PSE 406 - Lecture 17 1
Wood ChemistryPSE 406
Lecture 17Chemical Isolation and Analysis II Hemicelluloses and Lignin Analysis
PSE 406 - Lecture 17 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
PSE 406 - Lecture 17 3
How are hemicelluloses separated from cellulose and
lignin?
Generate Holocelulose Remember….in this procedure lignin is
removed through the action of sodium chlorite
PSE 406 - Lecture 17 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%
PSE 406 - Lecture 17 5
How are the hemicelluloses separated
from cellulose?
Cellulose is not soluble in almost any solvents.
What are hemicelluloses soluble in?» NaOH or KOH!!!!!
PSE 406 - Lecture 17 6
Isolation Scheme: Softwoods
Wood
HClO2
Holocellulose
KOH
Hemicellulose Mixture Residue
Soluble Insoluble
PSE 406 - Lecture 17 7
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 solubilize this material
PSE 406 - Lecture 17 8
Isolation Scheme: Softwoods
Wood
HClO2
Holocellulose
KOH
Hemicellulose Mixture Residue
Cellulose Crude Glucomannan
Soluble Insoluble
Ref: Timell: TAPPI 44, 88-96 1961
NaOH/Borate
SolubleInsolubleText
PSE 406 - Lecture 17 9
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?
PSE 406 - Lecture 17 10
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.
CHO
HHO
HHO
OHH
OHH
CH2OH
PSE 406 - Lecture 17 11
Isolation Scheme:Softwoods
Hemicellulose Mixture
Ba(OH)2
Soluble Insoluble
Mixture Crude Galactoglucomannan
Ba(OH)2
Galactoglucomann
Soluble InsolubleBa(OH)2
GalactoglucomannanDiscard
Soluble Insoluble
Ref: Timell: TAPPI 44, 88-96 1961
ArabinoGlucuronoxylan
Text
PSE 406 - Lecture 17 12
How is the composition of individual hemicelluloses
determined?
How can hemicelluloses be broken down into individual sugars.» Acid hydrolysis of glycosidic linkages.» Enzymatic hydrolysis
PSE 406 - Lecture 17 13
Chromatography
gas
liquid
Sample
Column
Packing material Detector
UV, RIFID,MS, Etc.
Compounds separate through:adsorption, size exclusion, boiling points
Time
The individual sugars are quantified using gas or liquid 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.
PSE 406 - Lecture 17 14
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
1. Maltotriose 2. Cellobiose 3. Glucose 4. Mannose 5. Arabinose 6. Adonitol 7. Arabitol 8. Xylitol
PSE 406 - Lecture 17 15
Lignin-Important Questions
How much lignin is in a sample?» Wood» Plant Material» Pulp
PSE 406 - Lecture 17 16
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 don’t
have a large sample to work with)» Acetyl bromide
PSE 406 - Lecture 17 17
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
PSE 406 - Lecture 17 18
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.
PSE 406 - Lecture 17 19
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 >5% 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.
PSE 406 - Lecture 17 20
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.
PSE 406 - Lecture 17 21
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.
PSE 406 - Lecture 17 22
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.
PSE 406 - Lecture 17 23
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
PSE 406 - Lecture 17 24
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.