Methyl modified MOF-5: A water stable hydrogen storage material

Article abstract of DOI:10.1039/c1cc11054c

Water stable methyl modified MOF-5s have been synthesized via a solvothermal route. Methyl- and 2,5-dimethyl-modified MOF-5s show the same topology and hydrogen uptake capability as that of MOF-5. The H₂ uptake capacity of MOF-5, however, drops

Full text of DOI:10.1039/c1cc11054c

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COMMUNICATION
Methyl modified MOF-5: a water stable hydrogen storage materialw
a
b
b
a
Jie Yang, Anna Grzech, Fokko M. Mulder and Theo J. Dingemans*
Received 22nd February 2011, Accepted 18th March 2011
DOI: 10.1039/c1cc11054c
Water stable methyl modified MOF-5s have been synthesized
via a solvothermal route. Methyl- and 2,5-dimethyl-modified
MOF-5s show the same topology and hydrogen uptake capability as
Structural decomposition, caused by the presence of water,
always leads to poor reproducibility and a decrease in gas
sorption capacities in MOFs. Discrepancies in hydrogen
uptake capacities have been found for MOF-5 and are thought
that of MOF-5. The H uptake capacity of MOF-5, however, drops
2
9
rapidly when exposed to the ambient air, whereas the H
2
uptake
to be due to the presence of water. High quality MOF-5 can
capacities of the methyl modified MOF-5s remain stable for 4 days.
only be obtained when the exposure to water and ambient air
9
was minimized. Much care must be taken to avoid the
Hydrogen, which has an exceptional energy density and little or
no harmful emissions, is currently under consideration as a
1
future energy carrier. However, the emergence of a hydrogen
structural collapse of MOFs, since it will limit their potential
commercial application. Therefore it is desirable to enhance
the structural stability of MOFs towards moisture, but at the
economy requires the development of new materials capable of
2
safely storing hydrogen in a compact and light weight fashion.
2
same time, the excellent H uptake capability of MOFs should
remain unaffected. Very recently, researchers have demonstrated
that it is possible to build water-stable MOF structures by
introducing hydrophobic functional groups into the frame-
Metal–organic frameworks (MOFs) are crystalline materials
consisting of metal ions, or metal ion clusters and organic
ligands, which are linked together to form extended porous
frameworks. Over the last decade, MOFs have attracted
considerable interest because of their large surface area,
adjustable pore size and controllable properties, as well as
their unusual thermal stability. These properties make them
interesting candidates for a wide range of applications, including
1
1,12
work structures.
Yet, no results have been published on
the hydrogen storage capability of such functionalized MOFs.
In this communication, the effect of moisture on the hydrogen
3 3
storage capability of methyl (CH ) and dimethyl (DiCH )
modified MOF-5 has been investigated. The obtained results
suggest that by simply introducing methyl groups the hydrogen
uptake capacity of MOF-5 remains uncompromised and
less sensitive to the presence of moisture, even after 4 days
exposure to air with a relative humidity of 32–37%.
3–6
gas sorption, catalysis, drug delivery and sensing. MOFs con-
taining Zn, Mn, Cr, and Cu show outstanding hydrogen adsorp-
tion behavior at 77 K and are among the most promising
7
hydrogen storage materials. Maybe the best known MOF to
The methyl and 2,5-dimethyl MOF-5 analogs were synthesized
using a well-known solvothermal route. The samples were
date is MOF-5. This three-dimensional network has a crystal
6+
structure where metal clusters [Zn O] are joined to an octa-
4
activated by immersing them in dried CHCl
heating at 160 1C in vacuum for 24 h. The samples thus
obtained were denoted as CH MOF-5 and DiCH MOF-5.
MOF-5 was synthesized for reference purposes according to a
3
followed by
hedral array of benzene-1,4-dicarboxylate (BDC) groups to form a
8
porous cubic Zn O(BDC) framework. MOF-5 displays an excess
4
3
3
3
H uptake capacity of 7.1 wt% at 77 K and 40 bar. At 100 bar, the
2
1
3
total hydrogen uptake of MOF-5 can reach up to 10 wt%,
À1 9
literature procedure. The samples exposed to ambient air for
4 days were labelled CH MOF-5-4d and DiCH MOF-5-4d.
The hydrogen uptake capacities of MOF-5, CH MOF-5 and
DiCH MOF-5 were measured on a homemade Sieverts setup.
corresponding to a record volumetric storage density of 66 g L
.
3
3
However, most MOFs, in particular zinc-based MOFs,
are moisture-sensitive because the relative weak metal–oxygen
coordination allows for attack by water molecules, resulting in
3
3
The elemental analysis of all samples and their hydrogen
uptake capacities are summarized in Table 1.
10
the phase transformation and decomposition of the framework.
The methyl modified MOF-5s show the same topology as
that of MOF-5, which is supported by the high degree of
correspondence between the PXRD patterns of methyl modified
MOF-5 and MOF-5 (Fig. S1, ESIw). MOF-5 samples were
exposed to ambient air and the structural stability under
ambient conditions was examined by PXRD. With respect
to MOF-5, our findings are in agreement with results reported
in the literature. When exposed to ambient air, with a relative
humidity of 32–37%, an extra peak at 2y = 8.81 appears after
only 45 min. In addition, the intensity of the original peaks
decreases dramatically with exposure time and after 1 day,
a
b
Faculty of Aerospace Engineering, Delft University of Technology,
Kluyverweg 1, Delft, The Netherlands.
E-mail: t.j.dingemans@tudelft.nl; Fax: +31 (0)152784472;
Tel: +31 (0)152784520
Department of Radiation, Radionuclides and Reactors,
Faculty of Applied Sciences, Delft University of Technology,
Mekelweg 15, 2629 JB Delft, The Netherlands.
E-mail: f.m.mulder@tudelft.nl; Fax: +31 (0)152783803;
Tel: +31 (0)152784870
w Electronic supplementary information (ESI) available: Full synthetic
procedure and characterization data including PXRD patterns and
hydrogen isotherms. See DOI: 10.1039/c1cc11054c
5
244 Chem. Commun., 2011, 47, 5244–5246
This journal is c The Royal Society of Chemistry 2011

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Table 1 Elemental analysis results (calculated values in parentheses) and hydrogen uptake capacities
2
À1
3
À1
a
Samples
MOF-5
C (%)
H (%)
SSABET/m g
V
p total/cm g
2
H uptake (wt%)
37.6 (37.5)
40.2 (40.0)
38.0
1.65 (1.56)
2.24 (2.22)
2.47
2750
2537
—
1927
—
1.15
1.03
—
0.82
—
1.44
1.42
0.97
1.29
1.25
CH
CH
3
MOF-5
b
b
3
MOF-5-4d
MOF-5
MOF-5-4d
DiCH
DiCH
3
41.0 (42.2)
41.0
2.89 (2.81)
2.93
b
b
3
a
b
7 K and 1 bar. Values are for reactivated samples.
7
the structure of MOF-5 completely transformed to ZnBDCÁ
9
,14
XH
2
O (Fig. S2, ESIw).
of CH MOF-5 remains unchanged even after 4 days exposure
Interestingly, the PXRD pattern
3
to air (CH MOF-5-4d). Similar results were obtained for
3
DiCH MOF-5, as shown in Fig. 1. Clearly, the structures of
3
CH MOF-5 and DiCH MOF-5 remain stable up to 4 days
3
3
1
1,12
after exposure to ambient air.
These results strongly imply
that introducing hydrophobic methyl groups enhances the
structural stability of MOF-5 type framework.
3 3
TGA curves of CH MOF-5 and DiCH MOF-5 both exhibit
weight loss events starting at B400 1C, which is due to
the decomposition of the organic linkers (Fig. 2). Compared
to the decomposition temperature of unsubstituted MOF-5, the
3
Fig. 2 TGA curves of (Di)CH MOF-5 before and after exposure to
3 3
decomposition temperatures for CH MOF-5 and DiCH MOF-5
ambient air. MOF-5 is included for reference purposes.
are decreased by B50 1C, suggesting that the thermal stability
of MOF-5 is reduced somewhat by introducing methyl (CH3)
groups on the terephthalic acid linker. A marginal difference in
functionalities increases (DiCH MOF-5-4d vs. CH MOF-5-4d)
3
3
decomposition behavior was observed between CH MOF-5
3
the water uptake decreases and stays well below 4 wt%. Therefore,
introducing hydrophobic CH groups is an effective approach
and DiCH
3
MOF-5, implying that the thermal stability of the
3
MOF-5 framework is insensitive to doubling the concentration
of methyl groups present in the framework.
towards the design of water-stable metal–organic frameworks.
It was demonstrated by us that methyl modified MOF-5,
very much like MOF-5, can load hydrogen within several
Compared with fresh samples, a weight loss of B3% was
observed for CH
3
MOF-5-4d at about 100 1C (inset Fig. 2), which
MOF-5-4d, on the other
minutes. CH
capacity as that observed for MOF-5 in the studied pressure
range (Fig. 3). The hydrogen uptake capacities of CH MOF-5
3
MOF-5 shows almost the same hydrogen uptake
is due to the removal of water. DiCH
3
hand, shows a loss of water of B0.25%, which is the result of
the more hydrophobic character of this MOF. Both MOFs
show some outgassing above 100 1C, which appears to be
diethylformamide (DEF). Both outgassing of water and DEF
was confirmed by mass spectroscopy. According to molecular
dynamics simulation results on the interaction of water and
MOF-5, MOF-5 is only stable at a very low water content but
3
and MOF-5 are 1.42 wt% and 1.44 wt% at 1 bar and 77 K,
respectively. DiCH MOF-5 demonstrates a lower hydrogen
3
uptake capacity of 1.29 wt% at 1 bar and 77 K. Previous
studies have shown that chemical modifications of the organic
linker have only little impact on the hydrogen uptake capacity
1
5
of MOF-5. The same seems to be true for our CH
3
MOF-5
10e
unstable when exposed to more than 4 wt% water. These results
are in line with our findings: when the concentration of CH
compound. The methyl group does not affect the hydrogen
3
Fig. 1 PXRD patterns of (Di)CH
3
MOF-5 before and after exposure
3
Fig. 3 Hydrogen uptake capacities of (Di)CH MOF-5 before and
to ambient air.
after 4 days exposure to ambient air.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5244–5246 5245

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uptake capacity of MOF-5, even though it was claimed that
the presence of a methyl group could have a positive effect
are readily available, this opens a route towards commercially
available moisture-stable MOFs.
1
6
on the hydrogen uptake capability of other MOFs. It is
noteworthy, however, that the decrease in hydrogen uptake of
We gratefully acknowledge financial support from the
NWO ACTS Sustainable Hydrogen Programme.
DiCH MOF-5 compared to MOF-5 is relatively less than the
3
lowering of surface area and pore volume.
Notes and references
Of significant interest is the hydrogen storage uptake
capability of the methyl and dimethyl modified MOF-5 after
exposure to ambient air. It is well documented that MOF-5
completely loses its hydrogen storage capability after exposure
to ambient air due to a total collapse of the framework
structure within 1 day (Fig. 3). The hydrogen uptake capacities
of the methyl modified MOF-5s, on the other hand, can be
recovered after exposure to ambient air with a relative humidity
1
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3
1
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capacity.
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Table 1, the H content in reactivated CH MOF-5-4d is higher
3
than that of a fresh sample, which is due to the absorbed water
and corresponds to a water content of about 3.2 wt%. This
CH MOF-5, with strongly adsorbed water, shows a lower H
3
2
uptake capacity as compared to a fresh sample. Although the
structure of CH MOF-5 remains stable at low water concen-
trations, distortions in the framework structure might occur as
(
3
´
1
0e
´
reported in a simulated study for MOF-5. Even 0.6 wt% of
water in the structure may lead to the distortion of the ZnO
4
8
9
tetrahedron and this will result in a reduced hydrogen uptake
capability. Introducing a second methyl group, as is the case
for DiCH MOF-5, the water uptake is reduced even further,
3
1
which can be concluded from the constant C/H ratio found for
DiCH MOF-5 and DiCH MOF-5-4d. This result confirms
¨
sgens,
3
3
¨
that incorporating hydrophobic methyl groups can indeed
improve the water stability of MOF-5 without seriously
compromising the hydrogen uptake capability. Even after a
total of 8 days exposure to ambient air, the original structures
(
of CH
to the PXRD patterns (see Fig. S3, ESIw). About 65% of the
hydrogen uptake capacities of CH MOF-5 and DiCH MOF-5
3 3
MOF-5 and DiCH MOF-5 are still maintained according
1
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3
3
could be retained (Fig. S4, ESIw). This exciting result further
demonstrates the potential application of methyl modified
MOF-5 as a hydrogen storage material.
1
1
1
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In summary, the crystal structure of MOF-5 is very sensitive
to the presence of (ambient) water and the hydrogen uptake
capability becomes compromised. Herein we have demonstrated
that by simply introducing one or two hydrophobic methyl
1
(
CH
significantly less sensitive to water without impairing the
hydrogen uptake capacity. We have shown that the H uptake
3
) functionalities on the BDC moiety, the structure becomes
1
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capacities of methyl modified MOF-5s can recover after 4 days
exposure to ambient air and even after 8 days exposure the
hydrogen uptake capacity was only reduced by 35%. Since
2
-methylterephthalic acid and 2,5-dimethylterephthalic acid
5
246 Chem. Commun., 2011, 47, 5244–5246
This journal is c The Royal Society of Chemistry 2011