Lignin and Lignans. Advances in Chemistry

Abstract

Since the realization of global sustainability depends on renewable sources of materials and energy, there is an ever-increasing need to use plant biomass to develop bio-based goods able to replace petroleum-based one. Biomass crops, woody species or grasses consist primarily of cell walls, composed of a complex matrix in which cellulose are embedded in a network of cross-linking hemicellulose polysaccharides. Lignin acts to harden the wall, providing structural rigidity and a physical barrier to fungi and insects, and assists movement and transport of water in plants. As complex compound, it forms a barrier for evaporation, helping to channel water to critical areas of the plant and, while its content and structure that depends on the type of tree, can vary within the same plant. Furthermore, lignin has an interesting redox potential containing more diverse reactive chemical groups, such as aromatic and aliphatic hydroxyl, carboxyl, and carbonyl moieties. However, it is a polymer that, built up by the combination of three basic monomer types (p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol), is formed from oxidative coupling of hydroxycinnamyl alcohols and related compounds such as hydroxycinnamaldehydes. The polymerization which occurs in the cell wall, especially in the secondary cell walls at the level of the water-conducting xylem vessels and sclerenchyma fibers, is mediated by peroxidases and/or laccases. Variation in lignin composition is a function of plant species, age, and tissue type. However, it is interesting to remember that lignin has historically been viewed as a waste product with little intrinsic value, while today, with the desire to maximize the use of all components of the biomass, efforts are being made to identify additional uses of this interesting polymer also. For all these considerations and because of the increasing economical interest to the global use of plant biomass, a deeper knowledge of lignin's chemistry has to be taken in great consideration. This the scope of Lignin and Lignans.