Pulping and BleachingPSE 476Lecture #8Kraft Pulping: Early Reactions andKraft Pulping Lignin Reactions

PSE 476: Lecture 7

AgendaBasic Chemical Pulping DiscussionLoss of Components During Kraft PulpingReactions in the Early Portion of the CookSaponificationNeutralization of ExtractivesInitial Lignin DiscussionKraft Pulping Lignin Reactions

PSE 476: Lecture 7

Wood ChemistryFor the students who do not recognize this molecule (did not take PSE 406), there is a short appendix at the end of this lecture to help you. Additionally, the class notes are available for review.

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PulpingThe goal of kraft pulping is to remove the majority of lignin from chips (or other biomass) while minimizing carbohydrate loss and degradation.Removal of lignin is accomplished through treatment of raw material with NaOH and Na2S at elevated temperatures.

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The Goal of Lignin Reactions in Kraft PulpingKraft PulpingSolubleFragmentsDuring kraft pulping, thelarge insoluble ligninmolecules are converted into small alkali soluble fragments.Carbohydrates are alsodegraded during pulping

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Yield of Wood Components After Kraft PulpingNotes* Yields = % of wood (pulp) components

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Initial Reactions: Low TemperatureCarbohydratesAlkaline hydrolysis of acetyl groups on xylan (see next slide).Removal of certain soluble carbohydrates.Certain galactoglucomannans.Arabinogalactans.ExtractivesAlkaline hydrolysis of fats (saponification), waxes, and other esters.Neutralization of extractives.There are a number of acidic extractives which consume NaOH.

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Alkaline Hydrolysis:Example Using Acetyl GroupsEsters are cleaved in alkaline solutions through hydrolysis reactions forming carboxylic acids and alcohols.Hydrolysis of acetyl groups occurs readily in alkaline solutions.Reaction occurs rapidly even at room temperature.Reaction consumes alkali.

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Saponification of Fats(Review slide from PSE 406)Treatment of fats with alkali converts them to fatty acids and glycerol through saponification.Once again this reaction consumes part of the alkalicharge.

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Acidic Extractive SpeciesResin AcidsLignansMonoterpenoids

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Consumption of AlkaliImpregnation zone

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Where Does All the Alkali Go?Spruce wood was soda pulped at a NaOH concentration of 19% (as Na2O).12.5% (or 66% of alkali) consumed to lower lignin content of wood to 2.8%.2.3-3% used in dissolution of lignin.1.3% for hydrolysis of acetyl and formyl groups.8.2-8.9% for neutralization of acidic products Some extractivesMostly carbohydrate degradation products (discussed later).

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Lignin Removal during Kraft Pulping This chart shows the lignin removal rate during a kraft cook. It is important to note that the rate of lignin removal is temperature dependent. What does this fact tell us about of lignin removal in this slide?

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Lignin RemovalIn the last slide, the rate of lignin removal appears to be linear over a large portion of the cook; even as the temperature increases.This means that lignin removal in the first portion of the cook is easier than as the cook proceeds.Lignin removal has been broken into three sections:Initial Phase (fast lignin removal reactions)Bulk Phase (slow lignin removal reactions)Residual Phase (really slow lignin removal)

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Kraft Pulping:Reaction Phases of Lignin RemovalBulk PhaseInitial PhaseImpregnation zoneResidual Phase70C70C137C170 CNotes

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Kraft Pulping Lignin Reactions

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Dissolution of LigninIn review the goal in kraft pulping is the cleavage of lignin into alkali soluble fragments.Cleavage is affected by the following factors:Type of linkagePresence of free phenolic hydroxyl groupFunctional groups (benzyl hydroxyl, carbonyl)Type and amount of nucleophiles (OH-, HS-)Reaction temperatureWe are going to first look at the chemical mechanisms of the reactions and then the kinetics.

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Sites for Nucleophilic AttackThe cooking chemicals used in kraft cooking (NaOH and Na2S: OH- and HS-) both act as nucleophiles* because of their free pair of electrons.Sites for nucelophilic attack in lignin are those areas of reduced electron density (partially positive sites).

* Notes

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Formation of Quinone MethideQuinone Methide(very reactive)These arrows indicate that a pair of electrons are moving

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Formation of Nucleophilic Attack SitesA free phenolic hydroxyl group is needed for the formation of a quinone methide.The oxygen of the quinone group (carbonyl) attracts the electron density on the double bond thus making the carbon more positive. This in turn shifts the electron densities of the other bonds on this conjugated system.

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Two Additional Examples of Nucleophilic Addition SitesConiferaldehyde type structuresThis structure contains an a-ketogroup. Notice that a free phenolichydroxyl groups is not needed!Notes

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Important Issues!!!!When learning about alkaline pulping mechanisms, remember to ask yourselves these questions!Which reactant are we concerned with: OH- or HS-?Does the lignin structure have a free phenolic hydroxyl group or is it etherified?Which linkage are you hoping to cleave?Is there an a-carbonyl or benzyl hydroxyl?

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Reactions of -O-4 LinkagePhenolic and EtherifiedIn kraft pulping, -O-4 linkages do not react with HS-Reaction with OH-Phenolic Units: -O-4 are very rapidly cleaved by alkali. This is the fastest of the lignin degradation reactions. (Will occur at low temperatures)Etherified Units: -O-4 linkages are stable (no reaction).Please work out reaction mechanism.

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Reactions of b-O-4 Linkages: Free Phenolic Hydroxyl/Benzyl HydroxylReaction with OH- aloneThe ether linkage is not cleaved; a vinyl ether structures is formed.Vinyl ether linkages are difficult to cleave.Reaction with HS- (OH- present)HS- is a very strong nucleophile which cleaves the -O-4 linkage.Reaction is very rapid even at lower temperatures.

* Mechanisms on following pages

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Kraft Reactions of b-O-4 Linkage (Free Phenolic Hydroxyl)Vinyl EtherFormaldehydeNotice that the b-O-4 bond isnot cleaved.Notes

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AppendixBasic Wood Chemistry

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What is the Chemical Makeup of Wood?* Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis

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CelluloseVery long straight chain polymer of glucose (a sugar): approximately 10,000 in a row in wood. Cotton is nearly pure cellulose.Think about a very long string of beads with each bead being a glucose molecule.Cellulose molecules link up in bundles and bundles of bundles and bundles of bundles of bundles to make fibers.Uncolored polymer.

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HemicellulosesBranched little uncolored sugar polymers (~ 50 to 300 sugar units)Composition varies between wood species.5 carbon sugars: xylose, arabinose.6 carbon sugars: mannose, galactose, glucose.Uronic Acids: galacturonic acid, glucuronic acid.Acetyl and methoxyl groups (acetic acid & methanol).Major hemicelluloses:Xylans - big in hardwoodsGlucomannans: big in softwoodsMinor hemicelluloses: pectins, others.

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Xylan Structure4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly4-O-Me--D-Glc -L-Araf

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Glucomannan StructureThere are different structured glucomannans in hardwoods and softwoods (and within softwoods)Glucomannans are mostly straight chained polymers with a slight amount of branching. The higher the branching, the higher the water solubility.

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LigninPhenolic polymer - the glue that holds the fibers together.Lignin is a very complex polymer which is connected through a variety of different types of linkages.Colored material.

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Lignin NomenclaturePhenylpropane UnitC9 }Common NamesSide ChainNotes

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Lignin Reactions:Linkage FrequenciesNotes

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Linkage

Softwood

%

Hardwood

%

-O-4

50

60

-O-4

2-8

7

-5

9-12

6

5-5

10-11

5

4-0-5

4

7

-1

7

7

-

2

3

ExtractivesThe term extractives refers to a group of unique chemical compounds which can be removed from plant materials through extraction with various solvents.Typically these chemicals constitute only a small portion of the tree (