Lecture 2
Lignin Isolation and
Acid Hydrolysis
Lignin
• Large macromolecule formed by various
types of substructures
Lignin
• In nature is very resistant to degradation.
• It is bonded in complex and various types of
carbohydrates.
LIGNIN
Lignin may be defined as an amorphous,
polyphenolic material arising from
enzymatic dehydrogenative polymerization
of three phenylpropanoid monomers,
namely, coniferyl alcohol, sinapyl alcohol,
and p-coumaryl alcohol. It is most
commonly derived from wood, and it is an
integral part of the secondary cell walls of
plants and some algae.
STRUCTURE
Lignin is a cross-linked racemic macromolecule
with molecular masses in excess of 10,000 u. It
is relatively hydrophobic and aromatic in nature.
The degree of polymerization in nature is difficult
to measure, since it is fragmented during
extraction and the molecule consists of various
types of substructures which appear to repeat in
a haphazard manner. Different types of lignin
have been described depending on the means of
isolation.
STRUCTURE
Structure of Lignin
Microscopy Image
LIGNIN ISOLATION
Several different enzymatic, chemical and
mechanical methods have been developed
for
the isolation of lignin from wood and pulp.
Some of these are;
1.Lignin Isolation by Enzymatic Hydrolysis of
Pulp.
2.Lignin Isolation by Acid Hydrolysis of Pulp.
Lignin Isolation by Enzymatic
Hydrolysis of Pulp
This procedure is based on selective
hydrolysis and dissolution of carbohydrates
in pulp by commercial cellulolytic enzymes,
leaving lignin behind as an insoluble
residue.
There are several drawbacks of utilizing
the enzymatic hydrolysis method for
isolating residual lignin.
Lignin Isolation by Enzymatic
Hydrolysis of Pulp
All residual lignin isolated with this
procedure contain some carbohydrates
that cannot be removed by prolonged and
repetitive enzymatic treatments or by
purification methods that are commonly
employed for milled wood lignins.
Lignin Isolation by Acid Hydrolysis
of Pulp
This technique is frequently employed
since it is relatively rapid and yields a lignin
of relatively high purity, free of
carbohydrate contamination.
Experimental Procedure for Acid
Hydrolysis Isolation of Lignin
1. Pulps are washed thoroughly with deionized
water and air dried prior soxhlet extraction with
acetone for 24 hours.
2. A 300mL three necked round-bottom flask
equipped with Friedrich’s condenser is charged
with 30.0 g of dry pulp.
3. The pulps are then refluxed with 0.100 N HCl in
p-dioxane at 4.00% consistency for two hours
under an argon atmosphere.
Experimental Procedure for Acid
Hydrolysis Isolation of Lignin
4. The solution is cooled and then filtered through
a coarse-sintering glass Buchner funnel
covered with filter paper to avoid plugging.
5. The resulting filtrate is passed through a fine
sintered glass Buchner funnel packed with a
celite® filter aid to remove fines.
Experimental Procedure for Acid
Hydrolysis Isolation of Lignin
6. The lignin/water/dioxane mixture is neutralized
with aqueous saturated solution of sodium
bicarbonate to an apparent pH value of 5.0-5.5
and concentrated at approximately under 35oc
under reduced pressure to approximately 10% of
the original volume.
Experimental Procedure for Acid
Hydrolysis Isolation of Lignin
7. Deionized water is added and the mixture is
concentrated again under reduced pressure to
remove the last traces of p-dioxane.
8. The resultant aqeous lignin solution is
transferred to a 1L beaker, diluted with deionized
water to approximately 750mL, acidified to a pH
of 2.0-2.5, and transferred to a 250mL capacity
centrifuge bottles.
Experimental Procedure for Acid
Hydrolysis Isolation of Lignin
9. The centrifuge bottles are frozen, thawed, and
centrifuged.
10. The supernatant is siphoned and fresh
acidified water is added to wash the lignin.
11. The lignin is freeze dried for 3 to 5 days, after
which
it is characterized by NMR, UV and methoxyl
content.
Experimental Procedure for Acid
Hydrolysis Isolation of Lignin
12. The yield of residual
lignin is calculated by
determining the
original pulp’s lignin
content by Kappa
number and acid
insoluble lignin
content.
Experimental Procedure for Acid
Hydrolysis Isolation of Lignin