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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

http://mail%20to:%[email protected]/

http://mail%20to:%[email protected]/



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-

30 X. Xu et al. / Microporous and Mesoporous Materials 62 (2003) 29–45



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

X. Xu et al. / Microporous and Mesoporous Materials 62 (2003) 29–45 31



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-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.




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.




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.




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.




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.




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




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

Â adsorption capacity of pure MCM-41Þ

þ ðPEI weight percentage in the adsorbent

Â 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.%).




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.




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

Â 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




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]




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




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.




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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