Review
Recent advances in carbon dioxide
capture with metal-organic
frameworks
Yangyang Liu, Zhiyong U. Wang and Hong-Cai Zhou, Texas A&M University,
College Station, TX , USA
Abstract: Uncontrolled massive release of the primary greenhouse gas carbon dioxide (CO2) into
atmosphere from anthropogenic activities poses a big threat and adversely affects our global climate
and natural environment. One promising approach to mitigate CO2 emission is carbon capture and
storage (CCS), in which ideal adsorbent materials with high storage capacity and excellent adsorption
selectivity over other gases are urgently needed. For practical applications in CO2 capture from flue
gas of power plants, the biggest single contributor of anthropogenic CO2 emission, the adsorbent
materials must also be chemically stable, be easy to regenerate with minimal energy input, and be
easily synthesized with low capital cost. Metal-organic frameworks (MOFs), highly crystalline porous
materials constructed by metal ions and organic ligands, have emerged as a class of excellent adsorbent materials for carbon capture. Great progress in MOF materials for CO2 capture has been made in
the past and reviewed accordingly, but new discoveries are constantly being made as the field quickly
grows. In this paper, we provide a short review on the most recent advances in using MOFs for CO2
adsorption, storage, and separation that are directly related to CO2 capture. Some of the important
properties of MOF adsorbents which are crucial for practical applications but are largely overlooked in
research carried out so far are discussed.
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd
Keywords: carbon dioxide capture; metal-organic frameworks (MOFs); adsorbent; flue gas; natural gas
upgrading
Introduction
he consumption of energy has been explosively
growing with the rapid increase of the global
population and industrialization. Due to the
ready availability of fossil fuels and mature techniques
to extract them, over 85% of our current global energy
demand is supported by burning fossil fuels which
releases large amounts of CO2 into the atmosphere.1
T
Over the last half-century, research results have shown
that the CO2 concentration in atmosphere has increased from about 310 ppm to over 390 ppm (Fig. 1);
this steep increase is unparalleled in human history.2
As the primary anthropogenic greenhouse gas, CO2
could result in the raise of the average temperature of
the Earth and disastrous global climate change if the
current rate of release is not stopped. Moreover, one
study has correlated increasing atmospheric CO2 level
Correspondence to: Hong-Cai Zhou, Department of Chemistry, Texas A&M University, College Station, TX 77842-3012, USA.
E-mail: zhouh@tamu.edu
Received April 18, 2012; revised June 10, 2012; accepted June 11, 2012
Published online at Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/ghg.1296
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
239
Y Liu, ZU Wang and H-C Zhou
Review: CO2 capture with metal-organic frameworks
Figure 1. Atmospheric CO2 concentration during 1958–2010
(at Mauna Loa Observatory), showing the continuing and
accelerating increase of CO2 in atmosphere. Reproduced
with permission from ESRL.2
with higher acidity of sea water.3 As more CO2 is
dissolved in sea water to generate carbonic acid, the
surface sea water pH could drop from a pre-industrial
value of about 8.2 to 7.8 by year 2095 in a worst case
scenario (Fig. 2). This will lead to dramatic detrimental consequences for the biological ecosystems in the
upper ocean.3 Effective techniques to reduce CO2
emission are thus urgently needed to maintain the
global climate and to protect our environment.
Carbon capture and storage (CCS) is an efficient way
to reduce CO2 concentration in the atmosphere. It is a
three-step process including separation of CO2 from
other emissions before entering the atmosphere, CO2
transportation, and its permanent storage. Among
them, the CO2 capture is the most challenging key
step in which new adsorbent materials need to be
developed. Conventional adsorbent materials rely on
either chemisorption or physisorption to capture CO2.
Amine scrubbing, which utilizes alkanolamines such
as monoethanolamine (MEA) in aqueous solutions as
the adsorbent, relies on the chemical reaction between
the amine group and CO2 to generate carbamate or
bicarbonate.4 Although amine scrubbing has been in
practice in industrial settings such as power plants for
decades, it is still considered the current state-of-theart because of its high efficiency (up to 98% capture).
The biggest problem with amine scrubbing, however,
is that large amounts of heat are needed to release
absorbed CO2 during adsorbent regeneration, consuming additionally 10–30% of the power plant’s
240
Figure 2. (Top and middle rows) National Center for
Atmospheric Research Community Climate System Model
3.1 (CCSM3)-modeled decadal mean pH at the sea
surface centered on the years 1875, 1995, 2050, and 2095.
(Bottom left) Global Ocean Data Analysis Project
(GLODAP)-based pH at the sea surface, nominally for
1995. (Bottom right) The difference between the GLODAPbased and CCSM based 1995 fields. Note the different
range of the difference plot. Deep coral reefs are indicated
by darker gray dots; shallow-water coral reefs are indicated
with lighter gray dots. White areas indicate regions with no
data. Reproduced with permission from Feely et al.3
energy output.5 Moreover, the amine scrubbing
solutions are corrosive and chemically unstable upon
heating. Because of their liquid form, they are also
difficult to contain and their handling is considerably
more difficult than that of solid adsorbents. Aminefunctionalized absorbents such as mesoporous
molecular sieves in solid form partially overcome
some of the above limitations, but the parasitic energy
waste is still pretty high.6,7 In contrast, physisorption
between solid adsorbents and CO2 molecules is a
reversible process that requires much less energy for
desorption. Traditional adsorbents such as zeolites
and activated carbons have been extensively studied
for CO2 capture.8,9 Zeolites are porous aluminosilicate
materials. Compared to alkanolamine solutions,
zeolites showed more rapid CO2 adsorption and lower
energy penalty during desorption in small-scale pilot
plants.10 But their usage is limited by low CO2 adsorption capacity and instability in the presence of water.
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
Review: CO2 capture with metal-organic frameworks
Figure 3. Single-crystal X-ray structures of (a) MOF-5 and
(b) IRMOF-6. Reproduced with permission from Lee et al.11
and Eddaoudi et al.,12 respectively.
Activated carbons have greater CO2 adsorption
capacities than zeolites especially at high pressure, but
they generally suffer from low CO2/N2 selectivity.
In the past two decades, metal-organic frameworks
(MOFs), also known as coordination polymers or
coordination networks, have attracted intense research
interest as novel functional materials. They are novel
hybrid materials that combine organic ligands and
metal ions or metal-containing clusters. Most MOFs
have robust 3D structures that are crystallographically
well-defined, and many of them possess superior
surface areas relative to those of traditional adsorbents
such as activated carbon and zeolites. A typical MOF,
MOF-5, which is constructed from zinc atoms as the
metal centers and terephthalic acid as the organic
linker, has its basic structural unit as shown in Fig. 3.11
After removal of the guest molecules such as solvents
in the open pores or channels, the 3D structure of the
MOFs can be usually retained and used for other
guest adsorption. Through judicious selection of metal
ions and organic linkers, the structure and properties
of MOFs can be systematically tuned and used for
specific applications. For instance, by systematically
changing the organic ligand in MOF-5, an isoreticular
series of IRMOFs with similar structures but different
pore sizes were obtained, of which IRMOF-6 showed
high methane storage capacity.12 Besides pre-design of
the ligands, the post-synthetic modification of MOFs
is a powerful way to tune the pore properties by
optimizing the pore size or adding functional groups
to the pore surface.13 The combined favorable properties of large surface area, permanent porosity and
tunable pore size/functionality have enabled MOFs as
ideal candidates for CO2 capture.
A number of reviews have summarized the work in
MOFs for gas adsorption applications including
hydrogen storage, methane storage and CO2
Y Liu, ZU Wang and H-C Zhou
capture.14–22 Most recently, Liu et al. summarized the
CO2 adsorption both at high pressures and selective
adsorption at approximate atmospheric pressures,23
and Sumida et al.20 contributed an extensive review
on CO2 capture from power plants using MOFs, in
which three main scenarios for CO2 capture from
power plants were discussed: post-combustion capture, pre-combustion capture and oxy-fuel combustion. As the MOF research field is quickly growing,
exciting new discoveries are still being made. In this
review, we will focus on the most recent (2011–2012)
advances in using MOFs as the adsorbent materials
for CO2 capture. The readers are referred to previous
reviews for more complete treatment of the earlier
literature. The new MOFs will be evaluated for two
most prominent applications: (i) post-combustion CO2
capture from flue gas of power plants and (ii) selective
removal of CO2 to upgrade the natural gas. As one of
the major contributors of anthropogenic CO2, massive
amounts of coals around the world are still being
combusted to generate electricity, producing 30–40%
of the total CO2 emission.24 If all the CO2 in flue gas
of power plants can be efficiently captured using MOF
adsorbents, significant CO2 mitigation will be
achieved immediately. Unlike CO2 emission from
separate individual sources such as vehicular exhausts, the centralized CO2 emission in flue gas from
each power plant makes it possible to capture significant amounts of CO2 at a single site. Compared to the
other options of pre-combustion capture and oxy-fuel
combustion,20 post-combustion CO2 capture with
appropriate adsorbents such as MOFs has the unique
advantage of being able to retrofit existing power
plants. So in cases when the CO2 capture unit fails to
function, the power plant could still keep working
without interruption. For these reasons MOFs could
potentially have significant social and environmental
impacts as flue gas adsorbents. In another important
area of application, MOFs might be particularly useful
for natural gas upgrade if they possess high adsorption selectivity of CO2 over methane. Crude natural
gases normally contain various amounts of inert CO2
and N2 in addition to the major component methane,
and removal of the inert gases is necessary to upgrade
the quality and commercial value of natural gases.
A typical flue gas from coal-fired power plants could
contain about 15% of CO2, 75% of N2, 5% of H2O, 3%
of O2, and various other trace amounts of gases
including sulfur oxides (SOx) and nitrogen oxides
(NOx).25 In order for an MOF adsorbent to be useful
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
241
Y Liu, ZU Wang and H-C Zhou
for CO2 capture from flue gas, the following criteria
must be met.
1. The adsorbent needs to have high adsorption
capacity for CO2. While gravimetric capacity is
normally studied whenever a new MOF adsorbent
is discovered, volumetric capacity is one parameter
that is often overlooked. The enormous scale of
continuous flue gas generation from power plants
dictates that large amounts of adsorbents need to
be used, and such adsorbents cannot take up
excessive space. MOF adsorbents with extremely
high surface area and pore volume tend to capture
more CO2, but they generally have low density
which would decrease the volumetric capacity. A
good CO2 adsorbent should thus have a balance
between gravimetric and volumetric capacities.
2. The adsorbent needs to have high selectivity for
CO2 against other gases. Because CO2 is the minor
component in flue gas, the MOF adsorbent should
have minimal adsorption of N2, O2 and H2O vapor.
3. The adsorbent needs to have good chemical and
mechanical stability. Many MOFs are sensitive
toward atmospheric moisture and would collapse
and lose their adsorption power upon extended
exposure to water-containing flue gas. Good
mechanical stability would allow the adsorbent to
be pulverized into fine particles and densely
packed for maximum volumetric capacity.
4. The adsorbent should be easily synthesized in large
scale with low cost. The ligand synthesis should
start from cheap starting materials and be
achieved in as few steps as possible. In the ideal
case, both the organic ligand and the metal salt
should be readily available from commercial
sources, and the MOF adsorbents could be conveniently synthesized in near quantitative yield.
5. The adsorbent should be able to be regenerated
with minimal additional energy input. The high
energy penalty needed for the regeneration of
amine scrubbing solutions is the primary drive for
developing alternative adsorbents including MOFs.
In order to minimize energy waste, a balance must
be achieved between efficient adsorption and easy
desorption. While high affinity of the MOF
adsorbents toward CO2 generally would enable
high capacity and selectivity, they could also result
in difficult desorption if the interaction between
adsorbents and CO2 is too strong. The CO2 affinity
is largely determined by the isosteric heat of
242
Review: CO2 capture with metal-organic frameworks
adsorption, and a moderately high value could
enable both efficient CO2 capture and subsequent
facile desorption to regenerate adsorbents.
6. The mass transfer and heat conductivity of the
MOF adsorbents should be good. When MOFs are
densely packed such as in a bed system, CO2
should be able to easily go in (adsorption) and out
(desorption) with appropriate stimuli such as
temperature or pressure swing. If heat is used to
regenerate the MOF adsorbents as in most cases,
good heat conductivity of the material is a must.
For the application of MOFs for natural gas
upgrade, the above criteria would still hold, and
high selectivity of CO2 over methane becomes a
paramount requirement.
In this review we will first present a few
representative MOFs with best-performing CO2
adsorption capacity. Then, we will summarize recent
development in MOFs for selective CO2 adsorption.
Next, we present simulation studies on the CO2
adsorption sites and binding nature. We will discuss
strategies to enhance CO2 adsorption, to improve
MOFs stability, and to address energy consumption
and other practical aspects associated with the
utilization of MOFs, followed by an outlook. Along
the way, we will also discuss some of the important
properties of MOF adsorbents which are crucial for
practical applications but are largely overlooked in
research carried out so far.
Adsorptive separation of CO2
in MOFs
MOFs with high capacity for CO2
adsorption
Like most porous materials, CO2 adsorption capacity
mainly depends on the surface area of the MOFs. A
lot of MOFs have higher surface areas than activated
carbons and zeolites, resulting in record-high CO2
uptake capacity. The CO2 gravimetric capacities of
selected MOFs are listed in Table 1. Since different
units are reported in the literature, for direct comparison of different MOFs, we here define gravimetric
capacity as the weight percentage of the adsorbed gas
to the total weight of the system, including the weight
of the gas. During recalculation of literature values in
which the volume of CO2 needs to be converted to the
mass, ideal gas behavior is assumed. The adsorbed
CO2 is assumed to be at standard temperature and
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
Review: CO2 capture with metal-organic frameworks
Y Liu, ZU Wang and H-C Zhou
Table 1. CO2 adsorption capacities in selected MOFs.
Chemical Formulaa
Common
Name
Zn2(BDC)2
MOF-2
Cu2(BPTC)
MOF-505
Surface Area (m2/g)
Capacity
Pressure
Temp
(wt %)
(bar)
(K)
345
12.3
35
298
26
1,547
31.0
35
298
26
BET
Langmuir
Ref.
Zn2(DOBDC)
Zn-MOF-74
816
31.4
35
298
26
Cu3(BTC)2
HKUST-1
1,781
32.0
35
298
26
Zn4O(HPDC)3
IRMOF-11
2,096
39.3
35
298
26
Zn4O(NH2BDC)3
IRMOF-3
2,160
45.1
35
298
26
Zn4O(C2H4BDC)3
IRMOF-6
2,516
46.2
35
298
26
Zn4O(BDC)3
IRMOF-1
2,833
48.8
35
298
26
Zn4O(BTB)3
MOF-177
4,508
60.0
35
298
27
Zn4O(BBC)2(H2O)3
MOF-200
4530
10400
70.9
50
298
27
Zn4O(BTB)4/3(NDC)
MOF-205
4460
6170
59.9
50
298
27
Zn4O(BTE)4/3(BPDC)
MOF-210
6240
10,400
70.6
50
298
27
Mg2(DOBDC)
Mg-MOF-74
1800
2060
35.2
1
298
28
(Ni2L2)(bptc)
SNU-M10
505
15.2
10
298
31
Cu3(TDPAT)(H2O)3
Cu-TDPAT
2608
25.8
1
298
48
5.4
15.5
298
51
1938
Mn5(btac)4(µ3-OH)2(EtOH)2
Cd(ANIC)2
Cd-ANIC-1
329.3
504.9
14.4
1
298
55
Co(ANIC)2
Co-ANIC-1
274.0
412.6
13.3
1
298
55
573
633
25.3
30
298
58
NJU-Bai3
2690
3100
21.4
1
273
60
61.1
20
273
61
(Me2NH2)In(NH2BDC)2·DMF·H2O
Cu3L2(H2O)5
(BTB6-)
Cu3
3288
(TATB6-)
Cu3
61.1
20
273
61
Zn5(dmtrz)3(IPA)3(OH)
MAC-4
3360
796
1151
6.7
1
298
62
Zn5(dmtrz)3(OH-IPA)3(OH)
MAC-4-OH
339
496
13.6
1
298
62
12.2
1
273
76
Al(OH)(NDC)
761
Zn(BIm)
STU-1
775
1225
12.2
1
273
77
Cu2(bttcd)
PCN-80
3850
3584
12.0
1
296
79
692.0
1011.2
7.8
1
298
86
[Ln2(TPO)2(HCOO)]·(Me2NH2)
a: See List of Abbreviations.
pressure such that 1 mol of CO2 would take up a
volume of 22.4 L.
The CO2 storage capacity of MOFs can be measured
at different temperatures and pressures, and positive
correlation between storage capacity and surface area
has been established at high pressure. Yaghi et al.26
carried out the first systematic study to explore the
relationship between surface area and CO2 capacity.
Nine MOFs were selected to examine their structural
and porous attributes. This lists include MOFs with
different features such as square channels (MOF-2),
pores with open metal sites (MOF-505 and
Cu3(BTC)2), hexagonally packed cylindrical channels
(MOF-74), interpenetrated (IRMOF-11), amino- and
alkyl-functionalized pores (IRMOFs-3 and -6) and the
extra-high porosity frameworks IRMOF-1 and
MOF-177. They found that MOF-177 has the highest
surface area among these materials and it also has the
highest CO2 uptake at high pressure, which is 60.0
wt% at 35 bar. Detailed data about the surface area
and CO2 uptake capacity of these nice MOFs is listed
in Table 1.
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
243
Y Liu, ZU Wang and H-C Zhou
Review: CO2 capture with metal-organic frameworks
triyl-tris(benzene-4,1-iyl))tribenzoate (BBC), MOF200 was obtained with a higher surface area than that
of MOF-177. The CO2 uptake at 298 K and 50 bar in
both MOF-200 and MOF-210 reaches ~71 wt%, which
is a new record among all porous materials. The
ultrahigh surface areas of MOF-200 and MOF-210 are
close to the theoretical upper limit for solid materials.
The CO2 storage capacity of MOFs at ambient
pressure is more relevant to flue gas CO2 capture, and
the capacity is not only affected by surface area but
more importantly dictated by adsorbent-CO2 interactions. The current best-performing MOF at ambient
pressure is Mg-MOF-7428 [Mg2(DOT); DOT: 2,5-dioxidoterephthalate], a framework with open Mg2+
sites, which has a high CO2 storage capacity of 35.2
wt% at 298 K and 1 bar. The open metal sites are
apparently essential in achieving the high capacity.
MOFs with high selectivity for CO2
adsorption
Figure 4. Zn4O(CO2) 6 unit (left) is connected with organic
linkers (middle) to form MOFs. Reproduced with
permission from Furukawa et al.27
Recently, Furukawa et al.27 prepared a series of
MOFs with ultrahigh porosity that are constructed
from Zn4O(CO2)6 unit with one or two organic
linkers (Fig. 4). Among them, MOF-210, in particular,
showed the highest BET (Brunauer-Emmett-Teller)
and Langmuir surface areas (6240 and 10,400 m2g-1,
respectively) and pore volume (3.60 cm3 g-1 and
0.89 cm3 cm−3 of MOF crystal) reported to date. The
ultrahigh porosity of the MOFs was mainly achieved
by expanding the organic linkers. For example, by
extending the size of the 1,3,5-benzenetribenzoate
(BTB) ligand in MOF-177 to 4,4’,4’’-(benzene-1,3,5-
244
CCS-related gas separation is primarily compromised
of CO2/N2 separation in post-combustion capture,
CO2/H2 separation in pre-combustion capture, air
(O2/N2) separation in oxy-combustion, and CO2/CH4
separation in the natural gas upgrading. In a selective
adsorption, both the capacity and selectivity are the
primary concerns. In most studies carried out so far,
single-component isotherms and the Ideal Adsorbed
Solution Theory (IAST)29 were used to calculate the
selectivity factor of a material. In the simplest treatment, the ratio of the adsorbed CO2 over other gases
at the same temperature and pressure is reported. In a
more realistic simulation of flue gas, the selectivity
factor is defined as the molar ratio of the adsorption
quantities at the relevant partial pressures of the
gases. It can be calculated from the following
expression:
S=
q1/q2
p1/p2
(1)
where S is selectivity factor, qi is the quantity adsorbed
of component i, and pi represents the partial pressure
of component i. Since this calculation is based on the
single-component adsorption isotherms, the selectivity factor from this method does not consider the
competition of gas molecules for the adsorption sites
on the pore surface. It therefore still does not represent the actual selectivity from the dosing of a mixed
gas. However, it provides a simple way of evaluating
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
Review: CO2 capture with metal-organic frameworks
Y Liu, ZU Wang and H-C Zhou
Table 2. CO2 selectivity in selected MOFs.
Chemical Formulaa
Common
Name
Mg2(DOBDC)
Mg-MOF-74
Zn2(BPDC)2(BPEE)
Selectivity*
CO2/N2
49
294
2
CO2/CH4
257
CO2 Concentration
Pressure
Ref.
(bar)
Temp
(K)
(%)
16
1
298
48
16
1
298
30
(Ni2L )(bptc)
SNU-M10
98
50
1
298
31
Cu3(TDPAT)(H2O)3
Cu-TDPAT
34
16
1
298
48
Cu3(TDPAT)(H2O)3
Cu-TDPAT
79b
10
1
298
48
Cu3L2(H2O)5
NJU-Bai3
25.1-60.8
13.7-46.6
50
0-20
298
60
34.3
8.6
50
1
273
61
Cu3(BTB6-), Cu3(TATB6-)
H3[(Cu4Cl)3(BTTri)8]
Cu-BTTri
21
50
1
298
68
Cu-BTTri-en
25
50
1
298
68
50
1
273
76
Al(OH)(NDC)
19.6
Cu2(bttcd)
PCN-80
[Ln2(TPO)2(HCOO)]·(Me2NH2)
4.4
11.8
50
1
296
79
28.2
50
1
298
86
*selectivity is calculated based on single-component gas adsorption isotherms.
a: See List of Abbreviations
b: Selectivity calculated based on IAST theory
the performance of different MOFs in terms of
selectivity. The selectivity factors for selected MOFs
are presented in Table 2.
A more accurate way to evaluate the separation
capacity of a CO2 adsorbent material is the breakthrough experiment, in which a bed packed with the
adsorbent is exposed to a mixed-gas stream, usually
two components and the appearance or ‘breakthrough’ of CO2 from the material is detected. MgMOF-7428 showed both high CO2 capacity and
excellent selectivity of CO2/CH4 (Fig. 5). The breakthrough experiment for Mg-MOF-74 was performed
Figure 5. Single crystal structure of Mg-MOF-74, formed by reaction of the
DOT linker with Mg(NO3 ) 2·6H2O. The structure consists of 1D inorganic rods
linked by DOT to form linear hexagonal channels. Reproduced with
permission from Britt et al.28
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
245
Y Liu, ZU Wang and H-C Zhou
Review: CO2 capture with metal-organic frameworks
Figure 6. A 20% mixture of CO2 in CH4 is fed into a bed of Mg-MOF-74.
Effluent concentrations are shown, indicating complete retention of CO2 until
saturation. CO2 breakthrough occurs at the dashed line. Reproduced with
permission from Britt et al.28
by Yaghi et al. by exposing this MOF to a mixture of
20% CO2 in CH4 (Fig. 6). Results demonstrated that
the adsorption of CO2 in this MOF is highly preferred
over CH4 with a dynamic capacity of 8.9% CO2
uptake. To evaluate the importance of the metal ion in
CO2 adsorption, the CO2 breakthrough experiments
were also carried on isostructural Zn-MOF-74.
Zn-MOF-74 only has a CO2 uptake of 0.35 wt%, which
is 96% reduced compared to that of Mg-MOF-74,
indicating the significance of correct metal ions in
CO2 binding. The interaction between CO2 and Mg2+
in Mg-MOF-74 is responsible for its high capacity.
A number of flexible MOFs have shown remarkable
selective adsorption of CO2 over other gases. A
flexible microporous MOF [Zn2(bpdc)2(bpee)]·2DMF
(bpdc = biphenyl-4,4′-dicarboxylate, bpee =1,2-bis(4pyridyl)ethylene) was prepared by Wu et al.30 It
showed remarkably high selectivity of CO2 over other
small gases at relatively low pressure and high temperature conditions. At 0.16 atm and 25 °C, the
separation ratios are 294, 190, 257, and 441 (v/v) for
CO2/N2, CO2/H2, CO2/CH4 and CO2/CO, respectively.
Another two flexible MOFs [(Ni2L2)
(bptc)]·6H2O·3DEF (DEF = N,N-diethylformamide)
and [(Ni2L4)(bptc)]·14H2O (structures shown in
Figs 7(b) and 8(a), respectively) were designed and
synthesized by Choi et al.31 These two flexible MOFs
are the first networks constructed from alkyl-bridged
Ni2+ bismacrocyclic complexes. They exhibit highly
selective CO2 adsorption over N2, H2 and CH4. The
channels inside the MOFs can open or close in
response to different gases. The CO2 adsorption
capacity of desolvated [(Ni2L2)(bptc)]·6H2O·3DEF at
195 K and 1 atm is 24.3 wt% and the CO2/N2
selectivity is 98/1 (v/v) at 298 K and 1 atm. The CO2
246
adsorption isotherms (Figs 7(c) and 8(b)) of both
MOFs showed gate opening phenomena and large
hysteretic desorption. In addition, these networks are
thermally stable up to 300 °C, and are air and water
stable.
The majority of MOFs that reported so far are
synthesized from non-renewable materials in toxic
organic solvents. Recently, Gassensmith et al.32
reported a green MOF made from renewable cyclic
oligosaccharide γ-cyclodextrin and RbOH (Fig. 9).
Th is MOF, namely CD-MOF-2, showed strong and
highly selective adsorption of CO2. The preparation
of this green MOF was performed in inexpensive
and low-toxicity solvent including water, methanol,
and ethanol. The CO2 gas-uptake experiments with
CD-MOF-2 revealed strong affi nity towards CO2 at
low pressure, indicative of a chemisorptive process.33
The selectivity for CO2 over CH4 at low pressure was
about 3000-fold, the highest value reported so far in
the literature.30,34-36 Th is high selectivity of this
MOF at low pressure was attributed to the free
hydroxyl groups in CD-MOF-2 which react with
adsorbed CO2 to from carbonic acid. The color
change of the framework upon removal of the CO2
source indicates that binding of CO2 to the framework is reversible and that the free alcohol groups
can be regenerated.
H2O effects on CO2 adsorption
Since the flue gas is saturated with water (5–7% by
volume),37,38 it is important to consider the humidity
effects on the CO2 adsorption capacity and CO2/N2
selectivity of MOF materials. In some cases, small
amounts of water have been shown to increase CO2
adsorption. Yazaydin et al.39 found that the water
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
Review: CO2 capture with metal-organic frameworks
Y Liu, ZU Wang and H-C Zhou
Figure 7. (a) L 2 alkyl-bridged Ni|| bismacrocyclic complexes and H4bpdc.
Design strategies for construction of 3D networks (b) The X-ray structure
and (c) gas adsorption/desorption isotherms of [(Ni2L 2)(bptc)]·6H2O·3DEF.
Reproduced with permission from Choi and Suh.31
molecules coordinated to open-metal sites of Cu-BTC
(HKUST-1) significantly increased the CO2 adsorption
of this framework. In most other cases, however,
water has been found to have detrimental effects on
CO2 adsorption of MOFs. Kizzie et al.40 evaluated the
humidity effects on the CO2-capture performance of
M/DOBDC series of MOFs (where M = Zn, Ni, Co
and Mg; DOBDC = 2,5-dioxidobenzene-1,4-dicarboxylate). In their study, significant decreases in the CO2
capacities were observed for Mg/DOBDC and Zn/
DOBDC which were regenerated after exposure to
70% relative humidity (RH). Only 16% and 22% of the
initial CO2 capacities can be recovered respectively. In
the cases of Ni/DOBDC and Co/DOBDC, however,
61% and 85% of the initial capacities were recovered,
respectively. The different degree of capacity retention
likely reflects the different stability of such MOFs
toward hydrolysis. Although the Mg/DOBDC has the
highest CO2 capacity under dry conditions, Co/
DOBDC might be a more suitable materials for CO2
capture from flue gas considering the added cost of
flue gas dehumidification. Recently, Liu et al.41 also
Figure 8. (a) The X-ray structure and (b) gas adsorption/desorption isotherms of [(Ni2L4 )
(bptc)]·14H2O. Reproduced with permission from Choi and Suh.31
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
247
Y Liu, ZU Wang and H-C Zhou
Review: CO2 capture with metal-organic frameworks
Figure 9. (a) Structural formula of γ-cyclodextrin (γ-CD) with the primary hydroxyl groups
colored red. (b) Stick representation of a single cubic (γ-CD) 6 unit of the extended
framework of activated CD-MOF-2. (c) Space-filling representation of the (γ-CD) 6 unit in
which the six γ-CD rings forming the sides of the cube are shown in different colors.
d) Gas adsorption isotherms for activated CD-MOF-2, illustrating the uptake of CO2
measured consecutively at 273 K (blue squares), 283 K (green circles), and 298 K (black
triangles) to be contrasted with the uptake of CH4 at 298 K (red diamonds). Solid
symbols indicate gas sorption and open symbols gas desorption. The initial steep rises
observed at very low CO2 pressures reach the same value of ~23 cc/g regardless of
temperature and are believed to be characteristic of a chemisorption process. Reproduced with permission from Gassensmith et al.32
evaluated the water effects on CO2 adsorption and
CO2/N2 selectivity of Ni/DOBDC in a fi xed-bed
system. They found that although trace amounts of
water can affect the performance of this framework,
Ni/DOBDC still possesses a CO2 capacity of 8.8 wt%
and CO2/N2 selectivity of 22 at 0.15 bar CO2 with 3%
RH, indicating it is a promising material for CO2
capture from flue gas. A nice treatment on the water
stability of various MOFs was presented in the recent
review by Liu and coworkers.23
Probing the adsorption sites and binding
nature of CO2 in MOFs
Determining the CO2 adsorption sites in MOFs and
the binding mechanisms would provide guidance in
rational design of new MOF materials tailored towards enhanced CO2 adsorption and separation.
Recently, Wu et al. studied the binding sites of CO2 on
two benchmark MOFs, Mg-MOF-74 and HKUST-1,
through neutron diffraction measurements.42 Both
Mg-MOF-74 and HKUST-1 have unsaturated metal
centers (UMCs) and neutron diffraction studies
showed that UMCs are the major CO2 adsorption sites
(Fig. 10). In both MOFs, the strongest binding between CO2 and UMCs are attributed to enhanced
electrostatic interaction. In Mg-MOF-74, all CO2
molecules bind to the open Mg2+ site. In the case of
248
Figure 10. (a) Real space Fourier difference scattering
length density (yellow regions) superimposed on the
Mg-MOF-74 structure, clearly indicating that the adsorbed
CO2 is located on top of the open Mg ions. (b) CO2
binding on Mg-MOF-74. (c) Real space Fourier difference
scattering length density (yellow region) superimposed
with the partial structure of HKUST-1, clearly indicating
that the adsorbed CO2 is primarily located on top of the
open Cu ions (left) and the window opening of the small
octahedral cage (right). Reproduced with permission from
Wu et al.42
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
Review: CO2 capture with metal-organic frameworks
HKUST-1, most CO2 molecules bind to the open Cu2+
sites at low CO2 concentration, and large amounts of
CO2 go into the small cage window only at high
loading. For adsorption sites other than UMCs, van
der Waals interaction holds the adsorbed CO2.
Accessible small cages and channels often provide
relatively stronger van der Waals interaction since the
gas molecules can interact with multiple ‘surfaces’.43
The CO2 adsorption in the small cage window sites in
HKUST-1 is a good example of this. Vibrational mode
analysis of the adsorbed CO2 based on first-principles
calculations reveals that CO2 molecules are attached
to the metal sites with one of its oxygen atoms and the
rest of the molecule is relatively free. Calculations also
indicated the CO2-metal interaction is still physisorptive and the MOFs can be completely regenerated. The
study demonstrates that UMCs is one of the most
important features to consider in the development of
new MOFs for CO2 capture application.
Vaidhyanathan et al.44 observed direct CO2 binding
with an amine-functionalized MOF Zn2(Atz)2(ox)
(Atz = aminotriazolate, ox = oxalate) through crystallographic resolution of CO2 molecules (Fig. 11). Two
independent CO2 binding sites were located. One is
close to the free amine group and the other is near the
oxalates. Accompanied with molecular simulation
studies, they confirmed that large uptake of CO2 at
low pressure in this MOF was due to the combination
Y Liu, ZU Wang and H-C Zhou
of appropriate pore size, strongly interacting amine
functional groups, and the cooperative binding of
CO2 guest molecules.
Chen et al. performed a systematic simulation study
on CO2 adsorption in cation-exchanged rho zeolitelike MOFs (ZMOFs).45 The cations studied include
monovalent Na+, K+, Rb+, Cs+, divalent Mg2+, Ca2+,
and trivalent Al3+. The isosteric heat of adsorption
and Henry’s constant at infinite dilution increase in
the following order Cs+ < Rb+ < K+ < Na+ < Ca2+
< Mg2+ < Al3+, in accord with the increasing order of
the charge-to-diameter ratio of the cations. The
cations were found to act as preferential sites for CO2
adsorption at low pressures.
Zhang et al.46 studied the cooperative effect of temperature and linker functionality on CO2 capture
through both molecular simulation and experimental
measurements of four Zr4+-based MOFs47 (UiO-66,
UiO-66-Br, UiO-66-NO2 and UiO-66-NH2). They
confirmed that at low to moderate pressures, the lower
the temperature, the larger the effect the functional
groups have on the performance of MOFs. It should be
noted that functional group incorporation not only
improves the affinity of MOF for CO2, it also reduces the
free volume and thus negatively affecting CO2 capture
efficiency. A balance between high affinity and size of
the functional group should be carefully considered in
the design of new MOF adsorbents for CO2 capture.
Figure 11. (a) X-ray structure and (b) gas adsorption/desorption isotherms of
the Zn2 (Atz) 2 (ox) (c) the CO2 biding (directly determined by X-ray structure
refinement at 173 K) within the pores of Zn2 (Atz) 2 (ox) 2. Reproduced with
permission from Vaidhyanathan et al.44
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
249
Y Liu, ZU Wang and H-C Zhou
Review: CO2 capture with metal-organic frameworks
Figure 12. (a) Structure of Cu-TDPAT . A portion of the (3,24)-connected
rht-net built on the shortest linker TDPAT is shown. (b) CO2 and N2 sorption
isotherms of Cu-TDPAT at 298 K (adsorption: filled; desorption: open; CO2:
red square; N2: blue triangles.) Reproduced with permission from Li et al.48
Strategies to enhance CO2
adsorption
For effective CO2 capture from flue gas, both high
CO2 capacity and high CO2 selectivity are requisite
attributes of the MOF adsorbents. While various
strategies have been studied to improve CO2 adsorption in MOFs, three approaches have been proven to
be particularly effective: incorporation of UMCs,
metal doping, and chemical functionalization.
making this MOF a promising candidate for CO2
capture from flue gases.
Han et al. prepared a Mn2+-based MOF
Mn5(btac)4(μ3-OH)2(EtOH)2·DMF·3EtOH·3H2O
(btac = benzotriazole-5-carboxylate) which has
exposed Mn2+ coordination sites.51 The unsaturated
Mn2+ centers enable significant H2 uptake (1.01 wt%)
at atmospheric pressure at 77 K, but the CO2
adsorption capacity (5.4 wt% at 15.5 atm and 298 K)
and isosteric heat of adsorption (22.0 kJ mol-1 at zero
surface coverage) are moderate.
Incorporation of unsaturated metal
centers (UMCs)
Metal doping
The UMCs are usually generated by removal of
coordinated solvents from metal centers. Li et al.
synthesized an rht-type MOF [Cu3(TDPAT)
(H2O)3]·10H2O·5DMA (Cu-TDPAT) that possesses a
high concentration of UMCs as shown in Fig. 12.48 It
shows high CO2 adsorption capacity at ambient to low
pressures. At 298 K, the CO2 uptake is 132 cm3 g–1
(STP; 20.6 wt%, 103 v/v) at 1.0 atm and 31.3 cm 3g–1
(STP, 5.8 wt%, 24.5 v/v) at 0.1 atm. In addition,
Cu-TDPAT has high adsorption selectivity of 79 for
CO2 over N2 at 298 K as calculated by the IAST
theory. Based on single-component gas adsorption
isotherms, the separation ratio for CO2/N2 is 16 v/v at
1 atm, which is even higher than MOF-74-Mg (12
v/v)49 and ZIF-78 (13 v/v)50. Besides the high density of
UMCs, the enhanced CO2 binding capacity and high
selectivity of this MOF are also attributed to the dual
functionalization of the framework and Lewis basic
sites. Compared to other similar rht-type MOFs, the
narrow pores in this MOF may also contribute to
stronger CO2-framework interactions. Moreover,
Cu-TDPAD is highly water and thermal-stable,
Computational studies showed that there is an
enhancement of CO2/CH4 selectivity in MOFs containing lithium cations.52 Chemical reduction and
cation exchange are two methods to incorporate Li+
into MOFs. Bae et al. first experimentally confirmed
the improvement of selectivity by incorporating Li+
with both strategies.53 In their work, three Zn based
mixed-ligand MOFs are investigated. Two of them
have the same two-fold catenated structure (Zn2(2,6NDC)2(diPyNI); 2,6-NDC = 2,6-naphthalenedicarboxylate and diPyNI = N,N-di-(4-pyridyl)-1,4,5,8naphthal-enetetracarboxydiimide) but are synthesized
by different methods (Fig. 13). The third one is a
non-catenated MOF with hydroxyl groups
(Zn2(TCPB)(DPG); TCPB = 1,2,4,5-tetrakis(4-carboxyphenyl)benzene and DPG = meso-1,2-bis(4-pyridyl)1,2-ethanediol). Experiments showed that both of the
two Li incorporation methods, chemical reduction
and cation exchange, can significantly enhance the
CO2/CH4 selectivity in these three MOFs. In the case
of lithium metal incorporation, the increase in CO2/
CH4 selectivity was attributed to the favorable
250
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
Review: CO2 capture with metal-organic frameworks
Figure 13. structure of (a) Zn2 (NDC) 2 (diPyNI) and (b)
Zn2 (TCPB)(DPG). Reproduced with permission from Bae
et al.53
displacement of catenated frameworks and pore-volume diminution. However, the enhanced selectivity in
Li+- exchanged MOFs was explained by enhanced
MOF-gas interaction between desolvated Li+ (charge)
and CO2 (quadrupole).
Besides metals, other inorganic materials can also be
doped into MOFs to improve their performance on
CO2 capture. Si and co-workers doped 1.3 wt% and
1.7 wt% of nanostructured boron nitride BNHx (x ≤ 1)
into MIL-53 and found the CO2 capacity steadily
increases with higher boron content from the original
13.7 wt% to 15.9 wt% and then to 16.8 wt% at
780 mmHg and 273 K.54
Chemical functionalization
CO2 has a high quadrupolar moment while N2, CH4
and H2 are non-polar or weakly polar. This difference
can be employed to separate CO2 from other gases by
introducing polar functional groups with high affinity
for CO2 into the pores of MOFs. Because of the high
affinity of amine groups toward CO2, amine functionalized ligands have been utilized in MOFs to enhance
both the adsorption capacity and selectivity for CO2.
Pachfule et al.55 synthesized two isostructural,
interpenetrated MOFs Cd-ANIC-1 and Co-ANIC-1
from ligand 2-aminoisonicotinic acid (ANIC). These
two MOFs showed high CO2 adsorption capacities
that outperform previously reported amino functionalized MOFs and ZIFs. Their favorable CO2 adsorption properties were attributed to the presence of the
Lewis basic amino groups of the ANIC ligand.
Another amino functionalized MOF NH2-MIL-101
(Al) prepared by Serra-Crespo et al.56 exhibited
higher CO2 adsorption capacity and selectivity than
the parent MIL-101 (Al). In a related MOF, the
separation of CO2 from methane, nitrogen, hydrogen,
Y Liu, ZU Wang and H-C Zhou
or a combination of these gases was demonstrated by
breakthrough experiments using pellets of NH2-MIL53(Al).57 Regeneration of the adsorbent can be
achieved by thermal treatment, inert purge gas
stripping, and pressure swing. Notably, the NH2-MIL53(Al) pellets retained their selectivity and capacity
after two-year exposure to ambient atmosphere
containing water. Yuan and co-workers synthesized a
microporous amine-functionalized MOF, (Me2NH2)
In(NH2BDC)2•DMF•H2O (NH2BDC = 2-amino
terephthalate), which possess high air and moisture
stability, acceptable capacity, and excellent selectivity
for CO2 over CH4.58 This MOF has a gravimetric
capacity of 25.3 wt% for CO2 at 3.0 MPa at 298 K, and
it has essentially no measurable uptake for CH4
(< 0.01 cm3 g-1) up to 3.0 MPa at the same temperature. The high selectivity was further confirmed by
breakthrough experiments.
Although amine-functionalization had proven to be
effective in many MOFs to improve CO2 affinity, there
appears to be some limit for this effect. Vaidhyanathan et al. demonstrated both computationally and
experimentally that higher degrees of amination are
not necessarily favorable for improving CO2 capacity
as excessive clustering of amine groups can actually
interfere with CO2 binding.59
The amide groups also turn out to be beneficial for
CO2 adsorption, although their chemical properties
are quite distinct from amines. Duan et al. incorporated the polar amide groups into an agw-type MOF,
[Cu3L2(H2O)5]•xGuest,60 which exhibits high CO2
capacity (21.4 wt% at 273 K and 1 bar) and high CO2/
N2 selectivity at the same time. Grand canonical
Monte Carlo (GCMC) simulations indicated that CO2
molecules prefer to be absorbed at both unsaturated
Cu2+ sites and the amide group sites. Zheng et al.
synthesized two expanded isoreticular rht-type MOFs
from nanosized triangular acylamide-bridging
hexacarboxylate linkers, [Cu3(BTB6–)]n and
[Cu3(TATB6-)]n (Fig. 14).61 The presence of amide
groups enables high CO2 uptake (61.1 wt% at 20 bar
and 273 K) and good selectivity for CO2/CH4 (8.6)
and CO2/N2 (34.3).
As previously discussed in CD-MOF-2,32 hydroxyl
group is another polar group that can be used to
enhance CO2 affinity. Ling and co-workers synthesized two isoreticular MOFs, namely ([Zn5(dmtrz)3
(IPA)3(OH)]·DMF·H2O)n (MAC-4) and
([Zn5(dmtrz)3(OH-IPA)3(OH)]·DMF·5H2O)n (MAC4-OH), using a combination of trinuclear-triangular
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
251
Y Liu, ZU Wang and H-C Zhou
Review: CO2 capture with metal-organic frameworks
Figure 14. Nanosized acylamide-bridging ligands (H6BTB
and H6TATB) and the 3D polyhedra (violet, cub-Oh ;
orange, T-Td ; green, T-Oh ) packing in single-crystal
structure of [Cu3 (BTB6- )] n. Reproduced with permission
from Zeng et al.61
and paddle-wheel SBUs.62 Although the introduction
of the hydroxyl groups to MAC-4 reduces its surface
area, the additional electrostatic interaction between
the hydroxyl groups and CO2 molecules significantly
enhances both the CO2 capacity and the CO2/N2
selectivity in MAC-4-OH (Fig. 15). The adsorption
results indicate that MAC-4-OH shows a significant
enhancement for CO2 uptake because the hydroxyl
groups provide additional active sites for interaction
with CO2, although its surface area is decreased
compared to that of MAC-4.
Fluorination of ligands in MOFs has been found to
be capable of either enhancing or decreasing H 2
adsorption affi nity.63–65 Pachfule et al. used isonicotinic acid (INA) and 3-fluoroisonicotinic acid
(FINA) with Co2+ to generate four MOFs by using
different reaction solvents.66 Two isoreticular pairs
of MOFs (Co-INA-1 ([Co3(INA)4(O)(C2H5OH)3]
[NO3]·C2H5OH·3H2O) vs Co-FINA-1 and Co-INA-2
([Co(INA)2]·DMF) vs Co-FINA-2) were prepared.
Fluorination was found to indeed improve the
framework stability, and moderate increase in both
H2 and CO2 adsorption capacity was observed.
The polar functional groups can not only be introduced by pre-design of ligands, but also by post-modification of existing MOFs. Bae et al.67 post-modified a
MOF by replacing coordinated solvent molecules with
a highly polar ligand py-CF3 to obtain Zn2(bttb)
(py-CF3)2. This functionalization resulted in a considerable enhancement of CO2/N2 selectivity at low
pressure. Long et al.68 post-synthetically modified the
UMCs in HCu[(Cu4Cl)3(BTTri)8] (Cu-BTTri) with
252
Figure 15. (a) The N2 adsorption isotherms of MAC-4 and
MAC-4-OH at 77 K and (b) their CO2 adsorption isotherms
at 298 K. Reproduced with permission from Ling et al.62
ethylenediamine (en) to obtain the en-functionalized
MOF as shown in Fig. 16. Although the surface area
of the MOF decreased after the modification, the
en-functionalized MOF showed higher CO2 uptake at
very low pressures compared to that of the parent
MOF, and a record high heat of adsorption of 90 kJ/
mol was reported.
Strategies to enhance MOF stability
Chemical and thermal stabilities are very important
parameters for MOFs to be considered for CO2
adsorption. While many MOFs are hydrolytically
unstable, 23 there are a number of MOFs which have
high chemical and thermal stability. Some of the
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
Review: CO2 capture with metal-organic frameworks
Y Liu, ZU Wang and H-C Zhou
Figure 16. (a) A portion of the structure of Cu-BTTri showing surface functionalization of a coordinatively unsaturated CuII site with ethylenediamine,
followed by attack of an amino group on CO2. (b) Adsorption isotherms for
the uptake of CO2 at 298 K in Cu-BTTri (black squares) and 1-en (red circles)
and for the uptake of N2 in 1 (black diamonds) and 1-en (red triangles). Filled
and open symbols represent adsorption and desorption, respectively. The
inset shows the higher uptake of CO2 for 1-en compared with 1 at low
pressures. Reproduced with permission from Long et al.68
most well-known examples include those generated
from either high-valency metal ions such as Al3+
(Al-MIL-11069), Cr3+ (Cr-MIL-10170) and Zr4+
(UiO-66 47) or various nitrogen-donor ligands
containing imidazole (ZIFs),71 pyrazole,72 triazole,68
and tetrazole.73,74 Abid et al. showed that ammonium hydroxide can be added to the reaction
medium for the preparation of UiO-66 without
modifying its crystal structure, and the resulting
Zr-MOF displayed slight increase in the CO2/CH4
selectivity.75 Zhang et al. prepared a Al3+-based
MOF, Al(OH)(1,4-NDC) (1,4-NDC = 1,4-naphthalenedicarboxylic acid), with great thermal stability
up to 500 oC.76 Th is MOF has a gravimetric capacity
of 12.2 wt% for CO2 at 273 K at 777 mm Hg, and
based on single gas isotherms the CO2/CH4, CO2/
N2 , and CO2/O2 selectivities are 4.4, 19.6, and 18.8,
respectively. Zhou et al. synthesized a series of
unusual gyroidal MOFs, termed STUs, with gie
topology from 1,2-bis((5H-imidazol-4-yl)methylene)
hydrazine and octahedral metal ions including Zn 2+,
Mn 2+, Cu 2+, and Ni 2+.77 As new members of the ZIF
family, the STUs have high thermal stability up to
420 oC in a N2 atmosphere and chemical stability in
boiling toluene and methanol-NaBH4. The Zn 2+
analogue, STU-1, exhibits high CO2 adsorption
capacity of 12.2 wt% at 273 K and atmospheric
pressure.
Utilization of dendritic ligands to generate MOFs is
a promising method to generate stable MOFs with
high porosity because of the ligands’ high connectivity. Yuan et al. synthesized an isoreticular MOF series
with the (3,24)-connected network topology from
dendritic hexacarboxylate ligands,78 of which PCN-68
showed high surface area (Langmuir 6033, BET 5109
m2/g) and high CO2 uptake of 57.2 wt% at 298 K and
35 bar. This series of MOFs have different extent of
stability, and a ligand size between 11.2 and 13.8 was
found to generate stable MOFs. Lu et al. constructed
PCN-80 ([Cu2(bttcd)]·2DMF, H8bttcd = 9,9′,9′′,9′′′([1,1′-biphenyl]-3,3′,5,5′-tetrayl)-tetrakis(9H- carbazole-3,6-dicarboxylic acid)) from the an octatopic
ligand H8bttcd featuring 90o-angle-dicarboxylate
moieties.79 Powder X-ray diffraction (PXRD) analysis
revealed that PCN-80 retained a robust structure after
high-pressure gas uptake measurements. PCN-80
exhibits remarkable CO2 uptake capacity at atmospheric pressure (19.3 and 12.0 wt% at 273 and 296 K,
respectively) which is among the highest for MOFs
based on dicopper(II) paddlewheel clusters. While
high H2 uptake is also observed for PCN-80, very
little N2 is adsorbed (0.73 and 0.34 wt% at 273 and
296 K, respectively). By determining the Henry’s Law
constants with single gas adsorption isotherms,80 the
CO2/N2 adsorption selectivity factors for PCN-80 are
18.7 and 11.8 at 273 and 296 K, respectively. Recently
Jia et al. developed a hexadendritic ligand TDCPB
(1,3,5-tris(3,5-di-(4-carboxy-phenyl -1-yl)phenyl-1-yl)
benzene) to synthesize three MOFs, namely JUC100,81 JUC-101 and JUC-102.82 JUC-101 was found to
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
253
Y Liu, ZU Wang and H-C Zhou
have a CO2 uptake capacity of 8.0 wt% at 273 K and
1 atm, but the selectivity over CH4 is low. One limitation with dendritic ligands is that long extended
ligands tend to counteract the stabilization effect of
high connectivity, so a balance between surface area
and stability needs to be tuned.
MOFs can be also dispersed into other porous
materials to improve its overall stability. Pachfule
et al. loaded a Zn-terephthalate based MOF (MOF-2)
in the inner cavity as well as on the outer walls of a
hollow carbon nanofiber (CNF) to create a MOF@CNF
hybrid.83 At 1 atm and 298 K, the gravimetric capacities of FCNF, MOF-2 and MOF@CNF hybrid are 3.0,
5.0, and 6.6 wt%, respectively. The hybrid material
shows improved thermal stability and higher CO2
uptake as compared to each individual component.
Strategies to address energy
consumption and other practical
aspects associated with the usage
of MOFs
Traditional MOF synthesis is usually performed
under solvothermal conditions in organic solvents
such as DMF. The process typically requires long
reaction time (days to weeks) and heavy energy
consumption, and it could become cost-prohibitive on
large scale. Microwave synthesis can dramatically
reduce the reaction time, enhance product yields, and
save energy. It has been successfully applied to MOF
synthesis.84,85 Lin et al. used microwave synthesis to
prepare a series of isoreticular lanthanide MOFs
formulated as [Ln2(TPO)2(HCOO)]·(Me2NH2)·(DMF)4
·(H2O)6 (H3TPO = tris(4-carboxylphenyl)phosphine
oxide) in as short as 30 min.86 For the best performing
MOF (Ln = Y) in this series, high CO2 adsorption
capacity of 7.8 wt% (298 K, 1 atm) and CO2/N2
selectivity of 28.2 (298 K, 1 atm) were observed.
CO2 release and adsorbents regeneration usually
employs pressure or temperature swing methods, both
of which would require significant energy input.
Recently Park et al. developed a novel strategy to
release adsorbed CO2 based on UV light irradiation.87
A photoactive azobenzene-bearing ligand, 2-(phenyldiazenyl)terephthalate, was reacted with Zn2+ to
construct PCN-123. The original trans-azobenzene
groups in the pristine PCN-123 sample partially
transformed into cis conformation upon UV
irradiation for 1 h (Fig. 17), resulting in overall decrease (release) of 53.9% originally-adsorbed CO2 after
254
Review: CO2 capture with metal-organic frameworks
Figure 17. (Top) Trans-to-cis isomerization of the ligand of
PCN-123 (PCN represents porous coordination networks)
induced by UV irradiation and the cis-to-trans isomerization induced by heat treatment. (Bottom) Schematic
illustration showing the suggested CO2 uptake in MOF-5,
PCN-123 trans, and PCN-123 cis. Reproduced with
permission from Park et al.87
5 h further standing at room temperature (Fig. 18).
Remarkably, the adsorbent can be easily regenerated
by standing at ambient conditions for a prolonged
period of time or by gentle heating (60 oC, 20 h) if fast
Figure 18. CO2 adsorption isotherms (at 295 K) of PCN-123
showing reversible conformational change: pristine sample
(red), right after the first UV irradiation (half-filled blue), 5 h
after the first UV irradiation (fully filled blue), after the first
regeneration process at 60 °C for 20 h in the dark (orange),
after the second UV irradiation (green), and after the
second regeneration process at 60 °C for 20 h in the dark
(yellow). In the figure, ‘UV’ represents ‘UV irradiation’.
Reproduced with permission from Park et al.87
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
Review: CO2 capture with metal-organic frameworks
regeneration is needed. The study opens the door for
an unprecedented strategy, namely, utilization of light
to achieve CO2 desorption. To be practically useful,
however, both the trans/cis isomerization kinetics and
the desorption efficiency need to be improved. Ideally
further optimization of the photoactive groups will
lead to a MOF adsorbent which can be efficiently
desorbed by the ubiquitous and free sunlight.
The majority of MOFs reported to date have micropores (<2 nm diameter). Yuan and co-workers recently
synthesized an iron-based MOF, PCN-53
(Fe3O(H2O)3(BTTC)2•xSolvent, H3BTTC = benzo(1,2;3,4;5,6)-tris(thiophene-2´-carboxylic acid), which
has both micropores and mesopores (2–50 nm
diameter).88 PCN-53 exhibits interesting stepwise
adsorption of CO2, and computational analysis
revealed that CO2 molecules first fill the micropores,
then get adsorbed along the mesopore walls, and
finally fill the mesopores. Optimized MOFs similar to
PCN-53 could be particularly useful for practical CO2
capture because their micropores will enable strong
interaction between adsorbent and CO2 while the
mesopores will enable facile mass transfer of CO2 in
(adsorption) and out (desorption) of the packed
adsorbents. To the best of our knowledge no detailed
studies on the mass transfer of CO2 in MOFs have
been performed up to date. It can be reasoned,
however, that an ideal MOF should have both mechanical stability and mass transfer properties.
Because a solid adsorbent would normally need to be
pulverized and densely packed in order to save space,
it is important that its favorable structure can be
retained after all the physical transformations.
The most convenient and feasible way to achieve
CO2 desorption and adsorbents regeneration is likely
through heat, in which case heat conductivity of
MOFs must be studied when they are densely packed
in a bed system. MOF-5 is the only MOF on which
heat conductivity measurements were performed
using the longitudinal, steady-state heat flow
method,89 but the limited data prevented comparison
either within MOFs or between MOFs and other
types of CO2 adsorbents. As an engineering problem,
more in-depth studies on heat conductivity of various
MOFs by material scientists are urgently needed.
Conclusion and outlook
The development of carbon capture and storage
technology is of paramount importance in
Y Liu, ZU Wang and H-C Zhou
maintaining the global climate and preserving our
environment. MOFs have emerged as promising
adsorbent materials for CO2 capture due to their high
surface area, large pore volume, tunable pore surface,
and crystalline structures.
Great progresses in MOF research have been made
to enhance the CO2 capacity and CO2 selectivity
against other gases. While MOFs with either high
capacity or high selectivity have been developed, there
are few MOFs which possess both of these desirable
properties. For MOFs to be practically useful as
adsorbents for either CO2 capture from flue gas or
natural gas upgrading, a number of aspects need to be
carefully evaluated in addition to capacity and
selectivity. These would include stability, costs of
preparation, adsorbent regeneration, energy input,
and heat conductivity. The high surface area and pore
volume of current record-holding MOFs such as
MOF-21027 have almost reached the theoretical upper
limit of solid sorbents. Such MOFs would provide
great templates to further incorporate various CO2philic functional groups such as metals, UMCs, and
polar groups through either pre-synthetic incorporation into the ligands or post-synthetic modifications.
Because functionalization generally would tend to
block the available pores, the high surface area parent
MOFs will have the best potential in providing
optimized MOFs with both reasonable residual pores
to achieve high CO2 capacity and CO2-philic active
sites to achieve high CO2 selectivity under ambient
conditions.
In the pursuit of ideal MOFs for CO2 capture, we
deem the following issues to be very important for
practical applications; they have been largely overlooked in MOF research so far. The mechanical
stability and volumetric capacity for CO2 of MOFs are
rarely studied. Breakthrough experiments of simulated flue gas or natural gas to evaluate CO2 selectivity
should be done whenever possible. Measurement of
heat conductivity and CO2 transfer in and out of
MOFs is rarely done, and appropriate models and
experimental techniques need to be established. When
am MOF adsorbent with all desirable properties is
developed, the final determining factor for its practical applications lies in the overall cost. So far only a
handful of MOFs are commercially available, and
reports on large-scale synthesis of MOFs are rare. The
overall cost would also include the energy needed to
regenerate the adsorbent. While the amine scrubbing
solutions have been estimated to consume additionally
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
255
Y Liu, ZU Wang and H-C Zhou
10–30% of the power plant’s energy output,5 no study
on the cost of MOF adsorbents regeneration has been
done yet because no current MOFs have been tested in
industrial scale CCS applications. Considering much
weaker interactions of physisorption compared to
chemisorption, however, it is safe to say that MOF
regeneration will cost significantly less than that of
amine scrubbing solutions. The synthesis of most
MOF ligands are still relatively complex, and synergistic efforts from organic, inorganic, and material
chemists are needed to make MOF synthesis in
kilogram-scale economically feasible.
The ability to design and tune the properties of
MOFs makes these adsorbent materials distinct from
other traditional adsorbents such as inorganic zeolites
and carbon materials. Analogous to designing drugs
in medicinal chemistry, the continuing enrichment of
knowledge concerning the structure-activity relationship (SAR) of MOFs will enable researchers to design
practically useful MOFs for CO2 capture. Many
challenges will still need to be solved, but we are
optimistic that MOFs with overall better performance
than the currently used amine scrubbing solutions
will be discovered.
Acknowledgements
We would like to thank financial support from US
Department of Energy (DOE DE-SC0001015 and
DE-AR0000073), the National Science Foundation
(NSF CBET-0930079), and the Welch Foundation
(A-1725).
Review: CO2 capture with metal-organic frameworks
BTTC
bttcd
BTTri
CCS
CD
DEF
diPyNI
DMF
dmtrz
DOBDC
DOT
DPG
en
Hdmtrz
INA
FINA
IPA
L2
1,4-NDC
2,6-NDC
MOF
NH2BDC
ox
PDC
py-CF3
TATB
TCPB
TDCPB
List of Abbreviations
ANIC
Atz
BBC
BDC
Bim
bpdc
bpee
BPTC
btac
BTB
BTC
bttb
256
2-amino-isonicotinic acid
aminotriazolate
4,4′,4′′-(benzene-1,3,5-triyl-tris(benzene4,1-iyl))tribenzoate
1,4-benzenedicarboxylate
1,2-bis((5H-imidazol-4-yl)methylene)
hydrazine
biphenyl-4,4′-dicarboxylate
1,2-bis(4-pyridyl)ethylene
3,3′,5,5′-biphenyltetracarboxylic acid
benzotriazole-5-carboxylate
4,4′4′′-benzene-1,3,5-triyl-tribenzoate
benzene-1,3,5-tricarboxylate
4,4′,4′′,4′′′-benzene-1,2,4,5-tetrayltetrabenzoic acid
TDPAT
TPO
UMCs
benzo-(1,2;3,4;5,6)-tris(thiophene-2′carboxylate
9,9′,9′′,9′′′-([1,1’-biphenyl]-3,3′,5,5′tetrayl)-tetrakis(9H-carbazole-3,6dicarboxylate)
1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene
carbon capture and storage
cyclodextrin
N,N-diethylformamide
N,N-di-(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide
N,N-dimethylformamide
3,5-dimethyl-1H-1,2,4-triazolate
2,5-dioxido-1,4-benzenedicarboxylate
2,5-dioxidoterephthalate
meso-1,2-bis(4-pyridyl)-1,2-ethanediol
ethylenediamine
3,5-dimethyl-1H-1,2,4-triazole
isonicotinic acid
3-fluoroisonicotinic acid
isophthalic acid
macrocyclic complexes shown in Fig. 7(a)
1,4-naphthalenedicarboxylate
2,6-naphthalenedicarboxylate
metal-organic framework
2-amino terephthalate
oxalate
pyrenedicarboxylic acid
4-(trifluoromethyl)pyridine
4,4′,4′′-s-triazine-2,4,6-triyl-tribenzoate
1,2,4,5-tetrakis(4-carboxyphenyl)
benzene
1,3,5-tris(3,5-di-(4-carboxy-phenyl -1-yl)
phenyl-1-yl)benzene
2,4,6-tris(3,5-dicarboxylphenylamino)1,3,5-triazine
tris(4-carboxylphenyl)phosphine oxide
unsaturated metal centers
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Yangyang Liu
Yangyang Liu obtained her BSc from
Nankai University in 2009. In the
same year, she joined Professor
Hong-Cai “Joe” Zhou’s group as a
graduate student at Texas A&M
University. Her research interests
include design and synthesis of
metal-organic frameworks, and their
applications in clean and renewable energy field.
Zhiyong Wang
Zhiyong U. Wang obtained his PhD in
2005 from Northwestern University
under the supervision of Dr Richard
B. Silverman. He has been doing
postdoctoral research, first at
University of Pittsburgh and then at
Texas A&M University. In 2011 he
joined Prof. Hong-Cai Zhou’s group
as an Assistant Research Scientist. He has a strong
interest in organic synthesis and his current research
focuses on the preparation of novel organic ligands to
construct metal-organic frameworks for clean energyrelated gas adsorptions including carbon capture,
hydrogen storage, and methane storage.
Hong-Cai ‘Joe’ Zhou
Hong-Cai ‘Joe’ Zhou obtained his
PhD in 2000 from Texas A&M
University under the supervision of
F. A. Cotton. After a postdoctoral
stint at Harvard University with
R. H. Holm, he joined the faculty of
Miami University, Oxford, in 2002. He
rose to the rank of full professor
within six years and moved to Texas A&M University in
2008. His research interest focuses on gas storage and
separations that are relevant to clean energy technologies. Recently, he served as a guest editor for Chemical
Reviews thematic issue focusing on metal-organic
frameworks.
© 2012 Society of Chemical Industry and John Wiley & Sons, Ltd | Greenhouse Gas Sci Technol. 2:239–259 (2012); DOI: 10.1002/ghg
259