Environmental Engineering Reference
In-Depth Information
OH
OH
OH
OH
O
O
HO
HO
O
O
O
HO
O
HO
O
O
OH
OH
OH
OH
Figure 6.1
Molecule of cellulose.
(a)
(b)
(c)
OH
OH
OH
O
CH 3
H 3 C
O
O
CH 3
OH
OH
OH
Figure 6.2 Molecules from which the characteristic units of lignin are obtained: (a) trans -
coniferyl alcohol; (b) trans -sinapyl alcohol; and (c) trans-p -cumaryl alcohol.
Cellulose is by far the most abundant component, especially in cotton fibres.
This biopolymer comprises cellobiose (repetitive unit), which is composed of two
glucose building blocks in inversed position with respect to the plane of the cycle
(Figure 6.1).
Such a configuration results from a β-1 → 4 glucosidic bond. Schematically
speaking, cellulose is a 'flat' molecule with its hydroxyl groups pointing out of
the 'ribbon'. The accessibility of OH groups favours the formation of intermo-
lecular hydrogen bonds and the creation of crystalline zones. A cellulose molecule
from a cotton fibre contains more than 7,000 glucose units. After extraction, the
degree of polymerisation is reduced to about 1,000 units.
Hemicelluloses comprise different hexoses and pentoses (glucose, mannose,
xylose, etc.). Since these heteropolysaccharides are often branched polymers,
they cannot constitute crystalline structures. However, their function in the consti-
tution of natural fibres is crucial. Together with lignin, they comprise the bonding
matrix of the cellulose microfibres.
Lignin is an amorphous non-polar macromolecule, comprising phenyl-propane
units. The structure of lignin depends on the source. Moreover, the extraction
method modifies the structure of the lignin prior to analysis. Recent studies [1, 2]
have shown fundamental differences with previously reported structures of lignin.
Lignin typically comprises three basic units (see Figure 6.2):
• the guaiacyl unit, derived from trans -coniferyl alcohol, which is abundantly
present in softwoods;
Search WWH ::




Custom Search