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Preparation and characterization of novel CO2
‘‘molecular basket’’ adsorbents based on polymer-modified
mesoporous molecular sieve MCM-41
Xiaochun Xu, Chunshan Song *, John M. Andreesen,Bruce G. Miller, Alan W. Scaroni
Clean Fuels and Catalysis Program, The Energy Institute, and Department of Energy and Geo-Environmental Engineering,
The Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802, USA
Received 22 October 2002; accepted 21 April 2003
Abstract
Novel CO2 ‘‘molecular basket’’ adsorbents were prepared by synthesizing and modifying the mesoporous molecular
sieve of MCM-41 type with polyethylenimine (PEI). The MCM-41-PEI adsorbents were characterized by X-ray powder
diffraction (XRD), N2 adsorption/desorption, thermal gravimetric analysis (TGA) as well as the CO 2 adsorption/de-
sorption performance. This paper reports on the effects of preparation conditions (PEI loadings, preparation methods,
PEI loading procedures, types of solvents, solvent/MCM-41 ratios, addition of additive, and Si/Al ratios of MCM-41) onthe CO2 adsorption/desorption performance of MCM-41-PEI. With the increase in PEI loading, the surface area, pore
size and pore volume of the PEI-loaded MCM-41 adsorbent decreased. When the PEI loading was higher than 30 wt.%,
the mesoporous pores began to be filled with PEI and the mesoporous molecular sieve MCM-41 showed a synergetic
effect on the adsorption of CO2 by PEI. At PEI loading of 50 wt.% in MCM-41-PEI, the highest CO2 adsorption capacity
of 246 mg/g-PEI was obtained, which is 30 times higher than that of the MCM-41 and is about 2.3 times that of the pure
PEI. Impregnation was found to be a better method for the preparation of MCM-41-PEI adsorbents than mechanical
mixing method. The adsorbent prepared by a one-step impregnation method had a higher CO 2 adsorption capacity than
that of prepared by a two-step impregnation method. The higher the Si/Al ratio of MCM-41 or the solvent/MCM-41
ratio, the higher the CO2 adsorption capacity. Using polyethylene glycol as additive into the MCM-41-PEI adsorbent
increased not only the CO2 adsorption capacity, but also the rates of CO2 adsorption/desorption. A simple model was
proposed to account for the synergetic effect of MCM-41 on the adsorption of CO 2 by PEI.
Ó 2003 Elsevier Inc. All rights reserved.
Keywords: Adsorption separation; Carbon dioxide; Characterization; CO2 adsorbent; Flue gas; MCM-41; Mesoporous molecular
sieve; Polyethylenimine; Preparation
1. Introduction
Existing energy utilization system is largely
based on combustion of fossil fuels in stationary
and mobile devices. It is likely that the world will
* Corresponding author. Tel.: +1-814-863-4466; fax: +1-814-
865-3248.
E-mail address: [email protected] (C. Song).
1387-1811/03/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved.
doi:10.1016/S1387-1811(03)00388-3
www.elsevier.com/locate/micromeso
Microporous and Mesoporous Materials 62 (2003) 29–45
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continue to rely on fossil fuels as the primary en-
ergy supply well into the 21st century [1–7]. Ex-
tensive studies have been conducted worldwide on
controlling the emission of pollutants (from com-bustion) such as SOx, NOx, and particulate matter
and trace elements. The availability of the tech-
nologies for using fossil fuels to provide clean,
affordable energy is essential for the prosperity and
the security of the world. On the other hand, the
increased CO2 concentration in the atmosphere
due to emissions of CO2 from fossil fuel combus-
tion has caused the concerns for global warming
[1–7]. Improving the efficiency of energy utilization
and increasing the use of low-carbon energy
sources are considered to be potential ways to re-
duce CO2 emissions [6,7]. Recently, CO2 capture
and sequestration have been receiving significant
attention and are being recognized as a third op-
tion [6,7]. Also, enriched CO2 streams can be an
important starting material for synthetic clean
fuels and chemicals [1,2,8,9]. For carbon seques-
tration, the costs for capture and separation are
estimated to make up about three-fourths of the
total costs of ocean or geologic sequestration [6,7].
It is therefore important to explore new ap-
proaches for CO2 separation [6–8].
There are chemical and physical methods forseparation of CO2 from gas mixtures. Adsorption
is one of the promising methods that could be
applicable for separating CO2 from gas mixtures
[10–40]. Various adsorbents, such as zeolites [10–
23], activated carbons [13,16,17,24–29,31–34,36],
carbon molecular sieves [16,33], pillared clays
[37,38] and metal oxides [30,35,39,40], have been
investigated. At lower temperatures (e.g., room
temperature), the zeolite-based adsorbents and
activated carbon have generally been found to
show higher adsorption capacity. However, theirselectivity to CO2 in the presence of other gases
(N2, etc.) is still low; their adsorption capacities
rapidly decline with increasing temperature above
30 °C, and become negligible at temperature in
excess of 200 °C. Siriwardane et al. [13] reported
that the CO2 adsorption capacity for zeolite 13X,
4A and activated carbon were about 160, 135 and
110 mg/g-adsorbent respectively at 25 °C and 1
atm CO2 partial pressure [30]. Van der Vaart et al.
[28] investigated the single and mixed gas adsorp-
tion equilibrium of CO2/CH4 on Nortit RBI acti-
vated carbon. The CO2 adsorption capacity was
108 mg/g-adsorbent at 21.5 °C. With increasing
temperature, the adsorption capacity decreased to77 mg/g-adsorbent at 30 °C, 56 mg/g-adsorbent at
56 °C and 40 mg/g-adsorbent at 75 °C respectively.
Meanwhile, the activated carbon also adsorbed
methane. The CO2/CH4 selectivity was around 2.
Berlier and Frere [24] and Heuchel et al. [27] also
found that CO2 adsorption capacity of activated
carbon decreased significantly when the tempera-
ture slightly increased and the CO2/CH4 selectivity
was low. Under 0.1 MPa CO2 partial pressure, the
adsorption capacity was 126 mg/g-adsorbent at 15
°C and 78 mg/g-adsorbent at 45 °C. The best CO2/
CH4 ratio (selectivity) was around 2. Since all the
gases are physically adsorbed into/onto these car-
bon and zeolite adsorbents, the separation factors
(e.g., CO2/N2 ratio) are low. In order to operate at
relatively high temperature and reach high sepa-
ration selectivity, chemical adsorption has been
studied. Unfortunately, the adsorbents only show
a low CO2 adsorption capacity at high tempera-
ture although the selectivity for CO2 is high.
Anand et al. [30] reported that MgO showed an
adsorption capacity of 8.8 mg/g-adsorbent at 400
°C. Ding and Alpay [37] investigated the adsorp-tion performance of hydrotalcite and this material
showed a CO2 adsorption capacity of 22 mg/g-
adsorbent at 400 °C and 0.2 atm CO2 partial
pressure. Both types of adsorbents need high
temperature operation and have a low adsorption
capacity, thus they are not suitable for practical
use for CO2 separation. For practical application,
selective adsorbents with high capacity are desired.
Many of the separations should preferably be
operated at elevated temperature, e.g., higher than
room temperature and up to $150°
C for the fluegas of power plants. Developing an adsorbent with
high CO2 selectivity and high CO2 adsorption ca-
pacity, which can also be operated at elevated
temperature, is desired for more efficient CO2
separation by adsorption method.
Recently, a new concept of CO2 ‘‘molecular
basket’’ has been proposed by our laboratory for
developing high-capacity, highly-selective CO2
adsorbents [41–43]. The CO2 ‘‘molecular basket’’
adsorbents can selectively ‘‘pack’’ CO2 in con-
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densed form in the mesoporous molecular sieve
‘‘basket’’ and therefore shows a high CO2 ad-
sorption capacity and a high CO2 selectivity. To
make the ‘‘molecular basket’’ adsorbent and tocapture a large amount of CO2 gas, the adsorbent
needs to have large-pore volume channels to serve
as the basket. In order to make the ‘‘basket’’ to be
a CO2 ‘‘molecular basket’’, a substance with nu-
merous CO2-affinity sites is loaded into the pores
of the support to increase the affinity between the
adsorbent and CO2 and, therefore, to increase the
CO2 adsorption selectivity and CO2 adsorption
capacity. In addition, the adsorption affinity to
CO2 by the CO2-philic substance increased in the
confined mesoporous environment and therefore
the mesoporous molecular sieve can have a syn-
ergetic effect on the adsorption of CO2 by the CO2-
philic substance. In our previous work [41–44], the
sterically branched polymer of polyethylenimine
(PEI), which has branched chains with numerous
CO2-capturing sites such as amine groups, was
loaded into the large pore volume porous material
of mesoporous molecular sieve MCM-41 type to
make the ‘‘molecular basket’’ adsorbent. The CO2
adsorption capacity was significantly increased
after loading the PEI. The novel CO2 ‘‘molecular
basket’’ adsorbent showed a CO2 adsorption ca-pacity of 215 mg/g-PEI at 75 °C and pure CO2
atmosphere, which was 24 times higher than that
of the MCM-41 and was two times that of the pure
PEI [41–43]. It clearly showed that the mesoporous
molecular sieve MCM-41 had a synergetic effect
on the adsorption of CO2 by PEI. The adsorption
of N2 was negligible and smaller than the appa-
ratusÕ measurement limit (<1.0 mg/g-adsorbent).
The novel adsorbent can also effectively adsorb
CO2, even at low CO2 concentration, e.g., 0.5%
CO2 in a CO2/N2 gas mixture. Satyapal et al. [44]used the PEI as CO2 adsorbent by coating the PEI
on the high surface area solid polymethyl meth-
acrylate polymeric support. The composite mate-
rial can effectively adsorb CO2 from gas mixtures
and has been successfully used in the space shuttle
for the removal of CO2 from the breathing air. By
loading the PEI into the large pore volume mate-
rial of MCM-41, the novel ‘‘molecular basket’’
adsorbent showed a higher adsorption capacity
than that of the PEI/polymer composite adsorbent
[41–43], which confirmed that it is the ‘‘molecular
basket’’ concept, not the high surface area mate-
rials, increase the adsorption capacity largely. In
this paper, we report the preparation approachesto further increase the CO2 adsorption capacity of
the novel ‘‘molecular basket’’ adsorbent (i.e., in-
fluence of PEI loading and method of loading on
MCM-41, effect of polymer additive; effect of Si/Al
ratio of MCM-41), and the synergetic effect of
MCM-41 on the adsorption of CO2 by PEI.
2. Experimental
2.1. Preparation of the adsorbents
Mesoporous molecular sieve MCM-41 was hy-
drothermally synthesized from a mixture with the
following composition: xAl2O3:50SiO2:4.32Na2O:
2.19(TMA)2O:15. 62(CTMA)Br:3165H2O ( x ¼ 0,
0.05, 0.25). The synthesis procedure was estab-
lished in our laboratory [45,46], which is based on
the method invented by researchers at Mobil
[47,48]. Cab-O-Sil fumed silica (Cabot Corpora-
tion), tetramethylammonium silicate solution (0.5
TMA/SiO2, 10 wt.% SiO2, Sachem Inc.), sodium
silicate (containing 14 wt.% NaOH and 27 wt.%SiO2, Aldrich), cetyltrimethyl-ammonium bromide
[(CTMA)Br, Aldrich], aluminum isopropoxide
(Aldrich) and deionized water were used as raw
materials. The synthesis was carried out at 100 °C
for 40 h. After the synthesis, the solid product was
recovered by filtration, washed several times with
deionized water, dried at 100 °C overnight and
calcined at 550 °C for 5 h to remove the template.
The polyethylenimine (PEI) modified MCM-41
was prepared by wet impregnation method. In a
typical preparation, the desired amount of PEI(viscous liquid) was dissolved in 8 g methanol
under stirring for about 15 min, after which 2 g
calcined MCM-41 was added to the PEI/methanol
solution. The resultant slurry was continuously
stirred for about 30 min. Then the slurry was dried
at 70 °C for 16 h under reduced pressure (700
mmHg). The as-prepared adsorbent was denoted
as MCM-41-PEI- X , where X represents the load-
ing of PEI as weight percentage in the sample. By
changing the preparation conditions, such as PEI
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loadings, preparation methods, preparation pro-
cedures, types of solvents, solvent/MCM-41 ratios,
Si/Al ratios of MCM-41, and adding additive of
polyethylene glycol (PEG), different adsorbentswere prepared. In one adsorbent, commercially
available silica gel (SiO2) (Merck, surface area
550 m2/g) was used as support instead of MCM-
41. Details on the adsorbents and their corre-
sponding preparation conditions are summarized
in Table 1.
2.2. Characterization of the adsorbents
The mesoporous molecular sieve MCM-41,
before and after modification, was characterized
by X-ray diffraction (XRD) and N2 adsorption/
desorption. The XRD patterns were obtained on
a Rigaku Geigerflex using CuKa radiation. The
N2 adsorption/desorption was carried out on a
Quantachrome Autosorb 1 automated adsorption
apparatus, from which the BET surface area, the
pore volume and the pore size were obtained. The
sample was out-gassed at 75 °C for 48 h on a high
vacuum line prior to adsorption. The pore volumeof the mesoporous molecular sieve was calculated
from the adsorbed nitrogen after complete pore
condensation ( P = P 0 ¼ 0:995) using the ratio of the
densities of liquid and gaseous nitrogen. The
pore size was calculated by using the BJH method.
The thermal chemical and physical properties of
MCM-41, PEI and MCM-41-PEI were character-
ized by thermal gravimetric analysis (TGA). The
TGA was performed on a PE-TGA 7 thermal
gravimeter. About 10 mg of the sample was heated
at 10 °C/min to 600 °C in airflow (100 ml/min).
2.3. Adsorption measurement
The adsorption and desorption performance of
the adsorbent was measured using a PE-TGA 7
Table 1
Preparation conditions and the adsorption/desorption performance of the adsorbents
Sample name Si/Al of
MCM-41
PEI loading
(wt.%)
Solvent Solution/
MCM-41
weight ratio
Adsorption
capacitya
(mg CO2/g-adsorbent)
Desorption
capacity (%)
Si-MCM-41 Pure silica – – – 8.6 103Al-MCM-41-100 100 – – – 7.6 101
Al-MCM-41-500 500 – – – 7.5 100
Si-MCM-41-PEI-5 Pure silica 5 Methanol 4:1 7.7 105
Si-MCM-41-PEI-15 Pure silica 15 Methanol 4:1 19.4 103
Si-MCM-41-PEI-30 Pure silica 30 Methanol 4:1 68.7 98
Si-MCM-41-PEI-50 Pure silica 50 Methanol 4:1 112 99
Si-MCM-41-PEI-75 Pure silica 75 Methanol 4:1 133 101
Si-MCM-41-PEI-25-25 b Pure silica 50 Methanol 4:1 96.8 97
Al-MCM-41-100-PEI-50 100 50 Methanol 4:1 127 100
Al-MCM-41-500-PEI-50 500 50 Methanol 4:1 121 99
Si-MCM-41-PEI-50-H 2O Pure silica 50 Water 4:1 111 100
PEI-30-Si-MCM-41 c Pure silica 30 Methanol 4:1 65.7 99
Si-MCM-41-PEI-50-M d Pure silica 50 – – 99 97
Si-MCM-41-PEI-50-S2 Pure silica 50 Methanol 2:1 97 98
Si-MCM-41-PEI-50-S8 Pure silica 50 Methanol 8:1 126 99
Si-MCM-41-PEI-30-
PEG-20e
Pure silica 30 Methanol 4:1 77.1 100
SilicaGel-PEI-50f – 50 Methanol 4:1 78.0 98
PEI – 100 – – 109 56
a The adsorption capacity was measured by TGA under pure CO 2 atmosphere at a flow rate of 100 ml/min at 75 °C.b The adsorbent was prepared by two-step impregnation method. In each step, 25 wt.% PEI was loaded.c The adsorbent was prepared by adding the PEI/methanol solution to the MCM-41 powder.d The adsorbent was prepared by mechanical mixing method.e PEG was added to the adsorbent of MCM-41-PEI-30. The weight percentage of PEG was 20 wt.%.f Silica Gel (SiO2 from Merck) was used as support material, instead of MCM-41.
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thermal analyzer. The weight change of the ad-
sorbent was followed to determine the adsorption
and the desorption performance of the materials.
In a typical adsorption/desorption process, about10 mg of the adsorbent was placed in a small
sample cell, heated up to 100 °C in N2 atmosphere
at a flow rate of 100 ml/min and held at that
temperature (about 30 min) until there was no
weight loss. Then the temperature was decreased
to 75 °C and the 99.8% bone-dry CO2 adsorbate
was introduced at a flow rate of 100 ml/min. After
the adsorption, the gas was switched to 99.995%
pure N2 at a flow rate of 100 ml/min to perform
desorption at the same temperature. The time for
both the adsorption and desorption was 150 min.
The adsorption/desorption temperature of 75 °C
and the adsorption and desorption time of 150 min
were selected because our previous investigation
showed that the MCM-41-PEI exhibited the best
adsorption/desorption performance at 75 °C; and
that the adsorption nearly reached equilibrium
and the desorption was complete after 150 min
[41,42]. Adsorption capacity in mg of adsorbate/g-
adsorbent and desorption capacity in percentage
were used to evaluate the adsorbent and were
calculated from the weight change of the sample
in the adsorption/desorption process. Desorptioncapacity in percentage was defined as the ratio of
the amount of the gas desorbed over the amount
of gas adsorbed.
3. Results
3.1. Preparation and characterization of MCM-41-
PEI
Si-MCM-41-PEI with different PEI loadingswere prepared and characterized by XRD, N2
adsorption/desorption and TGA. Fig. 1 shows the
XRD patterns of Si-MCM-41 and Si-MCM-41-
PEI with different PEI loadings. From comparing
the diffraction patterns of Si-MCM-41 with those
of Si-MCM-41-PEI with different PEI loadings,
the degree of Bragg diffraction angles were nearly
identical, indicating that the structure of Si-MCM-
41 was preserved after loading the PEI. However,
the intensity of the diffraction patterns of Si-
MCM-41 decreased after the PEI was loaded. Fig.
2 shows the diffraction intensity of Si-MCM-41
(100 plane) for Si-MCM-41-PEI with different PEI
loadings. With increasing PEI loadings, the inten-
sity of the diffraction peaks decreased. The inten-
sity of the diffraction peak of Si-MCM-41-PEI-50
and Si-MCM-41-PEI-75 was only 11% of that of
the Si-MCM-41 support. At the same time, the
degree of the Bragg diffraction angle of the 100
plane slightly increased from 2.265 for Si-MCM-
41 to 2.295–2.325 for Si-MCM-41-PEI. Thesechanges were possibly caused by the pore filling
effect of the MCM-41 channels and the PEI coat-
ing on the outer surface of the MCM-41 crystals.
Reddy and Song [45] reported that the XRD pat-
terns of MCM-41, after removal of the template,
exhibited peaks with increased intensity and a shift
to lower Bragg diffraction angle compared to the
template-containing MCM-41. Since the pore
volume of the Si-MCM-41 support is 1.0 ml/g and
the density of PEI is about 1.0 g/ml, the maximum
1 2 3 4 5 6 7 8 9 10
Si-MCM-41
Si-MCM-41-PEI-75
Si-MCM-41-PEI-50
Si-MCM-41-PEI-30
Si-MCM-41-PEI-15
Si-MCM-41-PEI-5
X 4
X 2
X 4
Fig. 1. XRD patterns of Si-MCM-41-PEI with different PEI
loadings as shown by the trailing digits. The top three curves
were shown at different Y -scales.
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PEI loading in the pores of Si-MCM-41 is 50 wt.%.
More PEI should be coated on the outer surface of
Si-MCM-41 crystals for Si-MCM-41-PEI-75 than
for Si-MCM-41-PEI-50. However, the diffraction
intensity of Si-MCM-41 for Si-MCM-41-PEI-50
and Si-MCM-41-PEI-75 was nearly the same,
which indicated that the PEI coating on the outer
surface of the crystals hardly influenced the
diffraction intensity of the Si-MCM-41 support.
Therefore, the decrease in the diffraction intensity
and the shift of the Bragg diffraction angle of the100 plane to high degree can be ascribed mainly to
the loading of PEI into the Si-MCM-41Õs pore
channels.
The pore size, surface area and pore volume of
Si-MCM-41 before and after loading the PEI were
obtained from the nitrogen adsorption/desorption
isotherms. Fig. 3 shows the pore size distributions
of Si-MCM-41-PEI with different PEI loadings
and Fig. 4 shows their surface areas and pore
volumes. The pore size of the Si-MCM-41 support
was 2.75 nm. After the PEI was loaded into itschannels, the pore size decreased. The pore size of
Si-MCM-41-PEI-5 was 2.47 nm, smaller than that
of the Si-MCM-41 support, which confirmed that
PEI was loaded into the Si-MCM-41 pore chan-
nels. With increasing PEI loadings, the pore size
further decreased. The pore sizes were 2.19 and
1.68 nm for Si-MCM-41-PEI-15 and Si-MCM-41-
PEI-30, respectively. When the PEI loading fur-
ther increased to 50 wt.%, the mesoporous pores
channels were completely filled with PEI, restrict-
ing the access of nitrogen into the pores at the
liquid nitrogen temperature. Therefore, informa-
tion on the pore size cannot be obtained from
the N2 adsorption/desorption isotherms for PEI
loading above 50 wt.%. This is also consistent with
the estimated maximum PEI loading of 50 wt.%inside the pores of Si-MCM-41 (based on the pore
volume of the Si-MCM-41 and the density of PEI).
The surface area and the pore volume of
Si-MCM-41, after loading the PEI, exhibited the
same trends as the pore size. The Si-MCM-41
support had a surface area of 1486 m2/g and a pore
volume of 1.0 ml/g. The surface area and the pore
volume decreased after PEI was loaded into its
channels. For example, when the PEI loading was
5.0 wt.%, the adsorbent showed a pore volume of
0
1000
2000
3000
4000
5000
6000
7000
8000
0 15 30 45 60 75
D i f f r a c t i o n i n t e n s i t y ( C P S )
Fig. 2. The influence of PEI loadings in Si-MCM-41-PEI on
the diffraction intensity of the (1 0 0) plane of MCM-41.
0
0.1
0.2
0.3
0.4
0.5
0.6
10 20 30 40 50 60
D ( v ) ( m l / Å / g )
Si-MCM-41
Si-MCM-41-PEI-5
Si-MCM-41-PEI-15
Si-MCM-41-PEI-30
Si-MCM-41-PEI-50
Si-MCM-41-PEI-75
Fig. 3. The pore size distribution of Si-MCM-41-PEI with
different PEI loadings.
0.1
1
10
100
1000
10000
100000
0 10 20 30 40 50 60 70 80
S u r f a c e a r e a ( m / g ) 2
0.0001
0.001
0.01
0.1
1
10
P o r e v o l u m e ( m l / g )
Surface Area
Pore Volume
Fig. 4. The surface area and pore volume of Si-MCM-41-PEI
with different PEI loadings.
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0.79 ml/g, smaller than that of the Si-MCM-41
support, which confirmed that PEI was loaded
into the Si-MCM-41Õs channels. With increasing
PEI loadings, the surface area and the pore volumefurther decreased. When the PEI loading was
higher than 30 wt.%, the surface area decreased
sharply, which indicated that some of the meso-
porous pores was completely filled with the PEI.
The surface area was only 4.2 m2/g and the resid-
ual pore volume was only 0.011 ml/g for Si-MCM-
41-PEI-50 at liquid nitrogen temperature. These
results correlate with the pore filling effect of the
PEI, which was also reflected by the XRD char-
acterization, and further confirms that PEI was
loaded into the pore channels of Si-MCM-41.
The thermal chemical and physical properties
of Si-MCM-41, PEI and Si-MCM-41-PEI were
measured by TGA. Fig. 5 shows the TGA results
and Fig. 6 shows the differential thermal gravi-
metric analysis (DTGA) results. As expected, there
was no weight loss for Si-MCM-41 till 600 °C,
which indicated that the hydrophobic Si-MCM-41
was free from adsorbed water or other gases at
room temperature. The PEI lost 3.8% of its origi-
nal mass at 100 °C, which can be mainly ascribed
to the desorption of CO2 and moisture. This was
confirmed by analyzing the effluent gas by gaschromatograph (GC). This also indicated that PEI
has a low vapor pressure and unlike the commer-
cially used amines such as diethanolamine (DEA),
which makes PEI suitable for long-term use at
relatively high temperature. The PEI began to
decompose above 150 °C and a sharp weight loss
appeared at 205 °C. When the temperature was
increased above 225 °C, the rate of weight loss
decreased, indicating that a different decomposi-
tion process took place. At 600 °C, the PEI was
completely decomposed and removed as volatiles.
After the PEI was loaded into the Si-MCM-41Õs
channels, the sharp weight loss of PEI took place
at a lower temperature, which indicated that the
decomposition temperature of PEI decreased. The
weight loss also took place in a narrower temper-ature range than that of the pure PEI. These
phenomena can be explained by the uniform dis-
persion of PEI into the nano-porous support
pores, since the melt or decomposition tempera-
ture will decrease when the particle size of a sub-
stance decreases [49]. In addition, when the PEI
loading was below 50 wt.%, Si-MCM-41-PEI
showed a $10.5% weight loss at 100 °C, which was
higher than that of the pure PEI and can be also
ascribed to the desorption of CO2 and moisture.
When the PEI loading was 75 wt.%, the weight losswas only $3.5% at 100 °C, similar with that of the
pure PEI, which indicated that PEI was coated on
the outer surface of the molecular sieve crystals
and the composite material behaved like the pure
PEI particles. The different weight loss at 100 °C
between the Si-MCM-41-PEI-50 and Si-MCM-41-
PEI-75 further confirmed that the PEI was loaded
into the MCM-41Õs channels. The weight loss of Si-
MCM-41-PEI also indicated that there was no PEI
loss during the preparation process. For example,
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500 600
W e i g h t ( % )
Si-MCM-41
Si-MCM-41-PEI-5
Si-MCM-41-PEI-15
Si-MCM-41-PEI-30
Si-MCM-41-PEI-50
Si-MCM-41-PEI-75
PEI
Fig. 5. TGA profile for Si-MCM-41-PEI with different PEI
loadings.
-30
-25
-20
-15
-10
-5
0
5
100 140 180 220 260 300o
D e r i v a t e w e i g h t l o s s
( % / m i n )
Si-MCM-41
Si-MCM-41-PEI-5
Si-MCM-41-PEI-15
Si-MCM-41-PEI-30
Si-MCM-41-PEI-50
Si-MCM-41-PEI-75
PEI
Fig. 6. DTGA profile for Si-MCM-41-PEI with different PEI
loadings.
X. Xu et al. / Microporous and Mesoporous Materials 62 (2003) 29–45 35
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the total weight loss for Si-MCM-41-PEI-50 was
$56% at 600 °C. If the adsorbed moisture or CO2
is excluded from the total weight, the PEI loading
is calculated to be about 50 wt.%, which is in ac-cordance with the designed PEI loading.
3.2. Adsorption performance of MCM-41-PEI
3.2.1. Effect of PEI loadings
The influence of PEI loading on the CO2 ad-
sorption performance of Si-MCM-41-PEI was in-
vestigated in pure CO2 atmosphere at 75 °C and
the results are shown in Fig. 7. Before the PEI was
loaded, the Si-MCM-41 support alone showed a
CO2 adsorption capacity of 8.6 mg/g-adsorbent.
The low adsorption capacity was caused by the
weak interaction between CO2 and Si-MCM-41 at
relatively high temperature. In order to strength
the interaction between CO2 and Si-MCM-41,
branched polymeric substances PEI with numer-
ous CO2-capturing sites, was loaded into the Si-
MCM-41 channels. Surprisingly, the PEI had no
contribution on the CO2 adsorption capacity at
low PEI loading. When the PEI loading was 5.0
wt.%, the CO2 adsorption capacity was only 7.7
mg/g-adsorbent, nearly identical to that of the Si-
MCM-41 support. When the PEI loading furtherincreased, the adsorption capacity increased. The
adsorption capacities were 19 mg/g-adsorbent and
69 mg/g-adsorbent for the PEI loading of 15 and
30 wt.%, respectively. At 50 wt.% PEI loading,
the adsorption capacity was 112 mg/g-adsorbent,
which is higher than that of the pure PEI of 110
mg/g-adsorbent. The mesoporous molecular sieve
of Si-MCM-41 showed a synergetic effect on the
adsorption of CO2 by PEI. The highest adsorptioncapacity of 133 mg/g-adsorbent was obtained when
the PEI loading was 75 wt.%.
The desorption was complete for all the Si-
MCM-41-PEI adsorbents as well as the Si-MCM-
41 support. However, the desorption for pure PEI
was slow and was not complete compared to the
desorption time of the Si-MCM-41-PEI absor-
bents. The fast desorption of CO2 from the Si-
MCM-41-PEI absorbents can be explained by the
high dispersion of the PEI into the Si-MCM-41
channels as shown by the XRD, N2 adsorption/
desorption and TGA characterizations.
3.2.2. Effect of preparation methods and preparation
procedures
Two methods, i.e., wet impregnation and me-
chanical mixing, were employed for the prepara-
tion of Si-MCM-41-PEI. For the mechanical
mixing method, PEI was solidified, grinded into
powder and mixed uniformly with Si-MCM-41 at
liquid nitrogen temperature. The powder mixture
was heated up to 70 °C and held at that temper-
ature for 16 h under reduced pressure (700mmHg). The PEI loading was 50 wt.%. The ad-
sorption performance of this absorbent was com-
pared with that of the adsorbent prepared by the
wet impregnation method in Table 1. The ad-
sorption capacity was 99 mg/g-adsorbent for the
adsorbent prepared by the mechanical mixing
method (Si-MCM-41-PEI-50-M) and 112 mg/g-
adsorbent for the adsorbent prepared by the wet
impregnation method (Si-MCM-41-PEI-50). The
adsorption capacity of Si-MCM-41-PEI-50-M was
lower than that of Si-MCM-41-PEI-50. The higheradsorption capacity for the wet impregnation
method was possibly caused by the uniform dis-
persion of PEI into the Si-MCM-41 channels.
However, if the PEI and the Si-MCM-41 were
simply mechanically mixed in Si-MCM-41-PEI-50-
M, the adsorption capacity of MCM-41-PEI-50-M
should be the linear sum of the adsorption ca-
pacity contributed from Si-MCM-41 and PEI, and
was calculated to be 58.9 mg/g-adsorbent. The
adsorption capacity of Si-MCM-41-PEI-50-M was
0
20
40
60
80
100
120
140
160
180
200
0 20 40 60 80 100
A d s o r p t i o n c a p a c i t y
( m g / g - a d s o r b e
n t )
10
20
30
40
50
60
70
80
90
100
110
D e s o r p t i o n c a p a c
i t y ( % )
Adsorption capacity
Desorption capacity
Fig. 7. The influence of PEI loadings on the CO2 adsorption
and desorption performance of Si-MCM-41-PEI.
36 X. Xu et al. / Microporous and Mesoporous Materials 62 (2003) 29–45
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higher than that value, which indicated that part
of the PEI may also be loaded into the Si-MCM-
41 channels or be coated on the external surface of
the Si-MCM-41 crystals during the thermal treat-ment. The desorption for both of the adsorbent
was complete.
Further, two different impregnation procedures
were examined. In the first set of experiments, the
adsorbent was prepared by adding the PEI-meth-
anol solution to Si-MCM-41 (PEI-30-Si-MCM-
41). Its adsorption performance was compared
with that of the adsorbent prepared by adding Si-
MCM-41 to the PEI-methanol solution (Si-MCM-
41-PEI-30) in Table 1. The adsorption/desorption
capacities of these two adsorbents were virtually
similar, which indicates that this impregnation
procedure did not influence the dispersion of PEI
into the Si-MCM-41 channels nor the adsorption
performance of the adsorbent.
In the second set of experiments, the adsorbent
was prepared by the following procedure: Si-MCM-
41 was first impregnated with 25 wt.% of PEI, then
dried under vacuum at 70 °C, and another 25 wt.%
of PEI was then loaded. The adsorption perfor-
mance of this adsorbent (Si-MCM-41-PEI-25-25)
was compared with that of the adsorbent loaded
with 50 wt.% PEI in one step (Si-MCM-41-PEI-50)in Table 1. The adsorption capacity of Si-MCM-41-
PEI-50 was higher than that of Si-MCM-41-PEI-
25-25, which indicates that one-step impregnation is
a better method than two-step impregnation. Since
PEI is hydrophilic in nature, the PEI impregnated
in the first step may adsorb the methanol and pre-
vent the PEI from entering the MCM-41 channels
in the next step of impregnation. Therefore, PEI
may coat the external surface of the support crystals
in the second impregnation and the adsorption ca-
pacity decreased.
3.2.3. Effect of solvent type and methanol/MCM-41
weight ratio
Since the Si-MCM-41 that has been used so far
is pure silica and is hydrophobic in nature, the
polarity of solvent for the dissolution of PEI may
influence the dispersion of PEI into the Si-MCM-
41 channels. Two solvents, i.e., methanol and
water, were used to examine the effect of solvent
on adsorbent performance. The PEI loading for
both adsorbents was 50 wt.% and the adsorption
results are shown in Table 1. The adsorption ca-
pacity was nearly the same for the two adsorbents,
which indicated that these two solvents had limitedinfluence on the adsorption performance of the
resulted adsorbent. The results also indicate that
PEI is more hydrophobic than both the water and
the methanol, and was therefore preferentially
adsorbed into the Si-MCM-41 channels from the
solution. Because methanol is easier to volatilize
than water and the Si-MCM-41 structure is not
stable under steam atmosphere at high tempera-
ture, methanol was selected as the solvent for the
dissolution of PEI in the subsequent study.
The influence of methanol/Si-MCM-41 weight
ratio on the adsorption performance of the resul-
tant adsorbents was also examined and the ad-
sorption results are shown in Table 1. From the
adsorption results, it can be concluded that the
adsorption capacity increased with increasing
methanol/Si-MCM-41 weight ratio. The highest
adsorption capacity of 126 mg/g-adsorbent was
obtained when the methanol/Si-MCM-41 weight
ratio was 8, which was 23% higher than that of the
adsorbent prepared with a methanol/Si-MCM-41
weight ratio of 2. When the methanol/Si-MCM-41
weight ratio was 2, the Si-MCM-41 cannot bewetted well by the PEI/methanol solution. In this
case, the PEI cannot be dispersed well and the
adsorption capacity was nearly equal to that of the
adsorbent prepared by mechanical mixing method.
When the methanol/Si-MCM-41 weight ratio was
increased to 4, MCM-41 can be wetted by the PEI/
methanol solution, allowing more PEI to be loa-
ded into the Si-MCM-41 channels and thus in-
creasing the adsorption capacity.
3.2.4. Effect of polyethyleneglycol as additiveThe influence of PEG on the adsorption per-
formance of Si-MCM-41-PEI was investigated.
Fig. 8 compares the adsorption and desorption
performance of Si-MCM-41-PEI-30 and Si-MCM-
41-PEI-30-PEG-20. The CO2 adsorption capac-
ity and the CO2 adsorption/desorption rate for
Si-MCM-41-PEI changed after adding the PEG
to Si-MCM-41-PEI. The adsorption capacity
of Si-MCM-41-PEI-30-PEG-20 was higher than
that of Si-MCM-41-PEI-30. In addition, the
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adsorption/desorption rates for Si-MCM-41-PEI-
30-PEG-20 were faster than those of for Si-MCM-
41-PEI-30. Satyapal et al. [44] also found that
adding PEG to the PEI/polymer adsorbent can
accelerate the CO2 adsorption and desorption
rates. The authors suggested that the enhancement
of the adsorption and desorption rate is related to
the preponderance of OH ions from the PEG
molecules. They also suggested that the PEG at-
tracts more water to the adsorbent due to thehydroscopic nature of the chemical. Since the ad-
sorbate was pure CO2 in our experiment and there
was no moisture in the gas, the enhanced adsorp-
tion capacity in this study must be from the CO2
alone. The increase in the adsorption capacity may
result from modifying the reaction route after
adding the PEG as discussed later.
3.2.5. Effect of the Si/Al ratio of the MCM-41
support
The above results were derived using pure silicaof MCM-41. To clarify whether aluminum incor-
poration into MCM-41 framework can affect the
property of the MCM-41-PEI adsorbent, we fur-
ther examined the influence of the Si/Al ratio of the
MCM-41 support on theadsorption performance of
the resultant adsorbents. Fig. 9 shows the adsorp-
tion capacity of MCM-41 and MCM-41-PEI-50
with different support Si/Al ratio. The adsorption
capacity was nearly identical for the MCM-41
support with different Si/Al ratio. However, the
adsorption capacity of MCM-41-PEI was influ-
enced by the Si/Al ratio of the MCM-41 support.
The lower the Si/Al ratio of the MCM-41 support,
the higher the adsorption capacity of the resultant
adsorbent. The adsorption capacity of Al-MCM-
41-100-PEI-50 was 127 mg/g-adsorbent and was
12% higher than that of Si-MCM-41-PEI-50.
4. Discussions
4.1. Effect of mesoporous molecular sieve of
MCM-41
In order to reveal any synergetic effects between
MCM-41 and PEI on CO2 adsorption, the linear
adsorption capacity and the synergetic adsorption
gain were defined as follows:
Linear adsorption capacity ðmg adsorbate=g-adsorbentÞ
¼ ½ðMCM-41 weight percentage in the adsorbent
 adsorption capacity of pure MCM-41Þ
þ ðPEI weight percentage in the adsorbent
 adsorption capacity of pure PEIÞ ð1Þ
Synergetic adsorption gain ðmg adsorbate=g-adsorbentÞ
¼ adsorption capacity of the adsorbent
À linear adsorption capacity ð2Þ
0
10
20
30
40
50
6070
80
90
100
0 50 100 150 200 250 300
W e i g h t c h a n g e ( m g / g - a d
s o r b e n t )
Si-MCM-41-PEI-30-PEG-20
Si-MCM-41-PEI-30
Fig. 8. The effect of PEG on the CO2 adsorption/desorption
performance of Si-MCM-41-PEI (PEI loading: 30 wt.%, PEG
loading: 20 wt.%).
Fig. 9. The influence of Si/Al ratio of the MCM-41 support on
the CO2 adsorption performance of MCM-41 and MCM-41-
PEI (PEI loading: 50 wt.%).
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The linear adsorption capacity and the syner-
getic adsorption gain were calculated and are
shown in Fig. 10. For Si-MCM-41-PEI-5 and Si-
MCM-41-PEI-15, the synergetic adsorption gainwas close to zero, indicating that the loading of
PEI into the Si-MCM-41 channels had no effect on
the increase in the CO2 adsorption capacity at
these low PEI loadings. When the PEI loading was
higher than 30 wt.%, a synergetic adsorption gain
was observed, which indicated that PEI modified
Si-MCM-41 began to act as a CO2 ‘‘molecular
basket’’. The highest synergetic gain was obtained
at the PEI loading of 50 wt.%, while the synergetic
adsorption gain decreased at the PEI loading of
75 wt.%.
By correlating the adsorption performance and
the pore structure of Si-MCM-41 and Si-MCM-
41-PEI, a simple model was proposed in Fig. 11 to
account for the synergetic effect of Si-MCM-41 on
the CO2 adsorption by PEI. Since the adsorption
was operated at a relatively high temperature and
the pore size of Si-MCM-41 is in the mesoporous
region, the CO2 adsorption capacity of Si-MCM-
41 alone is low. When the PEI, which has bran-
ched chains with numerous CO2 adsorption sites
of amine group, is loaded into the Si-MCM-41
channels, both the physical adsorption by capillarycondensation and the chemical adsorption by re-
action with PEI will contribute to the adsorption
capacity. It may be expected that the adsorption
capacity of Si-MCM-41-PEI would increase with
increasing amount of PEI loaded. However, only
with high PEI loading did Si-MCM-41-PEI show a
synergetic effect on the CO2 adsorption, indicating
a strong interaction between Si-MCM-41 and PEI.
When the PEI loading was low, the pore size of the
adsorbent slightly decreased, from 2.75 nm for Si-MCM-41 to 2.47 nm for Si-MCM-41-PEI-5. In
this case, the PEI was mostly adsorbed on the in-
ner pore wall of the Si-MCM-41 support (Fig.
11B). Both physical adsorption and chemical ad-
sorption contribute to CO2 uptake in this case.
The adsorption capacity was even smaller than
that of the linear adsorption capacity. With in-
creasing PEI loadings, the pore size further de-
creased. At the same time, because more PEI was
loaded into the channels, the chemical adsorption
of CO2 became much more dominant than that of with low PEI loadings. The physical adsorption on
the unmodified pore wall of Si-MCM-41 (and the
capillary condensation in the mesopore) became
negligible when compared with the chemical ad-
sorption force. In addition, mesoporous molecular
sieve Si-MCM-41 had a synergetic effect on the
adsorption of CO2 by PEI in the confined meso-
porous pores [42]. Therefore, the CO2 adsorption
capacity increased. The highest synergetic ad-
sorption gain (Figs. 10 and 11C) was obtained
-20
0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60 70 80 90 100
A d s o r p t i o n c a p a c i t y
( m g / g - a d s o r b
e n t )
Synergetic adsorption gain
Linear adsorption capacity
Adsorption capacity
Fig. 10. Synergetic effect of Si-MCM-41 on the adsorption of
CO2 by PEI as a function of PEI loading in MCM-41-PEI.
(A) (B)
Fig. 11. Schematic diagram of PEI loaded in the mesoporous
molecular sieve of MCM-41. (A) MCM-41 support; (B) low
PEI loading; (C) high PEI loading; (D) extremely high PEI
loading.
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when the mesoporous channels were completely
filled with PEI, i.e., about 50 wt.% of PEI loading.
When the PEI loading further increased to 75 wt.%
(Figs. 10 and 11D), the synergetic adsorption gainwas smaller than that with PEI loading of 50 wt.%.
Since the pore volume of Si-MCM-41 is about 1.0
ml/g and the density of PEI is about 1.0 g/ml, the
largest amount of PEI that can be theoretically
loaded into 1.0 g Si-MCM-41 is 1.0 ml, i.e., 50
wt.% PEI loading. For the adsorbent with PEI
loading of 75 wt.%, theoretically, only one third of
the PEI could be loaded into the Si-MCM-41
channels and two thirds of the PEI would be
coated on the external surface of the molecular
sieve particles. Interestingly, the highest adsorp-
tion gain weighed on PEI was also at the 50 wt.%
PEI loading, which indicated that only when the
PEI was loaded into the mesoporous molecular
sieve did the Si-MCM-41 and PEI show a strong
synergetic effect on the CO2 adsorption. When the
PEI was coated on the external surface of the
molecular sieve crystals, the synergetic effect was
limited. In order to distinguish the synergetic effect
of the PEI in the mesoporous channels and on the
external surface of the molecular sieve crystals, the
adsorption capacity of the PEI only in the ‘‘mo-
lecular basket’’ adsorbent was calculated as fol-lows:
PEI adsorption capacity ðmg adsorbate=g-PEIÞ
¼ ½adsorption capacity of the adsorbent-ðMCM-
41 weight percentage in the adsorbent
 adsorption capacity of pure MCM-41Þ
=ðPEI weight percentage in the adsorbentÞ
ð3Þ
The calculated PEI adsorption capacity is 215 mg/
g-PEI for Si-MCM-41-PEI-50 and 174 mg/g-PEIfor Si-MCM-41-PEI-75. Since the Si-MCM-41-
PEI-75 was prepared by 25 wt.% of Si-MCM-41
and 75 wt.% of PEI, we can assume that 25 wt.%
of the PEI is loaded into the pores of the Si-MCM-
41 and the other 50 wt.% of the PEI is coated on
the external surface of the Si-MCM-41. If we as-
sume that the PEI loaded into the Si-MCM-41
channel shows the same adsorption capacity as
that of the Si-MCM-41-PEI-50, the adsorption
capacity of the PEI that was coated on the external
surface of Si-MCM-41 for Si-MCM-41-PEI-75 can
be calculated and is found to be only154 mg/g-
PEI. The adsorption capacity for the PEI coated
on the external surface of the Si-MCM-41 wasmuch lower than that of the Si-MCM-41-PEI-50
(215 mg/g-PEI) when the PEI was fully filled in the
channels of the Si-MCM-41, which confirmed that
only when the PEI was loaded into the channels of
the mesoporous molecular sieve did the Si-MCM-
41 showed the highest synergetic effect and proved
that the ‘‘molecular basket’’ concept resulted in the
significant improvement on the CO2 adsorption by
PEI.
The adsorption capacity for the PEI coated on
the external surface of the crystal was slightly
greater than that of the pure PEI of 110 mg/g-PEI.
When the PEI was coated on the crystal surface,
more adsorption sites will expose to the adsorbate.
Therefore, the adsorption capacity will be higher
than that of the bulk PEI. This can also be verified
by the CO2 adsorption capacity of PEI coated on
other high-surface-area materials such as silica gel.
When the PEI was coated on a high-surface-area
silica gel (550 m2/g) and the PEI loading was 50%,
the CO2 adsorption capacity was only 156 mg/g-
PEI, slightly higher than that of the bulk PEI and
much lower than that of the ‘‘molecular basket’’adsorbent with the same PEI loading, which also
verified that only when the PEI was loaded into
the channels of the mesoporous molecular sieve
did the Si-MCM-41 showed the highest syner-
getic effect on the adsorption of CO2. The CO2
adsorption capacity of SilicaGel-PEI was nearly
identical to that of the calculated CO2 adsorption
capacity on the external surface of the MCM-41
crystals. Satyapal et al. [44] also observed the same
phenomenon when they coated the PEI on the
high-surface-area solid polymethyl methacrylatepolymeric support.
By employing the ‘‘molecular basket’’ concept,
the CO2 adsorption and desorption kinetics can
also be improved significantly. Fig. 12 compares
the CO2 adsorption curves of Si-MCM-41-PEI-50,
Si-MCM-41-PEI-75 and PEI in the first minute (60
s) of adsorption. The CO2 adsorption rate de-
creases in the following order: Si-MCM-41-PEI-
50 > Si-MCM-41-PEI-75 > PEI. If the linear part
of the adsorption curve was selected to calculate
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the CO2 adsorption rate, the CO2 adsorption rate
was 5.6 mg/s.g-PEI, 3.23 mg/s.g-PEI and 2.67 mg/
s.g-PEI for Si-MCM-41-PEI-50, Si-MCM-41-PEI-
75 and PEI, respectively. The CO2 adsorption rate
for Si-MCM-41-PEI-50 was much faster than that
of the Si-MCM-41-PEI-75 and PEI, which con-
firmed that only when the PEI was loaded into the
channels of the mesoporous molecular sieve did
the Si-MCM-41 showed the highest synergetic ef-
fect and proved that the ‘‘molecular basket’’ con-
cept resulted in the significant improvement on the
CO2 adsorption by PEI. Coating PEI on the high
surface area materials only slightly increased theCO2 adsorption rate. The CO2 desorption rate
showed the same trend as the adsorption. The CO2
desorption rate decrease in the following order: Si-
MCM-41-PEI-50 > Si-MCM-41-PEI-75 > PEI.The
CO2 desorption was complete for Si-MCM-41-
PEI-50 andSi-MCM-41-PEI-75 in 150min, whereas
only 56% of the adsorbed CO2 was desorbed when
the same desorption time as the Si-MCM-41-PEI
adsorbent was used.
Compared with conventional adsorbents, the
‘‘molecular basket’’ showed better CO2 adsorption
performance at relatively high temperature. Table
2 lists the CO2 adsorption performance of zeolite,
activated carbon, PEI/polymer composite and
‘‘molecular basket’’ adsorbent. The ‘‘molecular
basket’’ adsorbent shows superior CO2 selectivity
in terms of very high CO2/N2 ratios (>1000 for
MCM-41-PEI-50 vs 2–6 for aluminosilicates and
activated carbons). The high selectivity for CO2 is
particularly important when it was applied in the
separation of CO2 from flue gas [51].
y = 5.60x - 19.66
R = 1.002
y = 3.23x - 15.84R = 1.002
y = 2.67x - 16.87
R = 1.002
0
20
40
60
80
100
120
140
160
0 6 12 18 24 30 36 42 48 54 60
A d s o r p t i o n c a p a c i t y ( m g / g - P E I )
Si-MCM-41-PEI-50
Si-MCM-41-PEI-75
PEI
Fig. 12. Comparison of the CO2 adsorption kinetics of
Si-MCM-41-PEI-50, Si-MCM-41-PEI-75 and PEI.
Table 2
Comparison of CO2 adsorption performance of ‘‘molecular basket’’ adsorbent and other adsorbents
Adsorbents Temp
(°C)
Pressure
(atm)
Adsorption
capacity (mg CO2/g
adsorbent)
CO2/N2 or CO2/
CH4 selectivity
Ref.
Si-MCM-41 25 1 27.3 – This study
Si-MCM-41 75 1 8.6 – This study
Si-MCM-41 75 0.149 6.3 2.9 (CO2/N2) [51]
Al-MCM-41-100 75 1 7.6 – This study
Al-MCM-41-500 75 1 7.5 – This study
Si-MCM-41-PEI-50 25 1 32.9 – This study
Si-MCM-41-PEI-50 75 1 112 – This study
Si-MCM-41-PEI-50 75 0.149 89.2 >1000 (CO2/N2) [51]
Al-MCM-41-100-PEI-50 75 1 127 – This study
Al-MCM-41-500-PEI-50 75 1 121 – This study
Zeolite 13 X 25 1 168 – [13]
Zeolite 4A 25 1 135 – [13]
Activated carbon 25 1 110 – [13]
Norit RBI activated carbon 21.5 1 108 $2 (CO2/CH4) [28]
Norit RBI activated carbon 75 1 40 $2 (CO2/CH4) [28]
Activated carbon 20 1 88 $ 2 (CO2/CH4) [24]
Norit RBI activated carbon 25 1 140.8 $1.9 (CO2/CH4) [29]
PEI-silica gel 75 1 78.1 – This study
PEI-polymer $50 0.02 $40 – [44]
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4.2. Effect of preparation method
The adsorption capacity of the adsorbent pre-
pared by the wet impregnation method was higherthan that of the adsorbent prepared by the me-
chanic mixing method. For Si-MCM-41-PEI-50,
the mesoporous pores were completely filled with
PEI and high adsorption capacity was obtained
because most of the PEI is uniformly dispersed into
the Si-MCM-41 channels. In the case of Si-MCM-
41-PEI-50-M, whose morphology was similar to
that of Si-MCM-41-PEI-75, it is likely that only
some of the PEI was loaded into the support
channels during the thermal treatment and the re-
maining PEI may coat the outer surface of the
molecular sieve particles. As we discussed above,
only when the MCM-41 channels are filled with
PEI does the adsorbent show the highest synergetic
effect on the adsorption of CO2. When the PEI was
coated on the external surface of the molecular
sieve crystals, the synergetic effect was much lower
than that when the PEI was in the MCM-41
channels. Hence, the low adsorption capacity of Si-
MCM-41-PEI-50-M can be ascribed to the inho-
mogeneous dispersion of the PEI into the channels.
The low adsorption capacity for the adsorbent Si-
MCM-41-PEI-50-S2 and Si-MCM-41-PEI-25-25can also be explained for the same reason. The
homogeneous dispersion of PEI into the MCM-41
channels is critical for the preparation of this novel
CO2 ‘‘molecular basket’’ adsorbent.
4.3. Effect of PEG addition
CO2 adsorption capacity increased after adding
20 wt.% PEG into MCM-41-PEI-30. Chaffee and
co-workers [50] reported that the ratio of CO2
molecular per available N atom in the presence of hydroxyl group is approximately twice that with-
out the hydroxyl group. They suggested that the
CO2 chemical adsorption mechanism of the amine
changed in the presence of hydroxyl group. With-
out the hydroxyl group, the formation of carba-
mate is favored in the manner shown in Eqs.
(4)–(6). Two moles of amine groups react with 1
mol of CO2 molecule.
CO2 þ 2RNH2 () NHþ4 þ R2NCOOÀ ð4Þ
CO2 þ 2R2NH () R2NHþ2 þ R2NCOOÀ ð5Þ
CO2 þ 2R3N () R4Nþ þ R2NCOOÀ ð6Þ
In the presence of hydroxyl groups, the formation
of carbamate type zwitterions is stabilized in a
manner depicted in Eq. (7). One molar amine
groups react with one mole CO2 molecule. There-
fore, the adsorption capacity increased with the
same amine group.
ð7Þ
Although PEG cannot adsorb CO2, the presence
of OH groups in PEG may also influence the
chemical adsorption mechanism. In the presence
of hydroxyl group, the formation of carbamate
type zwitterions may be promoted and, therefore,
amine group will adsorb more CO2 molecular. The
change of adsorption mechanism is also suggested
by the adsorption and desorption rate as shown in
Fig. 9. The adsorption and desorption rates of Si-
MCM-41-PEI-30-PEG-20 increased after adding
the PEG into the Si-MCM-41-PEI-30.
4.4. Effect of Si/Al ratio of MCM-41 support
The Si/Al ratio of the MCM-41 support influ-
ences the adsorption performance of the resultant
MCM-41-PEI adsorbent. The lower the Si/Al ratio
of the MCM-41 support, the higher the CO2 ad-
sorption capacity of the MCM-41-PEI adsorbent.
When the Si/Al ratio of the MCM-41 support was
100, the highest CO2 adsorption capacity was ob-
tained and was 127 mg/g-adsorbent, which corre-spond to a CO2 adsorption capacity of 246 mg/
g-PEI. In order to clarify the effect of the Si/Al
ratio of the MCM-41 support, the pore structures
were measured for the MCM-41 with different Si/
Al ratio and their corresponding MCM-41-PEI
adsorbent with PEI loading of 50 wt.%. The results
are shown in Figs. 13–15, respectively. For the
MCM-41 support, the pore size and pore volume
increase with the decrease in Si/Al ratio. After the
PEI was loaded, the pore volume decreased. The
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lower the Si/Al ratio of the MCM-41 support, the
higher the residual pore volume of the MCM-41-
PEI adsorbent. It was possibly caused by the large
pore volume of the MCM-41 support with low Si/Al ratio. Since the PEI is a branched polymer, its
molecular size is relatively large compared to the
simple molecule, e.g., N2. Each of the pore chan-
nels of the MCM-41 support can only accommo-
date a limited number of PEI molecules. The pore
volume of Si-MCM-41-PEI-30 was 0.23 ml/g,
considerably smaller than that of the theoretical
value of 0.4 g/ml when we assume the PEI mole-
cules ‘‘pack’’ in a compact form in the channels as
they would in a neat PEI liquid. In the pore
channel of mesoporous molecular sieve, the PEI
molecules cannot ‘‘pack’’ as compact as in the neat
liquid or solid PEI, as suggested by the pore vol-
ume of Si-MCM-41-PEI-30 vs that of Si-MCM-41.
Therefore, although the maximum PEI loading in
the pore channel of Si-MCM-41 would be 50 wt.%,
a part of the PEI will be inevitably coated on the
external surface of the molecular sieve crystals.
The adsorption capacity for the PEI coated on the
external surface was much lower than that when
the PEI was filled in the channels of the MCM-41.
When the aluminum was incorporated into the
framework of MCM-41, the pore volume of MCM-41 increased allowing more PEI loading
into the channels of the mesoporous molecular
sieve. The lower the Si/Al ratio of MCM-41, the
higher the pore volume. For Al-MCM-41-100-
PEI-50, the pores were even not completely filled
with PEI as reflected by the residual pore volume
(Fig. 15). This can explain why the higher CO2
adsorption capacity was obtained with lower Si/Al
ratio of MCM-41 support.
It should be noted that the total volume of
adsorbed CO2 in MCM-41-PEI-50 was 114 ml(STP)/g-PEI for Si-MCM-41-PEI-50. Since the
adsorption mechanism is chemical adsorption for
the ‘‘molecular basket’’ adsorbent, CO2 reacts with
the PEI and is not adsorbed in the free space of the
pores. In addition, the pore size, pore volume, and
surface area were measured at liquid nitrogen
temperature. With the increase of temperature, the
polymer becomes more flexible and more CO2-
affinity sites will be exposed to the CO2 and
the adsorption capacity increases. The pore size of
Fig. 13. The influence of Si/Al ratio on the surface area and
pore volume of MCM-41.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
10 15 20 25 30 35 40 45 50 55 60
D v ( d ) ( m l / Å / g )
Si/Al=100
Si/Al=500
All silica
Fig. 14. The influence of Si/Al ratio on the pore size distribu-
tion of MCM-41.
Fig. 15. Comparison of the BET surface area and the pore
volume of the MCM-41-PEI-50 adsorbents with different Si/Al
ratio of MCM-41.
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the MCM-41-PEI-50 should increase to some ex-
tent with increasing temperature (e.g., 75 °C for
the CO2 adsorption compared to liquid nitrogen
temperature for pore size measurement). There-fore, the diffusion of CO2 in the MCM-41 channel
is faster at high temperature than at low temper-
ature.
5. Conclusions
1. Novel CO2 ‘‘molecular basket’’ adsorbents based
on polyethylenimine (PEI)-modified mesopor-
ous molecular sieve of MCM-41 type (MCM-
41-PEI) have been successfully developed. The
highest CO2 adsorption capacity of 246 mg/g-
PEI was obtained, which is 30 times higher than
that of the MCM-41 and more than twice that
of the pure PEI.
2. When the PEI loading was higher than 30 wt.%,
the mesoporous molecular sieve of MCM-41
showed a synergistic effect on the adsorption
of CO2 by PEI. The highest synergistic effect
was obtained when the PEI loading was 50
wt.%. When the PEI loading was 75 wt.%, the
PEI was coated on the outer surface of the mo-
lecular sieve crystals and the synergistic effectdecreased. The experiment results showed that,
although the dispersion of PEI on the high
surface area materials could increase the ad-
sorption capacity, it was the synergetic effect be-
tween PEI and MCM-41 pore channels (CO2
‘‘molecular basket’’) that contributes to major
increase in the CO2 adsorption capacity.
3. The uniform dispersion of the PEI into the pore
channels of the MCM-41 support is critical for
the preparation of this novel ‘‘molecular bas-
ket’’ adsorbent. The MCM-41-PEI adsorbentsprepared by the one-step impregnation method
showed a higher CO2 adsorption capacity than
the adsorbent prepared by the mechanical mix-
ing method and the two-step impregnation
method.
4. Proper incorporation of Al into the MCM-41
framework in the synthesis step can significantly
enhance the performance of the final MCM-41-
PEI adsorbents. The relatively lower Si/Al ratio
of MCM-41 support and relatively higher meth-
anol/MCM-41 weight ratio for adsorbent prep-
aration can lead to higher CO2 adsorption
capacity of MCM-41-PEI.
5. Adding PEG into the MCM-41-PEI adsorbentcan increase not only the CO2 adsorption and
desorption rate, but also the CO2 adsorption
capacity.
Acknowledgements
Funding for the work was provided by the
US Department of Defense (via an interagency
agreement with the US Department of Energy)
and the Commonwealth of Pennsylvania under
cooperative agreement no. DE-FC22-92PC92162.
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