Adsorption
of
Isopropyl
Alcohol
by
Carbon
Nanotubes
Paper # 1133
Shihchieh Hsu, Mengshan Lee and Chungsying Lu
Department of Environmental Engineering, National Chung Hsing University,
Taichung 40227, Taiwan
ABSTRACT
Carbon nanotubes known as a novel discovered form of carbon. They are also
expected to be excellent adsorbents for adsorption of trace pollutants from water or air.
The objective of this paper is to study adsorption characteristics of isopropyl alcohol
(IPA) vapor in the range of 40-800 ppmv from air stream with single-walled carbon
nanotubes (SWCNTs).
The test results indicated that the nature of the SWCNTs surface was greatly
improved after purification by concentrated hypochlorous acid, which made SWCNTs
become more hydrophilic and suitable for adsorption of IPA vapor from air stream. A
significantly higher adsorption capacity of IPA vapor was found with purified
SWCNTs than with raw SWCNTs. The BET isotherm is the best model to describe
adsorption behavior of IPA vapor onto purified SWCNTs, followed by the Freundlich
isotherm, and then the Langmuir isotherm.
Keyword: Single-walled carbon nanotubes, adsorption, isopropyl alcohol.
INTRODUCTION
Volatile organic compounds (VOCs) are among the most common pollutants emitted
by chemical process industries dealing with manufacture and process of chemicals.1
Isopropyl alcohol (IPA), kind of VOCs, a widely used solvent and drying agent was
chosen as a representative compound for low molecular weight, polar organic
molecules.2 With increasing environmental awareness and stringent regulations of
pollutants emission, adsorption offers an effective means to control VOCs emissions
in low concentration levels.3
Since carbon nanotibes (CNTs) were first discovered by Ijima,4 the interaction of
CNTs with their environmental, especially with gases or dopants adsorbed on their
interior or exterior surface, has attracted increasing attention due to the anticipated
influence on key properties of these materials.5 For the potential application of CNTs
in environmental field, they are relatively new adsorbents for adsorption of trace
pollutant from water or air. Long and Yang6 reported that a significantly higher dioxin
removal efficiency is found with CNTs than with activated carbon.
CNTs have been proved to possess great potential application in environmental
protection by foregoing investigator. However, the studies on the adsorption of trace
pollutants with CNTs are still limited in the literature. The aim of this paper was to
study adsorption characteristics of IPA vapor with single-walled nanotubes (SWCNTs)
in the concentration range of 40-800 ppmv.
MATERIAL AND METHOD
Preparation of purified SWCNTs
Single-walled carbon nanotubes (L-SWCNTs, Nanotech Port Co., Shenzhen, China),
with the diameter less than 2 nm and the length range 5-15 µm, were selected as
adsorbents in this study. The mass ratio of amorphous carbon was less than 5%.
The raw SWCNTs were dispersed into a 150 ml flask containing 70% concentrated
hypochlorous acid solutions. The volume ratio of acid solution to SWCNTs was kept
at 40. The mixed solution was refluxed using an ultrasonic cleaning bath (Model
D400H, Delta Instruments Co., USA) at 85oC for 3 h to remove metal catalysts and
amorphous carbon. After cooling, the SWCNTs were washed by deionized water.
Finally, the SWCNTs-containing solution was filtered by 0.45 µm glass-fiber filter to
obtain purified SWCNTs.
Experimental set-up
The experimental set-up for adsorption of IPA by SWCNTs is shown in Fig. 1. The
adsorption column was made of Pyrex and had a length of 30 cm and an internal
diameter of 1.5 cm. A 3-cm headspace was designed for the IPA inlet while a 3-cm
bottom space was designed for the outlet of treated air. The adsorption column was
filled with 0.7 g SWCNTs (packing height=2.3 cm) and hold by a glass supporter in
the middle of the column. The column was placed within a temperature control box
(Model CH-502, Chin Hsin, Taipei, Taiwan) to maintain gas temperature at 25oC.
Figure 1 Schematic diagram of experimental setup
Compressed air was passed first through a filtration device (LODE STAT compressed
air dryer, Model LD-05A, Taipei, Taiwan) to remove moisture, oil and particulate
matter. After purification, the minor air stream was passed through the first glass
bottle containing pure IPA solution (J. T. Baker, NJ, USA, 99.9% purity) to produce
IPA vapor. The IPA vapor was then mixed with the major air stream in the second
glass bottle and was passed downwards into the adsorption column. The influent IPA
concentration was controlled by regulating the minor air stream rate using mass flow
controller, while the empty-bed residence time (EBRT) was controlled by regulating
the major air stream rate using mass flow controller (Model 247C four channel
read-out and mass flow controllers, MKS instrument Inc., MA, USA). The variations
in the influent IPA concentration were within 10% and the air stream was controlled at
0.217 lpm (EBRT=1.12 s). The effluent air stream was then flowed into an auto
sampling system with a gas chromatograph (GC, Model SRI 8610C gas
chromatograph, SRI Instruments, CA, USA) equipped with a flame ionization
detector (FID).
Isopropyl alcohol adsorbed by SWCNTs was calculated as follows:
q=
1 t
Q ⋅ (C in − C eff
m ∫0
) dt
(1)
where q is the amount of IPA adsorbed by SWCNTs (mg/g); Q is the influent flow
rate (lpm); Cin is the influent IPA concentration (mg/l); Ceff is the effluent IPA
concentration (mg/l); m is the SWCNTs dosage (g) and t is the contact time (min).
Analytical methods
Concentration of isopropyl alcohol was determined using a GC-FID. A 15 m fused
silica capillary column with 0.32 mm inner diameter and 1.0 µm film thickness
(Supelco wax, Supelco Inc., PA, USA) was used for isopropyl alcohol analysis. The
GC-FID was operated at injection temperature of 150 oC, detector temperature of 200
o
C and oven temperature of 130 oC.
The total surface area, mean pore size, pore size distribution and pore volume of
purified SWCNTs were determined by a BET sorptometer (Model BET-202A, Porous
Materials Inc., NY, USA).
RESULTS AND DISCUSSION
Characterization of purified SWCNTs
Shape and size are essential as identifying characterization of adsorbents since these
parameters determine the specific surface area and thus influence adsorption rate and
sensitivity to environmental conditions. Figure 2 shows the scanning electron
microscope (SEM) images of raw and purified SWCNTs, respectively. As can be seen,
the cylindrical-shapes of isolated SWCNTs were not changed after purification
process. However, it is evident that there are many metal catalysts attached on the
surface of raw SWCNTs. Due to inter-molecular force, the isolated SWCNTs of
different size and direction form an aggregated structure. Figure 3 shows the high
resolution transmission electron microscope (HRTEM) images of raw and purified
SWCNTs. As can be seen, the aggregated structure of raw SWCNTs was destroyed
after purification process.
Figure 2 SEM images of CNTs: (a) raw SWCNTs, (b) purified SWCNTs.
Figure 3 HR-TEM images of CNTs: (a) raw SWCNTs, (b) purified SWCNTs.
Figure 4a and 4b exhibit the pore size distribution of raw and purified SWCNTs,
respectively. It is obvious that the pore size of raw SWCNTs is a bimodal distribution.
The major peak is located in the size range 4-50 nm while the minor peak is located in
the size range 1-3 nm. The 1-3 nm pores are the SWCNTs inner cavities and
responsible for around 25% of the total pore volume. The 4-50 nm pores are likely to
be contributed by aggregated pores which are formed by the confined space among
the isolated SWCNTs 7 and responsible for around 74% of the total pore volume.
After purification, the pore radius in 1-3 nm range increased from 25 to 67% of the
total pore volume while the pore radius in 4-50 nm range decreased from 74 to 30%
of the total pore volume. This is due to the isolated SWCNTs were separated apart
after purification process. Particularly, the pore volume for the pore radius less than 2
nm significantly increased after purification process, likely because of the removal of
metal catalysts and amorphous carbon. Table 1 lists the BET measurement results of
raw and purified SWCNTs. As can be seen, the specific surface area, the total pore
volume and the average pore radius decreased after purification process.
Table 1. BET measurement results of raw and purified SWCNTs
Specific
surface area
m2/g
Total pore
volume
cm3/g
Average pore
radius
nm
Raw SWCNTs
590.5418
1.1231
3.8037
Purified SWCNTs
423.1692
0.50423
2.0613
Figure 5 shows the Fourier transform infrared (FTIR) spectroscopy of raw and
purified SWCNTs. It is seen that the raw SWCNTs exhibit less significant peaks. In
contrast, the purified SWCNTs have more evident peaks at wavenumber of 1600,
1750 and 2850-2910 cm-1 which are associated with aromatic ring groups (quinone),
carboxylic acids and phenolic groups (O-H), asymmetric and symmetric C-H
stretching vibration in aliphatic, respectively.8, 9 It is evident that there are many
oxygen-containing groups attached on the surface of purified SWCNTs.
Figure 4a Pore size distribution of raw SWCNTs
Volume Fraction (cm3/g)
2.0
1.5
1.0
0.5
0.0
1
10
100
Proe Radius (nm)
Figure 4b Pore size distribution of purified SWCNTs
Volume Fraction(cm3/g)
2.0
1.5
1.0
0.5
0.0
1
10
Pore Radius (nm)
100
Figure 5 FTIR spectra of CNTs: (a) raw SWCNTs, (b) purified SWCNTs.
46
44
42
b
quinone
Transmission(%)
40
38
36
34
-COOH
32
30
C-H
a
28
26
24
22
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Effects of contact time
Figure 6a and 6b show the effect of contact time on the adsorption of 40 and 800
ppmv IPA vapor onto raw and purified SWCNTs, respectively. It is noted that the
adsorption of IPA vapor increases quickly with time and then reaches equilibrium.
The contact time for adsorption IPA onto raw SWCNTs to reach equilibrium is equal
to 480 min for C0=40 ppmv and equal to 120 min for C0=800 ppmv. The final
adsorption capacity of IPA reaches 16.21 mg/g for 40 ppmv and achieves 53.33 mg/g
for C0=800 ppmv. The contact time for adsorption IPA onto purified SWCNTs to
reach equilibrium exceeds 1020 min for C0=40 ppmv and equal to 150 min for
C0=800 ppmv. The final adsorption capacity of IPA reaches 43.12 mg/g for 40 ppmv
and achieves 79.36 mg/g for C0=800 ppmv. The longer contact time to reach
equilibrium for lower initial IPA concentration may be explained by the fact that
diffusion mechanism control the adsorption of IPA onto SWCNTs. Reid et al.10
indicated tat the mass diffusivity decreases with decreasing concentration under very
dilute solution and causes the decrease of diffusion flux of adsorbate onto the surface
of the adsorbent. As compared to the adsorption of IPA with raw and purified
SWCNTs, it is evident that it takes longer contact time for purified SWCNTs.
Furthermore, the adsorption capacity of IPA onto purified SWCNTs is higher than
onto raw SWCNTs. This can be attributed to more oxygen-containing groups attached
on the purified SWCNTs surface, which make purified SWCNTs become more
hydrophilic and suitable for adsorption of IPA vapor.
Figure 6a Effect of contact time on adsorption of IPA onto SWCNTs with the
initial concentration (C0) of 40 ppmv
45
40
Capacity (mg/g)
35
30
25
20
15
Purified S WCNTs
10
Raw S WCNTs
5
0
0
200
400
600
800
1000
1200
Contact time (min)
Figure 6b Effect of contact time on adsorption of IPA onto SWCNTs with the
initial concentration (C0) of 800ppmv
80
70
Capacity (mg/g)
60
50
40
30
Purifie d SWCNTs
Raw SWCNTs
20
10
0
0
30
60
90
Contact time (min)
120
150
180
Adsorption isotherms
Figure 7 shows the adsorption isotherms of IPA onto purified SWCNTs. The adsorbed
amounts of IPA vapor are equal to 43, 54, 68, 74 and 79 mg/g, respectively, for the
equilibrium concentrations of 40, 99, 291, 468 and 817 ppmv.
The experimental data for IPA adsorption onto SWCNTs could be approximated by
the isotherm models of Langmuir (2), Freundlich (3), BET(4)
q=
q m k a Ce
1 + bC e
q = K f Ce
q=
(2)
n
(3)
qm k B Ce
(1 − Ce / C s )(C s − Ce + k B Ce )
(4)
where q is the mass of IPA adsorbed by SWCNTs; Ce is the equilibrium IPA
concentration, Cs is the saturation concentration; a and b are Langmuir constants; Kf
and n are Freundlich constants; and kB is BET constants. The isotherm constants were
obtained from fitting the adsorption equilibrium data and listed in Table 2. As can be
seen, the BET isotherm is the best model (R2=0.9992) to describe the adsorption of
IPA vapor onto SWCNTs, followed by the Freundlich isotherm (R2=0.9858), and then
the Langmuir isotherm (R2=0.9706).
Figure 7 Adsorption isotherms for IPA with purified SWCNTs
90
80
Adsorption Capacity (mg/g)
70
60
50
40
30
20
10
0
0
200
400
600
Conccentration (ppmv)
800
1000
Table 2. Constants of Langmuir, Freundlich and BET isotherm models for adsorption
of IPA onto purified SWCNTs
Adsorption isotherm models
qm
Langmuir model
ka
R2
nF
Freundlich model
kF
R2
qm
BET model
kB
R2
Values
78.0858
0.0296
0.9706
0.2049
20.8113
0.9858
81.6622
913.3160
0.9992
CONCLUSIONS
The following conclusions could be drawn from this study:
1.The inner pore volume of SWCNTs increased and the aggregated pore volume of
SWCNTs decreased after purification by concentrated hypochlorous acid.
2.More oxygen-containing groups were found on the surface of purified SWCNTs,
which made purified SWCNTs become more hydrophilic and suitable for adsorption
of IPA vapor.
3.A significantly higher adsorption capacity of IPA vapor was found with purified
SWCNTs than with raw SWCNTs.
4.The BET isotherm is the best model to describe adsorption behavior of IPA vapor
onto SWCNTs, followed by the Freundlich isotherm, and then the Langmuir
isotherm.
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