Lignin is the second most abundant polymer in forest biomass. Compared to the ubiquitous use of cellulose, lignin is currently simply wasted. The reason for this lies in the challenging structural features and the still incomplete understanding of the correlation between structural features and polymer characteristics displayed by various lignins. This chapter will introduce the general characteristics and peculiarities of lignin as a biopolymer, present techniques specifically developed for its isolation, and describe promising ways toward its valorization, highlighting possibilities for using lignin in chemistry and material sciences. 11 Lignin biorefinery: structure, pretreatment and use biorefinery, biotechnology, spectrometry, spectroscopy, etc. The achievements have been summarized in several monographs [1][2][3][4][5]. Nevertheless, fundamental studies on lignin have not yet been concluded.
Origin and characteristics of lignin
Lignin occurrence and locationLignin, derived from lignum, Latin for wood, was introduced describing a not narrowly classified material early as 1819 by de Candolle, but not until Payen, in 1838, the term lignin was connected closely to a substance. Schulze, in 1865, chemically defined lignin as a polymer that was different from the cellulose components. [5]. Lignin accounts for 15-35% of the dry mass of wood, depending on the type of wood; it is thus the second most abundant natural polymer in forest biomass after cellulose [6]. The rather hydrophobic polymer lignin is present in plant cell walls, where it chemically and physically links the other two major matrix components of the cell walls, cellulose and hemicelluloses [7]: it is located in form of a mixture together with hemicellulose between the cellulose fibrils [1-8]. The tight interplay among these three major plant biopolymers results in an increased impermeability, mechanical strength, and rigidity of the plant cell walls and thus serves to give stability to the plants; it also gives the cells greater resistance to microbial attacks. The different functions of lignin in the plant cause its distribution to vary significantly within the different parts of the plant, i.e., among stem, branching points, branches, and leafs, and among the different walls of the plant cells themselves [9]. The concentration of lignin in the middle lamella and the primary wall is higher than the concentration in the secondary wall. Nonetheless, the majority of the total amount of lignin present in the plant, 75-85%, is located in the secondary wall, due to its considerably larger volume. The amount of lignin present in the plant varies from species to species, ranging from ca. 15% in monocots over ca. 20% in hardwoods to ca. 28% in softwoods and herbaceous angiosperms [2,7].
Lignin structureThe native structure of lignins suggests that it could play a central role as a new chemical feedstock, particularly in the formation of supramolecular materials and aromatic chemicals. It is the only aromatic large-volume renewable feedstock. Lignin, however,...