1 Lignin—Structure and Functions, Sources and Uses
1.1 Occurrence of Lignin and Its Composition and Natural
Roles
Lignin is, after cellulose, the second most abundant biopolymer on earth, comprising about 10–25% of the total biomass of plants and estimated to account for
30% of all non-fossil organic carbon on earth [1]. It is a highly branched polymer,
consisting of aromatic subunits that are connected into a complex network that
crosslinks cellulose and hemicellulose in the plant cell wall of softwood, hardwood,
and other non-herbaceous plants (Fig. 1). As integral part of lignocellulose, lignin
gives trees their rigidity and structure (e.g., [2, 3]), enabling them to grow to
considerable heights and sizes.
Lignin is built from a combination of three aromatic phenylpropanoid units, i.e.,
guaiacyl (G), syringyl (S) and p-hydroxyphenyl (H) propanoid, derived from the
monolignols coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol, respectively, assembled via reactive radical intermediates under presumed involvement of
enzymes such as laccases and peroxidases. The aromatic monomers are thereby
linked by C–C or ether bonds into a complex 3D structure, a process often referred
to as lignification [4, 5]. The ratio of the different monomers and the different
connecting bonds vary greatly, depending on both plant type and structure within
the plant. Hence there is no uniform ‘lignin structure’, but rather a spectrum of
diverse variants [6–9]. Polymerized lignin is deposited in the plant cell wall where,
besides structural function, it provides protection against invading pathogens [4]. In
addition, its hydrophobicity facilitates the transport of water and nutrients through
the plant's vascular system in both wood and bark.
1.2 Types and Sources of Lignin
The spectrum of lignins can be divided into (i) native lignin, referring to lignin
within the lignocellulose structures of plants without any alterations, and (ii) technical/processed/ pretreated lignin, extracted from plant biomass or recovered from
industrial processes [7, 9] (Fig. 1). Technical lignins are primarily derived from
industrial pulping processes, including kraft, sulfite, and soda pulping. However,
also Organosolv lignin and hydrolysis lignin are included under this category [4,
10]. Kraft lignin is a by-product of the kraft process, i.e., a traditional method used
in pulp and paper production. Here, the woody biomass is treated with NaOH and
Na 2 S at high temperature (150–170 °C), resulting in breakage of the ether bonds
and conversion of the complex rigid structures into smaller lignin fragments
(alkali-soluble lignin). This process typically results in solubility of up to 90% of
the lignin content. The liquidous mixture is then often referred to as ‘black liquor’,
from which highly condensed lignin can be recovered by precipitation upon
treatment with acid to a pH below 5.0 [7]. In contrast to the kraft process, sulfite
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G.-S. Nguyen et al.
1.1 Occurrence of Lignin and Its Composition and Natural
Roles
Lignin is, after cellulose, the second most abundant biopolymer on earth, comprising about 10–25% of the total biomass of plants and estimated to account for
30% of all non-fossil organic carbon on earth [1]. It is a highly branched polymer,
consisting of aromatic subunits that are connected into a complex network that
crosslinks cellulose and hemicellulose in the plant cell wall of softwood, hardwood,
and other non-herbaceous plants (Fig. 1). As integral part of lignocellulose, lignin
gives trees their rigidity and structure (e.g., [2, 3]), enabling them to grow to
considerable heights and sizes.
Lignin is built from a combination of three aromatic phenylpropanoid units, i.e.,
guaiacyl (G), syringyl (S) and p-hydroxyphenyl (H) propanoid, derived from the
monolignols coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol, respectively, assembled via reactive radical intermediates under presumed involvement of
enzymes such as laccases and peroxidases. The aromatic monomers are thereby
linked by C–C or ether bonds into a complex 3D structure, a process often referred
to as lignification [4, 5]. The ratio of the different monomers and the different
connecting bonds vary greatly, depending on both plant type and structure within
the plant. Hence there is no uniform ‘lignin structure’, but rather a spectrum of
diverse variants [6–9]. Polymerized lignin is deposited in the plant cell wall where,
besides structural function, it provides protection against invading pathogens [4]. In
addition, its hydrophobicity facilitates the transport of water and nutrients through
the plant's vascular system in both wood and bark.
1.2 Types and Sources of Lignin
The spectrum of lignins can be divided into (i) native lignin, referring to lignin
within the lignocellulose structures of plants without any alterations, and (ii) technical/processed/ pretreated lignin, extracted from plant biomass or recovered from
industrial processes [7, 9] (Fig. 1). Technical lignins are primarily derived from
industrial pulping processes, including kraft, sulfite, and soda pulping. However,
also Organosolv lignin and hydrolysis lignin are included under this category [4,
10]. Kraft lignin is a by-product of the kraft process, i.e., a traditional method used
in pulp and paper production. Here, the woody biomass is treated with NaOH and
Na 2 S at high temperature (150–170 °C), resulting in breakage of the ether bonds
and conversion of the complex rigid structures into smaller lignin fragments
(alkali-soluble lignin). This process typically results in solubility of up to 90% of
the lignin content. The liquidous mixture is then often referred to as ‘black liquor’,
from which highly condensed lignin can be recovered by precipitation upon
treatment with acid to a pH below 5.0 [7]. In contrast to the kraft process, sulfite
440
G.-S. Nguyen et al.