Methane storage in multi-walled carbon nanotube by
Moleular Dynamic simulation (MD)
Reza Alizadeh 1 and Masomeh shadi2
1-Assistant professor of behbahan khatam alanbia university of technology-.faculty of Environment.
khozestan- Iran
2- Behbahan khatam alanbia university of technology-faculty of Environment. khozestan- Iran
Abstract
Adsorption equilibria of methane on multi-walled carbon nanotube (MWCNT) were studied. two zeolites
(DAY and HSZ-320) Were used in the experiment to make a comparison of the adsorption properties.By
using a static volumetric method at 303.15 K, 313.15 K and 323.15 K at pressures up to 3 MPa, the
experimental data of zeolites were explained by the Sips isotherm while those of MWCNT were
correlated by the hybrid Langmuir-Sips equation. As a function of surface loading, the isosteric enthalpies
of adsorption of the MWCNT were calculated by the Clausius-Clapeyron equation.
introdauction
since the demand for energy for vehicles has increased, hydrogen and natural gas, nonpolution fuels,
become desired sources. consequently , the storage of those sources whose major component is methane
turns into a significant matter.normally, heavy steel cylinders having high pressure 20-30 MPa inside are
used to store the compressed natural gas while for adsorbed natural gas , only 4 MPa is required in
lightweight containers. obviously, adsorbed natural gas offers lower cost of storage[2]. There are many
types of porous media used to absorb natural gas such as molecular sieves, zeolit, activated
carbon,etc.recently, carbon nanotubes have become a popular porous media because of their exceptional
properties in terms of high tensile strength, uniform porosity, relative inertness and electrical
conductivity[1].
There are many researchers studing adsorption of methane on carbon nanotubefor example, Bekyarova et
al. observed the absorption of methane in a single-walled carbon nanotube (SWCNT).They measured
volumetric capacity which can reach 160 v/v[5]. Kaneko et al. applied a density functional theory method
to study the adsorption of methane on SWCNT. They discovered that SWCNT with disordered structure
could be successfully used as storage media for methane[6]. Cao et al. improved the SWCNT structure for
methane storage at room temperature[3].
Mostly, many studies focus on SWCNT used to adsorb methane because SWCNT can be fabricated in
diverse bundle sized and lengths with containing amorphous carbon. However, MWCNT is able to make
uniform lenghs without bundle formation and amorphous carbon. Therefore, the gas adsorption on
SWCNT and MWCNT appears to be different. In this paper, to understand the adsorption mechanisms
and improve adsorption models, the adsorption equilibria of methane on MWCNT were measure.
Discussion
The adsorption data of methane on MWCNT and zeolites at various temperatures, 303.15 K,
313.15 K and 323.15 K, are shown in Table 1, Table 2 and Table3. For the table, P is the
pressure used the system, T is the studied temperature and N is the amount of adsorbed methane.
From Result, it was found that at lower pressure, the Langmuir isotherm is used to demonstrate
the surface adsorption region. At high pressure, the Sips isotherm is used to describe the
capillary condensation because it is better than flexible fit of isotherm data.
By isotherms test used for the simulation of adsorption of methane on MWCNT, the hybrid
isotherm model of the Langmuir and Sips isotherm perfectly fits the experimental adsorption
data[8]. Langmoir-Sips:
bs P 1/ n
b P
N  qm ( L

)
1  b L P 1  bs P 1/ n
N = amount adsorbed, P = equilibrium pressure, qm, Bl, bs and n = isotherm parameters which are found
by a nonlinear least-squares fitting routine of the Nelder-Mead simplex method.
Table 4. Langmuir-Sips Isotherm Parameters for Methene onto MWCNT[7].
parameters
qm
bl
bs
n
T = 303.15 K
1.056
1.436
1.45 × 10−3
11.180
T = 313.15 K
0.967
1.025
3.886 × 10−4
11.070
T = 323.15 K
0.824
0.686
3.042 × 10−5
13.700
Table 1. Adsorption Isotherm Data for Methane onto HSZ-320 [7].
T = 303.15 K
P
N
(MPa)
(mmol.g−1)
0.045
0.075
0.098
0.140
0.163
0.246
0.282
0.391
0.466
0.563
0.606
0.673
0.748
0.758
0.949
0.859
1.201
0.961
1.517
1.050
1.920
1.127
2.632
1.231
T = 313.15 K
P
N
(MPa)
(mmol.g−1)
0.023
0.033
0.058
0.066
0.111
0.131
0.186
0.223
0.279
0.320
0.443
0.456
0.639
0.593
0.787
0.689
0.968
0.770
1.207
0.850
1.583
0.924
2.082
1.013
2.696
1.076
T = 323.15 K
P
N
(MPa)
(mmol.g−1)
0.038
0.047
0.087
0.091
0.173
0.163
0.302
0.272
0.490
0.399
0.790
0.553
1.038
0.657
1.341
0.733
1.710
0.8200
2.153
0.8864
2.618
1.9436
Table 2. Adsorption Isotherm Data for Methane onto DAY [7].
T = 303.15 K
P
N
(MPa)
(mmol.g−1)
0.029
0.029
0.069
0.074
0.119
0.126
0.206
0.225
0.312
0.349
T = 313.15 K
P
N
(MPa)
(mmol.g−1)
0.053
0.053
0.125
0.122
0.206
0.208
0.324
0.317
0.438
0.426
T = 323.15 K
P
N
(MPa)
(mmol.g−1)
0.038
0.015
0.089
0.058
0.133
0.101
0.207
0.168
0.278
0.229
0.416
0.552
0.740
1.001
1.291
1.653
2.079
2.607
0.462
0.597
0.773
0.988
1.197
1.416
1.588
1.774
0.633
0.992
1.169
1.584
1.851
2.141
2.568
0.593
0.866
0.994
1.204
1.333
1.451
1.611
0.390
0.558
0.790
1.095
1.422
1.873
2.533
0.332
0.461
0.634
0.840
1.011
1.196
1.449
Table 3. Adsorption Isotherm Data for methane onto MWCNT [7].
T = 303.15 K
P
(MPa)
0.032
0.081
0.156
0.255
0.367
0.559
0.798
1.114
1.546
1.948
2.376
2.745
N
(mmol.g−1)
0.019
0.136
0.227
0.256
0.403
0.438
0.518
0.698
0.862
1.481
1.818
1.886
T = 313.15 K
P
(MPa)
0.041
0.113
0.227
0.388
0.598
0.862
1.196
1.570
1.979
2.408
2.838
N
(mmol.g−1)
0.033
0.154
0.185
0.263
0.416
0.438
0.523
0.611
1.084
1.498
1.698
T = 323.15 K
P
(MPa)
0.038
0.092
0.169
0.292
0.441
0.627
0.878
1.189
1.551
1.968
2.507
2.836
N
(mmol.g−1)
0.043
0.064
0.091
0.175
0.224
0.244
0.341
0.336
0.378
0.684
1.252
1.350
Consider Table 1, Table 2 and Table 3. The adsorption capacity of methane on MWCNT and zellites
depends on temperature[4]. Methane tends to be more asorbed at the high temperature. However, the
adsorption of methane MWCNT decreases at low pressure especial lower than 1.5 MPa. To make it clear,
By isotherms test used for the simulation of adsorption of methane on HSZ-320 and DAY, the Sip
equation fits the experimental adsorption data[9].
Sips:
N 
qm, b, n = isotherm parameters
q m bp 1/ n
1  bp 1/ n
Table 5. Sips Isotherm Parameters for methane onto MWCNT[7].
adsorbate
HSZ-320
DAY
T
K
303.15
313.15
323.15
303.15
313.15
323.15
qm
mmol.g−1
1.567
1.315
1.299
2.923
3.049
2.616
b
MPa−1
1.288
1.426
0.958
0.514
0.400
0.420
n
0.928
0.857
0.940
0.867
0.921
0.869
error %
0.022
0.062
0.026
0.049
0.036
0.067
The isosteric enthalpy is an assessment of the interaction between adsorbent lattice atoms and adsorbate
molecules. It is sometimes used as an evaluation of the energetic heterogeneity of a solid surface. It has
been believed that surface heterogeneity results from the structural, geometric and energetic
heterogeneity. The isosteric enthalpies of adsorption can be found by the Clausius-Clapeyron equation[7].
  ln P 
Q st  R 

  (1 / T )  N
P = pressure, T = temperature, R = gas constant and Qst = isosteric enthalpy
The isosteric of MWCNT, HSZ-320 and DAY, are calculated.
Apowerful adsorption dynamic heterogeneity of MWCNT and HSZ-320 zeolite is noticed.
Particularly, the MWCNT adsorbents in the experiment have an actively heterogeneous surface.
The isosteric enthalpy is in the range between -15 kJ mol-1 and -50 kJ mol-1 for MWCNT and in
the range between -10 kJ mol-1 and -40 kj mol-1 for HSZ-320 zeolite, while it is an almost constat
value of -15 kj mol-1 for DAY zeolite.
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