Outline

INCREASE BRIGTHNESS PURIFY PULP AIM Bleaching chemicals INCREASE COLOUR STABILITY LOW ENVIRONMENTAL IMPACT

CHROMOPHORES REMOVABLE GROUPS RESIDUAL LIGNIN HEXA

Lecture 10 CHROMOPHORES SELECTIVITY HEMICELLULOSES CELLULOSE

REACTION KINETICS BLEACHING TIME PROCESS CONDITIONS REACTIVITY TEMPERATURE PH

BLEACHING SEQUENCES

BLEACHING AGENTS

NON-PHENOLIC STRUCTURES ELECTROPHILIC HEXA SURFACE LIGNIN (O2)

REACTIONS NUCLEOPHILIC CARBONYL STRUCTURES PHENOLIC STRUCTURES RADICAL HEXA (CLO2)

Bleaching agents Classification of bleaching agents

Designation Bleaching agent Formula

C Chlorine Cl2 RADICALS ELECTROPHILES NUCLEOPHILES - - - O Oxygen O2 ( HO , HOO ) O2 Cl2 HO2 (H2O2) - - D Chlorine dioxide ClO2 (Cl2, HOCl, ClO ) ClO2 HOCl ClO

Z Ozone O3 (H2O2) O3 Peroxyacids - - P Peroxide HO2 (HO ) Peroxyacids Paa Peracetic acid AcOOH E Alkali (extraction) HO- Reactions involve a transfer of Reactions involve a transfer of X Enzyme xylanase an unpaired electron (i.e radical) a pair of electrons + and these radicals are oxidants A Acid H3O • electrophiles accept a pair of electrons (oxidants) Q Chelant EDTA, DTPA • nucleophiles donate a pair of electrons (reductants) peracetic acid = peretikkahappo; chelate = kelaatti

1 Electrophiles Electrophiles

• According to IUPAC ”an electrophile is a • Electrophilic bleaching agents react with reagent that forms a bond to its reaction all unsaturated structures: partner by accepting both bonding electrons from that reaction partner” – Phenolic lignin structures – Non-phenolic • Main electrophilic bleaching agents are – Hexenuronic acid groups

– chlorine (Cl2), hypochlorous acid (HOCl), – Extractives peracetic acid (HOOAc) and ozone (O3) hypochlorous acid = alikloorihapoke

Reactivity of electrophilic bleaching Basic reaction mechanism of Cl2, agents HOCl and peroxyacids

Xσ+ -Yσ- X Compound Charge Leaving Acid X distribution group + HY + - + O3 O-O-O H2O OH Cl Clδ+-Clδ- Cl- HCl slow reaction fast reaction

Reactivity 2 HOCl HOδ+-Clδ- Cl- HCl • X is a electrophilic part –Cl(Cl2) or HO (AcOOH and HOCl) - δ+ δ- - 2- - HOOMoO3 HO - OMoO3 MoO4 HMoO4 • Y is a leaving group - δ+ δ- - 2- - HOOSO3 HO - OSO3 SO4 HSO4 –Cl(Cl2 and HOCl) or AcO (AcOOH) HOOAc HOδ+-δ-OAc AcO- AcOH • HCl is a stronger acid than AcOH HOCl is a better electrophile than AcOOH Affects bleaching conditions (temperature and time)

2 Reactions of electrophiles (Cl2, HOCl and Reactions of electrophiles (O3) and peroxyacids) and non-phenolic lignin structures unsaturad structures

OH OH OH

HO HO O • Formation of a otsonide, which is a OLignin OLignin OLignin X X I nucleophile + Depolymerization • Reactions of ozone are controlled by OMe OMe OMe of lignin OLignin OLignin OLignin diffusion Lignin Lignin

Lignin reactions are fast

I I + Chlorination or H OMe OMe hydroxylation • Depolymerization of lignin happens via X X OMe OLignin OLignin OLignin hydrolysis of ester Lignin Lignin Lignin Lignin Depolymerization

oxidation III of lignin + OH -MeOH Cleavage of OMe OMe OMe O X OLignin X OLignin -LigninOH methanol OLignin O

Reactions of electrophiles (O3) and unsaturad structures Nucleophiles

Lignin Lignin Lignin • According to IUPAC ”a nucleophile is a reagent that forms a bond to its reaction partner by + OMe donating both bonding electrons” OMe OMe O O LigninO O O + OLignin O O OLignin O O • Nucleophilic group of nucleophilic bleaching

O Lignin Lignin agents is oxygen atom

OH O OMe -MeOH O • Nucleophiles react with carbonyl structures of -LigninOH O O LigninO HO chromophores, such as quinones and stilbene quinones (selective reaction)

3 Reactivity of nucleophilic bleaching Reactions of nucleophilic bleaching agents agents

Bleaching agent Leaving group Lignin Lignin Lignin Lignin

- RO H2O Hypochlorite ClO- Cl- OH OH OH R -R O O O R O - - HO OH O Peroxyacids HOOSO3 SO4 O O HO Reactivity - - HOOMoO3 MoO4 • two-step nucleophilic reaction with conjugated carbonyl structures HOOAc AcO- – Step 1: nucleophilic addition – Step 2: cleavage of a leaving group - Hydrogen peroxide H2O2 HO -, - - - - – R is the leaving group (Cl SO4 ,, MoO4 ,AcO or HO )

Radicals Radicals

• IUPAC: ”a radical is a molecule possessing an • New radicals are also formed during bleaching unpaired electron” – Oxygen delignification . . • Radical bleaching agents: • hydroxy (OH ) and peroxy (HO2 ) radicals . Chlorine dioxide, ClO2 (O=Cl-O ) –ClO2 bleaching . . Reactivity •Cl. radicals Oxygen, O2 ( O-O ) • Radical bleaching agents react mainly with •Cl. and OH. radicals are very reactive phenolic lignin structures – reactions with lignin and carbohydrate structures

–ClO2 also reacts with HexA • oxidation and depolymerization of cellulose chains

4 Reaction mechanism, O2 Reaction mechanism, ClO2

R

R R R O R OR' O R OH OR' OR' R O R R OR' OH H O OR' O OCl O OR' ClO2 OH R R R O -ClO- O OR' - OR' OR' O2 R - R HO R R O R O -HO - O R O2 2 OR' OR' OO- O O ClO2 OH OR' O - O O2, -O2 OR' - OR' OCl -ClO2 O O - O O2, -O2 R R OR OR' OO. HO- ClO2 R R - O -ClO2 O ClO2 OR OR ClO2 O O OR' OR - - HO , ClO3

Auxiliary bleaching chemicals Bleaching sequences

Chemical Function • A single bleaching treatment or stage is not sufficient to remove all chromophores from pulp ACID Acid hydrolysis of HexA ⇒ bleaching sequence

ALKALI Solubilize lignin residue after • Order and type of bleaching stages has an acid stage effect on CHELANT Removal of transition metals – consumption of bleaching chemicals (Cu, Fe, Mn) which degrade – amount of organic material dissolved during peroxides bleaching XYLANASE ENZYME Degradation of LC complexes on the surface of fibres

5 Some examples of bleaching sequences for kraft pulps Bleaching sequences

• Conventional sequences • Which one of these bleaching sequences

–CD-EO-D-E-D is better and why? – C-E-D-EP-D • Elemental Chlorine Free (ECF) a) O – A – Z – Q/P – Paa

– O-D-EO-D-E-D

–D-EO-D-E-D • Total Chlorine Free (TCF) b) O –Z –A –Q/P –Paa – O-A-Z-Q/P-Paa

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