A microporous copper metal-organic framework with high H2 and CO2 adsorption capacity at ambient pressure

Article abstract of DOI:10.1002/anie.201102329

Fully accessible: Uptakes of 9.2 mmol g^-1 (40.5 wt %) for CO₂ at 273 K/0.1 MPa and 15.23 mmol g^-1 (3.07 wt %) for H₂ at 77 K/0.1 MPa are among the highest reported for metal-organic frameworks (MOFs) and are fou

Full text of DOI:10.1002/anie.201102329

Communications
DOI: 10.1002/anie.201102329
Microporous MOFs
A Microporous Copper Metal–Organic Framework with High H2 and
CO2 Adsorption Capacity at Ambient Pressure**
Daniel Lꢀssig, Jçrg Lincke, Jens Moellmer, Christian Reichenbach, Andreas Moeller,
Roger Glꢀser, Grit Kalies, Katie A. Cychosz, Matthias Thommes, Reiner Staudt, and
Harald Krautscheid*
Metal–organic frameworks (MOFs) as highly porous materi-
als have gained increasing interest because of their distinct
adsorption properties.[1–3] They exhibit a high potential for
applications in gas separation and storage,[4] as sensors[5] as
well as in heterogeneous catalysis.[6] In the last few years, the
H2 storage capacity of MOFs has been considerably
increased. Mesoporous MOFs show high adsorption capaci-
ties for CH4, CO2, and H2 at high pressures.[2,3,7–10] To increase
the uptake of H2 and CO2 by physisorption at ambient
pressure, adsorbents with small micropores as well as high
specific surface areas and micropore volumes are
required.[11,12] Such microporous materials seem to be more
appropriate for gas-mixture separation by physisorption than
mesoporous materials. For gas separation in MOFs the
interactions between the fluid adsorptive and “open metal
sites” (coordinatively unsaturated binding sites) or the ligands
are regarded as important.[13] Industrial processes, such as
natural-gas purification or biogas upgrading, can be improved
with those materials during a vapor-pressure swing adsorption
cycle (VPSA cycle) or a temperature swing adsorption cycle
(TSA cycle).[14] The microporous MOF series CPO-27-M
(M = Mg, Co, Ni, Zn), for example, shows very high CO2
uptakes at low pressures (< 0.1 MPa).[15,16] Concerning H2
adsorption, the microporous MOF PCN-12 offers with
3.05 wt% the highest uptake at ambient pressure and 77 K
reported to date.[17]
Herein, we present a novel microporous copper-based
MOF 13[Cu(Me-4py-trz-ia)] (1; Me-4py-trz-ia2ꢀ = 5-(3-
methyl-5-(pyridin-4-yl)-4H-1,2,4-triazol-4-yl)isophthalate)
with extraordinarily high CO2 and H2 uptakes at ambient
pressure, the H2 uptake being similar to that in PCN-12. The
ligand Me-4py-trz-ia2ꢀ (Figure 1a), which can be obtained
from cheap starting materials by a three-step synthesis in
good yield, combines carboxylate, triazole, and pyridine
functions and is adopted from a recently presented series of
linkers,[18] for which up to now only a few coordination
polymers are known.[19–22]
[*] D. Lꢀssig, J. Lincke, Prof. Dr. R. Glꢀser, Prof. Dr. H. Krautscheid
Universitꢀt Leipzig, Fak. fꢁr Chemie und Mineralogie
Johannisallee 29, 04103 Leipzig (Germany)
Single crystals of 1 that are suitable for X-ray crystal
structure analysis were prepared by diffusion of copper
sulfate and H2(Me-4py-trz-ia). Larger quantities of micro-
crystalline 1 are obtained not only by solvothermal synthesis,
but also in multigram scale by simple reflux of the starting
materials in water/acetonitrile (see Supporting Information).
According to the single crystal X-ray structure analysis, 1
crystallizes in the monoclinic space group P21/c (no. 14) with
four formula units per unit cell. The asymmetric unit contains
one linker anion and two crystallographically independent
Cu2+ ions residing on inversion centers. The copper ion Cu1 is
coordinated in a square-planar fashion by two monodentate
carboxylate and two pyridine functions in trans position
leaving two accessible open metal sites per Cu1 atom
(Figure 1b), whereas the second copper ion Cu2 is coordi-
nated by monodentate triazole and chelating carboxylate
groups forming a distorted octahedron (Figure 1c). For this
reason, both copper ions represent planar fourfold nodal
points and the ligands act as tetradentate linkers in a 3D
network with pts topology[23] and a 3D pore system (Fig-
ure 1d). With narrow channels of about 250 ꢀ 600 pm in
crystallographic c direction connecting the micropores with a
diameter of approximately 550 pm, the structure has a
calculated porosity of about 55% according to PLATON.[24]
Powder X-ray diffraction (PXRD) studies on the as-
synthesized microcrystalline sample 1a confirm both, the
agreement with the simulated powder pattern of 1 based on
E-mail: krautscheid@rz.uni-leipzig.de
J. Moellmer, Dr. A. Moeller
Institut fꢁr Nichtklassische Chemie e.V.
Permoserstrasse 15, 04318 Leipzig (Germany)
C. Reichenbach, Dr. G. Kalies
Universitꢀt Leipzig, Fak. fꢁr Physik u. Geowissenschaften
Linnꢂstrasse 5, 04103 Leipzig (Germany)
Dr. K. A. Cychosz, Dr. M. Thommes
Quantachrome Instruments
1900 Corporate Drive, Boynton Beach, FL 33426 (USA)
Prof. Dr. R. Staudt
Hochschule Offenburg
Fak. fꢁr Maschinenbau und Verfahrenstechnik
Badstrasse 24, 77652 Offenburg (Germany)
[**] We gratefully acknowledge financial support by Deutsche For-
schungsgemeinschaft (DFG SPP 1362-Porçse metallorganische
Gerꢁstverbindungen, STA 428/17-1, KR 1675/7-1, GL 290/6-1; and
KA 1560/3-2, 4-2), the University of Leipzig (PbF-1) and the
graduate school BuildMoNa is gratefully acknowledged. D.L. thanks
the Fonds der Chemischen Industrie for a fellowship, J.L. acknowl-
edges the ESF fellowship. We thank F. Kettner for DTA/TG-MS
analyses, Riaz Ahmad and Dr. Charlie Thibault (Quantachrome
Instruments, US) for help with the N2 (77 K), Ar (87 K) and H2 (77,
87 and 97 K) sorption experiments on sample 1d, and Dr. Angela
Puls (Rubotherm GmbH, Bochum (Germany)) for measuring the N2
(77 K) adsorption isotherms on sample 1a.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102329.
10344
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Angew. Chem. Int. Ed. 2011, 50, 10344 –10348

Figure 2. X-Ray powder patterns (Cu-Ka1) of microcrystalline samples
of 1: as-synthesized sample 1a, samples 1b–1d after postsynthetic
treatment and sample 1e after resolvation with methanol.
the activated phases 1b, 1c, and 1d show unit cell volumes of
87%, 70%, and 72%, respectively. This leads to the
conclusion that 1d has to expand during the adsorption
process to allow the observed high uptakes reported herein.
According to temperature-dependent X-ray powder dif-
fraction studies, the as-synthesized sample 1a is thermally
stable up to 1408C. Soxhleth extraction with methanol
significantly increases the stability up to 2508C (1d), also
shown by differential thermal analysis/thermogravimetry-
mass spectrometry (DTA/TG-MS) studies. Although 1d can
be activated at 708C for adsorption measurements, an
activation under vacuum at room temperature is sufficient
to reach the maximum capacity. In case of 1a the pores are
partially blocked, resulting in only poor adsorption capacities
(Figures S2 and S7). Hence for detailed adsorption studies, 1d
was used after activation at 258C for 48 h in vacuum.
N2 (77.4 K) and Ar (87.3 K) isotherms are shown in
Figure 3; Figure 3a shows the isotherms on a linear scale, and
the type I N2 isotherm (IUPAC classification[26]) indicates that
as expected only micropores are present. An equivalent BET
surface area of 1473 m2 gꢀ1 was obtained from the N2 (77 K)
isotherm (linear BET range p/p0: 0.004 to 0.03). The total
specific pore volume of 0.586 cm3 gꢀ1 determined from nitro-
gen adsorption (Table 1) is in perfect agreement with the
specific pore volume of 0.59 cm3 gꢀ1 calculated from crystal
structure data.[24] From the Ar and CO2 adsorption on 1d, a
comparable total pore volume is determined (Table 1)
indicating that the whole pore system of the MOF is open
for these adsorptives at the respective conditions.
Figure 1. a) The ligand [Me-4py-trz-ia]2ꢀ, b) and c) coordination
spheres of both crystallographically independent Cu2+ ions in 1,
d) schematic drawing of the platinum sulfide topology (pts) of 1 (blue:
square-planar Cu2+ nodal point; yellow: tetrahedral Me-4py-trz-ia2ꢀ
nodal point) and e) 3D crystal structure of 1 along [001]. Symmetry
codes: a: 1ꢀx, ꢀ0.5+y, 0.5ꢀz; b: x, 0.5ꢀy, 0.5+z; c: 1ꢀx, ꢀy, 1ꢀz;
d: x, 0.5ꢀy, ꢀ0.5+z; e: ꢀx, ꢀ0.5+y, 0.5ꢀz; f: ꢀx, ꢀy, ꢀz.
single crystal data and the phase purity of 1 obtained by
different synthesis methods (Supporting Information, Fig-
ure S1).
Depending on the postsynthetic treatment, 1 represents a
flexible network undergoing several phase transitions upon
removal of and exposure to guest molecules. As shown in
Figure 2, a treatment under vacuum at 258C (1b) and 708C
(1c) leads to powder patterns different from that of the as-
synthesized sample 1a. A further phase transition is observed
for a Soxhleth-extracted sample (MeOH) activated under
vacuum at 258C (1d). However, if the dehydrated samples
1b–1d are exposed to water or methanol, the powder pattern
of 1a is recovered (Figure 2). This indicates that the frame-
work of 1 undergoes reversible crystal-to-crystal structural
transformations.[25] The unit cell volumes determined from
the powder patterns reveal shrinking upon activation
(Table S1). In comparison to the as-synthesized 1a phase,
Figure 3b shows the N2 and Ar adsorption data in a
semilogarithmic plot, revealing details of the low-pressure
regions of these isotherms, which are associated with the
filling of micropores. The significant differences in the pore
filling pressures between the Ar (87 K) and N2 (77 K)
sorption isotherms is interpreted as specific interactions
between the quadrupolar N2 molecules and the polar groups
in the narrow pore channels of the MOF framework.[27] The
relative pressure region where Ar (87 K) fills the pores of this
copper MOF is for instance very similar to the relative
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Communications
As already indicated, the Ar (87 K) isotherms reveal a
sharp step at relative pressures of 0.05–0.06, which is
accompanied by a narrow, but intrinsic hysteresis loop (see
Figure 3b and inset). The hysteretic adsorption behavior in
that pressure range indicates a plausible structural trans-
formation of the non-rigid material.[20,21,25,29] This transition
appears also in the N2 (77 K) isotherm, however it is
reversible and smeared-out over a wider relative pressure
range from 0.005–0.1.
CO2 adsorption on 1d was studied at 273 K, 298 K, and
323 K and used to calculate the isosteric heat of adsorption
(Figures S8 and S9). All CO2 isotherms are of type I according
to the IUPAC classification.[26] The CO2 uptake of 1d was
determined gravimetrically: 9.2 mmolgꢀ1 (40.5 wt%) at
273 K/0.1 MPa and 6.1 mmolgꢀ1 (26.8 wt%) at 298 K/
0.1 MPa. Comparably high uptakes have been found so far
only for very few microporous MOFs, for example, for the
CPO-27-M series (M = Mg, Co, Ni, Zn).[15,31] For Ni- and Mg-
containing CPO-27-MOFs isosteric heats of 38–42 kJmolꢀ1
were measured calorimetrically showing the high interaction
energy between the quadrupolar CO2 molecules and the open
metal sites of the MOFs.[15] For the adsorption of CO2 on 1d, a
constant isosteric heat of about 30 kJmolꢀ1 was calculated
(see Supporting Information, Section 3.2.3), a similar value to
that of HKUST-1 with 29.2 kJmolꢀ1.[32]
To determine the hydrogen adsorption capacity, H2
adsorption on 1d (Figure 4) was measured at 77 K in the
low pressure region (up to 0.1 MPa) using an automatic
volumetric adsorption apparatus and in the high pressure
region using a magnetic suspension balance. At 0.1 MPa/77 K,
a remarkably high H2 uptake of 15.2 mmolgꢀ1 (3.07 wt%) of
1d was determined, emphasizing the extraordinary adsorp-
tion potential of 1d for H2.
Figure 3. a) Nitrogen (77 K, squares and circles) and argon (87 K,
triangles) adsorption isotherms on 1d (closed symbols=adsorption,
open symbols=desorption; (I) measurements performed by Quanta-
chrome Instruments Lab, US, (II) measurements performed by Univer-
sity of Leipzig, Germany). b) Semilogarithmic plot of nitrogen (circles)
and argon (diamonds) adsorption isotherms (data from (a)) on 1d at
77 K and 87 K (closed symbols=adsorption, open symbols=desorp-
tion). Inset: hysteresis loop of argon adsorption isotherm at 87 K.
Additionally we obtained a maximum uptake of about
4.1 wt% at 3 MPa. This uptake does only roughly correlate to
the established relation of 1 wt% H2/500 m2 gꢀ1 BET surface
Table 1: BETspecific surface area and pore volume of 1d calculated from
adsorption isotherms at given relative pressures by means of the Gurvich
rule.[30]
Adsorptive/Temperature SBET [m2 gꢀ1
]
p/p0
VPore [cmꢀ3 gꢀ1
0.586[b]
]
N2/77 K
Ar/87 K
1473
1429[a]
–
0.95
0.04[a]/0.95[b] 0.485[a]/0.574[b]
CO2/273 K
0.85
0.597[b]
[a] Before structural transformation. [b] After structural transformation.
pressure range where argon fills the narrow pores of a ZSM-5
zeolite.[28] Hence the effective degree of confinement for the
adsorbate is comparable to the situation in ZSM-5, and
accordingly the pore size distribution obtained by applying a
NLDFTAr (87.5 K) zeolite kernel based on a cylindrical pore
model results in a narrow pore size distribution centered
around 0.55 nm (Figure S4).
Figure 4. Hydrogen adsorption isotherm on 1d (triangles: excess
adsorption, using magnetic suspension balance (INC Leipzig),
squares: excess adsorption, using automatic volumetric adsorption
apparatus (Quantachrome) at 77 K (inset: excess hydrogen adsorption
up to 0.2 MPa).
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Angew. Chem. Int. Ed. 2011, 50, 10344 –10348

area[10] and leads to the conclusion that the surface area
determined from the N2 adsorption isotherm (77 K) is under-
estimated, because the nitrogen molecules cannot occupy the
inner surface on both sides of the micropore. Based on the H2
isotherm, we therefore assume a potentially higher surface
area.
this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
Received: April 4, 2011
Revised: July 7, 2011
Published online: September 16, 2011
The adsorption potential is confirmed by an isosteric heat
of about 6.5 kJmolꢀ1 calculated from the adsorption iso-
therms at 77 K, 87 K, and 97 K (Figure S6). The constancy of
the heat of adsorption is indicative of the homogeneity of the
H2 adsorption sites.[27] Therefore the strong interaction
between the hydrogen molecules and the framework of 1d
is mainly attributed to the narrow micropores and the polar
framework. To our knowledge, no MOF material has been
reported so far with a higher H2 uptake (15.2 mmolgꢀ1, 3.07
wt%) at ambient pressure at 77 K (Table 2).
Keywords: adsorption · coordination polymers · copper · metal–
organic frameworks · microporous materials
.
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PCN-12-Si[12]
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