Biomedical Engineering Reference
In-Depth Information
O
OH
O
OH
OH
OH
HO
HO
O
O
HO
O
O
HO
HO
OH
OH
OH
O
OH
O
Glucose
Cellobiose
Fig. 2.1
Molecular chain structure of cellulose [
8
]
form many hydrogen bonds, and these hydrogen bonds construct a huge network
that directly contributes the compact crystal structure [
7
]. In most conditions, the
cellulose is wrapped by hemicellulose (dry matter accounting for 20-35 %) and
lignin (dry matter accounting for 5-30 %). Cellulose has become an important raw
material for the pulp and paper, textile, and fibrous chemical industries. Predictably,
bioenergy generated from lignocellulosic materials will become clean energy in the
future.
2.3.1
Chemical Structure of Cellulose
Cellulose is a linear homopolymer composed of
D
-glucopyranose units linked by
“
-1,4-glycosidic bonds. It mainly contains carbon (44.44 %), hydrogen (6.17 %),
and oxygen (49.39 %). The chemical formula of cellulose is (C
6
H
10
O
5
)
n
;n,called
the degree of polymerization (DP), represents the number of glucose groups,
ranging from hundreds to thousands or even tens of thousands. In the twentieth
century, it was proved that cellulose consists of pure dehydrated repeating units of
D
-glucoses (as shown in Fig.
2.1
), and the repeating unit of the cellulose is called
cellobiose.
Sodium hydroxide solution at different concentrations and different temperatures
could dissolve cellulose with different DP. According to different solubilities under
specific conditions, cellulose can be divided into three types:
① '
-cellulose, which
is dissolved in 16.5 % NaOH at 20
ı
C;
-cellulose, which is deposition extracted
after neutralizing the acid solution and the remaining alkaline solution; and
② “
-
cellulose, which is the soluble remainder in the neutralized solution. Staudinger
used a viscosity method to measure the DP of these three celluloses. The results
indicated that the DPs of
③ ”
-cellulose were more than
200, between 10 and 200, and less than 10, respectively. In industry,
'
-cellulose,
“
-cellulose, and
”
'
-cellulose
usually is used to express the purity of cellulose. Traditionally,
-
cellulose are together called industrial hemicellulose.
Holocellulose
refers to all the
carbohydrates in natural cellulose material, which also is the sum of cellulose and
hemicellulose [
2
].
“
-cellulose and
”
