192 Introduction to Renewable Biomaterials
such as mono- and dicarboxylic acids by means of oxidation using different oxidants
including chlorine, chlorine dioxide, and oxygen. Oxidative materials can also break
the side chain of lignin monomeric units and generate components with three, two, or
one carbon atoms (Kirk-Otmer, 2001).
6.2.5 Hydroxyl Group
The hydroxyl group initiates substitution reactions. Under acidic conditions hydroxyl
group converted to aryl or allylic ether. Finally, ether substituted by an acid group
(e.g., sulfonic acid). The advantage of latter reaction is that the presence of the acid
group in the lignin polymer leads the polymer soluble in water and so it is named
lignosulfonates (Kirk-Otmer, 2001).
6.2.6 Ether Bond
The ether bonds are most interesting functional groups in lignocellulosic materials
because
1- thery are located between glucose monomers (glucosidic bond) and hold them in the
polymer chain;
2- they are the primary bonds in the lignin polymer.
Thus, by the break down of the ether bond, lignin separates from polysaccharide
matrix and then degradation of lignin and polysaccharides to lignin fragments and
monomer sugars takes place.
The cleavage of the ether bond is carried out by solvolytic reactions that take place
under acidic or alkaline conditions. The ether bond in lignin is converted to hydroxyl
and then converted to carbonyl or carboxyl (before it is finally fragmented to C 3 or C 2
molecules), under acidic conditions. Under alkaline conditions, the mechanisms are
different, and the aromatic ring is separated. Figure 6.8 shows an example of ether bond
cleavage of the lignin polymer under alkaline condition. Decomposition of the ether
bond increases with the addition of hydrosulfide. The ether bond of cellulose polymer
can cleave under both acidic and alkaline conditions. In acidic conditions, acid acts as
a catalyst protonating the oxygen atom. The charged group (ions) leaves the polymer
chain and is substitute by the hydroxyl group of water (Figure 6.9).
In alkaline conditions, the mechanism involves the formation of 1–2 anhydro config-
uration (Figure 6.10). The intermediate form is a type of epoxide and due to the ring that
is formed between the two carbon atoms and oxygen, allows via the SN2mechanismthe
nucleophilic substitution of hydrogen (Solomon, 1988). The use of concentrated alkali
and a minimum temperature of 150∘C are required to reach a satisfactory reaction rate.
6.2.7 Ester Bond
Ester bonds are recognized between lignin and polysaccharides as well as hemicellulose
polymer. The group of acetyl in hemicellulose is linked to the hydroxyl group of the
main chain of the polysaccharide by an ester bond. With respect to LCC, it is not
clear whether the ester bond is between lignin and cellulose or between lignin and
hemicellulose or between lignin and both polysaccharides (Faulonet al., 1994). In
general, hydrolysis is carried out to break the ester bond and results in carboxyl
