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7/24/2019 A Review on Solid Adsorbents for Carbon Dioxide Capture 2015 Journal of Industrial and Engineering Chemistry
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Introduction
Global warming and carbon economy
Humans are endangered by global warming caused by the
greenhouse effect. The greenhouse effect can be attributed to an
increase in the emissions of greenhouse gases such as carbon
dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluor-
ocarbons, and sulfur hexafluoride (SF6) since the beginning of
industrialization. The average world temperature has increased by
0.74% in the past 100 years and is expected to increase by another
6.4% by the end of the twenty-first century [1,2].
Global warming leads to droughts, floods, heat waves, and
destruction of ecosystems, and the economic loss due to climate
change is expected to be 520% of the worlds gross domestic
product. CO2 is a major greenhouse gas and is the main cause of
global warming [3,4].
Major sources of CO2 emissions are thermoelectric power
plants and industrial plants (such as steel mills and refineries),
which account for approximately 45% of global CO2 emissions.
According to a report in 2013 by the International Energy Agency
(IEA), the global atmospheric CO2 concentration has increased
from a preindustrial value of 280 parts per million by volume
(ppmv) to 394 ppmv in 2012. Most of the CO2emissions into theatmosphere originate from the combustion of fossil fuels (99% of
global annual CO2emissions of approximately 32 Gt) [5]. The IEA
and the Organization for Economic Co-operation and Development
(OECD) forecast that CO2capture and storage (CCS) could take care
of approximately 14%, the expected reduction in volume of CO2emission potentials [6]. Furthermore, the Intergovernmental Panel
on Climate Change (IPCC) reported that it would be possible to
achieve a more than 50% reduction in CO2 emissions from
2009 levels by 2050. According to their model for estimating
CO2 capture potential, it is estimated that 30 billion tons ofCO2 can
be captured and stored within the European Union (EU) by
2050. Globally, 240 billion tons of CO2could be captured by 2050
[7]. Without CCS technology, the cost of meeting a 50% global
reduction target by 2050 will be 70% higher [8].Because of these global concerns, strict global regulations of
CO2 emissions to the atmosphere have been imposed. With
enforcement of the Kyoto Protocol, there has been increased
interest in the atmospheric residence time of CO2 and its
contribution to the greenhouse effect, and self-reduction techni-
ques for CO2 generation rates and post-treatment of CO2 have
received significant attention [9,10]. Various industries need to
deal actively with these regulations in order to survive. CCS has
great potential to be one of the more important green technologies
in the future.
There is an urgent need to develop CO2reduction technologies,
andwebelieve thatCCS is themain technology that can reduce CO2emissions from the energy sector. However, with the exception of
developed countries, there is very little concern about, and
investment in, developing CCS. Fortunately, the world is now
paying more attention and aggressive efforts are being made to
commercialize CCS [1113]. A global CCS market has begun to
develop, along with certified emission reductions. The IEA/OECD,
EU, and United States forecast that the CCS market will grow to its
full capacity by 2020, and there is a need to obtain better insights
into the techno-economic possibilities of CO2 capture.
CO2 capture and storage (CCS)
The basic concept of CCS is to capture CO2 emissions without
releasing them into the atmosphere; they include sequestration or
storage, as shown in Fig. 1. CCS is the process of capturing and
compressing CO2 generated by existing large sources of high-
density CO2, transporting and depositing it safely in the ground or
an ocean-bedrock sediment layer, and long-term monitoring [14
16]. Transportation of CO2 refers to transporting captured and
compressed CO2to a storage site via a pipeline or other means oftransport [17,18]. Storage of CO2 includes post-management: the
observation and prediction of the movement of the bedrock layer
after CO2storage and evaluation of its effect on the environment,
and evaluation of the bedrock layers characteristics for depositing
captured CO2(basins, oilfields, etc.) in the ground or below the sea
[19,20]. Technologies for capture, transportation, and storage of
CO2are summarized in Table 1.
All the above processes are called CCS. In addition, CCS
technologies allow the continuing usage of fossil fuels and
stabilize the density of greenhouse gases. Moreover, CCS may
decrease the total cost of CO2reduction and offer different ways to
reduce CO2emissions [2123]. CCS is a comprehensive technology
for direct reduction of CO2using the above-mentioned processes.
CO2 capture technology
Classification
of
CO2 capture technology
CO2 capture is a core technology and accounts for 7080% of the
total costs of CCS technologies. It is classified at (i) post-
combustion, (ii) pre-combustion, and (iii) oxy-fuel combustion
Fig.
1.
Concept
and
summary
of
CO2 capture
and
storage
(CCS).
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technologies. Schematic diagrams of the three types CO2capture
process are presented in Fig. 2.
Post-combustion capture technology involves collecting CO2from the emission gases of a power plant [2426]. Pre-combustion
capture technology is mainly used in fertilizer and hydrogen
production and can be used in integrated gasification combined
cycle plants [2729]. Although the fuel conversion process (e.g.,
gasification
of
coal)
prior
to
combustion
is
more
complex
and
costsmore, the density and pressure of CO2 in the gases allow easy
separation. Oxy-fuel combustion, or oxygen-fired combustion, is in
a trial phase in a pilot, plant and uses high-purity oxygen instead of
air [3032]. Thus, the density of CO2 in the gas flow, i.e., the
emission gas, becomes high, resulting in easy separation of CO2;
however, the separation of oxygen from the air consumes more
energy.
Post-combustion capture technology
Among the currently available technologies, post-combustion
capture, a technology for capturing CO2 from post-combustion
emission gases, is the most easily applied technology for existing
sources of emissions. Post-combustion capture uses wet/dry
adsorbents, which are used for gas separation, and separates
and
collects
CO2 by
adsorption/desorption.
The
classification
oftechnologies for post-combustion capture of CO2 is shown in Fig. 3.
In general, post-combustion capture technologies include wet
absorption [3335], dry adsorption [3638], membrane-based
technologies [3941], and cryogenics [42,43]. Current research
focuses on dry adsorption systems using dry adsorbents. Wet
absorption is good for treating large emission volumes from
combustion and is very useful for changing the density of CO2;
Fig.
2.
Schematic
diagrams
of
three
types
of
CO2capture
process:
(a)
post-combustion,
(b)
pre-combustion,
and
(c)
oxy-fuel
combustion
technologies.
Table 1
Technologies for capture, transportation, and storage of CO2.
Technologies Classification Applied areas
CO2 capture technologies Post-combustion
Pre-combustion
Oxy-fuel combustion
Direct capture from (thermoelectric) power plants and
industries (steel mills, refineries, etc.)
CO2 transportation technologies CO2compression and transportation Transportation of captured CO2to a storage site via a
pipeline or other means of transport
CO2 storage technologies Characterization and evaluation of bedrock
layers Drilling and injection
Observation and prediction of movement of
bedrock layers
Evaluation of the effect on the environment
and post management
Deposit to a bedrock layer (basins, oilfields, etc.) in the
ground
and
below
the
sea
and
its
management
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however, it requires high energy for absorbent regeneration.
Additional drawbacks are (i) the use of heated absorbents, (ii)
erosion of materials, and (iii) slow solidgas reactions [44,45]. A
dry adsorption process has the following advantages: it requires a
simple device, is easy to operate, and has excellent environmental
effects and energy efficiency; however, its performance is poor in
the treatment of large emission volumes and its separation
efficiency is also poor [46,47]. A membrane-based process has the
following advantages: a simple device, easy operation, and low
energy consumption; however, it requires a high-cost module, isnot suitable for treating large volumes of emission gases, and is not
very durable [48,49]. Finally, cryogenics is a suitable process
because of its low investment cost and high reliability; however, it
is not suitable because of its energy consumption [50].
Recent research has focused on improving adsorbents and
developing efficient processes for cost-effective capture perfor-
mance. The capture and separation of CO2from a large volume of
combustion gases are still expensive and have high energy
consumption; therefore, more efficient adsorbents for relatively
concentrated streams need to be developed.
CO2 capture using dry adsorbents
The
process
of
capturing
CO2 using
a
dry
adsorbent
involvesselective separation of CO2based on gassolid interactions [51]. In
general, universal dry adsorbents such as activated carbons and
molecular sieves are used in packed columns [52]. The important
variables in a dry adsorption process are the (i) surface tension and
pore size of the adsorbent and (ii) temperature andpartialpressure
during theadsorptionprocess [53]. Theprocess involvesa repeated
cycle of adsorption and desorption (regeneration).
The different types of adsorption are as follows: (i) pressure-
swing adsorption (PSA) [5456]; (ii) temperature-swing adsorp-
tion (TSA) [57,58], where two processes are combined, i.e.,
adsorption at a low temperature followed by desorption/regener-
ation by heating or lowering the pressure; (iii) electric-swing
adsorption (ESA) [59], i.e., adsorption/desorption by varying the
electricity
supply,
with
a
low-voltage
current
passing
through
the
adsorbent; and (iv) vacuum swing adsorption (VSA) [60]. Further-
more, adsorption-based technologies such as pressure/vacuum-
swing adsorption (PVSA)have frequently been investigated because
of their low energy requirements and relative simplicity [6163].
It is essential to improve the CO2 capture selectivity and
adsorptive capacity of adry adsorbent for treating large volumes of
combustion emissions from many different sources. The CO2adsorption can be improved and stabilized by introducing
functional groups (mainly amine groups) with high affinities for
CO2onto the surface of the adsorbent material to react with CO2,which can then be selectively adsorbed using the wide specific
surface area and pore structure of the adsorbent [6469].
To achieve high capture performance and high selectivity, the
incorporation of various amine groups into solid materials used as
CO2capture sorbents is expected to improve polarization and CO2capture. Such adsorbents have several advantages such as
potential elimination of corrosion problems and lower energy
costs for regeneration. The interactions between CO2 and amine
functional groups produce ammonium carbamates under anhy-
drous conditions as follows [70]:
1CO22RNH2 ! RNHCOORNH3
2CO22R2NH ! R2NCOOR2NH2
The adsorption process is significantbecause it is reversible and
the adsorption efficiency can be improved by modifying the
structure of the adsorbent materials. The CO2 adsorption efficiency
can therefore be improved by selecting an appropriate adsorbent
material. At present, the main commercially available adsorbents
are activated carbons [71,72], zeolites [73], hollow fibers, and
alumina. Each material has a different pore structure, specific
surface area, and surface functional groups, and their application
fields are highly specific. Some typical non-carbonaceous dry
adsorbents for CO2 capture are listed in Table 2 and will be
described
in
the
following
section.
Fig. 3. Classification of application technologies for post-combustion capture of CO2.
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Non-carbonaceous
dry
adsorbents
Zeolites
Zeolites are typical naturally occurring microporous crystalline
silicate framework materials; they can also synthesize in the
laboratory. They have uniform pore sizes of 0.51.2 nm, forming
networks of interconnecting channels or cages for adsorbing/
capturing gas molecules [74]. They have therefore been widely
used for gas separation and purification [7577]. Zeolites have
been extensively investigated for CO2 capture because of their
molecular sieving effect and the strong dipolequadrupole(electrostatic) interactions between CO2 and alkali-metal cations
in the zeolite frameworks [78]. The cations in zeolites, such as Li,
Na, and Al, influence the heat of adsorption of CO2; the heat of
adsorption increases with increasing monovalent charge density
(of negative charges) [79,80].
Many groups have reported the synthesis and characterization
of zeolites for CO2 capture, mainly zeolite 13X and zeolite 5A,
which gave CO2 capture performances of 325 wt.% at room
temperature and a CO2pressure of 100% [8184]; they also gave a
CO2capture performance of 212 wt.% at room temperature and a
CO2partial pressure of 15% [8587].
Cavenati et al. [88] investigated zeolite 13X as a solid adsorbent.
Zeolite 13X exhibited a CO2 adsorption capacity of 28.7 wt.% and
CO2/N2separation
capacity
of
3.65
at
298
K
and
10
bar.Jadhav et al. modified zeolite 13X via monoethanol amine
(MEA) impregnation to improve the CO2adsorption capacity. The
CO2 adsorption capacity was better than that of the unmodified
zeolite by a factor of approximately 1.6 at 303 K, and at 393 K, the
efficiency improved by a factor of 3.5. The higher capacity of MEA-
modified zeolite 13X at 393 K indicated that the chemical
interactions between CO2 and amine groups might play a key
role in CO2 sorption, despite the reduced pore volume and lower
surface area resulting from MEA impregnation [89].
In the case of zeolite 5A, a typical CO2 adsorbent, the CO2adsorption capacity and CO2/N2 selectivity were 20.8 wt.% and
8.45, respectively, at 298 K and 1 bar. At a higher pressure of
10 bar, the CO2 adsorption capacity of zeolite 5A was 22.3 wt.%
[90].
Recently, binderless zeolite NaX microspheres for CO2capture
were prepared using chitosan-assisted synthesis. The obtained
microspheres had auniformparticle size of approximately1.3 mm,
a specific surface area of 931 m2/g, and a moderate crushing
strength of 0.46 MPa. The CO2adsorption capacities were 22.7 and
31.2 wt.% at 1 and 10 bar, respectively, at 298 K [91].
Most of the researchusing zeolites forCO2 capture from flue gas
has focused on PSA or VSA processes [9294]. The CO2 capture
performances of zeolites are greatly influenced by the temperature
and pressure. Zeolites are typically used at pressures above 2 bar
and require very high regeneration temperatures, often above
573 K; therefore, CO2 regeneration results in huge energy loss[95,96]. In addition, the adsorption performances of zeolites
decrease because they absorb moisture easily; therefore, many
studies have been conducted improve this [9799].
Silica materials
Many studies on the synthesis and modification of silica
materials containing a large amount of mesoporous materials for
efficient CO2 capture have been reported [100111].
An amine-rich nano-silica adsorbent was synthesized using
poly(acrylic acid) as a multi-functional bridge for amine
immobilization on the silica nanoparticle surfaces, followed
by treatment with polyethylenimine (PEI) as the amine source.
The
CO2 adsorption capacity of the sample increased withincreasing temperature and reached a maximumCO2adsorption
capacity of 16.7wt.% at 313 K and 1 bar. The low adsorption
capacity at low temperatures was caused by kinetic limitations.
When the temperature was increased to 353 K, the adsorption
capacity decreased to only 4.4 wt.%, because the reaction
between CO2 and amines is an exothermic process (van t Hoff
behavior) [112].
Le et al. reported that tetraethylenepentamine (TEA)-function-
alized/monodispersed porous silica microspheres were success-
fully prepared using tetraethoxysilane as the precursor and
dodecylamine as the templating agent and hydrolysis catalyst,
followed by calcining at 873 K, and functionalization with TEA.
They found that for the optimal TEA loading of 34 wt.% on the silica
microspheres,
the
maximum
CO2 adsorption
capacity
was
Table 2
Comparison between major non-carbonaceous dry adsorbents for CO2capture.
Adsorbents Advantages Disadvantages
Zeolites, silica
materials
Low production cost
Large micropores/mesopores
Medium CO2adsorption
(at
298
K
and
1
bar)
Poor performance of CO2adsorption due to easy
moisture absorption
Heavy energy consumption during CO2desorption
(poor
economic
feasibility)
Renewal difficulties
Metal organic
frameworks
(MOFs)
Large specific surface area (over 10,000m2/g) and regularpore
distributions Ease of controlling pore sizes
Possible improvement in CO2selectivity according to various
combinations of metal clusters and organic ligands
Poor performance at the partial pressure of CO2
Poor
economic
efficiency
due
to
high
production
cost Complicated synthetic process
Moisture-sensitive (possible structure failure due to
moisture absorption during CO2 capture)
Unsuitable for use at high temperature. Mostly VSA
process (poor economic feasibility)
Alkali-based dry
adsorbents
(K-, Na-, etc.)
Possible adsorption and desorption at a low temperature, i.e.,
313-343
K
(similar
to
amine-based
absorption)
Possible CO2collection under wet conditions
Absorption and renewal under 473K and possible operation at
atmospheric
pressure
(high
economic
efficiency)
Low adsorption capability (311wt.%)
High-temperature reactions
Decrease in the collection ratio of CO2 because of stable
products (e.g., Na2CO3and NaHCO3)
Requires high temperatures during desorption
(high energy consumption)
Complicated operation
Metal oxides-based
adsorbents
(CaO, MgO, etc.)
Dry chemical absorbents
Adsorption/desorption at medium to high temperatures
(>673K)
Popular as a pre-combustion absorbent
High consumption of energy due to adsorption/desorption
at medium to high temperatures (>673K)
High cost for regeneration
Demand for continuous addition of absorbents
Complicated process
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18.8 wt.% at 348 K and 1 bar. The sample showed relatively good
thermal stability in CO2adsorption in the TSA process (desorption
temperature 393 K) [113].
Xu et al. reported that PEI-modified mesoporous MCM-41-type
molecular sieves (MCM-41-PEI) can serve as molecular baskets
for condensed CO2. MCM-41 has a synergetic effect on CO2adsorption by PEI. The CO2 adsorption capacities of MCM-41-PEI-
50 (PEI loading 50 wt.%) and MCM-41-PEI-75 (PEI loading 75 wt.%)
were 11.2 and 13.3 wt.%, respectively, at 348 K and 1 bar [114].
Hicks et al. synthesized covalently tethered hyperbranched
aminosilica-based SBA-15 materials capable of adsorbing CO2reversibly, with very high CO2 capacities of 13.6 wt.% at 298 K.
They suggested that the advantages of this adsorbent are its large
CO2 capacity and multicycle stability. The material was recycled by
thermally desorbing the CO2from the surface with essentially no
changes in the capacity [65].
Sanz-Perez et al. tested tetraethylenepentamine (TEPA)-
functionalized SBA-15 (SBA-15-TEPA) using a simulated gas
mixture similar to that from a coal-fired thermal power plant.
The CO2adsorption capacity of SBA-15-TEPA (50) after 10 adsorp-
tiondesorption cycles was 86% of the initial value, which was
11.35% under 100% CO2at 318 K; it also reached a CO2adsorption
capacity of 16.2% in a humid stream with 15% CO2 at 318 K and
1 bar. They concluded that the presence of 5% moisture enhancedthe CO2adsorption capacity [115].
Kjdary et al. suggested the incorporation of Cu, Fe, Ag, and Au
nanoparticles intomercaptopropyl-modified silica forCO2 capture.
The CO2adsorption capacities of the metal ions and nanoparticle
nanocomposites increased significantly, by 20 and 100%, respec-
tively. Cu-MP-S showed the maximum capacity, 22.9 wt.% at 323 K
and 1 bar [116].
Mello et al. synthesized amine-modified MCM-41 mesoporous
silica for CO2 capture. They showed that at a low pressures of
0.1 bar, the CO2 adsorption capacity of MCM-41-NH2 was
3.08 wt.%, which is much higher than that of 0.53 wt.% for
MCM-41. This is because of the great affinity of CO2, a Lewis acid,
for basic amine sites at low pressures. However, at high pressures
of 2.1 bar, the difference is not so pronounced; the CO2adsorptioncapacities of MCM-41-NH2 and MCM-41 were 5.06 and 4.4 wt.%,
respectively. From the results, they concluded that the most
reactive amine groups were bound to CO2 at low pressures, but the
available pore volume was filled at high pressures [117].
Besides the incorporation of amine-functional groups or metal
nanoparticles on silica supports, various silica materials with
controlled pore diameters and arrangements have been synthe-
sized and they showed significantly enhanced CO2 capture
performances [118122].
Metal-organic
frameworks
(MOFs)
and
porous
polymers
Numerous MOFs have been synthesized from combinations
of
inorganic (metal
clusters) and organic units (ligands) bystrong bonding (reticular synthesis). The flexibilitywith which
the geometries, sizes, and functionalities of the constituents
can be varied has led to more than 20,000 differentMOFs being
reported and studied; their specific surface areas typically
range from 1000 to 10,000m2/g [123,124]. They are popular
because of their excellent selectivities and superior adsorption
capacities, particularity for H2 adsorption [125127], CH4adsorption [128,129], and adsorption of some toxic gases
[130,131], and they are used as catalysts for fine chemical
synthesis [132,133].
The structures of MOF adsorbents for CO2 capture have
available spaces at the center because the molecules are
intertwined. Such a structure has a large body volume because
the
pores
are
attached
to
organic
molecules
and
knots
of
metallic
ions. MOFs are widely used as storage media for various gases
because the pore diameters can be easily controlled [134138].
McDonald et al. reported that N,N0-dimethylethylenediamine/
H3[(Cu4Cl)3(BTTri)8(CuBTTti; H3BTTri = 1,3,5-tri(1H-1,2,3-triazol-
4-yl)benzene] composites adsorbed 15.4 wt.% of CO2at 298 K and
1 bar and 9.5 wt.% of CO2 at 298 K and 0.15 bar CO2/0.75 bar N2,
with a selectivity of 327, determined using the ideal adsorbed
solution theory (IAST). They emphasized that amines tethered to
solid adsorbents have considerable advantages over aqueous
alkanolamines, because of their quick regeneration using mild
temperature swings [139].
Bao et al. reported that Mg-based MOF-74 had a very high CO2adsorption capacity of 37 wt.% at 298 K and 1 bar [140] and Caskey
et
al. reported that it had a considerable adsorption capacity of
23.6 wt.% at 296 K and 0.1 bar [141]; they investigated the
selectivities for CO2 and CH4, but not for CO2/N2. They suggested
that the exceptionally high performance of Mg-based MOF-74 for
CO2may be attributed to strong interactions between the oxygen
lone pair orbitals of CO2 with coordinatively unsaturated metal
cations, suggesting that it is a promising adsorbent for CO2 capture
[142].
MOFs show high CO2 storage capacities at 298 K and 42 bar.
Gravimetric CO2 isotherms for various MOFs such as MOF-2,
MOF-505, Cu3(BTC)2, MOF-74, IRMOFs-11, -3, -6, and -1, andMOF-177 were reported by Yaghis group. They confirmed that
the amine functionalities of the IRMOF-3 pores increased the
affinity for CO2. The highest CO2 adsorption capacity was
147 wt.%, for MOF-177 [143].
Yan et al. reported high enhancement of CO2 uptake by an
HKUST-1 MOF via a simple chemical treatment, to improve theCO2capture performance. It had a maximum CO2adsorption capacity
of 51 wt.% at 273 K and 1 bar, which is a significant increase of 61%
compared with the original MOF sample [144]. Llewellyn et al.
reported chemical treatment of MIL-101 with ethanol and then
NH4F aqueous solution; the CO2 adsorption capacity of the
chemically treated MIL-101 increased from 79.2 to 176 wt.% at
304 K and 50 bar [145].
More recently, conjugated microporous polymers (CMPs) haveattracted much interest because of their porous structures, formed
from organic functionalities; they have applications such as gas
storage, capture, and separation [146148]. In particular, the
incorporation of metal-organic species in CMPs provides new
materials capable of CO2capture and its simultaneous conversion
for cost-effective reduction of CO2. Xie et al. reported that the CO2sorptionproperties of theCo-CMP andAl-CMP were comparatively
high, although these materials have a relatively low specific
surface areas compared with other MOF materials. The CO2adsorption capacity and specific surface area of Co-CMP were
7.93 wt.% and 965 m2/g, respectively; those of Al-CMP were
7.65 wt.% and 798 m2/g, respectively. The co-catalysts Co/Al-
CMP displayed exceptionally high catalytic activity in the
conversion
of
propylene
oxide
and
CO2 to
propylene
carbonateat room temperature and atmospheric pressure [149].
MOFs have very high CO2capture capacity at high pressure.
However, most MOFs have poor CO2 capture performances
compared to other solid physical sorbents at a low partial
pressure of CO2, and maintenance of a high partial pressure is
not economically feasible (process unit costs). An economically
viable MOF needs to be developed for the efficient storage and
separation of large volumes of CO2, up to 1 ton/day. Further-
more, the synthesis of MOF, a metal complex, from an organic
ligand is in many cases very expensive, and the synthetic
process is complicated. MOFs suffer from durability and
mechanical strength problems because of moisture absorption
during CO2 capture. Theuse of MOFs in powerplants is therefore
limited.
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Alkali-metal-based materials
The use of alkali-metal (such as Na, K, Al) carbonates as
renewable dry adsorbents for capturing CO2 from flue gas at
operating temperatures below 473 K, relatively moderate condi-
tions, with independent supports has been reported [150152]. In
this process, the alkali-metal carbonates are added to various
inorganic supports such as carbon materials, zirconia, ceramics,
silica, and alumina; CO2 absorption is achieved by reaction with
moisture (carbonation; (Eq. (3))), and the absorbent is renewed by
decarbonation (Eq. (4)) [153,154]:
M2CO3H2O CO2@2MHCO3M Na;K (3)
DH = 135 kJ/mol and 141 kJ/mol for M = Na and K, respec-
tively.
2MHCO2@M2CO3H2O CO2 (4)
It is well known that CO2 and H2O normally react with a
carbonate sorbent at 333383 K to form alkali-metal bicarbonates
in an adsorption process, as in Eq. (3), and the bicarbonate
regenerates the alkali-metal carbonate at 373473 K, with release
of CO2. The theoretical CO2 adsorption capacities of Na2CO3 and
K2CO3 are 41.5 and 31.8 wt.%, respectively.Li-based zirconate (Li2ZrO3) and Li-based silicate (Li4SiO4) are
also promising CO2 captors and can be used for direct CO2separation from flue gas at high temperatures, 700900 K
[155,156]. Li4SiO4, in particular, has great potential as a CO2captor because of itshighCO2 sorption capacity of36.7 wt.% and its
small volume change during the CO2adsorptiondesorption cycle
[157,158].
Kato et al. investigated the CO2 sorption capacities of Li2ZrO3and Li4SiO4at low CO2 concentrations, i.e., 50 ppv, and concluded
that the CO2 sorption capacity of Li4SiO4 is more than 30 times
greater than that on Li2ZrO3. Moreover, silica materials are cheaper
than zirconia materials [159,160]. More recently, Seggiani
et
al. reported that Li4SiO4 with addition of 30 wt.% K2CO3 or
Na2CO3showed a CO2sorption capacity of 23 wt.%, at an optimumsorption temperature of 853 K and low CO2 partial pressure of
0.04 bar, corresponding to a Li4SiO4 conversion of about 80%
[161]. Duran-Munoz et al. proposed Li8SiO6 as an alternative dry
adsorbent for CO2capture, it showed a very high sorption capacity
of about 51.9 wt.% over a wide temperature range, with an
efficiency of 71.1% [162].
These alkali-metal-based materials are technically and eco-
nomically attractive for post-combustion CO2 capture at high
temperatures and low concentrations, because they do not need
further cooling processes; however, problems with the long-term
stabilities and sustained performances of these adsorbents under
real flue gas conditions in post-combustion applications need tobe
solved.
Metal
oxide
carbonate
materials
Mineral carbonation technology uses metal oxides such as CaO
and MgO as dry chemical adsorbents; a chemical reactionfixesCO2as an insoluble carbonate [163168]. MgO has been shown to be
good a particularly candidate for CO2capture adsorbents because
of its low cost, abundance, and low toxicity. However, MgO has a
very low sorption capacity of 0.57 wt.% at moderate temperatures
in dry environments [169].
Han et al. suggested novel MgO-based mesoporous composites
with concrete-like structures, which were synthesized using a
facile coprecipitation method, for trapping CO2 in flue gas in the
high temperature range 423673 K. Microcrystalline MgO present
in
the
alumina
frameworks
had
CO2 adsorption
capacities
of
7.7 and 13.1 wt.% in the absence or presence of water vapor,
respectively, at 473 K. Regeneration was performed at 873 K, and
stable cyclic adsorption was achieved [170].
Bhagiyalakshmi et al. synthesized mesoporous MgO using
mesoporous carbon, CMK-3, obtained from mesoporous SBA-15, as
an exotemplate. They confirmed that the pore structure of the
mesoporous MgO sample resembled those of SBA-15 and CMK-3.
The CO2adsorption capacities of mesoporous MgO were 8 wt.% at
298 K and 10 wt.% at 373 K, whereas that of non-porous MgO was
12 wt.% at 298 K [171].
Liu et al. reported that MgO sorbents doped with alkali-metal
carbonates had enhanced CO2sorption capacities at both low and
moderate operating temperatures, compared with pure MgO
sorbents. MgO-doped Cs2CO3 had a maximum CO2 sorption
capacity greater than 8.36 wt.% at 573 K [172].
However, the chemical reaction is too slow and requires high
consumption of energy. Moreover, the regeneration process is
performedat6731073 K, i.e., it alsohashighenergy consumption.
Therefore, although CO2capture using metal-oxide carbonates has
a high absorption volume, this technology may be inappropriate
for CO2 storage.
Carbonaceous
adsorbents
Although carbonaceous materials consist of a single element,
they have many advantages such as high thermal/chemical
stabilities, electrical and heat conductivities, strengths, elasticities,
and bio-affinities [173177]. They are particularly good for
applications for gas adsorption or storage, because they are
lightweight, and have very high specific surface areas and large
pore volumes [178180]. They also have advantages for CO2capture: (i) carbon materials are not moisture sensitive; (ii) the
cost of carbon materials is reasonable; (iii) the adsorption/
desorption temperatures are below 373 K; (iv) they can be used
at atmospheric pressure; and (v) the energy consumption is low;
all of these factors influence current studies in this field.
Activated carbonsCO2 adsorption capacity is strongly sensitive to the textural
properties and surface groups of carbon-based adsorbents [181
183]. The pore size distributions of activated carbons vary from
micropore to macropore; therefore, activated carbons are inap-
propriate for selective adsorption of a specific gas. Generally,
pristine carbon-based adsorbents have weak affinities for CO2,
with adsorption heats of less than 25 kJ/mol [184]. The typical CO2adsorption capacity of an activated carbon is 5 wt.% at 298 K and
0.1 bar [185,186].
Thepore structures of activated carbons canbe easily controlled
by varying the preparation and activation conditions [187
190]. Moreover, the functional groups on the activated carbon
surface can also be easily controlled using various treatments
[191194].
The
incorporation
of
various
basic
groups
on
activatedcarbon has been actively investigated for enhancing the CO2affinity by increasing the CO2 adsorption capacity [195,196].
Zhao et al. designed and prepared a novel microporous carbon
material (KNC-A-K) with a high N-doping concentration, above
10.5 wt.%, and extra-framework K+ cations, which gave CO2capture performances of 7.1 wt.% (0.1 bar) and 17.8 wt.% (1 bar)
at 298 K, and a CO2/N2 selectivity of 48, determined using IAST
method. They emphasized that its outstanding low-pressure CO2adsorption ability makes KNC-A-K a promising candidate for
selective CO2 capture from flue gas. The K+ ions played a key role in
promoting CO2 adsorption via electrostatic interactions [197].
Lee et al. prepared heat-treated carbons by pyrolysis of
poly(vinylidene fluoride) at various heat-treatment temperatures
and
evaluated
the
CO2 adsorption
capacities.
The
CO2 adsorption
S.-Y. Lee, S.-J. Park/Journal of Industrial and Engineering Chemistry 23 (2015) 111 7
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7/24/2019 A Review on Solid Adsorbents for Carbon Dioxide Capture 2015 Journal of Industrial and Engineering Chemistry
8/11
capacity increased with increasing heat-treatment temperature up
to 873 K (15.5 wt.% at 298 K and 1 bar) and then decreased at
973 K; this correlated with their micropore volumes. Interestingly,
they emphasized that themicropore volumehad the greatesteffect
on CO2 adsorption at1 bar; meanwhile, the pore sizes also strongly
affected the CO2 adsorption behavior at pressures below 0.3 bar. In
addition, it was found that ultra-micropores (
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7/24/2019 A Review on Solid Adsorbents for Carbon Dioxide Capture 2015 Journal of Industrial and Engineering Chemistry
9/11
adsorption capacities. They demonstrated that the ACF pore
structure can be designed to enhance the CO2adsorption capacity
by controlling the concentration of KOH intercalated into the
graphitic lattice. The highest CO2 adsorption capacity was
25.0 wt.% at 298 K and 1 bar, achieved using a materials heat-
treated with 3:1 KOH/ACF. It was also found that the CO2adsorption capacity was strongly affected by the ultra-micropore
size distribution rather than by the specific surface area or
micropore volume [230]. This is because ultra-micropores have a
high adsorption potential, which enhances the adsorption of CO2molecules [231,232].
Thiruvenkatachari et al. prepared large honeycomb-shaped
carbon fiber composite (HMCFC) adsorbents for large-scale CO2capture tests (the adsorbent mass in one column was 4.486 kg).
The average CO2 adsorption capacity was 11.9 wt.% for a simulated
fluegas consisting of13%CO2,5.5%O2, and thebalanceN2, at293 K.
They also showed that the CO2 capture efficiency of HMCFC
adsorbents is more effective in a combined vacuum and thermal
decomposition process [233].
Graphene
Graphene oxide (GO) is a derivative of graphene and can be
synthesizedwith various functional groups on thebasalplanes and
edges [234]. The surface modification of GO with variousfunctional groups and the synthesis of new types of GO-like
derivatives with lightweight frameworks has been widely
researched for applications such as gas storage and separation,
energy conversion, and sensors [235240].
Zhao et al. demonstrated that ethylenediamine (EDA)-interca-
lated GO had a CO2adsorption capacity of 4.65 wt.% at 303 K and
1 bar for CO2/N2 mixed gases [241].
Lee et al., prepared isothermally exfoliated GO using a new CO2pressure swing method. They suggested that the CO2 pressure
swing method exerted sufficient pressure to overcome the GO-
interlayer van der Waals binding energy and expand the GO, thus
promoting GO exfoliation by enabling large pores to develop. The
results showed that the best sample had a specific surface area of
547 m2
/g and total pore volume of 2.468 cm3
/g, resulting in a CO2adsorption capacity of 28.2 wt.% at 298 K and 30 bar [242].
Meng et al. reported thermally exfoliated graphene nanoplates
as novel high-efficiency sorbents for CO2 capture. The prepared
graphene nanoplates had high capture capacities, 248 wt.%, at
298 K and 30 bar. The improved CO2 capture capacity of the
graphene nanoplates was attributed to the larger inter-layer
spacing and high interior void volume [243].
Aminated GO for CO2 adsorption was synthesized by the
intercalation reaction of GO with amines such as EDA, diethylene-
triamine (DETA), and triethylene-tetramine (TETA). The results
showed that the adsorption performance of the sample with 50%
EDA covalently attached to GO was better than those of GO
modified with DETA or TETA. The EDA-modified sample had a CO2
adsorption
capacity
of
4.65
wt.%
at
303
K
in
15%
CO2/N2 mixedgases at a flow of 40 mL/min [241].
Conclusions
The performances of currently available adsorbents for CO2capture technologies need to be improved in terms of working
adsorption capacity, cycle lifetime, and multicycle durability. The
development of new highly efficient adsorbents for CO2capture is
necessary to obtain systems that are techno-economical. It is
therefore very important to acquire data on adsorption reactors,
regeneration processes, and overall process integration of capture
systems in power plants.
Furthermore, through thedevelopment of advanced adsorbents
for
CO2 capture,
highly
techno-economical
systems
will
be
achieved by combining advanced CO2 capture technologies with
related systems such as hydrogen generation in the steam
reforming reaction, water gas conversion, electricity generation
using hydrogen, fuel cells, and water treatments.
Acknowledgements
This work was supported by the Carbon Valley Project of the
Ministry
of
Knowledge
Economy
and
the
Energy
Efficiency
&Resources of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP) Grant funded by the Ministry of Commerce,
Industry, Trade and Energy, Korea.
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7/24/2019 A Review on Solid Adsorbents for Carbon Dioxide Capture 2015 Journal of Industrial and Engineering Chemistry
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7/24/2019 A Review on Solid Adsorbents for Carbon Dioxide Capture 2015 Journal of Industrial and Engineering Chemistry
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