213
11
5 These three building blocks can be linked
together in different ways to form an
inhomogeneous and widely cross-linked
polymer. Polymerization takes place
radically, e.g. ether bonds, aryl bonds or
cyclic structures form.
5 Lignins of different plants can be
characterized by the relative number
of monomeric building blocks and the
number of linking patterns.
5 Traditionally, lignin is obtained as a
by-product of pulp production using
the sulfate and sulfite process. There
it is separated from the cellulose as
waste liquor in a soluble form with the
hemicelluloses.
5 In the basic sulfate process, lignin is
found in the black liquor and is present
as lignin phenolate. In the acidic
sulfite process, lignin is produced as
lignosulfonate.
5 In addition to the two classical processes
for the recovery of lignin, there are a
number of alternative processes which
Summary (Take-Home Messages)
5 Lignin is a phenolic macromolecule
that is a major component of wood,
along with cellulose and hemicelluloses.
Embedded in this composite structure,
it ensures the necessary stability of the
wood by forming a three-dimensional
and thus rigid network.
5 Lignin is present in different proportions
in all vascular plants. Typically, the lignin
content is 20–30% based on the dry
substance of the plant.
5 The chemical structure of lignin is not
uniform, and each plant has its own
characteristic lignin.
5 All lignins have in common that
they are composed of three building
blocks which come from the group of
phenylpropanoids. They differ in the
number of methoxy groups on the
phenol ring: Cumaryl alcohol has none,
coniferyl alcohol one and sinapyl
alcohol two methoxy groups in ortho
position to the phenolic OH group.
BOX: Biosolvents from Lignin?
In the following, a concept is
presented on how solvents
can be produced from lignin,
which in turn can be used for
the decomposition of the lignin
itself:
1. In the first step,
depolymerization of lignin
into monoaromatic products,
such as catechol, vanillin
or coumaric acid, occurs.
Such degradation can be
catalyzed either strongly
basic or acidic; there are also
oxidative, metal catalyzed
and enzymatic variants. With
caustic soda as catalyst, a
conversion rate of (only)
11.5% has been achieved
at high temperatures
(240–340 °C) and pressures
(250 bar), whereby a very
broad mixture of aromatics
containing hydroxyl groups
is obtained.
2. In the second step, these
hydroxyaromatics are
reacted with choline.
Choline is the industrially
easily accessible
(2-hydroxyethyl)trimethylammonium chloride.
Together with the
hydroxyaromatics (phenols)
from the first step acting
as H donors, it acts as
H-acceptor and forms a
mixture whose melting
point is considerably lower
than that of the individual
starting materials. Because
of this strongly lowered
eutectic point, the liquids
formed in this way are called
“Deep Eutectic Solvents
(DES)”. The exact bonding
conditions in these DES
have not yet been clarified;
it is assumed that the
phenolic OH groups form
strong hydrogen bonds
to the choline. These DES
behave in the same way as
“ionic liquids”, but they are
very simple and therefore
inexpensive to produce.
3. Part of the DES can be
used for the degradation
of biomass. The biomass,
e.g. switchgrass, is heated
for 3 h at 160 °C in a DES. In
this degradation process,
the carbohydrates are
partially split off from the
lignin.
The carbohydrates can be
processed in a biorefinery (see
7 Chap. 20), for the production
of fuels, e.g. bioethanol.
The separated lignin can be
returned to steps 1 and 2 for the
production of new “biosolvents”.
If this concept, which has so
far only been investigated on
a laboratory scale, leads to an
economical process in the long
term, a closed-loop biorefinery
would be conceivable. However,
there are still many open
questions to be clarified.
11.4 · Use of Lignin
11
5 These three building blocks can be linked
together in different ways to form an
inhomogeneous and widely cross-linked
polymer. Polymerization takes place
radically, e.g. ether bonds, aryl bonds or
cyclic structures form.
5 Lignins of different plants can be
characterized by the relative number
of monomeric building blocks and the
number of linking patterns.
5 Traditionally, lignin is obtained as a
by-product of pulp production using
the sulfate and sulfite process. There
it is separated from the cellulose as
waste liquor in a soluble form with the
hemicelluloses.
5 In the basic sulfate process, lignin is
found in the black liquor and is present
as lignin phenolate. In the acidic
sulfite process, lignin is produced as
lignosulfonate.
5 In addition to the two classical processes
for the recovery of lignin, there are a
number of alternative processes which
Summary (Take-Home Messages)
5 Lignin is a phenolic macromolecule
that is a major component of wood,
along with cellulose and hemicelluloses.
Embedded in this composite structure,
it ensures the necessary stability of the
wood by forming a three-dimensional
and thus rigid network.
5 Lignin is present in different proportions
in all vascular plants. Typically, the lignin
content is 20–30% based on the dry
substance of the plant.
5 The chemical structure of lignin is not
uniform, and each plant has its own
characteristic lignin.
5 All lignins have in common that
they are composed of three building
blocks which come from the group of
phenylpropanoids. They differ in the
number of methoxy groups on the
phenol ring: Cumaryl alcohol has none,
coniferyl alcohol one and sinapyl
alcohol two methoxy groups in ortho
position to the phenolic OH group.
BOX: Biosolvents from Lignin?
In the following, a concept is
presented on how solvents
can be produced from lignin,
which in turn can be used for
the decomposition of the lignin
itself:
1. In the first step,
depolymerization of lignin
into monoaromatic products,
such as catechol, vanillin
or coumaric acid, occurs.
Such degradation can be
catalyzed either strongly
basic or acidic; there are also
oxidative, metal catalyzed
and enzymatic variants. With
caustic soda as catalyst, a
conversion rate of (only)
11.5% has been achieved
at high temperatures
(240–340 °C) and pressures
(250 bar), whereby a very
broad mixture of aromatics
containing hydroxyl groups
is obtained.
2. In the second step, these
hydroxyaromatics are
reacted with choline.
Choline is the industrially
easily accessible
(2-hydroxyethyl)trimethylammonium chloride.
Together with the
hydroxyaromatics (phenols)
from the first step acting
as H donors, it acts as
H-acceptor and forms a
mixture whose melting
point is considerably lower
than that of the individual
starting materials. Because
of this strongly lowered
eutectic point, the liquids
formed in this way are called
“Deep Eutectic Solvents
(DES)”. The exact bonding
conditions in these DES
have not yet been clarified;
it is assumed that the
phenolic OH groups form
strong hydrogen bonds
to the choline. These DES
behave in the same way as
“ionic liquids”, but they are
very simple and therefore
inexpensive to produce.
3. Part of the DES can be
used for the degradation
of biomass. The biomass,
e.g. switchgrass, is heated
for 3 h at 160 °C in a DES. In
this degradation process,
the carbohydrates are
partially split off from the
lignin.
The carbohydrates can be
processed in a biorefinery (see
7 Chap. 20), for the production
of fuels, e.g. bioethanol.
The separated lignin can be
returned to steps 1 and 2 for the
production of new “biosolvents”.
If this concept, which has so
far only been investigated on
a laboratory scale, leads to an
economical process in the long
term, a closed-loop biorefinery
would be conceivable. However,
there are still many open
questions to be clarified.
11.4 · Use of Lignin