Lignin. delignification. Outline. kraft pulping. Reaction of non-phenolic structures

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1 utline Lignin reactions in bulk delignification Aim Disintegrate fibres High yield Low lignin content Pulping conditions EATIN KINETIS initial, bulk and residual delignification Time Temperature ph eaction of non-phenolic Sulphate pulping, delignification Lignin Initial delignification Bulk delignification Phenolic arbonyl Non-phenolic L complexes Initial delignification Peeling Hydrolysis of esther Peeling Polysaccharides Bulk delignification Generation of HexA Hydrolysis of glycosidic linkages Hemicelluloses Degradation of ellulose Extractives L linkages Kraft pulping alkaline method Pulping chemicals: NaH and Na S Nucleophilic reactions (H - and HS - ions) Three pulping stages I Initial delignification impregnation phase temperature < 0 º HS - ion concentration important high carbohydrate losses (hemicelluloses) II Bulk delignification temperature >0 º (0-80 º) H - and HS - concentrations important III esidual delignification High carbohydrate losses (cellulose) I II III eactions of wood constituents during kraft pulping LIGNIN Degradation cleavage of ether linkages demethoxylation ondensation reactions formation of new carbon-carbon linkages Formation of chromophores HEMIELLULSES AND ELLULSES Degradation and stabilization peeling reaction alkaline hydrolysis deacetylation formantion of hexenuronic acid groups of xylan eadsorption of xylan on the fibres EXTATIVES Hydrolysis of fatty esters Evaporation => Tall oil soaps and crude turpentine degradation = hajoaminen ; cleavage = lohkeaminen ; deacetylation = deasetyloituminen ; peeling = päätepikkoutuminen ; tall oil = mäntyöljy ; crude turpentine = tärpätti

2 Typical composition of softwood and hardwood kraft pulps (Sjöström, E., Tappi 0(9): (9)) Percentage of original wood Pine Birch omponents Wood Pulp Wood Pulp Lignin - terminology Native lignin = lignin in wood Pulp yield Lignin ellulose Kraft lignin = dissolved and degraded lignin in black liquor Glucomannan Xylan Pitch esidual lignin (L) = remains of lignin in the kraft pulp after the cooking thers - - Linkages between phenylpropane units of native lignin (Sjöstr ström, E., Wood hemistry Fundamentals and Applications,, nd ed. Academic Press, 99) Linkage type β Ο α Ο β Ο β β β Softwood % Hardwood % 0-8 γ β α β - α - β β - -- β β Degradation and fragmentation of lignin in alkaline pulping arbon-carbon (-, β-β, β- and β-) linkages are usually resistant to chemical attacks. Degradation is limited mainly to cleavages of ether units. α-- linkages β-- linkages The amount of α- and β-- linkages between phenyl propane units of lignin is large. 8

3 H H eactions of lignin during alkaline pulping Me Phenolic unit H H Me Me Me Non-phenolic unit H Me arbonyl unit Me I Initial delignification Phenolic and carbonyl react 0 % of lignin degrades due to these reactions in the initial stage II Bulk delignification Main part of lignin is removed during the bulk delignification non-phenolic react Slow reaction Bulk delignification First the focus is on bulk delignification eason: reactions are more simple in the main pulping phase than in the initial phase of delignification phenolic = fenoli- ; carbonyl = karbonyyli ; 9 0 eactions of non-phenolic leavage of β-- linkage leavage of β-- linkage Non-phenolic lignin In alkaline media hydroxyl groups in α or γ carbon are ionized Ionized hydroxyl group acts as a nucleophile attacking β-carbon Formation of an epoxide leavage of β-- linkage Degradation of lignin α aryl ether (α--) linkages are stable in non-phenolic H Me Me H H α γ β Me Me H K ion Me Me k H Me Me hydroxyl = hydroksyyli ; ionized = ionisoitunut

4 leavage of β-aryl ether (β--) linkage Why does this reaction happen? Usually ethers are relatively inert substances However the lone pairs of oxygen are a source of reactivity Increased polarity of the - bond makes the neighbouring β-carbon atom sensitive to nucleophilic attacks neighbouring benzene ring is more stable due to the resonance structure H H α β Non-bonding lone pairs of electrons γ Ö Me Me eactions of non-phenolic leavage of β-- linkage Since reaction happens via ionization the rate of reaction is dependent on the degree of ionization = ionization of hydroxyl groups in α- orγ-carbon Ionization equilibrium: K ion =[ - ]/[H][H - ] Degree of ionization: x ion =[ - ]/([H]+[ - ])=K ion [H - ]/(+K ion [H - ]) ate of reaction is therefore: dc/dt=-kx ion c=-kck ion [H - ]/(+K ion [H - ]) ionization = ionisoituminen ; degree of ionization = ionisoitumisaste ; rate = nopeus ; ionization equilibrium = ionisoitumistasapaino eactions of non-phenolic eaction kinetics: Transition state theory eactions of non-phenolic rate constant I) k = (kt/h)e S/ e - H/T II) k = (kt/h)e - G/T where where k is Boltzmann constant G is Gibbs energy T is temperature (K) h is Planck s constant is gas constant S is entropy of activation where H is entalphy of activation E is energy of activation III) k = (kt/h)e S/ e -E/T Entropy of intramolecular reaction is large since reactive component are constantly close to each other However quantities of H and E are not quite the same and the relationship depends on the type of reaction Activation energy (E) of a reaction is the amount of energy needed to start the reaction. It is the minimum energy needed to form a complex during a collision between reactants E depends on chemical properties of reactants In nucleophilic substitution: Nucleophilicity of nucleophile nucleophilicity is a measure of a reagent s ability to cause a substitution reaction hemical character of leaving group a good leaving group is a very weak base => the conjugated acid is a strong acid rate constant = nopeusvakio ; transition state theory = siirtymätilateoria, intramolecular = molekyylinsisäinen ; energy of activation = aktivoitumisenergia

5 Energy of activation (E) S N reaction (bimolecular nucleophilic substitution) Energy of activation (E) S N reaction (unimolecular nucleophilic substitution) Transition state Transition states Potential energy (E) eactants H E Products Potential energy (E) eactants E E H Products Progress of reaction Progress of reaction 8 eactions of non-phenolic oncurrent reactions - demethylation S - is stronger nucleophile than - and therefore it can demethylate methoxy groups of lignin during bulk delignification Degree of demethylation is appr. 0% Formation of highly volatile and malodorous sulphur compounds eactions of non-phenolic oncurrent reactions S N of HS - HS - is able to cause a nucleophilic substitution in β linkage (cf. ionization mechanism) The leaving group is similar in both reactions However methyl group is sterically less hindered than β-carbon leavage of β linkages is a slow reaction H H Me Me - SH H H Me MeS - HS - H H Me Me H H SH Me H Leaving group demethylation = metyyliryhmän poistaminen, 9 cf.= confer (lat.) = vrt; sterically hindered = steerisesti estynyt 0

6 ate of reactions cleavage of β-- linkage ate of reaction eaction... H large small large S large small small eactions of non-phenolic ate of hydrolysis of β-- linkages Influence of ionization (main reaction) and S N reaction (concurrent reaction) on hydrolysis of β-- linkages and degradation of lignin during bulk delignification:. HS - H. H - H. Me H Me dc/dt = -c { kk ion [H - ]/(+K ion [H - ])+ k [HS - ]} Me Me Summary of key points Ionization (alkaline induced ionization of α or γ hydroxyl groups) leavage of the β-aryl ether linkage (degradation of lignin) Substitution (intermolecular or intramolecular) H H Me Me Demethylation (Formation of malodorous, volatile compounds; mercaptans)