Page 28 - Lignocellulosic Biomass to Liquid Biofuels

8 Lignocellulosic Biomass to Liquid Biofuels

measuring unit for the determination of extent of a particular polymeric
cellulose usually expressed as (C 6 H 10 O 5 ) n , where n is the degree of poly-
merization (DP). DP of cellulose hangs on the type of plant or microor-
ganism from which it is isolated and also the method of isolation. For
some of the native polymers the DP is estimated to be from 2000 to
14,000 glucan units, DP of the wood pulp in the range of 650 1500
units per glucan chain.
Due to the impending of monomer unit of cellulose to formulate
three hydrogen bonds with a monomer of neighboring chain, the chains
are fit tightly together for the formation of larger units known as microfi-
brils. The result is a very stable configuration—essentially free of intersti-
tial spaces, making it anhydrous and quite recalcitrant to hydrolysis by
acid, base, or enzyme action. In a polymer chain of cellulose, low chain
disturbance is occurred in the crystalline region named micelles. Regions
without extensive interchain hydrogen bonding are consequently less
structured (amorphous). Amorphous parts are less susceptible to hydrolysis
comparative to crystalline part. Natural cellulose consists of cellulose fibrils
bound together by an amorphous matrix comprising pectin, hemicellu-
lose, and lignin [14].
The composition of hemicellulose depends on the source biomass or
species type. Hardwoods, annual plants, and cereal are predominated by
xylan hemicellulose, whereas softwoods are predominated by glucomannan
hemicellulose [15]. Hemicelluloses are linked with cellulose by inter- and
intramolecular hydrogen bonds and with lignin by ester and ether bonds
[16]. Hemicellulose also forms covalent associations with lignin, a complex
aromatic polymer, whose structure and organization within the cell wall are
not completely understood yet. However, chemical properties of several
hemicelluloses of LCB s have been discovered as shown in Table 1.2.
Lignin molecule is made by phenylpropane units allied in a three-
dimensional structure, which is very complex. Lignin is considered to be
the glue holding the cellulose fibrils together and difficult to completely
remove. Lignin provides both stiffness to the cell and protection against
the microbial attack because lignin molecules contain several types of
chemical bonds and linear polymer chain which increase the mechanical
properties [14]. Plant lignin can be broadly divided into three classes: soft-
wood (gymnosperm), hardwood (angiosperm), and grass or annual plant
(graminaceous) lignin. Three different phenylpropane units, or mono-
lignols, are responsible for lignin biosynthesis. Guaiacyl lignin is composed
principally of coniferyl alcohol units, while guaiacyl syringyl lignin