Article
pubs.acs.org/EF
Sulfuric Acid Modified Bentonite as the Support of
Tetraethylenepentamine for CO2 Capture
Weilong Wang,†,‡ Xiaoxing Wang,‡ Chunshan Song,‡ Xiaolan Wei,§ Jing Ding,*,† and Jing Xiao*,§
†
Center for Energy Conservation Technology, School of Engineering, Sun Yat-sen University, Guangzhou, 510006, China
Clean Fuels and Catalysis Program, EMS Energy Institute, and Department of Energy & Mineral Engineering, Pennsylvania State
University, University Park, Pennsylvania 16802, United States
§
Department of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
‡
S Supporting Information
*
ABSTRACT: In this work, an inexpensive and commercially available bentonite was modified by sulfuric acid and explored as
the new type of support to immobilize tetraethylenepentamine (TEPA) for CO2 capture from flue gas. By applying sulfuric acid
treatment, the textural properties, in particular, pore volume and surface area of bentonite, were significantly improved. Bentonite
treated with 6 M sulfuric acid (Ben_H2SO4_6M) can reach a pore volume of 0.77 cc/g from that of the parent bentonite of 0.15
cc/g. With the maximum TEPA loading of 50 wt % onto the Ben_H2SO4_6M sorbent, the maximum CO2 breakthrough
sorption capacity reached 130 mg of CO2/g of sorbent at 75 °C under a dry condition. With an addition of moisture to the
simulated flue gas, the CO2 sorption capacity can be further improved to 190 mg of CO2 at 18 vol% of moisture addition sorbent
due to the bicarbonate formation under a wet condition. The TEPA/Ben_H2SO4_6M sorbents show a good regenerability and
thermal stability below 130 °C. The high CO2 sorption capacity, positive effect of moisture addition, and low capital cost of the
raw bentonite materials imply that TEPA/Ben_H2SO4_6M could be a promising sorbent for cost-efficient CO2 capture from flue
gas. The sulfuric acid treatment was demonstrated as an effective method for bentonite modification to immobilize TEPA for
CO2 capture.
1. INTRODUCTION
Reducing anthropogenic CO2 emission and lowering the
concentration of greenhouse gases in the atmosphere have
quickly become one of the most urgent environmental issues,1
as the increasing CO2 emissions are thought to be one
contributor to global warming.2 Carbon capture and storage
(CCS) is a viable strategy for mitigating CO2 emissions while
retaining the continuous use of fossil-fuel-based energy. In the
industrial scale, amine scrubbing, using aqueous solutions of
amine, that is, monoethanolamine (MEA) and diethanolamine
(DEA), to selectively absorb CO2 from flue gases, has been
applied in conventional absorber/stripper systems in power
plants for effective CO2 capture.3 However, one major concern
of amine scrubbing technology is the high energy penalty, as
well as equipment corrosion,2,4−6 and the amine degradation
especially in the presence of oxygen and/or sulfur dioxide,3 etc.
Therefore, less energy-intensive technologies should be
developed for postcombustion CO2 capture.
A new concept of “molecular basket” sorbents (MBS),4,7,8
which selectively capture CO2 molecules onto a functional
“basket”, has been proposed for CO2 capture. The MBS-type of
sorbents are prepared by immobilizing an amine-functional
polymer, that is, polyethyleneamine (PEI), onto a porous
supporting material. MBS has shown great advantages for CO2
capture, including superior sorption−desorption characteristics
(i.e., sorption capacity, selectivity, regenerability, stability, and
kinetics, etc.), no or less corrosion, and a lower energy
consumption9 compared to the conventional amine scrubbing.
Various mesoporous silica molecular sieves, that is, MCM41,10,11 MCM-48,12 and SBA-15,7,13,14 have been studied as the
© 2013 American Chemical Society
supporting materials for PEI for CO2 capture. However, most
of them are not commercially available and also expensive to
prepare in an industrial scale. It was pointed out that the
support material accounts for over 70% of the total capital cost
for sorbent preparation.15 Moreover, mesoporous silica
molecular sieves can be degradable due to their poor
hydrothermal stability. Therefore, developing MBS with lowcost and/or rich natural abundance supporting materials and
good hydrothermal stability guide a new direction for CO2
capture. So far, studied supporting materials include mesoporous silica gel,16 and activated carbon.9
Bentonite is one of the most common raw clay materials,
consisting mainly of the montmorillonite group. The inner
layer is composed of one octahedral alumina sheet placed
between two tetrahedral silica sheets. Due to the isomorphous
substitutions within the layers of Al3+ for Si4+, the surface of
bentonite is negatively charged.17 The sorption features over
clay are believed to be related to the nature of the parent clay,
and an attempt18 to rationalize the fact was made considering
the Si/Al ratio, together with textural characteristics. Taking the
advantages of low cost (∼$50/ton), easy availability, and high
mechanical and chemical stability, bentonite has been widely
used for industrial separation processes, that is, sorption
materials for the adsorption of ethofumesate19 and dibenzofuran20 in water treatment processes. However, the microstructure, in particular, the micropore size distribution, limits
Received: December 17, 2012
Revised: February 17, 2013
Published: March 12, 2013
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TEPA/Ben sorbent, or 11, 25, 43, 67, 100, and 150 wt % of TEPA
loading referring to the weight of the bentonite support, were prepared
and further studied.
2.4. Characterization of TEPA/Ben Sorbents. 2.4.1. N2
Adsorption Test. The textural structure of the adsorbents were
characterized using a Micromeritics ASAP2020 surface area and
porosimetry analyzer. BET surface area was calculated from adsorption
isotherms using the standard Brunauer−Emmett−Teller (BET)
equation. The t-plot method was applied to derive the micropore
surface area. Pore size distributions (PSDs) were determined using
density functional theory (DFT) based on statistical mechanics.22
2.4.2. SEM. To investigate the physical and chemical compatibility
of the TEPA/Ben sorbents, the morphology was investigated using
SEM (model S-520/ISIS-300, Hitachi/Oxford). The samples were
prepared by fracturing the specimen in liquid nitrogen and then
casting it with gold (AU) powder for SEM imaging.
2.4.3. FT-IR. The functionalities on the TEPA/Ben sorbents were
characterized using FT-IR (model Nicolet/Nexus 670) in the
frequency range of 4000−400 cm−1.
2.4.4. TGA. TGA (model of Q600SDT) was used for the thermal
stability study of the TEPA/Ben sorbents in comparison to the TEPA.
All the samples were ground to powder and scanned within a
temperature range of 50−800 °C.
2.5. Evaluation of Sorption/Desorption Performance.
2.5.1. TGA Tests. The sorption−desorption performance of prepared
MBS samples was evaluated using a thermogravimetric analyzer (TGA
Q600SDT) on the basis of the weight change during the sorption and
desorption. The analysis is described as below: About 10 mg of the
sample was placed into the sample pan, and the temperature was
increased at a rate of 10 °C/min from 30 to 100 °C and equilibrated at
100 °C for 40 min under N2 (99.999%) with a flow rate of 100 mL/
min to remove the possible moisture, solvent, or other adsorbates from
the samples. The temperature was then cooled down to the desired
temperature of 75 °C, and the gas was switched from N2 to pure CO2
(99.99%) and maintained at this temperature for 40 min for CO2
sorption. After that, the temperature was increased to 110 °C, and the
gas flow was switched from CO2 to N2 for desorption. The mass-based
CO2 sorption capacity (mg of CO2/g of sorb) was calculated
according to the weight change of the sample measured by TGA in the
sorption/desorption process. Here, it should be pointed out that the
sorption capacity measured by TGA in this study is not an equilibrium
sorption capacity, as the weight of the sample still showed an
increasing trend after 40 min sorption due likely to the slow sorption
rate.8,23
2.5.2. Fixed-Bed Flow Sorption Tests. The CO2 sorption
performance of some samples from a simulated flue gas was also
investigated in a fixed-bed flow sorption system. Scheme 1 shows the
its potential use in many catalytic and separation processes. To
extend the application ranges of bentonite, the textural and
chemical properties, that is, Si/Al ratio, can be tuned by
different activation methods, that is, mechanochemical
activation, intercalation, thermochemical activation, chemical
activation, etc., which may possibly contribute to the generation
of a new porous structure and the creation of new/more active
sites on the surface, resulting in improved adsorptive or
catalytic behaviors of the modified bentonite. Among them,
acid activation has been studied as a chemical treatment
method for the alternation of the textural (i.e., surface area,
pore volume) and chemical properties (i.e., catalytic,
adsorptive) of clays. The activation process includes the
following steps: (i) leaching of the clays with inorganic acids,
(ii) causing disaggregation of clay particles, (iii) elimination of
mineral impurities, and (iv) dissolution of the external layers.21
The acid treatment is beneficial in terms of increased surface
area, porosity, and number of acid sites with respect to the
parent bentonite, which could be potentially a good supporting
material for the immobilization of a basic amine-functionalized
polymer (macromolecule) as a new MBS for CO2 capture. To
the best of our knowledge, no work has been carried out in this
area.
The objective of this study is to explore the use of
inexpensive and commercially available bentonite (∼$50/ton)
modified by sulfuric acid as a support to prepare the bentonitesupported PETA (TEPA/Ben) MBS for CO2 capture. The
textural properties and morphology of the bentonite-supported
MBS were determined by N2 adsorption tests, scanning
electron microscope (SEM), and Fourier transfer-infrared
spectroscopy (FT-IR). The CO2 sorption/desorption performance was evaluated in a thermogravimetric analyzer (TGA) and
a fix-bed flow sorption system. The effects of TEPA loading,
sorption temperature, and H2O addition on CO2 sorption were
examined and discussed. A 10-cycle sorption−desorption test
was carried out to study the regenerability of the TEPA/Ben,
and its thermal stability was investigated in a TG experiment.
2. EXPERIMENTAL SECTION
2.1. Materials. Bentonite was provided by JiangXi Yushan
Bentonite Company, China. Tetraethylenepentamine (TEPA) was
purchased from Shanghai Wuhua Company. Ethanol with a purity of
99.8% was purchased from Guangzhou Guanghua Chemicals for using
as a solvent in the preparation of TEPA/Ben sorbents. Sulfuric acid
(concentrated) was provided by Guangzhou Chemicals Company,
which was further diluted in the lab for different concentrations.
2.2. Bentonite Modification by Acid Treatment. To increase
the porosity of the supporting materials, bentonite was modified by
sulfuric acid at different concentrations (3, 6, and 9 M), which were
noted as Ben_H2SO4_3M, Ben_H2SO4_6M, and Ben_H2SO4_9M.
The desired amount of bentonite was added into the sulfuric acid
solution at a fixed ratio of 10 cc of sulfuric acid to 1 g of bentonite. The
mixture was then heated at 95 °C in an oil bath, stirring at 600 rpm for
4 h. After that, the suspension was filtered and washed with a large
amount of distilled water and dried in an oven at 100 °C overnight.
Finally, the powder was heated continuously under vacuum for 24 h.
2.3. Preparation of TEPA/Ben Sorbents. The modified
bentonite-supported MBS were prepared by the wet impregnation
method. About 3 g of TEPA was dissolved in 30 g of ethanol under
stirring for 30 min. The sulfuric acid modified bentonite was then
added into the above solution and further stirred for 3 h at room
temperature. After that, the slurry was dried in a vacuum oven at 45 °C
for 12 h. To optimize the TEPA loading onto the sulfuric acid
modified bentonite, different amounts, including 10, 20, 30, 40, 50, and
60 wt % of TEPA loading referring to the weight of the modified
Scheme 1. Schematic Diagram of the Fixed-Bed Flow System
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Table 1. Textural Properties (Vtotal, SBET, dave), Mass Loss Percentage (Mi/Mo), and Maximal CO2 Sorption Capacity of the
Parent Bentonite and the Modified Bentonites by Sulfuric Acid at Different Concentrations of 3, 6, and 9 M
samples
Vtotal (cc/g)
SBET (m2/g)
dave (nm)
Mi/Mo (%)
max CO2 sorption cap (mg of CO2/g of sorb)
parent bentonite
Ben_H2SO4_3M
Ben_H2SO4_6M
Ben_H2SO4_9M
0.15
0.53
0.77
0.41
118
307
316
265
4.2
5.7
6.9
6.5
0
2.7
4.9
8.7
26 (15 wt % of TEPA)
80 (38 wt % of TEPA)
135 (50 wt % of TEPA)
62 (30 wt % of TEPA)
schematic diagram of the fixed-bed flow sorption system for CO2
capture. Three gas lines connect to the nitrogen gas, simulated flue gas,
and water bubbler, respectively. In the system, there is also a furnace
with a temperature controller, flow controllers, and an online SRI
8610C gas chromatography (GC). The gas flow controllers were
calibrated by a soap-film flow meter. The sorbent sample was packed
into the stainless column with a 80 mm length and 5 mm inner
diameter. The column was placed in the furnace and heated at 100 °C
for 5 h under an ultra-high-purity (UHP) nitrogen flow of 50 mL/min
to remove the existing moisture and/or carbon dioxide in the samples.
After the column was cooled down to the desired sorption
temperature of 75 °C under the nitrogen flow, the simulated flue
gas (15 vol % CO2, 4.5 vol % O2 in N2) was introduced at the desired
flow rate of 20 mL/min. The concentration of CO2 in the effluent was
monitored by online GC at an interval of 2.5 min. After the sorbent
was saturated, the simulated flue gas was changed back to the UHP
nitrogen gas at a flow rate of 50 mL/min, and simultaneously the
sorbent bed temperature was increased to 100 °C and held at this
temperature for 1 h to perform the desorption. The sorption capacity
of CO2 was calculated based on the breakthrough curve, which is
described in our previous study.8,24
modification of textural properties by sulfuric acid may vary
with the nature of the bentonite.
According to the literature,29 the reaction between bentonite
and sulfuric acid can be described by the following chemical
equation: Al2O3·2SiO2·2H2O + 3H2SO4 = Al2(SO4)3 + 2SiO2 +
5H2O. As the content of the acid increases, the Al2O3, MgO,
CaO, and K2O contents presented in the parent bentonite may
be dissolved from the octahedral layer, resulting in the creation
of new pores on modified bentonites, as shown in Scheme 2.
Scheme 2. Simplified Illustration of Bentonite Modification
by Sulfuric Acid
3. RESULTS AND DISCUSSION
3.1. Effect of Acid Modification on the Textural
Properties of Bentonite. Textural properties, including
pore volume, pore size, and surface area, of the supporting
material play a critical role for the immobilization of
tetraethylenepentamine (TEPA) for CO2 capture. It was
found out that large pore volume of supporting materials can
load more amine-functionalized polymer, resulting in a higher
CO2 sorption capacity. It was also reported that the pore
volume and pore size are key factors for MBS performance for
CO2 capture.23 In the case of bentonite, even though the parent
bentonite may have little surface area and pore volume,19,25 its
textural properties can be improved by acid treatment. As
sulfuric acid was reported as an effective acid to enhance the
textural properties of bentonites,21 it was chosen as the
modification agent of bentonites in this work. Table 1 lists the
textural properties of modified bentonite with sulfuric acid at
different concentrations (3, 6, and 9 M), where less than 5% of
variance in textural parameters (as shown in the Supporting
Information, Table 1), including pore volume, surface area, and
pore size, suggested that the modification method and results
can be reproduced. In comparison to the parent bentonite, the
acid-modified bentonites show larger pore volumes and surface
areas, suggesting that new pores may be generated during
sulfuric acid treatment. In addition, all of the acid-modified
bentonites show increased pore sizes, which can be ascribed to
the leaching/dissolving of metal ions located on the surface of
the smaller pores, and/or the generation of relatively larger new
pores during acid treatment. It should be addressed that the
commercial bentonite has a range of surface areas and pore
volumes, that is, a surface area of 20−250 m2/g and pore
volume of 0.1−0.3 cc/g,26−28 due to its nature, as well as the
element compositions, that is, the Si/Al ratio and the ratios of
other metals, such as Mg, Ca, K, etc. The effectiveness of the
Besides the changes in the textural properties, the composition
of the modified bentonite, that is, the Si/Al ratio, would also
change considerably due to the leaching of the Al3+ ions and
other basic mental ions. Table 1 listed the weight loss of
bentonite modified by sulfuric acid at different concentrations
of 3, 6, and 9 M, which were 2.7, 4.9, and 8.7%, respectively,
suggesting more weight loss or metal leaching by a more
concentrated sulfuric acid. It should also be mentioned here
that the surface area and pore volume of Ben_H2SO4_9M are
smaller than those of Ben_H2SO4_6M, which can be ascribed
to the pore collapse during strong acid treatment when the
concentration of sulfuric acid was 9 M. Table 1 also lists the
maximal CO2 sorption capacity at the optimized loading of PEI.
It can be noted that, for the parent bentonite, the maximal CO2
sorption capacity only reached 26 mg of CO2/g of sorbent at
15 wt % of TEPA loading. In contrast, the maximal sorption
capacity of sulfuric acid modified bentonite reached 135 mg of
CO2/g of sorbent at 50 wt % of TEPA loading, which should
mainly be ascribed to its maximal pore volume of 0.77 cc/g. On
the basis of the screening experiments, Ben_H2SO4_6M was
selected as the supporting material for MBS preparation and
further studied for CO2 capture in this work.
3.2. Morphology of the Bentonite-Supported MBS
Samples. Figure 1 shows the SEM images of the bentonitesupported MBS samples, including Ben_H 2 SO 4 _6M,
TEPA(50)/Ben_H2SO4_6M, and TEPA(60)/
Ben_H2SO4_6M. Almost no TEPA was observed in the SEM
image of TEPA(50)/Ben_H2SO4_6M, indicating that all the
immobilized TEPA went into the bentonite pores. The result
further suggested that TEPA prefers to fill in the pores of the
modified bentonite support, which may be ascribed to the
capillary action of pores, or the amine-philic functional groups
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peak position in FT-IR spectra between the TEPA and the acidmodified bentonite-supported TEPA was observed, indicating
an interaction between TEPA and the internal surface of
bentonite,24,35 which may be ascribed to the interaction
between the basic −NH2 groups on TEPA and the acidic
sites on the acid-modified bentonite support, that is, Al3+ ions,
Si-OH, and others.
3.4. Effect of TEPA Loading over the Acid-Modified
Bentonite on CO2 Sorption. In the acid-modified bentonitesupported MBS, the accessible amine sites on TEPA play a key
role for CO2 sorption as the sorption goes through an
interaction between amine groups and CO2. Therefore, the
TEPA loading can greatly affect the CO2 sorption capacity of
the prepared MBS. It was observed during the sorbent
preparation that all the acid-bentonite-supported TEPA
sorbents were fine powders, except the one with the TEPA
loading as high as 60 wt %, which showed a sticky appearance.
Therefore, semiempirically, the Ben_H2SO4_6M support can
accommodate a maximum TEPA amount of around 50 wt %,
where the largest amount of TEPA entered the channels and/or
pores of the supporting material. To further confirm the
assumption, the CO2 sorption performance of the TEPA/
Ben_H2SO4_6M sorbents at different TEPA loadings was
evaluated by TGA, with the adsorption capacity as shown in
Figure 3. It can be seen that, with the TEPA loading lower than
Figure 1. SEM images of (a) Ben_H2SO4_6M support, (b)
TEPA(50)/Ben_H2SO4_6M, and (c) TEPA(60)/Ben_H2SO4_6M.
inside the pores. By increasing the TEPA loading, more TEPA
can be observed in the SEM images of bentonite-supported
TEPAs. It can be noted that some TEPA was present on the
external surface of the TEPA(60)/Ben_H2SO4_6M particles,
which may result in the partial glomeration of TEPA. The SEM
results further suggested the presence of amine-philic sorption
sites inside the pores of the Ben_H2SO4_6M support, which
attracted TEPA to fill in the pores at a lower TEPA loading
(<50 wt %). The amine-philic sorption sites may be weakened
or blocked by further increasing the TEPA loading (>60 wt %),
which was further confirmed by the N2 adsorption test of
TEPA(60)/Ben_H2SO4_6M; almost no pore volume (0.02 cc/
g) and surface area (10 m2/g) were measurable.
3.3. FT-IR Study of the Bentonite-Supported MBS
Samples. The FT-IR spectra of the Ben_H2SO4_6M and the
Ben_H2SO4_6M supported TEPA sorbents at different TEPA
loadings are shown in Figure 2. In the spectrum of the
Figure 3. Effect of TEPA loading onto Ben_H2SO4_6M on CO2
sorption capacity and amine efficiency at 75 °C.
50 wt % on the Ben_H2SO4_6M support, the CO2 sorption
capacity increases with the increase of the TEPA loading
amounts. The TEPA/Ben_H2SO4_6M sorbents reached the
maximal CO2 sorption capacity of 135 mg of CO2/g of sorbent
at the 50 wt % of TEPA loading. It was noticeable that, when
further increasing the TEPA loading to 60 wt %, the CO2
sorption capacity dropped significantly, suggesting a decreased
amount of accessible −NH2 sites for CO2 capture. This can be
due to the TEPA agglomeration when in an excess loading
amount. The result further suggested the presence of hard-toaccess amine sites in TEPA/Ben_H2SO4_6M for CO2 due to
the great diffusion barrier. It can be noted in Figure 3 that the
amine efficiency (mole of CO2 absorbed per mole of amine
functional group on the amine sorbent (TEPA/Ben) of TEPA/
Ben_H2SO4_6M) follows the same trend as the CO2 sorption
capacity, further suggesting that the optimal TEPA loading
amount over the Ben_H2SO4_6M support was 50 wt %. It
should be mentioned here that, theoretically, the maximal
Figure 2. FT-IR spectra of TEPA/Ben_H2SO4_6M sorbents at
different TEPA loadings: (a) 0, (b) 10, (c) 30, (d) 50, (e) 60 wt %.
Ben_H2SO4_6M support, the peak at 3433 cm−1 was assigned
to Si-OH,30 while peaks appeared at 789 cm−1 due to the
bending vibration of the Si-O-Si group. Another peak at 1094
was caused by the C−O stretching. In the spectra of the
Ben_H2SO4_6M supported TEPA sorbents, the bands at 2819
and 2943 cm−1 can be ascribed to the stretching vibration of
−CH2 in the TEPA chain.31−33 Two bands at 1566 and 1450
cm−1 were caused by symmetric and asymmetric bending
vibrations of −NH2, while another band at 1660 cm−1 was
attributed to the bending vibration of −N(R)H in TEPA. With
the increase in the TEPA loading,34 the bands for these specific
functional groups of TEPA became greater, which suggests that
more TEPA molecules are dispersed inside the pores and/or
over the inner surface of Ben_H2SO4_6M. A slight shift of the
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also shown in Figure 4. It can be seen that the maximal amine
efficiency of 0.23 can be reached at a sorption temperature of
75 °C. It should be mentioned that it is most energy efficient to
adsorb CO2 at or close to the real flue gas temperature. In a
typical coal-fired power plant, the temperature of the outlet flue
gas is around 120−160 °C. After an installed denitrogenation
and desulfurization process, the flue gas temperature is reduced
to around 50−80 °C,38 where the optimized CO2 sorption
temperature of the TEPA(50)/Ben_H2SO4_6M sorbent fits in
well as the CO2 desorption only requires 25 °C higher than the
sorption temperature. Therefore, TEPA(50)/Ben_H2SO4_6M
can be an energy-efficient sorbent for CO2 capture after the
denitrogenation/desulfurization process from flue gas.
3.6. Effect of Moisture Addition on CO2 Sorption from
a Simulated Flue Gas. When considering the removal of CO2
from the real flue gas, it is of great importance to clarify the
effect of moisture on the CO2 sorption capacity because water
is always present in the flue gas. The effect of moisture on the
CO2 sorption over the TEPA(50)/Ben_H2SO4_6M sorbent
was evaluated in the fixed-bed flow sorption system using a
simulated flue gas containing 15 vol % CO2 and 4.5 vol % O2 in
N2. The breakthrough curves for CO2 sorption on the
TEPA(50)/Ben_H2SO4_6M sorbent in the presence and
absence of moisture in the simulated flue gas are shown in
Figure 5. In the dry condition, the breakthrough capacity of the
amine efficiency is 0.5 mol of CO2/mol of amine. However, due
to the presence of inaccessible amine sites on the TEPA/
Ben_H2SO4_6M sorbents, the practical maximal amine
efficiency (0.23 mol of CO2/mol of amine) was lower than
0.5. The result also hinted that the CO2 sorption performance
of MBS may be further improved by facilitating the CO2
diffusion in MBS by various methods, that is, addition of CO2neutral surfactant to amine.36
3.5. Effect of Sorption Temperature on CO2 Sorption.
It is well-known that sorption temperature is a crucial
parameter for the sorption performance, that is, capacity and
kinetics, of various sorbents. In this study, TEPA(50)/
Ben_H2SO4_6M with the maximal CO2 sorption capacity,
was selected to investigate the effect of the sorption
temperature on the CO2 sorption capacity. Figure 4 shows
Figure 4. Effect of sorption temperature on CO2 sorption capacity and
amine efficiency of the TEPA/Ben_H2SO4_6M sorbent.
the effect of sorption temperature, 30, 50, 60, 75, 85, and 100
°C, on CO2 sorption capacity of TEPA/Ben_H2SO4_6M. At
30 °C, the CO2 sorption capacity was 79.5 mg of CO2/g of
sorbent. The CO2 sorption capacity increases with an increase
in sorption temperature. The MBS sample gave the highest
sorption capacity of 135.0 mg of CO2/g of sorbent at 75 °C. By
further increasing the sorption temperature from 75 to 100 °C,
the sorption capacity decreased to 110 mg of CO2/g of sorbent.
Compared to some other MBS-type sorbents, such as PEI(50)/
SG16 and PEI(50)/SBA-1535 in our group, a similar trend can
be observed on the sorption temperature effect, which may
suggest different dominancy on CO2 sorption at the two
temperature ranges. At a sorption temperature from 30 to 75
°C, the diffusion of CO2 to reach a greater amount of amine
sites in the sorbent dominates the sorption. Therefore, a higher
diffusion rate at a higher temperature results in a higher CO2
sorption capacity. It should be mentioned that the amine−CO2
chemisorption occurs from almost room temperature (30 °C)
to 100 ◦C, suggesting that the activation barrier of the reaction
is quite low, which is different from some chemisorption cases,
that is, the adsorption of organic thiophenic compounds over
Ni0-based sorbent occurs at around 200 °C37 as it is required to
overcome a high activation barrier. When the sorption
temperature is higher than 75 °C, even with a higher diffusion
rate of CO2, the CO2 sorption is unfavorable thermodynamically as the CO2 desorption dominates; therefore, the CO2
capacity decreased. The amine efficiency of the TEPA(50)/
Ben_H2SO4_6M sorbent at different sorption temperatures is
Figure 5. CO2 sorption breakthrough curves over the TEPA(50%)/
Ben_H2SO4_6M sorbent under (a) dry and (b) wet conditions from a
simulated flue gas containing 15 vol % CO2 and 4.5 vol % O2 in N2 (a)
with and (b) without an addition of 3 vol % of moisture. (Inset: effect
of moisture concentration in the simulated flue gas on CO2 sorption
capacity.)
TEPA(50)/Ben_H2SO4_6M sorbent was 130 mg of CO2/g of
sorbent, whereas the breakthrough capacity of the TEPA(50)/
Ben_H2SO4_6M sorbent increased to 169 mg of CO2/g of
sorbent in the wet condition, corresponding to an increased
amine efficiency of 0.29 mol of CO2/mol of amine from 0.23
mol of CO2/mol of amine. The results suggested that moisture
addition in the flue gas promoted the CO2 sorption over the
TEPA(50)/Ben_H2SO4_6M sorbents, which can be due to the
formation of bicarbonate with an ideal amine efficiency of 1
under a wet condition, rather than carbamate with an ideal
amine efficiency of 0.5 under a dry condition.39 Generally, the
water content in the flue gas is 8−20%.40 Different water
amounts were introduced into the simulated flue gas by
controlling the water bubbling temperature as the saturated
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vapor pressure varies with temperature. The inset in Figure 5
shows the effect of different water contents in the simulated flue
gas on the CO2 sorption performance of the prepared MBS
from the flow system tests. For 0, 3, 6, 10, and 18 vol % of
water content in the simulated flue gas (15 vol % CO2), the
CO2 sorption capacities were 130, 169, 182, 189, and 190 mg of
CO2/g of sorbent, respectively, indicating an increase in the
CO2 sorption capacity with the increase in the water content in
the simulated flue gas until the ratio of CO2 to H2O reached a
desirable value, here, in between 0.83 and 1.5, in good
agreement with the theoretical value of 1 for complete
bicarbonate formation.
It was noted that the sorption capacities were 130 mg of
CO2/g of sorbent for 15 vol % CO2 (flow system test) and 135
mg of CO2/g of sorbent for 100 vol % CO2 (TGA test),
respectively. The quite close CO2 sorption capacity at different
CO2% suggested that the CO2% on the sorption capacity is not
significant, which is probably due to the strong amine−CO2
interaction, that is, heat of sorption of 70−90 kJ/mol-CO241 for
the amine adsorbents. The CO2 sorption capacity of the
TEPA(50)/Ben_H2SO4_6M sorbent is close to that of
mesoporous molecular sieve supported PEI sorbents, that is,
PEI/MCM-41 (133 mg of CO2/g of sorb),39 PEI/SBA-15 (140
mg of CO2/g of sorb),7 and other supported MBS, that is, PEI/
silica gel (138 mg of CO2/g of sorb)16 and PEI/carbon black
(154 mg of CO2/g of sorb),9 suggesting the inexpensive acidmodified bentonite (The cost of parent bentonite was ∼$50/
ton; the cost of sulfuric acid is ∼$7/ton,42 where the desired
amount of the sulfuric acid required was 18.4 g (10 cc) of
concentrated sulfuric acid per gram of bentonite. Hence, the
total cost of acid-modified supporting materials was ∼$180/
ton) can be a potential supporting material of MBS for CO2
capture. Reported other types of sorbents include MgO (8.8
mg of CO2/g of sorbent, 400 °C),43 activated carbon (135 mg
of CO2/g of sorbent, 25 °C),44 nitrogen-doped template
carbon (176 mg of CO2/g of sorbent, 25 °C),45 etc.
It should be also mentioned that real flue gas is a complex
mixture; besides CO2, O2, N2, and moisture, trace amounts of
some coexisting acidic gases, that is, NOx and SOx, may even
adsorb strongly over amine-based sorbents due to their strong
acidity. Generally, less than 400 ppm of NOx is present in flue
gas after the denitrogenation process, where 80−90% is NO40
(barely affects the CO2 sorption capacity over the amine
sorbents). Meanwhile, hundreds of parts per million of SO2 is
present in real flue gas. It was reported that the coadsorbed
acidic NO2 (rather than NO) and SO2 can cause the
degradation of MBS due to the formation of the heat-stable
and irreversible amine salts,7 that is, sulfite/sulfate and nitrate
amine salts (O2 in real flue gas may contribute to the oxidation
of SOx/NOx), while orders of 10 ppm of SOx and NOx are
desirable46 to avoid more severe degradation of amine sorbents
for CO2 capture. Therefore, the strong acidic gases should be
removed prior to CO2 capture by amine-based sorbents from
flue gas.
3.7. Regenerability and Thermal Stability of MBS
Samples. For the practical application, the sorbents should
possess not only high sorption capacity and selectivity but also
good regeneration and stability. In this study, the regenerability
of the TEPA/Ben_H2SO4_6M sorbent was investigated in 10
sorption−desorption cycles using TGA. The sorption temperature was set at 75 °C, and the desorption temperature was 100
°C, respectively. The sorption capacities in the 10 sorption−
desorption cycles are shown in Figure 6. It can be noted that
Figure 6. Recovery percentage of the CO2 sorption capacity of the
TEPA/Ben_H2SO4_6M sorbent as a function of the cycle number
during the multiple sorption−desorption cycles.
the TEPA/Ben_H2SO4_6M sorbent can be regenerated at 100
°C, suggesting a reversible CO2 sorption−desorption cycle.
However, the sorption capacity dropped by around 4.4% from
112 to 107 mg of CO2/g of sorbent after 10 cycles of
regeneration, indicating a loss of accessible −NH2 sites for CO2
sorption while increasing of the sorption−desorption cycle
numbers. The loss of accessible −NH2 sites may be due to the
loss (evaporation) of ethyleneamine oligomers with a lower
molecular weight,16 which may be improved by arching TEPA
onto the surface of the supporting material using adhesives, that
is, a silane coupling agent.47
It is essential to have fast adsorption/desorption kinetics for
CO2 capture under the operating conditions, as it controls the
cycle time of a fix-bed adsorption system.48 Even a material
with a high sorption capacity will have little applicability if it
sorbs the carbon dioxide too slowly. Figure 7 shows the CO2
Figure 7. Rate of CO2 sorption/desorption on the TEPA/
Ben_H2SO4_6M sorbent.
sorption/desorption rate of the TEPA/Ben_H2SO4_6M
sorbent in a CO2 sorption/desorption cycle from the TGA
test. Assuming diffusion into porous spheres, the transient
fraction uptakes can be described by the following equation in
the region with fractional uptake qt/qe of less than 70%49
qt
qe
≅
6
rc
DM t
π
(1)
2
where DM (cm /s) is the intracrystalline diffusion coefficient,
and rc (cm) is the crystal radius. In this case, the diffusion
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constant (DM/rc2, s−1) was obtained from the slope of qt/qe
versus √t.
By regression of the uptake curves with eq 1, the diffusion
constant (DM/rc2, s−1) for CO2 adsorption is calculated to be
∼7.1 × 10−4 s−1 for the initial 90% of CO2 sorption, and ∼7.9 ×
10−5 s−1 for the initial 80−90% of CO2 desorption. In the
literature, the diffusion constant of some sorbents for CO2
sorption at 25 °C were reported, that is, 1.3 × 10−3 s−1 for
MOF-5,50 and 3 × 10−3 s−1 for chromium terephthalate MIL101,51 and 8.1 × 10−3 s−1 for a N-doped template carbon,45 etc.
It should be mentioned that the diffusion rate for CO2
sorption/desorption decreased significantly at a later close-toequilibrium stage, that is, ∼2.3 × 10−6 s−1 for CO2 sorption,
and ∼1.2 × 10−5 s−1 for CO2 desorption, which can be
attributed to the great diffusion barrier to reach certain amine
sites in the TEPA(50)/Ben_H2SO4_6M sorbent.
The thermal stabilities of Ben_H2SO4_6M and TEPA(50)/
Ben_H2SO4_6M were also investigated in this work. Figure 8
improved (to as high as 0.77 cc/g from 0.15 cc/g). With a
maximal TEPA loading of 50 wt %, the CO2 breakthrough
sorption capacity can reach as high as 130 mg of CO2/g of
sorbent with the amine efficiency of 0.23 mmol of CO2/mmol
of amine at 75 °C under a dry condition. With moisture
addition in the fuel gas, the CO2 breakthrough sorption
capacity can be further improved to 190 mg of CO2 at 18 vol%
of moisture addition sorbent with the amine efficiency of 0.29
mmol of CO2/mmol of amine. Moreover, the TEPA/
Ben_H2SO4_6M sorbents show a good regenerability in 10
sorption−desorption cycles, and a good thermal stability below
130 °C. The high CO2 sorption capacity, positive effect of
moisture, and low capital cost of the raw bentonite materials
suggest that TEPA/Ben_H2SO4_6M could be a promising
cost-effective sorbent for CO2 capture from flue gas. The
sulfuric acid treatment was demonstrated as an effective
method for bentonite modification to immobilize TEPA for
CO2 capture.
■
ASSOCIATED CONTENT
* Supporting Information
S
Textural properties with variance of the parent bentonite and
the modified bentonites. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: dingjing@sysu.edu.cn (J.D.), cejingxiao@scut.edu.cn
(J.X.).
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We are pleased to acknowledge the support by the NSFCGuangdong Joint Fund Project (U1034005), the National
Natural Science Foundation of China (51106185), and the
National Basic Research Program of China (2012CB720404).
Figure 8. Thermal stability of the Bentonite_H2SO4_6M, TEPA, and
TEPA/Ben_H2SO4_6M.
■
shows the weight loss of the samples from room temperature to
800 °C. It can be observed that the Ben_H2SO4_6M sample
was quite stable at the temperature less than 800 °C. The
sorbent weight only decreased by 3.5 wt %, possibly due to the
water evaporation in the network. A weight loss on
unsupported TEPA was observed at 120 °C, and it was further
decomposed completely at 250 °C. For TEPA (50)/
Ben_H2SO4_6M, a sharp weight loss was observed between
130 and 300 °C, which should be also due to the
decomposition of TEPA. It should be mentioned here that,
compared to unsupported TEPA, the Ben_H2SO4_6M
supported one showed an increased initial decomposition
temperature by 10 °C, indicating that the Ben_H2SO4_6M
support had a positive effect on the thermal stability of TEPA.
The results further suggested the presence of the amine-philic
sites on the Ben_H2SO4_6M support to stabilize the supported
TEPA. Overall, the TEPA (50)/Ben_H2SO4_6M sorbent
should be thermally stable below 120 °C.
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