First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 185
H
HH H
HO
O
O
O
O
O
O
H H
H
H
O
O
O
O
O
O
HO
OH
O
O
O
Intra-and intermolecular
hydrogen bonds
H
OH
OH
OH
HO
CH 2 OH
H
HO
O
O
CH O
2 OH CH 2 OH
CH 2 OH
CH 2 OH
CH 2 OH
H
HH HH
HO
O
O
O
O
O
O
H H
H
H
O
O
O
O
O
O
HO
OH
O
O
O
H
OH
O
CH 2 OH HO OH
H
HO
O
O
CH O
2 OH CH 2 OH
CH 2 OH
CH 2 OH
CH 2 OH
Figure 6.3Demonstration of the hydrogen bonding that allows the parallel arrangement of the
cellulose polymer chains.
at low temperature. The solubility of the polymer depends strongly on the degree of
hydrolysis achieved. As a result, factors affecting the hydrolysis rate of cellulose also
affect its solubility, however, with the molecule being in a different form than the
native one. It becomes soluble at higher temperatures, since the energy provided is
adequate to break the hydrogen bonds holding the crystalline structure of the molecule.
Cellulose is also soluble in concentrated acids, but severe degradation of the polymer by
hydrolysis is turning. In alkaline solutions, extensive swelling of cellulose takes place in
addition to dissolution of the low-molecular-weight fractions of the polymer (DP<200)
(Krassig and Schurz, 2002). Different solvents of cellulose that have been applied in
industrial or laboratory practice consist of uncommon and complex systems, such
as cupriethylenediamine hydroxide or the cadmium complex cadoxen. Furthermore,
aqueous salt solutions, such as zinc chloride, dissolve limited amounts of cellulose
(Kirk-Otmer, 2001). Cellulose does not melt with temperature, but its decomposition
starts at 180∘C.
6.1.1.3 Hemicellulose
Hemicelluloses are complex heterogeneous polysaccharides composed of monomeric
residues including d-glucose, d-galactose, d-mannose, d-xylose, l-arabinose,
d-glucuronic acid, and 4-O-methyl-d-glucuronic acid that are found in the plant
cell wall and have different composition and structure depending on their source and
the extraction method. Hemicelluloses have a DP lower than 200, side chains, and can be
acetylated. Hemicelluloses are classified according to main sugar that exists in the back-
bone of the polymer, for example, xylan (β-1,4-linked xylose) or mannan (β-1,4-linked
mannose). Plants belonging to the grass family (Poaceae), for example, rice, wheat, and
switchgrass have hemicelluloses that are composed of mainly glucuronoarabinoxylans.
In softwoods such as fir, pine, and spruce, galactoglucomannans are the principal hemi-
celluloses, while arabinoglucuronoxylans are second most abundant. In hardwoods
species, such as birch, poplar, and so on, 4-O-methyl-glucuronoxylans are the most
abundant hemicellulose units. Hemicelluloses with glucomannans are the second most
