First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 183
wet oxidation, etc.) is needed to soften the material and break down its structure to
make it more susceptible to enzymatic attack before fermentation (Mtui, 2009). All
pretreatments have severe reaction condition and high equipment and processing
cost, and special problems related to the recovery of expensive catalyzer. During the
pretreatment process, degradation compounds of pentoses and hexoses primarily
furfural and 5-hydroxymethyl furfural (5-HMF) are formed. These components are
toxic and inhibit the subsequent enzymatic and fermentative processes. Therefore,
they must be removed or neutralized before the fermentation process (Palmqvist
and Hahn-Hägerdal, 2000). After initial pretreatments on lignocellulosic materials
and obtaining access to cellulose, enzymatic hydrolysis should be conducted. It is
executed by cellulose enzymes such as endoglucanase, exoglucanase, andβ-glucosidase.
Enzymatic hydrolysis improve by optimization of substrate concentration, amount of
enzyme, using surfactant and enzymatic mixtures. In the following section, pretreat-
ment is discussed, but at the first it is better to discuss about lignocellulosic materials.
6.1.1 What Is Lignocellulosic Material?
6.1.1.1 Lignocellulosic Materials
Lignocellulosic materials are most abundant biopolymers on the earth. They formed
from some basic constituents that have in their turn, also complicated internal structure
(Chunpinget al., 2008). For a clear understanding about lignocellulosic materials, an
analysis of the structure and properties of lignocellulosic materials and its components
are explained in this section. The physical properties of each of the components of
lignocellulosic materials and how each of these components contributes to the behavior
of the complex structure as a whole are addressed. As previously mentioned, the
principle function of pretreatment process is to break down the biomass structure and
the use of its constituent for producing fermentable sugars. Figure 6.1 shows the general
structure of lignocellulosic biomass.
6.1.1.2 Cellulose
Cellulose, the most abundant constituent of the plant wall is a homopolysaccharide
composed entirely of d-glucose linked together withβ-glycosidic bonds and with
a degree of polymerization (DP) up to 10,000 or higher (Keshwani, 2009). Indeed
cellulose is theβ-1,4-polyacetal of cellobiose (4-O-β-d-glucopyranosyl-d-glucose).
Cellulose is more commonly considered as a polymer of glucose because cellobiose
consists of two molecules of glucose. The chemical formula of cellulose is (C 6 H 10 O 5 )n,
and the schematic structure of one chain of the polymer is given in Figure 6.2.
Many characteristics of cellulose depend on its DP, that is, the number of glucose
units that make up one polymer molecule. The DP of cellulose can extend to a value
of 17,000, even though more normally a number of 800–10,000 units are encountered
(Kirk-Otmer, 2001). For example, cellulose from wood pulp has a DP between 300 and
1700.
The nature of the bond between the glucose molecules (β-1,4-glucosidic) allows
the polymer to be arranged in long straight chains. The latter arrangement of the
molecule, together with the fact that the hydroxides are evenly scattered on both sides
of the monomers, leads to the formation of hydrogen bonds between the molecules
of cellulose. The hydrogen bonds in turn result in the formation of a compound that
