CH4 storage and CO2 capture in highly porous zirconium oxide based metal-organic frameworks

Article abstract of DOI:10.1039/c2cc34714h

A series of porous Zr oxoclusters-based MOFs was computationally explored for their gas storage/capture performances. The highly porous UiO-67(Zr) and UiO-68(Zr) solids show exceptionally high CH₄ and CO₂ adsorption capacities under operating conditions that make these thermal, water and mechanical resistant materials very promising for physisorption-based processes. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc34714h

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COMMUNICATION
CH4 storage and CO2 capture in highly porous zirconium oxide based
metal–organic frameworkswz
Qingyuan Yang,ab Vincent Guillerm,c Florence Ragon,c Andrew D. Wiersum,d
Philip L. Llewellyn,d Chongli Zhong,a Thomas Devic,c Christian Serrec and
Guillaume Maurin*b
Received 2nd July 2012, Accepted 14th August 2012
DOI: 10.1039/c2cc34714h
A series of porous Zr oxoclusters-based MOFs was computationally
explored for their gas storage/capture performances. The highly
porous UiO-67(Zr) and UiO-68(Zr) solids show exceptionally high
CH4 and CO2 adsorption capacities under operating conditions that
make these thermal, water and mechanical resistant materials very
promising for physisorption-based processes.
Through a judicious choice of these organic and inorganic moieties,
many MOFs were designed with ultra-high porosity, including
MIL-101,5a MOF-177,5b MOF-205,3a PCN-14,5c UMCM-2,5d that
have further shown high CO2 and/or CH4 uptakes.3a,5c,6 However,
the majority of these promising materials suffers either from a
relatively poor stability under humidity or would require a high
energy cost for regeneration that are both severe drawbacks for
practical applications.3c,d Indeed, tailoring more stable, chemically-
resistant MOF frameworks combined with high adsorption perfor-
mance is still nowadays a great challenge for the MOF community.
Here, we focus on an emerging class of zirconium-based
MOFs derived from the UiO-66(Zr) topology. This isoreticular
series includes the existing UiO-67(Zr),7a UiO-68(Zr)7a and
Zr-AzoBDC7b solids where 4,40-BPDC, TPDC and AzoBDC
ligands substitute the parent BDC organic node of UiO-66(Zr),
respectively, that are completed by a newly synthesized
Zr-MOF involving Cl2AzoBDC as a linker (Fig. 1). The
hydroxylated forms of these analogue materials are built up
from the inorganic block Zr6-octahedra [Zr6O4(OH)4] bounded
to twelve organic linkers, leading to a three dimensional periodic
structure in which each centric octahedral cage (Fig. 1a) is
connected to eight corner tetrahedral cages (Fig. 1b) through
triangular windows. The parent UiO-66(Zr) has been recently
shown to be stable up to 450 1C under air, unaltered upon water
adsorption8a and mechanical pressure,7a easily regenerable8a,b
and selective for the CO2–CH4 mixture with a high working
capacity.8c The four other analogues are also porous MOFs (see
the structural features provided in Table S3 in the ESIz). The
solvothermal synthesis of the UiO-67(Zr), Zr6-AzoBDC and its
3,30,4,40-Cl2AzoBDC functionalized analogue have been carried
The anthropogenic emission of CO2 from fossil fuel burning in
power plants and automobile transport is a growing environ-
mental and economic issue. CH4 is another strategic gas of
considerable interest. Apart from its negative greenhouse
effect, this latter species, which is the main component of the
natural gas, is also considered as a cleaner energy carrier than
petroleum oil because of its higher hydrogen-to-carbon ratio
and a resulting much lower carbon emission.1 Indeed, the
search for safe and high-capacity CH4 storage and CO2
capture systems has been a long-standing challenge that has
triggered tremendous studies with the aim to improve the
existing related technologies. It is well established that the
physisorption-based processes involving porous solids offer an
efficient storage/capture alternative with the ability to release
the adsorbed gases easily.2 Among this class of materials,
porous metal–organic frameworks (MOFs) have attracted
substantial attention during the past decades in the field of
gas adsorption/separation both experimentally3 and theoreti-
cally.4 Such an intriguing family of coordination materials
corresponds to crystalline hybrid media built up from inorganic
metal-ion subunits connected by polytopic organic ligands.
a State Key Laboratory of Organic-Inorganic Composites,
Beijing University of Chemical Technology, Beijing 100029, China
b Institut Charles Gerhardt Montpellier, UMR CNRS 5253, UM2,
Place E. Bataillon, 34095 Montpellier Cedex 05, France.
E-mail: guillaume.maurin@univ-montp2.fr
Institut Lavoisier, UMR CNRS 8180-Universite de Versailles
c
´
StQuentin-enYvelines, 45 avenue des Etats-Unis, 78035 Versailles,
France
´
Madirel UMR CNRS 7246, Universites Aix-Marseille - CNRS,
Centre de Saint Je´roˆme, 13013 Marseille, France
d
w This article is part of the ChemComm ‘Metal-organic frameworks’
web themed issue.
z Electronic supplementary information (ESI) available: Details of
experiments and molecular simulations. CCDC 889529–889533. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cc34714h
Fig. 1 Illustration of the Zr-MOFs crystalline structure: (a) octahedral
cage, (b) tetrahedral cages. H atoms were omitted for clarity. The large
spheres represent the void regions inside the cages (Zr polyhedra: red for
octahedral cage, green for tetrahedral cages; C, gray; O, red). The extended
organic linkers are also presented.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9831–9833 9831

out and the resulting solids activated and characterized. As a
further step, their thermal, water and mechanical stabilities have
been assessed (see ESIz), revealing that this class of solids is stable
up to 450 1C, in water at room temperature overnight and exhibits
a fair mechanical stability upon application of uniaxial pressure.
In light of such favourable porosities and properties, a step
further consisted of exploring via a predictive approach the
adsorption capacities of this extended series of Zr-MOFs for
CH4 and CO2 under industrially relevant conditions. This
modelling effort is based on a preliminary computational
assisted structure determination strategy conducted in tandem
with X-ray powder diffraction (XRPD) experiments to propose
plausible structure candidates for each investigated Zr-MOF.
Indeed, starting with the parent UiO-66(Zr), the crystallographic
structure for each Zr-MOF analogue was first built using a
ligand replacement strategy9 based on the unit cell parameters
deduced from XRPD experiments. The resulting structures were
further refined using a density functional theory (DFT) geometry
optimization procedure (details of the computational method-
ology are described in the ESIz). The geometry optimized
structures and their corresponding structural features are
provided in Fig. S8 and Table S3 (see ESIz). The theoretical
accessible surface areas and pore volumes vary from 1020 to
Fig. 2 Simulated adsorption isotherms for (a) CH4 and (b) CO2 in the
five Zr-MOFs at 303 K. The experimental data are also provided for
UiO-66(Zr) as a comparison.
MOFs, activated carbons and zeolites. As indicated in this
table, among these Zr-MOFs, UiO-67(Zr) shows the highest
storage capacity for CH4 (146 cm3 (STP) cmꢀ3) at 35 bar. It
should be noted that the target set by the U.S. department of
Energy (DOE) for such an application is 180 cm3 (STP’) cmꢀ3
where STP’ denotes 1 bar and 298 K.11 To match the current
IUPAC definition used in this work, this objective corresponds
to a value of 167 cm3 (STP) cmꢀ3 11a Together with the results
.
shown in Fig. 2, one can argue that the storage capacities of
both UiO-67(Zr) and Zr6-Cl2AzoBDC are only slightly below the
DOE requirement. Further, Table 1 emphasizes that UiO-67(Zr)
outperforms most of the well-known MOFs reported so far except
the Cu(II)-paddlewheel based Cu-BTC, UTSA-20 and PCN-14,
and the CPO-27(Ni) containing Ni(II) coordinatively unsaturated
sites (cus).
4240 m2 gꢀ1 (0.45 cm3 gꢀ1 to 1.82 cm3 ꢀ1) when one goes
g
from UiO-66(Zr) to UiO-68(Zr) and further decrease for
Zr6-AzoBDC (3640 m2 gꢀ1 and 1.38 cm3 gꢀ1) and Zr6-Cl2AzoBDC
(2885 m2 gꢀ1 and 1.13 cm3
g
ꢀ1). Except from the parent
Indeed, while these three last MOFs show the best performances,
their structures are known to be dramatically altered upon water
adsorption3c,12 in contrast to what have been demonstrated
UiO-66(Zr) where a very good agreement is obtained between the
experimental and predicted geometric features, some discrepancy
can be noticed for the other solids (see ESIz) mainly due to the
presence of residual Zr oxide and/or incomplete activation of the
sample. This observation strongly suggests that the optimal adsorp-
tion performances of these Zr-MOFs can be only achieved via a
computational approach. This information will further orientate
the synthesis effort to be deployed for optimizing the activation
procedure of the most promising solids.
Table 1 Adsorption uptakes for CH4 (35 bar)–CO2 (40 bar) and
working capacity predicted in the Zr-MOFs at ambient temperature
vs. the values in other adsorbents including MOFs, zeolites and
activated carbons
a
Vadsorption uptake
(cm3 (STP) cmꢀ3
Vworking capacity
(cm3 (STP) cmꢀ3
)
)
Grand Canonical Monte Carlo simulations (see ESIz) were
performed to predict the optimal adsorption uptakes for CH4
and CO2 in this series of Zr-MOFs for a wide range of pressure at
303 K. For gas uptake and delivery purpose, the total adsorbed
amount is a more relevant quantity compared to the excess one.4b
Further from an application standpoint, it is more meaningful to
express the adsorption uptakes of different materials in terms of
volume than mass as the densities of the adsorbents vary.10 For this
reason, the adsorbed amounts were converted into cm3 (STP) cmꢀ3
(STP defined by IUPAC: 1 bar and 273 K) using the crystallo-
graphic density of each MOF. Primarily, UiO-66(Zr) was selected
as a model material since its experimental and simulated geometric
features concur very well (see Table S1, ESIz).
Materials
T/K CH4
CO2
CH4
CO2
UiO-66(Zr)
UiO-67(Zr)
UiO-68(Zr)
Zr6-AzoBDC
303
303
303
303
127
146
123
124
135
205 105
301 138
333 118
223 122
293 127
116
266
313
209
267
Zr6-Cl2AzoBDC 303
Mesoporous MOFs
MIL-101(Cr)6a
MOF-2003a
MOF-2053a
UMCM-2
303
298
298
303
298
298
103
61
114
128
117
––
340
267
310 111
357 123
281 112
327 ––
64
59
197
263
305
346
264
310
PCN-6814a
NU-1004b
Microporous MOFs
MOF-53a,6b
MOF-1773a,6b
MIL-47(V)14b,e
Cu-BTC14c
298
126
124
145
180
230
214
195
326/293 124
357/332 122
250b 132
312/283
349/328
214
298
303
298
290
298
300
Fig. 2 shows a very good agreement between the adsorption
isotherms simulated for this material and the corresponding
experimental data obtained by gravimetry measurements (see
ESIz). Such a validation step allowed us to confidently carry
out further explorations for the whole series of Zr-MOFs. Fig. 2
thus also reports the absolute adsorption isotherms for CH4
and CO2 in the remaining four Zr-MOFs. Table 1 summarizes
the predicted adsorption capacities at the pressures considered
in practical applications (CH4: 35 bar,5c CO2: 40 bar4b) that are
compared with the performances of other adsorbents including
278 153
200
PCN-145c
––
210
331 160
323c 173
––
CPO-27(Ni)14d
UTSA-2014f
142
226
Other conventional adsorbents
AC Maxsorb13a
Zeolite 13X13b
298
298
74
128
166
64
147
69
185d 112
a
Calculated from the difference of the adsorption amounts adsorbed
in MOFs at 1 and 10 bar for CH4 and at 1 and 40 bar for CO2.
b
c
Measured at 20 bar. Measured at 35 bar. Measured at 32 bar.
d
c
9832 Chem. Commun., 2012, 48, 9831–9833
This journal is The Royal Society of Chemistry 2012

above for UiO-67(Zr). In addition, this Zr-MOF shows a
storage capacity for CH4 which is much higher than other
conventional adsorbents such as the activated carbon (AC)
Maxsorb13a and the zeolite 13X,13b thus UiO-67(Zr) is particularly
promising for such an application.
support of the Beijing Nova (2008B15), ‘‘Hubert Curien Cai
Yuanpei’’ (24038XC) programs and Natural Science Foundation
(21136001) is also greatly appreciated.
Notes and references
The situation slightly differs for the CO2 capture with UiO-68(Zr)
showing the highest adsorption capacity (333 cm3 (STP) cmꢀ3
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9831–9833 9833