PSE 406 - Lecture 171 Wood Chemistry PSE 406 Lecture 17 Chemical Isolation and Analysis II...

PSE 406 - Lecture 17 1

Wood ChemistryPSE 406

Lecture 17Chemical Isolation and Analysis II Hemicelluloses and Lignin Analysis


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


How are hemicelluloses separated from cellulose and

lignin?

Generate Holocelulose Remember….in this procedure lignin is

removed through the action of sodium chlorite


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%


How are the hemicelluloses separated

from cellulose?

Cellulose is not soluble in almost any solvents.

What are hemicelluloses soluble in?» NaOH or KOH!!!!!


Isolation Scheme: Softwoods

Wood

HClO2

Holocellulose

KOH

Hemicellulose Mixture Residue

Soluble Insoluble


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


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


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?


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


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


How is the composition of individual hemicelluloses

determined?

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


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.


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


Lignin-Important Questions

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


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


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


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.


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.


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.


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.


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.


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


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.

PSE 406 - Lecture 171 Wood Chemistry PSE 406 Lecture 17 Chemical Isolation and Analysis II...

Documents

Transcript of PSE 406 - Lecture 171 Wood Chemistry PSE 406 Lecture 17 Chemical Isolation and Analysis II...