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Environmental Science & Technology is published by the American Chemical Society.1155 Sixteenth Street N.W., Washington, DC 20036Published by American Chemical Society. Copyright © American Chemical Society.However, no copyright claim is made to original U.S. Government works, or worksproduced by employees of any Commonwealth realm Crown government in the courseof their duties.
Article
Adsorption of CO2, CH4, and N2 on Ordered Mesoporous Carbon:Approach for Greenhouse Gases Capture and Biogas Upgrading
Bin Yuan, Xiaofei Wu, Yingxi Chen, Jianhan Huang, Hongmei Luo, and Shuguang DengEnviron. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es4000643 • Publication Date (Web): 22 Apr 2013
Downloaded from http://pubs.acs.org on May 4, 2013
Just Accepted
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1
Adsorption of CO2, CH4, and N2 on Ordered Mesoporous Carbon: Approach for
Greenhouse Gases Capture and Biogas Upgrading
Bin Yuana, Xiaofei Wua, Yingxi Chena, Jianhan Huanga, b, Hongmei Luoa, Shuguang
Denga, *
a Chemical Engineering Department, New Mexico State University, Las Cruces, New
Mexico, 88003, U.S.A.
b School of Chemistry and Chemical Engineering, Central South University, Changsha,
Hunan 410083, China
___________________
* Corresponding author
E-mail address: [email protected] (S. Deng), Phone: 1-575-646-4346; Fax:
1-575-646-7706.
To be submitted to Environmental Science & Technology (Online)
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Abstract:
Separation of CO2 and N2 from CH4 is significantly important in natural gas
upgrading, and capture/removal of CO2, CH4 from air (N2) is essential to greenhouse gas
emission control. Adsorption equilibrium and kinetics of CO2, CH4, and N2 on an ordered
mesoporous carbon (OMC) sample were systematically investigated to evaluate its
capability in the above two applications. The OMC was synthesized and characterized
with TEM, TGA, small-angle XRD, and nitrogen adsorption/desorption measurements.
Pure component adsorption isotherms of CO2, CH4 and N2 were measured at 278, 298,
and 318 K and pressures up to 100 kPa, and correlated with the Langmuir model. These
data were used to estimate the separation selectivities for CO2/CH4, CH4/N2, and CO2/N2
binary mixtures at different compositions and pressures according to the ideal adsorbed
solution theory (IAST) model. At 278 K and 100 kPa, the predicted selectivities for
equimolar CO2/CH4, CH4/N2, and CO2/N2 are 3.4, 3.7, and 12.8, respectively; and the
adsorption capacities for CH4 and CO2 are 1.3 mmol/g and 3.0 mmol/g, respectively. This
is the first report of a versatile mesoporous material that displays both high selectivities
and large adsorption capacities for separating CO2/CH4, CH4/N2, and CO2/N2 mixtures.
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Introduction
Natural gas, compared with other kinds of fossil fuels such as coal and petroleum,
produces less CO2 per energy unit, and is therefore regarded as a cleaner energy carrier.
The presence of N2 and CO2 impurities could reduce the heating value of the natural gas,
and cause equipment and pipeline corrosion (1). The pipeline specification requires that
the proportion of N2 and CO2 in the natural gas should be lower than 4% and 2%,
respectively (2). Separation of N2 and CO2 from natural gas (CH4) is inevitably
demanded in order to utilize the low quality natural gas, such as biogas.
Greenhouse gases (CO2 and CH4) contribute significantly to the global warming.
About 60% of the global warming effect is caused by the CO2 (3), most of which is
released from the flue gases (typically contains ~70% N2 and 15% CO2) of the industrial
plants (4, 5). Therefore, the CO2 capture/separation from the flue gas (N2) is important to
limit its release to the atmosphere. CH4 has much higher global warming potential (GWP)
than that of CO2 (6). Landfill gas (LFG) among others, is a principal source of the CH4
emission to the atmosphere (7). The N2 level in the LFG is particularly high (~20%) in
some cases (7). CH4 adsorption and CH4/N2 separation are essential to the reduction of
CH4 emission and upgrading of N2-contaminated LFG.
To date, various technologies have been developed for gas separation/purification,
such as cryogenic distillation, absorption, membrane separation, and adsorption. Among
these, adsorption has received intense interest due to its great advantages: high energy
efficiency, ease of control, low capital investment costs (2, 8). The main adsorbents
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been evaluated for the adsorptive separation of CO2/CH4, CH4/N2, and CO2/N2 binary
mixtures include zeolites (9–11), metal organic frameworks (MOFs) (1, 12–17), silicas
(18–21), carbon-based materials (8, 22–25), and clays (26, 27). MOF-177 shows a
CH4/N2 selectivity of 4 with a low CH4 adsorption capacity of 0.6 mmol/g at 298 K and
100 kPa (28). The CO2 uptake capacity on ASMS-3A silica molecular sieve at 283 K and
1 atm is as low as ~0.8 mmol/g (18). ETS-4, ETS-10, and their derivatives are attractive
adsorbents for natural gas upgrading (11, 29). However, the synthesis processes of these
adsorbents are very complex and time-consuming (30). It was also reported that
considerable heat was required to regenerate some zeolite adsorbents (31). Development
of robust adsorbents with adequate adsorption capacity, enough selectivity, and facile
synthesis and regeneration remains challenging.
Recently, various kinds of mesoporous materials have been studied for gas
adsorption and separation (19, 32, 33). For example, Katsoulidis et al. investigated
mesoporous polymeric organic frameworks for C2H6/CH4 separation (33). Ordered
mesoporous carbon (OMC) are of great research interest among the mesoporous materials,
owing to their exceptional properties, such as ease of synthesis, large specific surface area,
huge pore volume, tunable pore texture et al. (34, 35). These features lead them to great
potential applications in various fields including adsorption, catalysis, electrochemistry
(36, 37). The main objective of the present study is to investigate the potential application
of OMC, prepared via a soft template method, in gas adsorption and separation.
Adsorption equilibrium and kinetics of CO2, CH4, and N2 on the OMCs were determined.
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The isotherm data were used to predict the adsorptive separation of CO2/CH4, CH4/N2,
and CO2/N2 mixtures by the IAST (38, 39). Isosteric heats of adsorption and diffusion
time constants of CO2, CH4, and N2 were also calculated and carefully analyzed.
Materials and Methods
Synthesis of Ordered Mesoporous Carbon. The OMC studied in this work was
prepared via a soft template approach following a previously reported procedure (40).
Poly(propylene oxide)-b-poly(ethylene oxide)-b-poly(propylene oxide) triblock
copolymer Pluronic F127, tetraethyl orthosilicate (TEOS, 99+%), formalin solution (37
wt% formaldehyde), NaOH (98+%), HF( 47~51%), and ethanol (99.9%) were purchased
from Sigma-Aldrich. Phenol (99+%) and HCl (37%) were purchased from Acros Corp.
All chemicals were used as received without any further purification. Water used in all
experiments was deionized. Briefly, 2.08 g of TEOS was hydrolyzed in a solution
containing 4.0 g of ethanol and 1.0 g of HCl (0.2M). Then, it was mixed with 8.0 g of
ethanol, 1.6 g of F127, and 5.0 g of 20 wt% phenolic resin (pre-synthesized by the
procedures described in (40)) under stirring. After a few minutes, the mixture was
transferred into dishes for ethanol evaporation and then polymerized at 100 °C for 24 h.
Calcination was carried out at 350 °C for 5 h and 900 °C for 4 h under nitrogen protection
with a heating rate of 1 °C/min. The OMC product was obtained after the removal of
silica by HF etching. It is referred to as sOMC in the following text, where “s” denotes
soft template approach.
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Material Characterization. The pore structure of the synthesized sOMC was examined
by transmission electron microscopy (TEM) images taken by Hitachi H-7650.
Thermogravimetric analysis (TGA) was performed on Pyris 1 TGA from room
temperature to 950 °C in air with a heating rate of 10 °C/min. The small angle X-ray
diffraction (XRD) pattern was measured on Bede D1System X-ray diffractometer with a
Cu Kα source (40 kV and 40 mA). The hexagonal lattice parameter (a0) was calculated by
a0 = 2d100/√3 (nm), where d = 0.15418/(2sinθ) from Bragg’s law. The nitrogen
adsorption/desorption isotherms on the adsorbent at 77 K were determined via
Micromeritics ASAP 2020. Prior to the adsorption measurement, the sample was
degassed under a vacuum at 250 °C for over 12 h to remove the guest molecules in the
sample.
Adsorption measurements. The adsorption equilibrium data of CO2, CH4 and N2 on the
sOMC were measured by the Micromeritics ASAP 2020 volumetrically at three
temperatures (278, 298, and 318 K) and gas pressure up to 100 kPa. Ultrahigh-purity CO2,
CH4, and N2 were used as received. As aforementioned, the degas procedure was carried
out prior to the adsorption measurement.
The adsorption kinetic data were also recorded during the process of adsorption
equilibrium data determination. Typically in this procedure, an adsorbate gas was first
conducted into the Micromeritics ASAP 2020 adsorption unit at a designated dose.
Subsequently, the adsorbate gas pressure was measured continuously at fixed intervals. It
was then converted to gas uptake quantity as a function of time automatically, which
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gives the adsorption kinetics data.
Results and Discussion
Characterization of Ordered Mesoporous Carbon. The TGA curve of the sOMC is
shown in Figure S1 in the Supporting Information. It is evident from the 100 % weight
loss at high temperature (~900 °C) in the TGA plot that the silica component was
thoroughly removed from the carbon framework by HF etching. The initial weight loss
(~3%) before 100 °C corresponds to the loss of water and other guest molecules adsorbed
by the sample. It needs to be noted here that the sOMC synthesized in this work is stable
up to about 500 °C in air, which is much more stable than MOFs and mesoPOFs (15, 33).
The mesoporous structure of the as-synthesized carbon adsorbent was characterized
by the TEM images shown in Figure S2 in the Supporting Information. Typical highly
aligned stripe-like and hexagonally arranged structure with spherical and uniform pores
was clearly observed, indicating that the carbon adsorbent possesses a well ordered 2D
hexagonal mesostructure with 1D channels (41). The ordered mesostructure was further
confirmed by the well-resolved diffraction peaks at 2θ < 5° in the small-angle XRD
pattern shown in Figure S3 in the Supporting Information. The strong and narrow peak at
0.92° (2θ) can be indexed to (10) diffraction of ordered 2D hexagonal mesostructure (40),
from which the lattice parameter was calculated to be 11.1 nm.
Figure S4 in the Supporting Information shows the nitrogen adsorption/desorption
isotherms at 77 K and the pore size distribution curve of the sOMC sample. The nitrogen
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sorption isotherms Figure S4 (a) are of type IV with a clear capillary condensation step at
the relative pressure (P/P0) of 0.6-0.8, implying a narrow pore size distribution with large
mesopores, as further confirmed by the pore size distribution in Figure S4 (b). This is also
consistent with the TEM analysis result. The sOMC adsorbent with a bimodal pore size
distribution centered at 6.8 nm and 2.3 nm exhibits a considerable BET specific surface
area (2255 m2/g) and pore volume (2.17 cm3/g).
Adsorption Isotherms of CO2, CH4, and N2. The pure component
adsorption/desorption isotherms of CO2, CH4, and N2 on the sOMC at three temperatures
(278, 298, and 318 K) and pressure up to 100 kPa are given in Figure 1. All the isotherms
show excellent reversibility without hysteresis, indicating that the adsorbed gas molecules
can be completely removed during the desorption process. Thus, the sOMC adsorbent can
be easily regenerated by vacuum. This property makes the sOMC superior to a few
zeolite and MOF materials (18). In addition, neither gas reaches its saturated adsorption
capacity throughout the entire pressure range studied here. The isotherms for CO2 and
CH4 have modest curvatures, whereas the isotherms for N2 are almost linear. These also
suggest good regenerability of the adsorbent (42). CO2 is most favorably adsorbed
presumably owing to its significant quadrupolar moment. CH4 is preferentially adsorbed
over N2, which is most likely because the polarizability of CH4 is higher than that of N2
(10, 14).
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0 20 40 60 80 1000.0
0.5
1.0
1.5
2.0
2.5
3.0 278 K 298 K 318 K
CO
2 upt
ake
(mm
ol/g
)
Pressure (kPa)
(a)
0 20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
278 K 298 K 318 K
CH
4 upt
ake
(mm
ol/g
)
Pressure (kPa)
(b)
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0 20 40 60 80 1000.0
0.1
0.2
0.3
0.4
0.5 278 K 298 K 318 K
N2 u
ptak
e (m
mol
/g)
Pressure (kPa)
(c)
Figure 1. Adsorption (solid) and desorption (open) isotherms of CO2 (a), CH4 (b), and N2
(c) on the sOMC.
Adsorption capacity is one of the key factors to assess the gas separation capability
of an adsorbent. The CO2 uptake capacities at 100 kPa on the sOMC at 278 and 298 K are
3.0 and 2.0 mmol/g, respectively. These values are higher than those obtained on many
well-known ordered mesoporous adsorbents: MCM-41 (~0.75 mmol/g), SBA-15 (~0.6
mmol/g) and CMK-3 (~1.7 mmol/g) at 298 K and 100 kPa (19, 21, 24). They are also
superior to those of many other adsorbents studied for CO2/CH4 and CO2/N2 separations.
For example, the CO2 uptake is ~1.3 mmol/g on open ended CNx at 273 K (32); 0.4–1.2
mmol/g on clays at 298 K (26, 27); ~0.8 mmol/g on silica molecular sieve at 283 K (18);
0.89 mmol/g on commercial AC at 298 K (22); and 0.8–1.6 mmol/g on many MOF
materials at 298 K (17, 28). The sOMC also exhibits high CH4 adsorption capacities of
1.3 and 0.9 mmol/ g at 278 and 298 K respectively at the pressure of 100 kPa. These
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values outperform the ones on various kinds of adsorbents studied for CH4/N2 separation
such as zeolite 5A and ZIF-68/69 that shows CH4 uptake of ~0.8 and ~0.5 mmol/g,
respectively at 298 K (9, 16, 26, 28). It is worth noting that the CH4 uptake capacities on
the sOMC at 298 K and 100 kPa are about 2 times the reports for MOF-177, UMCM-1,
and ZIF-8 with huge BET specific surface areas (1300-2900 m2/g) (15).
Separation of Binary Mixtures. IAST was widely used to predict the gas mixture
adsorption behavior in a number of adsorbents (15, 43), including mesoporous materials
(33). Here, IAST was used to examine the selectivities of the binary mixtures (CO2/CH4,
CH4/N2, and CO2/N2) on the sOMC from the experimental pure-component adsorption
isotherms. These isotherms are fitted by the Langmuir model as equation 1.
� �����
���, (1)
where q (mmol/g) is the adsorbed gas amount at pressure P (kPa), am (mmol/g) is the
monolayer uptake capacity, and b (kPa-1) is the Langmuir isotherm constant. The fitted
Langmuir equation parameters (am and b) are summarized in Table 1. Henry’s constants
(K), calculated from the product of am and b, are also listed in the table. As shown in
Figure 1, the Langmuir model correlates all the isotherms very well (R2 > 0.998). The
fitted parameters were applied to perform the IAST calculation following the reported
procedures (26, 33). The selectivity of components i and j in a binary mixture Si/j is
defined as (xi/yi)/(xj/yj), where xa and ya are respectively the mole fractions of component
a (a = i, j) in the adsorbed and bulk phases.
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0 20 40 60 80 1000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
278 K 298 K 318 K
Sele
ctiv
ity C
O2/
CH
4
Pressure (kPa)
(a)
0 20 40 60 80 1000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
278 K 298 K 318 K
Sele
ctiv
ity C
H4/
N2
Pressure (kPa)
(b)
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0 20 40 60 80 1000
2
4
6
8
10
12
278 K 298 K 318 K
Sele
ctiv
ity C
O2/N
2
Pressure (kPa)
(c)
Figure 2. IAST predicted adsorption selectivities for equimolar binary mixtures of
CO2/CH4 (a), CH4/N2 (b), and CO2/N2 (c).
Table 1. Summary of parameters for the Langmuir isotherm
model and Henry’s constants (K)
Adsorbate T (K) am (mmol/g) b (kPa-1) K (mmol/g kPa)
CO2 278 6.429 0.00845 0.0544
298 5.005 0.00654 0.0327
318 4.182 0.00482 0.0201
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CH4 278 3.248 0.00630 0.0205
298 2.758 0.00475 0.0131
318 2.143 0.00414 0.00888
N2 278 2.687 0.00211 0.00568
298 2.099 0.00173 0.00364
318 1.900 0.00134 0.00255
The selectivities for each equimolar binary mixture at 278, 298, and 318 K are
plotted as a function of total bulk pressure in Figure 2. For a binary mixture of CO2 and
CH4, the selectivity increases with the pressure, reaching about 3.4 (278 K) and 2.9 (298
K) at 100 kPa. The CO2/CH4 selectivity displayed by the sOMC is much higher than the
ones reported on MaxsobAC and NoritAC (21), CMK-3 and CMK-5 (20, 23), and many
MOFs and COFs (15, 12) which display CO2/CH4 selectivity in the range 2-2.4 at 298 K,
comparable to the those of chabazite, Linde 4A, and H+ mordenite (commercial zeolites)
which were reported as 2.8–3.7 at 273 K (9). It is lower than the values found on SBA-15
(~5.5) and MCM-41 (~5.5), however the CO2 uptake capacity on the sOMC, as
mentioned above, is significantly larger than that of the mesoporous silica under similar
condition (20, 21). Figure 2b shows that the CH4/N2 selectivity slightly increases with the
increase in pressure. At 298 K and 100 kPa, a CH4/N2 selectivity of 3.8 is obtained, which
is about twice the selectivity on CMK-5 (23). It also surpasses the value reported for
IRMOF-1 (~2) and ZIF-69 (~3), and is comparable to that shown by Cu-BTC, IRMOF-11,
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and ZIF-68 whose selectivities of CH4 over N2 range from 3.5 to 3.8 (16, 26, 43). When it
comes to the CO2/N2 separation, the selectivity of CO2 over N2 gradually increases as the
pressure increases, similar to the case of CO2/CH4 selectivity, as shown in Figure 2 (a)
and (c). At 100 kPa, the CO2/N2 selectivity reaches 12.8 (278 K) and 11.3 (298 K). It is
larger than or comparable to those found on a variety of adsorbents at similar conditions
as well including MIL-47(v) (9 at 298 K) and nitrogen doped hierarchical carbons (5.7–
8.4 at 298 K) (43, 44) These comparisons suggest the great potential application of the
as-synthesized ordered mesoporous carbon in gas adsorptive separation.
0.0 0.2 0.4 0.6 0.8 1.00.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
278 K 298 K 318 KSe
lect
ivity
CO
2/C
H4
y CH4
(a)
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0.0 0.2 0.4 0.6 0.8 1.00.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
278 K 298 K 318 K
Sele
ctiv
ity C
H4/
N2
y N2
(b)
0.0 0.2 0.4 0.6 0.8 1.00
2
4
6
8
10
12
14
16
18
278 K 298 K 318 K
Sele
ctiv
ity C
O2/N
2
y N2
(c)
Figure 3. IAST predicted selectivities of CO2/CH4 (a), CH4/N2 (b), and CO2/N2 (c) at
total bulk pressure of 100 kPa.
The separation efficacy of the ordered mesoporous carbon adsorbent was further
explored by the selectivities at different binary compositions with a bulk pressure of 100
kPa, as shown in Figure 3. The selectivities of CO2/CH4 and CH4/N2 keep nearly constant
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in a wide composition range (0.05-0.95), which is an attractive feature of an absorbent
(15). Although the selectivity for CO2/N2 decreases gradually with the yN2 (mole fraction
of N2 in the gas phase), it is still around 10 at 298 K and 100 kPa even when yN2 equals
0.95, which is still higher than or comparative to the reports for lots of other adsorbents,
such as ZnDABCO (8.5) and CMK-5 (4.5), under similar conditions (13, 23).
Isosteric Heat of Adsorption. To design and operate a gas adsorption process, the
isosteric heat of adsorption (Qst) is always taken into account to estimate the temperature
change in the adsorption process. In addition, the Qst is an indicator of the regenerability
of an adsorbent. The energetic heterogeneity of the surface of an adsorbent can be also
investigated by the Qst. The single component isosteric heat of adsorption as a function of
surface loading can be determined by the Clausius-Clapeyron equation as
�� = � R���� ���
���� (2)
where Qst (kJ/mol) is the isosteric heat of adsorption, T (K) is the temperature, P (kPa) is
the pressure, R is the gas constant, and q (mmol/g) is the adsorbed amount. Based on the
general assumption that the isosteric heat of adsorption is independent of the temperature,
integration of equation 2 gives,
ln � ����
�� !"#$%&#%. (3)
In this study, the isosteric heats of adsorption of CO2, CH4, and N2 were calculated
via the slopes of the linear plots of ln P versus 1/T by using the equilibrium isotherm data.
The resulting values of the isosteric heats of adsorption are shown in Figure 4.
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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40
5
10
15
20
25
CO2 CH4 N2
Qst (
kJ/m
ol)
Amount adsorbed (mmol/g)
Figure 4. Isosteric heats of adsorption for CO2, CH4, and N2 on the sOMC.
It can be observed in Figure 4 that the isosteric heat of adsorption of each gas
increases gently as the surface coverage increases within the experimental range, which
can be attributed to an increase in the interaction between the adsorbate molecules (lateral
interaction) with increasing loading. This indicates that the synthesized ordered
mesoporous carbon has a homogeneous surface for the adsorption of CO2, CH4, and N2.
Activated carbon, generally having an adsorption energetic heterogeneity, is in a different
case (45). The limiting isosteric heats of adsorption at zero loading for CO2, CH4, and N2
were calculated, from the slopes of the van’t Hoff plots, to be 18.2, 15.4, and 14.7 kJ/mol
respectively (see Supporting information). They are in good agreement with the values
obtained by extrapolation of the isosteric heat of adsorption curves to the zero loading.
These values are lower than those reported on activated carbon (46, 47), presumably due
to the larger pore size of the ordered mesoporous carbon (25). They are also lower than
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the values found on CMK-5 (23), zeolite 5A (48), silicalite (49) et al. The relatively low
isosteric heat of adsorption is another indication of the good regeneration of the sOMC
adsorbent. The combination of the high thermal stability, large adsorption capacity,
sufficiently high selectivity, and facile regeneration, demonstrate that the sOMC studied
in this work is a promising candidate for the selective separation of CO2/CH4, CH4/N2,
and CO2/N2 binary mixtures.
Adsorption Kinetics. Adsorption kinetics data of CO2, CH4, and N2 on the as-made
ordered mesoporous carbon were measured at three different temperatures (278, 298, and
318 K) and at a low pressure (~2 kPa). The fractional uptake curves are plotted in Figure
5. At 298 K, CH4 and N2 reached the equilibrium in a shorter time (~10 s) as compared
with CO2 (~40 s). In addition, it took slightly shorter time to get the equilibrium
adsorption at higher adsorption temperature for each gas.
0 20 40 60 800.0
0.2
0.4
0.6
0.8
1.0
Frac
tiona
l upt
ake
Time (s)
278 K 298 K 318 K
(a)
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0 20 40 60 800.0
0.2
0.4
0.6
0.8
1.0
Frac
tinal
upt
ake
Time (s)
278 K 298 K 318 K
(b)
0 20 40 60 800.0
0.2
0.4
0.6
0.8
1.0
Frac
tiona
l upt
ake
Time (s)
278 K 298 K 318 K
(c)
Figure 5. Fractional uptake of CO2 (a), CH4 (b), and N2 (c) on the sOMC.
TABLE 2. Summary of diffusion time constants of CO2, CH4,
and N2 on the sOMC at different temperatures
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T (K) Carbon dioxide
Dc/rc2 (10-2 s-1)
Methane
Dc/rc2 (10-2 s-1)
Nitrogen
Dc/rc2 (10-2 s-1)
278 1.69 3.80 3.30
298 2.18 4.60 4.11
318 2.88 5.28 4.68
The diffusion time constants can be extracted by fitting the fractional uptake curve
with a proper diffusion model. When the fractional uptake is larger than 0.7, it can be
expressed by the following equation (50, 51).
1 �)�
)∞�
*
+,exp��
01
21, 3
�%� (4)
where )�
)∞ is the fractional uptake, and
01
21, is the diffusion time constant. The slope of the
linear plot of ln(1 �45
4∞) versus t was used to determine the diffusion time constants for
each gas on the sOMC at different temperatures (Table 2). It can be observed from Table
2 that the difference between the diffusion time constants of CO2, CH4, and N2 is small,
implying an effective kinetic based adsorptive separation is difficult to achieve on the
sOMC. This is because the pore size of the carbon adsorbent is fairly large compared with
the kinetic diameters of the adsorbates. As shown in Table 2, the diffusion time constant
increases gently as the temperature increases for each gas. The diffusion activation
energies were calculated to be 9.78, 6.06, and 6.42 kJ/mol for CO2, CH4, and N2,
respectively, based on the Arrhenius equation (Supporting Information).
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Acknowledgments
This project was partially supported by U.S. Air Force Research Laboratory
(FA8650-11-C-2127), U.S. Department of Energy (DE-EE0003046), U.S. National
Science Foundation (EEC 1028968), and New Mexico State University Office of Vice
President for Research (GREG award for X. Wu). We appreciate Mr. Kirill Shcherbachev
and Dr. Ilya Krechetov (National University of Science and Technology, “MISiS”, Russia)
for assisting with the small-angle XRD data measurement for this work. The XRD data
were measured in the Joint Research Center of “Material Science and Metallurgy” (NUST,
MISiS, Russia) that was funded by The Ministry of Education and Science of the Russian
Federation. S. Deng is grateful for the U.S. Department of State for the Fulbright award
(Distinguished Chair in Energy Conservation) and his host institute (NUST, MISiS) in
Moscow, Russia.
Supporting Information Available
TGA curve; TEM images; small-angle XRD pattern; nitrogen adsorption/desorption at 77
K; van’t Hoff plots; Arrhenius plots. This information is available free of charge via the
Internet at http://pubs.acs.org/.
References
(1) Bae, Y.-S.; Mulfort, K. L.; Frost, H.; Ryan, P.; Punnathanam, S.; Broadbelt, L. J.;
Hupp, J. T.; Snurr, R. Q. Separation of CO2 from CH4 using mixed-ligand
Page 22 of 29
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Environmental Science & Technology
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23
metal−organic frameworks. Langmuir 2008, 24, 8592–8598.
(2) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. Removal of carbon dioxide from natural
gas by vacuum pressure swing adsorption. Energy Fuels 2006, 20, 2648–2659.
(3) Yamasaki, A. An overview of CO2 mitigation options for global
warming-Emphasizing CO2 sequestration options. J. Chem. Eng. Jpn. 2003, 36,
361–375.
(4) Wang, Z.; Zhang, L.; Ge, M.; Xie, F.; Wang, Y.; Qiao, W.; Liang, X.; Ling, L. Pith
based spherical activated carbon for CO2 removal from flue gases. Chem. Eng. Sci.
2011, 66, 5504–5511.
(5) Chandra, V.; Yu, S. U.; Kim, S. H.; Yoon, Y. S.; Kim, D. Y.; Kwon, A. H.; Meyyappan,
M.; Kim, K. S. Highly selective CO2 capture on N-doped carbon produced by
chemical activation of polypyrrole functionalized graphene sheets. Chem. Commun.
2012, 735–737.
(6) Lohila, A.; Laurila, T.; Tuovinen, J.; Aurela, M.; Hatakka, J.; Thum, T.; Pihlatie, M.;
Rinne, J.; Vesala, T. Micrometeorological measurements of methane and carbon
dioxide fluxes at a municipal landfill. Environ. Sci. Technol. 2007, 41, 2717–2722.
(7) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. Upgrade of methane from landfill gas
by pressure swing adsorption. Energy Fuels 2005, 19, 2545–2555.
(8) Peng, X.; Wang, W. C.; Xue, R. S.; Shen, Z. M. Adsorption separation of CH4/CO2
on mesocarbon microbeads: Experiment and modeling. AIChE J. 2006, 52, 994–1003.
(9) Jensen, N. K.; Rufford, T. E.; Watson, G.; Zhang, D. K.; Chan, K. I.; May, E. F.
Page 23 of 29
ACS Paragon Plus Environment
Environmental Science & Technology
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960
24
Screening zeolites for gas separation applications involving methane, nitrogen, and
carbon dioxide. J. Chem. Eng. Data 2012, 57, 106–113.
(10) Harlick, P. J. E.; Tezel, F. H. Adsorption of carbon dioxide, methane and nitrogen:
pure and binary mixture adsorption for ZSM-5 with SiO2/Al2O3 ratio of 280. Sep.
Purif. Technol. 2003, 33, 199–210.
(11) Kuznicki, S. M.; Bell, V. A.; Nair, S.; Hillhouse, H. W.; Jacubinas, R. M.; Braunbarth,
C. M.; Toby, B. H.; Tsapatsis, M. A titanosilicate molecular sieve with adjustable
pores for size-selective adsorption of molecules. Nature 2001, 412, 720−724.
(12) Liu, Y.; Liu, D.; Yang, Q.; Zhong, C.; Mi, J. Comparative study of separation
performance of COFs and MOFs for CH4/CO2/H2 Mixtures. Ind. Eng. Chem. Res.
2010, 49, 2902–2906.
(13) Mishra, P.; Mekala, S.; Dreisbach, F.; Mandal, B.; Gumma, S. Adsorption of CO2,
CO, CH4 and N2 on a zinc based metal organic framework. Sep. Purif. Technol. 2012,
94, 124–130.
(14) Bae, Y.-S.; Farha, O. K.; Hupp, J. T.; Snurr, R. Q. Enhancement of CO2/N2
selectivity in a metal-organic framework by cavity modification. J. Mater. Chem.
2009, 19, 2131−2134.
(15) Xiang, Z. H.; Peng, X.; Cheng, X.; Li, X. J.; Cao, D. P. CNT@Cu3(BTC)2 and
metal–organic frameworks for separation of CO2/CH4mixture. J. Phys. Chem. C 2011,
115, 19864–19871.
(16) Liu, B.; Smit, B. Molecular simulation studies of separation of CO2/N2, CO2/CH4,
Page 24 of 29
ACS Paragon Plus Environment
Environmental Science & Technology
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960
25
and CH4/N2 by ZIFs. J. Phys. Chem. C 2010, 114, 8515–8522.
(17) Mu, B.; Li, F.; Walton, K. S. A novel metal-organic coordination polymer for
selective adsorption of CO2 over CH4. Chem. Commun. 2009, 2493–2495.
(18) Morishige, K. Adsorption and separation of CO2/CH4 on amorphous silica
molecular sieve. J. Phys. Chem. C 2011, 115, 9713–9718.
(19) Liu, X.; Li, J.; Zhou, L.; Huang, D.; Zhou, Y. Adsorption of CO2, CH4 and N2 on
ordered mesoporous silica molecular sieve. Chem. Phys. Lett. 2005, 415, 198–201.
(20) Saini, V. K.; Andrade, M.; Pinto, M. L.; Carvalho, A. P.; Pires, J. How the
adsorption properties get changed when going from SBA-15 to its CMK-3 carbon
replica. Sep. Purif. Technol. 2010, 75, 366–376.
(21) Belmabkhout, Y.; Sayari, A. Adsorption of CO2 from dry gases on MCM-41 silica at
ambient temperature and high pressure. 2: Adsorption of CO2/N2, CO2/CH4 and
CO2/H2 binary mixtures. Chem. Eng. Sci. 2009, 64, 3729 –3735.
(22) Ma, X.; Cao, M.; Hu, C. Bifunctional HNO3 catalytic synthesis of N-doped porous
carbons for CO2 capture. J. Mater. Chem. A 2013, 1, 913–918.
(23) Peng, X.; Cao, D.; Wang, W. Adsorption and separation of CH4/CO2/N2/H2/CO
mixtures in hexagonally ordered carbon nanopipes CMK-5. Chem. Eng. Sci. 2011, 66,
2266–2276.
(24) Zhou, L.; Liu, X. W.; Li, J. W.; Wang, N.; Wang, Z.; Zhou, Y. P. Synthesis of
ordered mesoporous carbon molecular sieve and its adsorption capacity for H2, N2, O2,
CH4 and CO2. Chem. Phys. Lett. 2005, 413, 6–9.
Page 25 of 29
ACS Paragon Plus Environment
Environmental Science & Technology
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960
26
(25) Himeno, S.; Komatsu, T.; Fujita, S. High-pressure adsorption equilibria of methane
and carbon dioxide on Several Activated Carbons. J. Chem. Eng. Data 2005, 50, 369–
376.
(26) Pires, J.; Saini, V. K.; Pinto, M. L. Studies on selective adsorption of biogas
components on pillared clays: Approach for biogas improvement. Environ. Sci.
Technol. 2008, 42, 8727 –8732.
(27) Pinto, M. L.; Pires, J.; Rocha, J. Porous materials prepared from clays for the
upgrade of landfill gas. J. Phys. Chem. C 2008, 112, 14394–14402.
(28) Saha, D.; Bao, Z.; Jia, F.; Deng, S. Adsorption of CO2, CH4, N2O, and N2 on
MOF-5, MOF-177, and Zeolite 5A. Environ. Sci. Technol. 2010, 44, 1820–1826.
(29) Al-Baghli, N. A.; Loughlin, K. F. Adsorption of methane, ethane, and ethylene on
titanosilicate ETS-10 zeolite. J. Chem. Eng. Data 2005, 50, 843–848.
(30) Marathe, R. P.; Mantri, K.; Srinivasan, M. P.; Farooq, S. Effect of ion exchange and
dehydration temperature on the adsorption and diffusion of gases in ETS-4. Ind. Eng.
Chem. Res. 2004, 43, 5281–5290.
(31) Surble, S.; Millange, F.; Serre, C.; Duren, T.; Latroche, M.; Bourrelly, S.; Llewellyn,
P. L.; Ferey, G. Synthesis of MIL-102, a chromium carboxylate metal−organic
framework, with gas sorption analysis. J. Am. Chem. Soc. 2006, 128, 14889–14896.
(32) Shen, Y.; Bai, J. A new kind CO2/CH4 separation material: open ended nitrogen
doped carbon nanotubes formed by direct pyrolysis of metal organic frameworks.
Chem. Commun. 2010, 1308–1310.
Page 26 of 29
ACS Paragon Plus Environment
Environmental Science & Technology
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960
27
(33) Katsoulidis, A. P.; Kanatzidis, M. G. Mesoporous hydrophobic polymeric organic
frameworks with bound surfactants. Selective adsorption of C2H6 versus CH4. Chem.
Mater. 2012, 24, 471−479.
(34) Huang, Y.; Cai, H.; Feng, D.; Gu, D.; Deng, Y.; Tu, B.; Wang, H.; Webley, P. A.;
Zhao, D. One-step hydrothermal synthesis of ordered mesostructured carbonaceous
monoliths with hierarchical porosities. Chem. Commun. 2008, 2641–2643.
(35) Lu, A.-H.; Spliethoff, B.; Schuth, F. Aqueous synthesis of ordered mesoporous
carbon via self-assembly catalyzed by amino acid. Chem. Mater. 2008, 20, 5314–
5319.
(36) Ji, L.; Liu, F.; Xu, Z.; Zheng, S.; Zhu, D. Adsorption of pharmaceutical antibiotics
on template-synthesized ordered micor- and mesoporous carbons. Environ. Sci.
Technol. 2010, 44, 3116–3122.
(37) Cui, X.; Shi, J.; Zhang, L.; Ruan, M.; Gao, J. PtCo supported on ordered mesoporous
carbon as an electrode catalyst for methanol oxidation. Carbon 2009, 47, 186–94.
(38) Myers, A. L.; Prausnitz, J. M. Thermodynamics of mixed-gas adsorption. AIChE J.
1965, 11, 121–127.
(39) Myers, A. L. Equation of state for adsorption of gases and their mixtures in porous
materials. Adsorption 2003, 9, 9–16.
(40) Zhuang X.; Wan, Y.; Feng, C.; Shen, Y.; Zhao, D. Highly efficient adsorption of
bulky dye molecules in wastewater on ordered mesoporous carbons. Chem. Mater.
2009, 21, 706–716.
Page 27 of 29
ACS Paragon Plus Environment
Environmental Science & Technology
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960
28
(41) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky,
G. D. Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300
angstrom pores. Science 1998, 279, 548–552.
(42) Bhatia, S. K.; Myers, A. L. Optimum conditions for adsorptive storage. Langmuir
2006, 22, 1688–1700.
(43) Liu, B.; Smit, B. Comparative molecular simulation study of CO2/N2 and CH4/N2
separation in zeolites and metal−organic frameworks. Langmuir 2009, 25, 5918–
5926.
(44) Gutierrez, M. C.; Carriazo, D.; Ania, C. O.; Parra, J. B.; Ferrer, M. L.; del Monte, F.
Deep eutectic solvents as both precursors and structure directing agents in the
synthesis of nitrogen doped hierarchical carbons highly suitable for CO2 capture.
Energy Environ. Sci. 2011, 4, 3535–3544.
(45) Choi, B.-U.; Choi, D.-K.; Lee, Y.-W.; Lee, B.-K. Adsorption equilibria of methane,
ethane, ethylene, nitrogen, and hydrogen onto activated carbon. J. Chem. Eng. Data
2003, 48, 603-607.
(46) He, Y.; Yun, J. H.; Seaton, N. A. Adsorption equilibrium of binary methane/ethane
mixtures in BPL activated carbon: Isotherms and calorimetric heats of adsorption.
Langmuir 2004, 20, 6668-6678.
(47) Lopes, F. V. S.; Grande, C. A.; Ribeiro, A. M.; Loureiro, J. M.; Evaggelos, O.;
Nikolakis, V.; Rodrigues, A. E. Adsorption of H2, CO2, CH4, CO, N2 and H2O in
activated carbon and zeolite for hydrogen production. Sep. Sci. Technol. 2009, 44,
Page 28 of 29
ACS Paragon Plus Environment
Environmental Science & Technology
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960
29
1045–1073.
(48) Liu, Z.; Liu, C. A.; Liu, P.; Yu, J.; Rodrigues, A. E. Adsorption and desorption of
carbon dioxide and nitrogen on zeolite 5A. Sep. Sci. Technol. 2011, 46, 434–451.
(49) Dunne, J. A.; Mariwala, R.; Sircar, S.; Gorte, R. J.; Myers, A. L. Calorimetric heats
of adsorption and adsorption isotherms. 1. O2, N2, Ar, CO2, CH4, C2H6, and SF6 on
silicalite. Langmuir 1996, 12, 5888-5895.
(50) Ruthven, D.M. Principles and Adsorption and Adsorption Processes, Wiley
Interscience, 1984.
(51) Crittenden, B.; Thomas, W. J. Adsorption Technology and Design,
Butterworth/Heinemann, Oxford, 1998.
Page 29 of 29
ACS Paragon Plus Environment
Environmental Science & Technology
123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960