First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 187
α-L-Arabinofuranosyl residue
4-O-methyl α-D-glucuronyl residue
Acetyl residue
Figure 6.5A schematic structure of the hemicellulose backbone.
The most important aspects of the structure and composition of hemicellulose are the
lack of crystalline structure, mostly because of the highly branched structure, and the
existence of acetyl groups linked to the polymer chain (Kirk-Otmer, 2001).
Hemicellulose produced from plants have a high degree of polydispersity,
polydiversity, and polymolecularity (a wide range of size, shape, and mass characteristics).
However, the DP does not top to the 200 units, while the minimum limit can be
approximately 150 monomers.
Hemicellulose is insoluble in water at low temperature, while its hydrolysis begins at
a lower temperature than cellulose, which renders it soluble at elevated temperatures.
Its solubility in water improves in the presence of acid.
6.1.1.4 Lignin
Lignin is an aromatic and natural polymer with very complicated structure and nearly
forms about 1/4 to 1/3 dry weights of trees. Lignin is composed of phenyl propane
units and constitutes the most abundant nonpolysaccharide fraction in lignocellulosic
biomass. The three monomers in lignin arep-coumaryl alcohol, coniferyl alcohol, and
sinapyl alcohols (Figure 6.6) that are joined together through alkyl–aryl, alkyl–alkyl, and
aryl–aryl other bonds and form amorphous three-dimensional polymer. Lignin embeds
the cellulose thereby offering protection against microbial and chemical degradation.
Furthermore, lignin is able to form different covalent bonds with some hemicelluloses,
for example, benzyl ester bonds with the carboxyl group of 4-O-methyl-d-glucuronic
acid in xylan. More stable ether bonds, also known as lignin carbohydrate complexes
(LCCs) can be formed between lignin and arabinose or galactose side groups in xylans
and mannans.
Figure 6.6p-Coumaryl-, coniferyl-, and
sinapyl alcohol: major building
components of the three-dimensional
polymer lignin.
HO
OH
Coumaryl alcohol
HO
OH
OMe
Coniferyl alcohol
HO
OH
MeO OMe
Sinapyl alcohol
