First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 191
separated. The separation of polymers leads to alternation of their original structure,
and consequently linkages are not determined clearly. Three types of hydrogen bonds
connecting lignin to cellulose and hemicellulose have been found. In addition, the
existence of covalence bonds between lignin and polysaccharides has been confirmed.
It is also known that hemicellulose connects to lignin by ester bond, and there are ether
bonds between lignin and cellulose or between lignin and hemicellulose. The hydrogen
bonds between hemicellulose and cellulose are also identified, but these bonds are
not strong enough due to the fact that hemicellulose does not have a primary alcohol
functional group outside of the pyranoside ring.
6.2.3 Functional Groups and Chemical Characteristics of Lignocellulosic Biomass
Components
The important ingredients to produce sugar monomers and finally alcohol or other
compositions of lignocellulosic materials are
- functional groups and the ingredient that are involved in the hydrolysis of polysac-
charides to their monomers also the consequent degradation reactions of these
monomers to furfural
- functional groups and the ingredient that are involved in lignin de polymerization in
order to increase cellulose accessibility for hydrolyzing enzyme.
The functional groups of all three components of lignocellulosic biomass are listed in
Table 6.3.
The hydrogen bond is not a functional group, since its reaction does not lead to
chemical change of the molecule. However, it changes the solubility of the molecule,
and it is therefore important for the breakdown of lignocellulosic materials. Concerning
the cellulose polymer, the principal goal of pretreatment is to break down the glucosidic
(ester) bond that produces sugar monomers. The reactions that occur using functional
groups of Table 6.3 are discussed in the following.
6.2.4 Aromatic Ring
Chlorination and nitration reactions are carried out via the electrophilic substitution
mechanism. Finally, substitution of the aromatic ring of lignin is performed with
chlorine or nitrate groups. This type of substitution is not achieved in the same way.
Aromatic ring can be converted to cyclic structures and finally smaller molecules
Table 6.3Functional groups of lignocellulosic biomass.
Functional group Lignin Cellulose Hemicellulose
Aromatic ring *
Hydroxyl group *
Carbon to carbon bond *
Ether bond * * *
Ester bond *
Hydrogen bond * *
