Large H2 storage capacity of a new polyhedron-based metal-organic framework with high thermal and hygroscopic stability

Article abstract of DOI:10.1039/b909250a

Two metal-organic frameworks (MOFs) based on metal-organic cuboctahedra were prepared using a rigid C₃ symmetric ligand, where Zn polyhedron-based MOF (PMOF-2(Zn)) did not show any significant gas sorption behavior, whereas the isostructural Cu

Full text of DOI:10.1039/b909250a

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Large H2 storage capacity of a new polyhedron-based metal–organic
framework with high thermal and hygroscopic stabilityw
Seunghee Hong,a Minhak Oh,a Mira Park,a Ji Woong Yoon,b Jong-San Changb
and Myoung Soo Lah*a
Received (in Cambridge, UK) 11th May 2009, Accepted 29th June 2009
First published as an Advance Article on the web 24th July 2009
DOI: 10.1039/b909250a
Two metal–organic frameworks (MOFs) based on metal–
organic cuboctahedra were prepared using a rigid C3 symmetric
ligand, where Zn polyhedron-based MOF (PMOF-2(Zn)) did
not show any significant gas sorption behavior, whereas the
isostructural Cu polyhedron-based MOF (PMOF-2(Cu))
showed a large surface area of B4180 m2 gꢀ1, high hydrothermal
stability, and very promising H2 sorption properties.
MOCs could be covalently linked in triangular arrangement
for a close packed structure (Scheme S1, ESIw).
The solvothermal reaction of Zn(NO3)ꢁ6H2O with
the ligand in N,N0-dimethylformamide (DMF) led to
cube-shaped colorless crystals that could be analyzed by
single crystal X-ray diffraction analysis.z The crystal
structure of [Zn24L8(H2O)12], 1 (PMOF-2(Zn), Fig. 1 and
Fig. S1, ESIw), was isoreticular with the reported structure
prepared using
a C3-symmetric hexacarboxylic ligand,
Development of an on-board hydrogen storage system is one
of the key technologies needed for transportation application
based on fuel cells. Porous metal–organic frameworks (MOFs)
with large surface area have received the attentions of
researchers for their potential as hydrogen storage materials.1
Currently the highest material-based gravimetric and volumetric
excess H2 uptake values reported for MOF-1772 at 77 K are
7.5 wt% and 43.9 g Lꢀ1, respectively. However, these storage
capacities are material-based but not system-level, and the
MOFs are not stable enough at hygroscopic conditions. The
storage capacities and the stabilities of the MOFs still need to
be improved. For high H2 uptakes, the high surface area of the
frameworks is the single factor of utmost importance. The
pore size/geometry and the presence of strongly interacting
functional groups, such as exposed metal sites in high site
density, also play important roles in high H2 uptake.3
5,50,500-[1,3,5-benzenetriyltris(carbonylimino)]tris-1,3-benzenedi-
carboxylic acid.7
The bdc moieties in 1 were involved in the formation of
MOCs with a cavity of B1.2 nm in diameter (Fig. 1 and
Fig. S1a, ESIw), which in turn were interconnected via
quadruple covalent linkages to a cubic close-packing arrangement
(Fig. S2, ESIw). This in turn led to the formation of two
different kinds of supercages, one face-directed edge-linked
superoctahedron (Fig. 1 and Fig. S1b, ESIw) having a cavity
of B2.0 nm in diameter, and another supertetrahedron
(Fig. 1 and Fig. S1c, ESIw) having a cavity of B1.4 nm in
diameter. The cavities of these three cages were interconnected
through three different windows of triangular (B3.7 A in
MOFs based on metal–organic polyhedra (MOP) have been
demonstrated to be very effective for the construction of the
MOFs of high surface areas and large H2 storage capacities.4–6
In this study we synthesized two isostructural MOP-based
MOFs (PMOFs) using
a C3 symmetric hexacarboxylic
ligand, 1,3,5-tris(3,5-dicarboxylphenylethynyl)benzene (H6L),
containing three 3,5-benzenedicarboxylate (bdc) units well
known as a primary building unit for the construction of a
paddle-wheel-based edge-directed corner-linked metal–organic
cuboctahedron (MOC) with potential exposed metal sites.3b–f,7
The introduction of a sterically less demanding but rigid 1,3,5-
triethynylbenzene group in the ligand could lead to the
formation of a highly porous and rigid PMOF, where the
a Department of Chemistry and Applied Chemistry,
Hanyang University, Ansan, Kyunggi-do, 426-791, Korea.
E-mail: mslah@hanyang.ac.kr; Fax: 82 31 436 8100;
Tel: 82 31 400 5496
b Korea Research Institute of Chemical Technology, Daejeon, 305-600,
Korea
Fig.
1 A schematic packing diagram of PMOF-2(Zn), where
w Electronic supplementary information (ESI) available: Experimental
procedure, crystallographic details and X-ray crystallographic files in
CIF format, PXRD, IR spectra, and gas sorption data. CCDC 707996
and 722583. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b909250a
cuboctahedra (red polyhedra) are interconnected to a cubic close
packing arrangement, which leads to the generation of two
different types of supercages, superoctahedra (yellow polyhedra) and
supertetrahedra (blue polyhedra).
ꢂc
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Chem. Commun., 2009, 5397–5399 | 5397

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diameter), square (B6.6 A in diameter) and rectangular shape
(5.5 ꢃ 7.4 A2), respectively (Fig. S3, ESIw). These inter-
connections of the three different types of pores lead to a
PMOF with 3D porosity. Although the MOCs were in the
cubic close packing arrangement in the framework, the total
solvent cavity volume of the framework 1 was extremely large,
59 607 A3 per unit cell (i.e. 75.7% of the total unit cell
volume, where all the solvent, including the ligated solvent
molecules were excluded from the framework for the solvent
cavity volume calculation). The presence of the sterically
non-demanding but rigid 1,3,5-triethynylphenyl group in the
ligand probably renders such a large solvent cavity in the
framework.
temperature, showed significant reduction in peak intensity.
The PXRD pattern of the freshly harvested 2 was also very
similar with that simulated from the single crystal structure of 2.
The sharp PXRD pattern of sample 2a, which was prepared by
soaking 2 in DMF for 2 days, in methanol for 2 days, and then
vacuum-drying overnight at 150 1C, indicated the high thermal
stability of the framework in contrast to that of 1. The
variable-temperature PXRD (VT PXRD) of PMOF-2(Cu)
(Fig. S4, ESIw) also indicated the high thermal stability of
the framework up to 250 1C, and the framework decomposed
around 300 1C. The PXRD pattern of sample 2b, which was
further soaked in water overnight and dried at ambient
temperature, indicated the complete loss of crystallinity.
However, sample 2c, which was further soaked in DMF
overnight, completely recovered its crystallinity when the
sample was reactivated by vacuum-drying at 150 1C overnight.
PMOF-2(Cu) is even stable at hygroscopic conditions and
could be recovered to its original state by the activation.
Because the PXRD of the activated sample 1a indicated
some degree of crystallinity, 1a was subjected to gas
adsorption using N2 and H2 at 77 K. The estimated
Brunauer–Emmett–Teller (BET) surface area from the N2
The copper analogue of 1, [Cu24L8(H2O)24]
(PMOF-2(Cu)), could also be obtained in a crystalline form
under very similar reaction conditions. solvothermal
2
A
reaction of Cu(NO3)2ꢁ3H2O and H6L in DMF in the presence
of a small amount of HCl forms crystalline material 2
(see ESIw). X-Ray diffraction analysis of a single crystal
revealed that the copper analogue is isostructural with
PMOF-2(Zn), the only difference being the replacement of
zinc metal ions in 1 with copper metal ions in 2.z
The similarity of the powder X-ray diffraction (PXRD)
pattern of the freshly harvested bulk sample of 1 with that
simulated from the single-crystal structure of 1 indicated that
the single crystal is representative of the bulk sample (Fig. 2).
1a, which was prepared by soaking 1 in DMF, methylene
chloride, and then vacuum-drying overnight at ambient
sorption isotherm, 72 m2 ꢀ1, and the H2 uptake, 0.2 wt%
g
at 77 K and 1 atm, were much smaller than the plausible values
expected from the solvent cavity size of the single crystal
structure of PMOF-2(Zn) (Fig. S5w). These poor sorption
behaviors might come from inappropriate treatment of
the framework. Despite extensive efforts, we have not yet
succeeded finding the appropriate activation conditions for
gas sorption on 1.
PMOF-2(Cu) was also subjected to gas adsorption using N2
and H2. The N2 sorption isotherm on 2a at 77 K is presented in
Fig. 3. The adsorbed N2 amount was approximately 960 mL gꢀ1
(at STP) and the estimated Langmuir surface area was
B4180 m2 gꢀ1 (estimated BET surface area B3730 m2 gꢀ1),
which was smaller than the highest reported value for
MIL-101 (B5900 m2 gꢀ1 1i or MOF-177 (B5250 m2 gꢀ1 by
)
gravimetric measurement and B5640 m2 gꢀ1 by volumetric
Fig. 2 PXRD patterns of MOFs. (a) A simulated PXRD pattern from
the single-crystal structure of 1, (b) 1, as-synthesized, (c) 1a, wet-ground
in DMF, then vacuum-dried at ambient temperature overnight. (d) A
simulated PXRD pattern from the single-crystal structure of 2, (e) 2,
as-synthesized, (f) 2a, wet-ground in DMA, then vacuum-dried at
150 1C overnight, (g) 2b, soaked in water overnight, then dried at
ambient temperature, (h) 2c, prepared by soaking 2a in water overnight,
in DMA overnight, then vacuum-drying at 150 1C overnight.
Fig. 3 N2 sorption isotherm on 2a at 77 K. (inset) High-pressure H2
sorption isotherms on 2a at 77 K, where blue filled and open circles
represent N2 adsorption and desorption isotherms, respectively, and
green filled triangles and rhomboids represent the total and excess H2
sorption capacities, respectively.
ꢂc
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measurement),2 but was comparable to that of MOF5
(B4400 m2 gꢀ1).8
R1 (wR2)
R1(wR2) = 0.1228 (0.1846) for all 3375 reflections.
y The IR spectrum of 2a (Fig. S7w) indicates the presence of some
carboxylic acid in the activated sample. We could not succeed in removing
this carboxylic acid from the framework even though the sample was
extensively washed and/or soaked using various organic solvents.
= 0.0944 (0.1721) for 2860 reflections [I 4 2s(I)],
The H2 uptake of PMOF-2(Cu) obtained using a volumetric
sorption measurement method was 2.29 wt% at 77 K and
1 atm. The isosteric heat of adsorption of PMOF-2(Cu) was
calculated using a modified version of the Clausius–Clapeyron
equation by fitting a second H2 adsorption isotherm at 87 K
(Fig. S6a and S6bw).9 The initial isosteric heat of adsorption
at 0.02 wt% H2 loading on 2a, 9.2 kJ molꢀ1, decreased to
5.0 kJ molꢀ1 at a loading of 1.46 wt% H2. This range of the
isosteric heat of adsorption was similar to other MOFs having
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A
high-pressure H2 sorption study was performed on 2a using
the volumetric measurement method. The inset of Fig. 3 shows
the excess and total H2 adsorption isotherms at 77 K. The
excess gravimetric H2 adsorption capacity of 2a reached its
maximum value of 5.0 wt% around 30 bar and the total
gravimetric H2 uptake was 7.0 wt% at 50 bar, values that were
smaller than those of MOF-5 with a similar surface area. The
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also smaller than that of MOF-5. This reduced efficiency of
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cavity diameter of the MOF11 and/or the incomplete removal of
the non-reacted or partially reacted reactants in the pores.y
In conclusion, we prepared two isostructural MOFs based
on covalently interconnected metal–organic cuboctahedra.
Although the structural elements resembled one another, the
stability and sorption behaviors of the MOFs were completely
different. The framework of PMOF-2(Zn) was not stable when
the sample was activated. When the solvents in the cavity were
removed, its N2 sorption and the corresponding surface area
were very small, probably because of the collapse of the cavity
caused by the instability of the Zn(II) paddle-wheel secondary
building unit. In contrast, the framework of PMOF-2(Cu) was
stable up to 250 1C. The N2 sorption study of the activated
sample of PMOF-2(Cu) revealed extremely large BET and
Langmuir surface areas, B3730 and B4180 m2 gꢀ1, respectively.
The framework with exposed metal sites was thermally and
hygroscopically stable and showed high adsorption enthalpy.
PMOF-2(Cu) is a new type of MOF having very interesting H2
sorption properties. Further studies on the sorption behaviors
of this MOF are currently in progress.
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This work was supported by KRF (KRF-2008-
313-C00424), KOSEF (R01-2009-0052888) and CBMH.
The KRICT researchers are grateful to ISTK through the
Institutional Research Program (KK-0904-A0) for the
financial support. The authors also acknowledge PAL for
beam line use (2009-1063-06).
Notes and references
z Crystal data for 1: Zn24C288H144O120: M = 7092.91, cubic, space
3
group Fm3m, a = 42.854(5), V = 78 699(16) A , T = 100(2) K,
ꢀ
Z
=
4, m(synchrotron,
l
=
0.77489 A)
=
0.752 mmꢀ1
,
147 398 reflections were collected of which 4347 were unique
(Rint = 0.0576). R1 (wR2) = 0.0653 (0.2344) for 3890 reflections
[I 4 2s(I)], R1 (wR2) = 0.0699 (0.2434) for all 4347 reflections.
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2005, 44, 4670–4679; (b) J. L. Belof, A. C. Stem, M. Eddaoudi and
B. Space, J. Am. Chem. Soc., 2007, 129, 15202–15210; (c) M. Dinca
and J. R. Long, J. Am. Chem. Soc., 2005, 127, 9376–9377;
(d) S. Ma, J. Eckert, P. M. Forster, J. W. Yoon, Y. K. Hwang,
J.-S. Chang, C. D. Collier, J. B. Parise and H.-C. Zhou, J. Am.
Chem. Soc., 2008, 130, 15896–15902.
Crystal data for 2: Cu24C288H144O120: M = 7048.99, cubic, space
3
group Fm3m, a = 42.833(3) A, V = 78 583(10) A , T = 173(2) K,
ꢀ
Z
=
4, m(Mo-Ka,
l = 0.71073 A) = , 95 044
0.670 mmꢀ1
reflections were collected of which 3375 were unique (Rint = 0.1675).
ꢂc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5397–5399 | 5399