Page 31 - Lignocellulosic Biomass to Liquid Biofuels

10 Lignocellulosic Biomass to Liquid Biofuels

contains monomeric units from coniferyl and sinapyl alcohol
(Scheme 1.2). In general, guaiacyl lignin is found in softwoods, while
guaiacyl syringyl lignin is present in hardwoods. Graminaceous lignin is
composed mainly of p-coumaryl alcohol units [18].

Scheme 1.2 Monolignols: (A) p-coumaryl alcohol (hydroxyphenyl unit), (B) coniferyl
alcohol (guaiacyl units), and (C) sinapyl alcohol (syringyl unit).

Lignin is bonded to the cellulose and hemicellulose by polysaccharides
and serves as a strengthening and hydrophobic agent, complicating bio-
mass breakdown. The schematic diagram (Scheme 1.3) shows possible
covalent cross-links between polysaccharides and lignin in cell walls. Since
lignin is the hardest part of the plant cell wall and not easy to convert to
biofuels, it is essential to find out alternative uses to maximize the energy
yield from biomass.

Scheme 1.3 Linkage of lignin with polysaccharides.