Carbohydrate Complexes (RLCC)
It is well known that lignin and carbohydrates are linked in wood, and that new
linkages are formed during a kraft cook.
During oxygen delignification of pine pulp, the polysaccharides dissolve together
with lignin in the form of lignin–carbohydrate complexes (LCC) [247]. The
structures of these dissolved polysaccharides from pine and birch kraft pulps
treated under oxygen delignification conditions [247], when determined by using
methylation analysis [248], included 1,4-linked xylan, 1,3(,6)-linked and 1,4-linked
galactan, 1,5-linked arabinan, and notable amounts of a 1,3-linked glucan,
whereas the glucose-containing polysaccharide in the pine pulp effluent was 1,3-
linked glucan and not cellulose [247]. From the birch pulp mainly xylan, but also
traces of arabinan, 1,3-linked galactan and 1,4-linked glucan have been removed
[247].
Softwood kraft pulps with a kappa number between 50 and 20 and oxygendelignified
to a similar lignin content (kappa ~6) led to the isolation of LCCs using
a method based on selective enzymatic hydrolysis of the cellulose, and quantitative
fractionation of the LCC [63]. The large majority (85–90%) of the residual lignin
in the unbleached kraft pulp, and all of that in the oxygen-delignified pulps,
when isolated as LCC, was found as one of three types of complex, namely xylan–
lignin, glucomannan–lignin–xylan and glucan–lignin. Most of the lignin was
linked to xylan in high-kappa number pulps, but to glucomannan when the pulping
was extended to a low kappa number. Lawoko et al. [63] reported that, with
increasing degree of oxygen delignification, a similar trend in the delignification
666 7Pulp Bleaching
rates of LCC was observed; thus, the residual lignin was increasingly linked to glucomannan.
From this it was concluded that complex LCC network structures
appear to be degraded into simpler structures during delignification. Two excellent
schemes for the degradation of hemicellulose networks during pulping, and
possible differences in the accessibility of lignin under alkaline conditions between
a xylan–lignin complex and a glucomannan–lignin complex, were
described by Lawoko et al. [63]. Moreover, the chemical structure of the residual
lignin bound to xylan was different from that bound to glucomannan.
Enzymatically isolated residual lignin–carbohydrate complexes (RLCC) from
spruce and pine pulp (kappa number ca. 30) contained 4.9–9.4% carbohydrates,
with an enrichment of galactose and arabinose compared to the original pulp
samples. The main carbohydrate units present in the RLCC were 4-substituted
xylose, 4-, 3- and 3,6-substituted galactose, 4-substituted glucose, while 4- and 4,6-
substituted mannose were assigned to carbohydrate residues of xylan, 1,4- and
1,3/6-linked galactan, cellulose and glucomannan [65]. The comparison of RLCC
of surface material and the inner part of spruce kraft pulp fiber revealed that the
1,4-linked galactan was the major galactan in RLCC of fiber surface material of
spruce kraft pulp, and towards the inner part the proportion of 1,3/6-linked galactan
increased relative to 1,4-linked galactan [65]. It has been suggested that 1,3/6-
linked galactan structures may have a role in restricting lignin removal from the
secondary fiber wall. The RLCC of three different alkaline pine pulps studied by
Lawoko et al. [65] before and after oxygen delignification revealed small differences
in the carbohydrate structures of the unbleached pulps resulting from the
cooking method [conventional kraft pine pulp, a polysulfide/anthraquinone (AQ)
pine pulp and a soda/AQ pine pulp]. These authors found that all RLCC of oxygen-
delignified pulps had more nonreducing ends and less 1,3/ 6-linked galactan
than the corresponding RLCC of the unbleached pulps. Moreover, the oxygendelignified
soda/AQ pulp had a higher ratio of 1,4-galactan to 1,3/6- linked galactan
and shorter xylan residues than the RLCCs of oxygen-delignified conventional
kraft pine pulp and polysulfide/AQ pulps [65]. From the above results and the calculated
degree of polymerization, conclusions were drawn on the possible positions
of lignin–carbohydrate bonds (Fig. 7.27).
These authors concluded that xylan residues were partly bound to lignin via the
reducing end-groups, and that the RLCC contained either long galactan chains or
bonds linking galactans to lignin via the reducing ends [65]. Oxygen delignification
shortened the oligosaccharide chains present in RLCC and removed preferably
the 1,3/6- linked galactan compared to 1,4-linked galactan structures connected
to residual lignin. The RLCC of oxygen-delignified soda/AQ pulp differed
from those of the other two pulps after oxygen- delignification in that it had a
higher ratio of 1,4- to 1,3/6-linked galactan, and shorter xylan residues. However,
even this detailed analysis did not reveal any major differences in the soda/AQ
pulp that could explain its poor bleaching response. It is possible that factors other
than the chemical composition and interactions between lignin and carbohydrates
affect the bleachability of the pulps. These factors may be physical rather than
chemical [65].
7.3 Oxygen Delignification 667
gal-(1 4)-gal-(1 4)-gal-(1 4)-gal-(1 4)-gal-1
n
Cellulose and 1.3-glucan residues
glc-(1 3)-glc-(1 3)-glc-(1 3)-glc-(1 3)-glc-1
1-6
xyl-(1 4)-xyl-(1 4)-xyl-(1 4)-xyl-1
1-3