Paula C. Rodrigues Pinto, Eduardo A. Borges da Silva and Alirio E. Rodrigues

12.1 Lignin, a Fascinating Complex Polymer

Lignin is a three-dimensional phenolic macromolecule that constitutes roughly 15-25% of vegetal biomass acting as structural and cohesion components of the cell walls in vascular plants [1]. Following cellulose, lignin is the most abundant natural biopolymer and contains about 30% of non-fossil organic carbon on Earth [2].

The principle of lignin biosyntheses is the polymerization by dehydrogenation of the hydroxycinnamyl alcohols, the monolignols p-coumaryl, coniferyl, and sinapyl alcohols [2-6]. Each of these monolignols gives rise to one subunit type in lignin structure, p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), respectively, differing between them in the methoxylation of the aromatic nuclei as depicted in Fig. 12.1.

The general structural unit of lignin is commonly called a phenylpropane unit or, briefly, ppu. Although with some exceptions, softwood lignins primarily con­tain G units and a small proportion of H units: G lignin. Pinus and spruce are some examples of trees containing this lignin. Hardwood lignins contain both S and G units, with a very small proportion of H units: GS lignin. Some examples of hardwoods are birch, eucalyptus, beech, and aspen. The lignin of some crop plants, palm trees, and banana plants are composed by all the three subunits, although with the predominance of H type: HGS lignin.

Lignin ppus are linked by ether and carbon-carbon bonds either in aliphatic and/or aromatic moiety [1, 7]. Types and frequencies of the most abundant dilignols in softwood and hardwood lignins are summarized in Table 12.1.

P. C. Rodrigues Pinto • E. A. Borges da Silva • A. E. Rodrigues (H)

Laboratory of Separation and Reaction Engineering—LSRE, Associate Laboratory LSRE/ LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal e-mail: arodrig@fe. up. pt

C. Baskar et al. (eds.), Biomass Conversion,

DOI: 10.1007/978-3-642-28418-2_12, © Springer-Verlag Berlin Heidelberg 2012

The respective letters are shown in Fig. 12.2, representing one fragment of hardwood lignin. The numbering system for the ppu is also shown in Fig. 12.2.

The dilignol b-O-4 (A) (arylglycerol-b-aryl ether) is, by far, the most frequent dilignol, accounting for more than 50% of the structures. It is also the one most easily cleaved, providing a basis for industrial processes, such as chemical pul­ping, and several methods in lignin chemical analysis. The other linkages are all more resistant to chemical degradation [1, 3]. The proportion of each linkage depends on the relative contribution of a particular monomer to the polymerization process. For example, G-type lignins (softwood lignins), contain more resistant linkages as those involving the C5 of aromatic nuclei (b-5 (C), 5-5′(E) and 4-O — 5′(F)) than SG lignins (hardwood lignins) due to the availability of the C5 position for coupling. This is the reason for the higher condensation degree (frequency of C-C linkages between aromatic rings) for softwood than for hardwood lignins. This fact has implications on lignin reactivity. In spite of these common features, chemical structure of lignin cannot be described by a simple structural formula due to the enormous and apparently random possibilities of combination between units in the macromolecule. Monolignols can also form bonds to other cell wall poly­mers as polysaccharides in a complex three-dimensional network [3, 8].

It is widely recognized that lignin content and composition differ between the major groups of higher plants (as evidenced by the data in Table 12.1) and also between species and even between trees and morphological parts of the tree [1]. This fact denotes the flexibility of the combinatorial polymerization reactions allowing significant variations in the final structure and high number of possible isomers.

This is a natural and powerful tool of higher plants to adaptive response to the various environmental conditions stresses, for example, the lignin formed in compression wood [3]. Simultaneously, it reveals an unexploited opportunity to engineer the lignin structures to modify their proprieties for required applications [11, 12].