207
11
of the wood and the total production of pulp,
considerable quantities of by-products are produced and their utilization is an important criterion for the economic profitability of the overall
process. Due to the different conditions and
chemicals used in these two processes, lignin is
available in two different forms after digestion:
5 In the sulfate process, the wood is digested
under basic conditions with NaOH and Na 2 S
(sodium sulfide). Lignin is decomposed into
smaller fragments by deprotonation and
sulfur-induced rearrangement reactions. The
phenolic hydroxyl groups are present as salts
(phenolates) under these conditions, and
the resulting soluble lignin phenolate can
be separated from the pulp with the black
liquor. It has a molecular weight of about
2000–3000 Dalton.
5 In the sulfite process, the wood is usually
decomposed at an acid pH value using
calcium or magnesium hydrogen sulfite. The
lignin is “dissolved” by the addition of sulfonic acid groups mainly to the carbon atom
of the phenylpropane units. The ether bonds
are largely stable under acidic conditions, so
that hardly any depolymerization takes place.
Rather, the molar mass increases to around
20,000–50,000 Dalton due to further condensation reactions. The resulting lignin is
separated from the pulp as ligninsulfonates
with the sulfite liquor.
These two processes result in about 55 million
tons of lignin as a by-product of cellulose production. What both disintegration processes
have in common, however, is that the lignin is
not pure because it is contaminated with sulfur
compounds, among other things. The possibilities for the targeted further processing of these
lignins into chemicals are therefore severely limited. For this reason, alternative disintegration
processes have been developed in recent decades
which isolate lignin in a particularly pure form.
11.3.2 Alternative Wood Pulping
Methods for Lignin Recovery
Wood pulping processes that do not rely on classical inorganic pulping chemicals in aqueous
media were developed at the end of the 1960s.
linkages with 40–50% β-O-4 bonds and 11–16%
α-O-4 bonds (. Fig. 11.6).
The analysis of the compositions is carried
out on the one hand by targeted degradation
reactions in alkaline potassium permanganate
solutions and goes back to works by K. J.
Freudenberg (. Fig. 10.3) in the 1950s and 1960s.
The protocol was modified several times and
contains several steps at the end of which fragments of lignin remain, which can be examined
by gas chromatography and which provide information about the ratios of the three different
phenylpropanoids. Another important method
of lignin analysis is NMR spectroscopy, which
can be used to identify different types of bonds.
For this purpose, either lignin as such can be
examined by solid-state NMR spectroscopy or a
lignin derivative obtained, for example, by acetylation can be examined by NMR spectroscopy in
solution.
11.3 Lignin Recovery
In wood, together with cellulose and hemicelluloses, lignin forms a composite material from
which lignin cannot simply be separated. The
production of lignin is therefore always associated with the breaking down of these structures,
a process known as disintegration. Native lignin
is almost insoluble in all solvents and must first
be converted into a soluble form for effective
separation from cellulose. Depending on which
digestion variant is selected, lignin is present in
different forms after disintegration.
11.3.1 Classical Wood Pulping
Processes
The most important wood pulping processes in
terms of quantity have already been discussed in
7 Chap. 7: the sulfate (also Kraft-) and the sulfite
processes. Both processes are primarily used to
obtain cellulose from wood, e.g. for paper production. The aim is to convert the hemicellulose
and lignin into a soluble form so that they can be
separated from the solid cellulose. Lignin leads
to yellowness in paper, which is an undesired
property. The quality of lignin as a by-product
plays only a minor role. Due to the composition
11.2 · Structure of Lignin
11
of the wood and the total production of pulp,
considerable quantities of by-products are produced and their utilization is an important criterion for the economic profitability of the overall
process. Due to the different conditions and
chemicals used in these two processes, lignin is
available in two different forms after digestion:
5 In the sulfate process, the wood is digested
under basic conditions with NaOH and Na 2 S
(sodium sulfide). Lignin is decomposed into
smaller fragments by deprotonation and
sulfur-induced rearrangement reactions. The
phenolic hydroxyl groups are present as salts
(phenolates) under these conditions, and
the resulting soluble lignin phenolate can
be separated from the pulp with the black
liquor. It has a molecular weight of about
2000–3000 Dalton.
5 In the sulfite process, the wood is usually
decomposed at an acid pH value using
calcium or magnesium hydrogen sulfite. The
lignin is “dissolved” by the addition of sulfonic acid groups mainly to the carbon atom
of the phenylpropane units. The ether bonds
are largely stable under acidic conditions, so
that hardly any depolymerization takes place.
Rather, the molar mass increases to around
20,000–50,000 Dalton due to further condensation reactions. The resulting lignin is
separated from the pulp as ligninsulfonates
with the sulfite liquor.
These two processes result in about 55 million
tons of lignin as a by-product of cellulose production. What both disintegration processes
have in common, however, is that the lignin is
not pure because it is contaminated with sulfur
compounds, among other things. The possibilities for the targeted further processing of these
lignins into chemicals are therefore severely limited. For this reason, alternative disintegration
processes have been developed in recent decades
which isolate lignin in a particularly pure form.
11.3.2 Alternative Wood Pulping
Methods for Lignin Recovery
Wood pulping processes that do not rely on classical inorganic pulping chemicals in aqueous
media were developed at the end of the 1960s.
linkages with 40–50% β-O-4 bonds and 11–16%
α-O-4 bonds (. Fig. 11.6).
The analysis of the compositions is carried
out on the one hand by targeted degradation
reactions in alkaline potassium permanganate
solutions and goes back to works by K. J.
Freudenberg (. Fig. 10.3) in the 1950s and 1960s.
The protocol was modified several times and
contains several steps at the end of which fragments of lignin remain, which can be examined
by gas chromatography and which provide information about the ratios of the three different
phenylpropanoids. Another important method
of lignin analysis is NMR spectroscopy, which
can be used to identify different types of bonds.
For this purpose, either lignin as such can be
examined by solid-state NMR spectroscopy or a
lignin derivative obtained, for example, by acetylation can be examined by NMR spectroscopy in
solution.
11.3 Lignin Recovery
In wood, together with cellulose and hemicelluloses, lignin forms a composite material from
which lignin cannot simply be separated. The
production of lignin is therefore always associated with the breaking down of these structures,
a process known as disintegration. Native lignin
is almost insoluble in all solvents and must first
be converted into a soluble form for effective
separation from cellulose. Depending on which
digestion variant is selected, lignin is present in
different forms after disintegration.
11.3.1 Classical Wood Pulping
Processes
The most important wood pulping processes in
terms of quantity have already been discussed in
7 Chap. 7: the sulfate (also Kraft-) and the sulfite
processes. Both processes are primarily used to
obtain cellulose from wood, e.g. for paper production. The aim is to convert the hemicellulose
and lignin into a soluble form so that they can be
separated from the solid cellulose. Lignin leads
to yellowness in paper, which is an undesired
property. The quality of lignin as a by-product
plays only a minor role. Due to the composition
11.2 · Structure of Lignin