Wood Chemistry PSE 406

1
Wood ChemistryPSE 406

• Lecture 17
• Chemical Isolation and Analysis II Hemicelluloses
  and Lignin Analysis

2
Class Agenda

• How are hemicelluloses separated from cellulose
  and lignin?
• How are individual hemicelluloses separated?
• How is the composition of individual
  hemicelluloses determined?
• How are the linkages determined?
• Lignin analysis

3
How are hemicelluloses separated from cellulose
and lignin?

• Generate Holocelulose
• Remember.in this procedure lignin is removed
  through the action of sodium chlorite

4
Cellulose Isolation

• A Tappi Standard procedure for cellulose
  isolation from holocellulose is as follows
• Extract holocellulose with 5 and then 24 KOH to
  remove hemicelluloses. The remaining material is
  termed alpha-cellulose
• This results in cellulose of reduced molecular
  weight and some yield loss. Typical recoveries
  are 40-60

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How are the hemicelluloses separated from
cellulose?

• Cellulose is not soluble in almost any solvents.
• What are hemicelluloses soluble in?
• NaOH or KOH!!!!!

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Isolation Scheme Softwoods
Wood
HClO2
Holocellulose
KOH
Soluble
Insoluble
Hemicellulose Mixture
Residue
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What is in the residue?

• Cellulose
• It is not soluble in much of anything
• Galactoglucomannan (not the water soluble)
• It turns out that this hemicellulose is not all
  that alkali soluble at this level of KOH
• It takes the addition of NaOH and borate to
  solublize this material

8
Isolation Scheme Softwoods
Wood
HClO2
Holocellulose
KOH
Soluble
Insoluble
Hemicellulose Mixture
Residue
NaOH/Borate
Soluble
Insoluble
Text
Crude Glucomannan
Cellulose
Ref Timell TAPPI 44, 88-96 1961
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What makes up the rest of the hemicellulose
mixture?

• Xylans
• Galactoglucomannans (water soluble)
• Maybe some pectins, a little glucans, and who
  know what else
• We are mainly concerned with the top two.
• How do we separate the xylans from the
  galactoglucomannans?

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Barium

• Because of the orientation of the C2 and C3
  hydroxyl groups in mannose, it will form an
  insoluble complex with barium ions.
• Therefore the addition of Ba(OH)2 will cause
  glucomannans to precipitate out of solution

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Isolation SchemeSoftwoods
Hemicellulose Mixture
Ba(OH)2
Soluble
Insoluble
Mixture
Crude Galactoglucomannan
Ba(OH)2
Ba(OH)2
Soluble
Insoluble
Soluble
Insoluble
Galactoglucomann
Arabino Glucuronoxylan
Galactoglucomannan
Discard
Text
Ref Timell TAPPI 44, 88-96 1961
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How is the composition of individual
hemicelluloses determined?

• How can hemicelluloses be broken down into
  individual sugars.
• Acid hydrolysis of glycosidic linkages.
• Enzymatic hydrolysis

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Hemicellulose Analysis

• The individual sugars are quantified using gas or
  liquid chromatography.
• Often the individual components require
  derivitization before analysis.
• Other analytical techniques are used to
  positively identify components

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Chromatography

• Chromatography is the process in which chemicals
  are transported by liquid or gas past a
  stationary phase. The individual components are
  attracted to different degrees to the stationary
  phase and thus travel at different speeds and are
  separated.

Sample
UV, RI
FID,MS, Etc.
Column
gas
Detector
Packing material
liquid
Compounds separate through adsorption, size
exclusion, boiling points
Time
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Derivitization

• Gas Chromatography - Chemicals to be analyzed
  must be volatile Sugars and uronic acids are not
  volatile.
• Blocking hydroxyl groups will make chemicals
  volatile.
• Derivitization procedures
• Methylation
• Acetylation
• Silylation

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Chromatography

• Once the components are separated, they are
  detected by a number of different types of
  systems.
• A chromatograph is produced in which the
  components are seen as peaks.
• Quantification is accomplished by measuring the
  peak area

Poorly reproduced figure from Supelco
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Determination of Linkages

• How is it possible to determine how the
  individual sugars are linked?
• Methylation of the free hydoxyl groups
• Acid Hydrolysis
• Chromatographic determination of products

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Pyranose? Furanose?

• Mild hydrolysis of hemicellulose results in the
  presence of monomers, dimers, trimers, etc. of
  the hemicelluloses.
• These materials can be separated by
  chromatography and compared to known dimers.

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Lignin-Important Questions

• How much lignin is in a sample?
• Wood
• Plant Material
• Pulp

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Quantification of Lignin

• Wood and non-woody materials
• Acid Insoluble lignin (along with acid soluble
  lig)
• Pulp
• Kappa number
• Other non woody materials (or I dont have a
  large sample to work with)
• Acetyl bromide

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Acid Insoluble Lignin (Klason)

• Goal is separate carbohydrates from lignin
• Lignin condenses (reacts with lignin) to become
  very water insoluble (it becomes very large)
• Acid cleaves glycosidic linkages in carbohydrates
  forming individual sugars.
• Sugars dissolve in water (acid) and lignin does
  not

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Klason Procedure

• Wood meal (or pulp) is treated with 72 H2SO4 for
  2 hours. The material is then diluted to 3
  H2SO4 and then boiled for 4 hours.
• These two steps dissolves the carbohydrates
  leaving chunks of lignin floating in the acid
• The lignin is filtered, washed and weighed.

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Acid Soluble Lignin

• A certain percentage of the lignin is soluble in
  the Klason lignin procedure
• This amount is very small with softwoods but
  higher gt5 in hardwoods and grasses.
• The filtrate from the Klason procedure is
  collected and the UV absorbance is checked.
• Lignin absorbs UV light, sugars do not
• The amount of lignin that is soluble is
  determined through comparing the UV absorbance to
  a standard.

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Lignin Content of Pulp

• Pulps contain only small amounts of lignin so a
  different (and quicker) method is used the kappa
  number.
• This procedure is based upon the fact that lignin
  reacts very quickly with KMnO4 while
  carbohydrates (mostly) react very slowly.

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Kappa Number Procedure

• Pulp is dissolved in water and reacted with a
  KMnO4 solution for 10 minutes under very
  controlled conditions.
• The goal is to consume 50 of the KMnO4 in this
  time.
• Excess KMnO4 is consumed with potassium iodide
  forming I2 (iodine).
• The iodine is titrated with sodium thiosulfate to
  a starch endpoint.

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Kappa Number Information

• This method is typically used with pulp
  containing low amounts of lignin (chemical
  unbleached pulp).
• It was found about 15 years ago that hexenuronic
  acids formed during kraft pulping from uronic
  acids consume KMnO4 thus giving false kappa
  numbers (if based only on lignin).
• A typical kappa number for an unbleached kraft
  pulp is around 20.

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Kappa to Klason

• Correction factors have been developed to convert
  kappa numbers to Klason lignin. These factors
  are different for different processes and
  species.
• Kraft pulps Klason kappa number 0.15
• Sulfite pulps Klason kappa number 0.167
  (or 0.187 depending on who did the work).
• Kappa number 20 3 lignin

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Acetyl Bromide Procedure

• This procedure was developed to measure lignin
  content in small samples.
• Samples are dissolved by reaction with acetyl
  bromide (with a little perchloric acid) in acetic
  acid.
• The solution is analyzed by UV (remember lignin
  absorbs, carbohydrates do not).
• The amount of lignin in the sample is determined
  by comparison against standards.
• Every material seems to have a different standard
  number.