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Synthesis and characterization of carboxymethyl cellulose with high degree substitution from Vietnamese pineapple leaf

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Physical sciences
|
Chemistry
13
SEPTEMBER 2022
Volume 64 Number 3
Introduction
CMC is one of the most common derivatives obtained by
the carboxymethylation of the hydroxyl groups of cellulose.
CMC exhibits a great potential as thickening additives,
film former, binder, suspending aid, and biodegradable
materials [1-4]. In order to obtain CMC, first, cellulose
was swollen in a NaOH solution, and then reacted with
monochloroacetic acid in alcohol [5]. In this reaction, the
sodium carboxymethyl groups substitutes the hydroxyl
groups in C-2, C-3, and C-6 of the anhydro-glucose unit.
It seems that substitution in the C2 position is slightly
more dominant [6]. The solubility of CMC in water is a
key parameter in their applications and a higher DS will
normally improve the solubility of the CMC. Theoretically,
the maximum DS is 3. CMC is soluble in water when DS
is higher than 0.4. Most research [5-7] has achieved a DS
ranging from 0.5 to 2.0. The DS of commercially available
CMC is in the range of 0.4-1.4. Recently, many researchers
are trying to find a way to achieve CMC with higher DS in
order to improve commercial products. It has been shown
that cellulose sources have a very important role since the
crystalline content and the size of cellulose are the most
crucial parameters for attaining CMC with a high DS [5].
Finding raw materials based on agricultural by-products to
produce CMC has been obtaining more and more interest
from researchers. For example, the use of cellulosic sources
as an alternative to virgin softwood pulp to synthesize
CMC has been reported [4-10]. N. Haleem, et al. (2014)
[7] obtained cellulose fibre with sizes of 15-20 μm from
cotton waste by acid hydrolysis with 10 M H
2
SO
4
at 70-
80
o
C for 1 h. Generally, cellulose extraction is a complicated
process, and several steps have to be performed to gain a
high degree of substitution. Thus, finding new, available,
and cheap cellulose sources for CMC preparation is of great
significance.
Pineapple is one of the most popular tropical fruits in
Vietnam. During harvesting, pineapple leaves are discarded.
Their release into the environment, in turn, leads to pollution
of our living environmental system [11, 12]. However,
pineapple leaves are an abundantly available and potential
source of cellulose. These leaves contain about 65-70%
dry weight of cellulose [11, 13]. The process of extracting
cellulose from pineapple leaf is simple [14-18], and the
extracted cellulose has relatively low crystal content as
compared to that of cotton waste [7], paper sludge [8], rice
straw [19, 20], and other sources [3, 4, 6, 19]. These two
factors positively affect
the possibility of synthesis of CMC
with high DS.
The purpose of this work is to confirm the potential
of Vietnamese pineapple leaf waste as a raw material
for industrial production of CMC with high degree of
substitution.
Synthesis and characterization of carboxymethyl cellulose with
high degree substitution from Vietnamese pineapple leaf waste
Thi Dieu Phuong
Nguyen, Nhu Thi Le, Tien Manh
Vu, Truong Sinh
Pham, Thi Dao
Phan,
Ngoc Lan
Pham, Thi Tuyet Mai
Phan
*
Faculty of Chemistry, University of Science, Vietnam National University, Hanoi
Received 15 June 2021; accepted 8 September 2021
*
Corresponding author: Email: [email protected]
Abstract:
In this work, cellulose
was successfully extracted from pineapple leaf waste by 0.75 M NaOH at 90
o
C and
5 M HNO
3
at 70
o
C for 1.5 h and 5 h, respectively. The obtained cellulose fibres, with average diameters of
150-300 nm, were converted to
carboxymethyl cellulose (CMC) by esterification. The pure cellulose was soaked
in a solution mixture of isopropanol and NaOH for 2 h. It was then reacted with chloroacetic acid (MCA) at
60
o
C for 1.5 h. The optimum conditions for carboxymethylation were found to be 5 g cellulose, 1.5 g MCA,
and 15 ml 16% w/v NaOH. The obtained CMC had a high degree of substitution (DS) of 2.3. The properties of
CMC were determined.
Keywords:
carboxymethyl cellulose,
cellulose degree of substitution, Vietnamese pineapple leaf waste.
Classification number:
2.2
DOI : 10.31276/VJSTE.64(3).13-18
Physical
s
ciences
|
Chemistry
14
september 2022
Volume 64 Number 3
Materials and methods
Materials
The pineapple leaves were collected from the pineapple
Dong Giao farm, Tam Diep, Ninh Binh, Vietnam. The
pineapple leaves were cut into 5 mm using a grinding
machine, then dried in an oven at 60
o
C for 24 h. The samples
were kept in zipper polyethylene bags.
For this study, the following acids, such as nitric acid
65%, monochloroacetic acid (MCA) (UK) 99.7%, acetic
acid 99.9% and sodium hydroxide 99.9% (Merck), as well as
methanol 99.8% and ethanol 99.9% from Xilong Chemical,
isopropanol 99.7% (Merck), and acetone 99.8% (Merck)
were used. They were of high purity.
Methods
Cellulose extraction:
The extraction process of cellulose
from pineapple leaf waste is illustrated in Fig. 1.
Fig. 1. Schematic illustration for the cellulose extraction process
from pineapple leaf waste.
The d
ry pineapple leaf waste powder was treated with
0.75 M NaOH at 90
o
C and 5 M HNO
3
at 70
o
C for 1.5 and
5 h, respectively. This mixture was then centrifuged at
3000 rpm for 20 min to remove large particles and washed
with warm distilled water until the indicator paper did not
change colour. The residue was dried in an oven at 60
o
C
overnight until the weight remained constant. Finally, the
dried cellulose was ground and kept in a polyethylene bag
for the next process modification.
The yield of the cellulose was gravimetrically determined
and expressed as the weight of the extracted dried cellulose
to 100 g of the dried pineapple leaf used for extraction. This
was repeated 3 times for each extraction condition and the
yield average and the standard deviation were calculated.
Equation (1) below was used for the determination of the
yield of cellulose:
The dry pineapple leaf waste powder was treated with 0.75
M NaOH at 90
o
C and 5
M HNO
3
at 70
o
C for 1.5
and 5 h, respectively. This mixture was then centrifuged at 3000
rpm for 20 min to remove large particles and washed with warm distilled water until
the
indicator paper did not change colour. The residue was dried in an oven at 60
o
C overnight
until
the we
ight remained constant.
Finally, the dried cellulose was ground and kept in a
polyethylene bag for the next process modification.
The yield of the cellulose was gravimetrically determined and expressed as
the
weight of the extracted dried cellulose to 100 g of the dried pineapple leaf used for
extraction.
This
was repeated 3 times for each extraction condition
and the yield average
a
nd the standard deviation were calculated.
Equation (1
)
below
wa
s used for the determination of the yield of cellulose:
H(%
) =
m
m
0
×
100 (1)
where m
0
is the weight of initial dried pineapple leaf powder,
m
is the weight of obtained
cellulose,
and H
is the yield of cellulose (named as HC
).
S
ynthesis of CMC:
f
ive grams of extracted cellulose from Vietnam’s pineapple
leaf
powder was added to 150 ml
of isopropanol under continuous stirring for 60 min. Then,
15 ml of 16% NaOH solution was
dripped
into the mixture and further stirred for 1
h at
room temperature. The carboxymethylation was started when
y grams
of MCA (
y=
0.5,
1.0, 1.5, and 2.0 g)
were
added under continuous stirring for another 90 min at 60
o
C. The
solid part was neutralized with acetic acid to pH=7.0 and washed two times by soaking
in 20 ml
of ethanol to remove undesirable by
products. The obtained CMC was filtered
and dried at 60ºC un
til it
reached constant weight
, and it was then kept in the polyethylene
bag. Equation (1) above is also used to determine the yield of the CMC (HCMC) where
m
is the weight of the obtained CMC, and
m
0
is the weight of the cellulose used for the
CMC synthesis.
(1)
where m
0
is the weigh
t of initial dried pineapple leaf powder,
m is the weight of obtained cellulose, and H is the yield of
cellulose (named as HC).
Synthesis of CMC:
Five grams of extracted cellulose
from Vietnam’s pineapple leaf powder was added to 150
ml of isopropanol under continuous stirring for 60 min.
Then, 15 ml of 16% NaOH solution was dripped into the
mixture and further stirred for 1 h at room temperature. The
carboxymethylation was started when y grams of MCA
(y=0.5, 1.0, 1.5, and 2.0 g) were added under continuous
stirring for another 90 min at 60
o
C. The solid part was
neutralized with acetic acid to pH=7.0 and washed two
times by soaking in 20 ml of ethanol to remove undesirable
by-products. The obtained CMC was filtered and dried at
60ºC until it reached constant weight, and it was then kept
in the polyethylene bag. Equation (1) above is also used to
determine the yield of the CMC (HCMC) where m is the
weight of the obtained CMC, and m
0
is the weight of the
cellulose used for the CMC synthesis.
Infrared
spectroscopy:
FTIR analysis of the
obtained cellulose and CMC were performed by a FT/
IR-6300 spectrometer using KBr pellet methods. The
spectral
resolution was 4 cm
-1
and the absorption region
was
600-4000 cm
-1
.
X-ray diffraction:
The crystallinity index (CrI) of the
obtained cellulose and CMC were analysed by Shimadzu
XRD-6100 diffractometer. The diffraction angle ranged
from 5 to 80° (0.05°/min). The measurement was carried
out at 30 kV and 15 mA under Cu K
α
radiation. The CrI of
the samples was calculated by Eq. (2):
Infrared spectroscopy:
FTIR analysis of the obtained cellulose and CMC were
performed by
a
FT/IR
6300 spectrometer using KBr pellet method
s. The
spectral
resolution was 4 cm
1
and
the absorption region was
600
4000 cm
1
.
X-ray diffraction:
t
he crystallinity index (CrI) of the obtained cellulose and CMC
were analysed
by Shimadzu XRD
6100
diffractometer. The diffraction angle ranged from
5 to
8
(0.05°/min). The measurement was carried out at 30
kV and 15
mA under Cu K
α
radiation. The
CrI of the samples was calculated by Eq. (2
)
:
CrI (%)
=
I
002
−I
am
I
002
×
100 (2)
where
I
002
:
(2θ=
22.8°) and
I
am
:
(2θ=
18°) correspond to the crystalline and amorphous
regions, respectively [21].
Particle size measurement:
The particle size of the obtained cellulose was measured by
a
Shimadzu S
ald-
2001
Analyser. First, the cellulose suspension was diluted to 0.05
0.2 wt% concentration
.
Then
, it was measured in a container.
Scanning electron microscopy (SEM):
The surface of the separated cellulose is observed by the SEM images. The SEM
images were done on a
Hitachi S4800
NHE scanning electron microscope (Hitachi Co.,
Ltd., Japan).
Determination of Degree of Substitution (DS):
d
egree of Substitution
of CMC is
determined according to ASTM 1994 [22].
Sample preparation:
350 m
l
of ethanol was added to a 500
ml
conical flask
containing 5 g of CMC to the nearest 0.1
mg. The s
uspension in the flask was shaken for
30 min, then filtered through a porous funnel. The solvent was removed by heating at
100°C for 60 min. The sample was dried in an oven at 110°C until
a
constant weight
was
reached
.
(2)
where I
002
: (2θ=22.8°) and I
am
: (2θ=18°) correspond to the
crystalline and amorphous regions, respectively [21].
Particle size measurement:
The particle size of the
obtained cellulose was measured by a Shimadzu Sald-2001
Analyser. First, the cellulose suspension was diluted to 0.05-
0.2 wt% concentration. Then, it was measured in a container.
Scanning electron microscopy (SEM):
The surface of the
separated cellulose is observed by the SEM images. The
SEM images were done on a Hitachi S4800-NHE scanning
electron microscope (Hitachi Co., Ltd., Japan).
Determination of Degree of Substitution (DS):
Degree
of Substitution of CMC is determined according to ASTM
1994 [22].
Physical sciences
|
Chemistry
15
SEPTEMBER 2022
Volume 64 Number 3
Sample preparation:
350 ml of ethanol was added
to a 500 ml conical flask containing 5 g of CMC to the
nearest 0.1 mg. The suspension in the flask was shaken for
30 min, then filtered through a porous funnel. The solvent
was removed by heating at 100°C for 60 min. The sample
was dried in an oven at 110°C until a constant weight was
reached.
Procedure:
2 g of the dried obtained substance to the
nearest 0.1 mg was put to a tared porcelain crucible. The
crucible was carefully charred with a small flame, then with
a large flame for 10 min. The cooled residue was moistened
with 3-5 ml of concentrated sulfuric acid. Next, the crucible
was cautiously heated until the fuming was finished. Then,
the crucible was cooled to room temperature. About 1 g
of ammonium carbonate was added. The powder was
distributed over the content of the entire crucible. It was
heated again with a small flame until the fuming stopped,
and then was maintained at a dull red heat for 10 min.
The treatment procedure was repeated with sulfuric acid
and ammonium carbonate if the residual sodium sulphate
still contained some carbon. The crucible was cooled in
a desiccator and weighed. The sodium content, A, was
calculated by Eq. (3):
Procedure: 2 g of the dried obtained substance to the nearest 0.1
mg was put to a tared
porcelain crucible.
T
he crucible
was carefully charred
with a small flame, then with a
large flame for 10
min
. The cooled
residue was moistened with 3
5 ml
of concentrat
ed
sulfuric acid. Next
,
the crucible was cautiously heated until the fuming
wa
s finished.
The
n, the
crucible was cooled to room temperature.
About 1 g of ammonium carbonate
was added. The powder was distributed over the
content of the entire
crucible. It was
heated again with a small flame until the fuming stopped
,
and then
was
maintained at a
dull red heat for 10 min. The treatment procedure
wa
s repeated with sulfuric acid and
ammonium carbonate if the residual sodium
sulphate
still contain
ed
some carbon. The
crucible was cooled in a desiccator and weighed. The sodium content
,
A
,
was calculated
by
Eq.
(
3
)
:
A
(
%
)
=
a
×
32
.
28
b
(
3
)
where
a
is the weight of the sodium
sulphate
residue
and
b
is the weight of the dry sample.
The degree of substitution was calculated by
Eq. (
4
)
:
DS
=
162
×
A
2300
80
×
A
(
4
)
w
here 162 is the molecular weight of the glucose unit and 8
0
is the net increment in the
anhydrous glucose unit for every substituted carboxymethyl group.
Results and discussion
Extraction
of
cellul
ose
from Vietnam’s pineapple leaf waste
The extracted cellulose yield was 55±1.75 wt.%
.
This yield value is much higher
than that of cellulose extracted from other agricultural biomass
es
such as 37
.
67 wt.%
fr
om the
Baobab
fruit shell [18] and 32 wt.% fr
om
rice straw [19]. The high cellulose
content
would guarantee a lower price for cellulose derivatives.
The morphology of
the
obtained cellulose is shown in Fig
.
2.
(3)
where a is the weight of the sodium sulphate residue and b is
the weight of the dry sample.
The degree of substitution was calculated by Eq. (4):
Procedure: 2 g of the dried obtained substance to the nearest 0.1
mg was put to a tared
porcelain crucible.
T
he crucible
was carefully charred
with a small flame, then with a
large flame for 10
min
. The cooled
residue was moistened with 3
5 ml
of concentrat
ed
sulfuric acid. Next
,
the crucible was cautiously heated until the fuming
wa
s finished.
The
n, the
crucible was cooled to room temperature.
About 1 g of ammonium carbonate
was added. The powder was distributed over the
content of the entire
crucible. It was
heated again with a small flame until the fuming stopped
,
and then
was
maintained at a
dull red heat for 10 min. The treatment procedure
wa
s repeated with sulfuric acid and
ammonium carbonate if the residual sodium
sulphate
still contain
ed
some carbon. The
crucible was cooled in a desiccator and weighed. The sodium content
,
A
,
was calculated
by
Eq.
(
3
)
:
A
(
%
)
=
a
×
32
.
28
b
(
3
)
where
a
is the weight of the sodium
sulphate
residue
and
b
is the weight of the dry sample.
The degree of substitution was calculated by
Eq. (
4
)
:
DS
=
162
×
A
2300
80
×
A
(
4
)
w
here 162 is the molecular weight of the glucose unit and 8
0
is the net increment in the
anhydrous glucose unit for every substituted carboxymethyl group.
Results and discussion
Extraction
of
cellul
ose
from Vietnam’s pineapple leaf waste
The extracted cellulose yield was 55±1.75 wt.%
.
This yield value is much higher
than that of cellulose extracted from other agricultural biomass
es
such as 37
.
67 wt.%
fr
om the
Baobab
fruit shell [18] and 32 wt.% fr
om
rice straw [19]. The high cellulose
content
would guarantee a lower price for cellulose derivatives.
The morphology of
the
obtained cellulose is shown in Fig
.
2.
(4)
where 162 is the molecular weight of the glucose unit and 80
is the net increment in the anhydrous glucose unit for every
substituted carboxymethyl group.
Results and discussion
Extraction of cellulose from Vietnam’s pineapple leaf
waste
The extracted cellulose yield was 55±1.75 wt.%. This
yield value is much higher than that of cellulose extracted
from other agricultural biomasses such as 37.67 wt.% from
the Baobab fruit shell [19] and 32 wt.% from rice straw [20].
The high cellulose content would guarantee a lower price
for cellulose derivatives.
The morphology of the obtained cellulose is shown in
Fig. 2.
Fig. 2.
SEM images of pineapple leaf cellulose at (
a) 10,000 x
magnification (5 μm size bar) and (
b
) 35,000 magnification (1 μm
size bar).
As can be seen in from the SEM images, the obtained
cellulose showed uniform size with average diameters of
150-300 nm, which was similar to that of another reported
work [23]. It is worth mentioning that the separation of
cellulose in this work is easier and the cellulose obtained
had a significantly higher yield compared to that of previous
reports [7, 14, 15, 16, 17]. Of course, this comparison is
only relative because cellulose yield depends on the method
and conditions of separation. The FTIR spectroscopy of
obtained cellulose is displayed in Fig. 3.
Fig. 3. FTIR spectroscopy of extracted cellulose and CMC from
pineapple leaf waste.
As for cellulose, as shown in Fig. 3, there is a large band
at 3329 cm
-1
corresponding to the OH group. The peak at
2899 cm
-1
represents the C-H stretching vibrations. The
peak at 1159 cm
-1
can be assigned to C-O-C stretching of
the β(1,4)-glycosidic linkage. Besides, the peaks at 1367
and 1427 cm
-1
are attributed to the -C-H and -C-O bending
vibrations, respectively, in the polysaccharide rings. The
vibration of the -C-O group of secondary alcohols in
the cellulose chain backbone appears at 1105 cm
-1
. The
absorption band range of 879-1051 cm
-1
is assigned to the

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