Angewandte
Chemie
DOI: 10.1002/anie.200806203
Metal–Organic Frameworks
Improving the Hydrogen-Adsorption Properties of a Hydroxy-
Modified MIL-53(Al) Structural Analogue by Lithium Doping**
Dieter Himsl, Dirk Wallacher, and Martin Hartmann*
[
6]
To realize an energy-efficient and sustainable transportation
system, hydrogen as a fuel in combination with fuel cell
electric vehicles (FCEV) is one of the most promising energy
not suitable for our purpose. Therefore, our first task was
the synthesis of a MOF which fulfils the above-mentioned
requirements. It should be noted that it is not straightforward
to synthesize a hydroxy-modified MOF by using a hydroxy-
modified linker because the hydroxy groups are often found
[
1a]
carriers. Using solid-state materials as hydrogen carriers
could provide a higher hydrogen density than the current
state-of-the art automotive systems employing compressed
to be coordinated to metal centers after synthesis because of
their nucleophilic properties.
showed that it is possible to synthesize isoreticular forms of
MIL-53(Fe), MIL-88B(Fe), and MIL-101(Fe) carrying a free
amino group at the linker 2-aminoterephthalic acid. The
synthesis conditions were determined by high-throughput
[1]
[7,5d–g]
hydrogen at 70 MPa. Within this field of research, porous
metal–organic frameworks (MOFs) have been shown to be
very suitable candidates for future applications in hydrogen-
Recently, Bauer et al.
[2]
storage technology. Recently, a number of theoretical works
dealing with this topic proposed that doping MOFs with
lithium significantly enhances the hydrogen-adsorption prop-
[8–11]
methods.
Since an amino group has nucleophilic proper-
[
3]
erties of a given material. However, in the majority of the
calculations, doping with lithium atoms or cations which are
situated above the usually planar core of the linker is
ties similar to those of the hydroxy group, we assumed that the
synthesis of a hydroxy-modified MIL-53(Al) analogue is
feasible by employing 2-hydroxyterephthalic acid as the
linker.
[
3b–h]
investigated.
This situation is hard to realize experimen-
tally because there is no a clear relation, in terms of molecular
fragments, to the chemical structure of the linker. Therefore,
it is not surprising that only a few examples can be found for
Typically, MIL-53(Al) is synthesized under hydrothermal
conditions by treating terephthalic acid with aluminum
nitrate at 2208C for 72 h in an autoclave. Our first attempt
to synthesize the hydroxy-functionalized MIL-53(Al) was to
simply substitute terephthalic acid by 2-hydroxyterephthalic
[
4]
the reactivity of MOFs with elemental lithium. An exper-
imentally more feasible suggestion was made by Klontzas
et al. who proposed to establish lithium-doping of MOFs via
[8a]
acid (H BDC-OH) in the standard synthesis procedure.
2
[3a]
addition of lithium alkoxide groups to the organic linker.
But unfortunately we were not able to find suitable hydro-
thermal conditions for this case (Figure S1, see supporting
information for details). Thus, we developed a novel synthesis
route to hydroxy-modified MIL-53(Al) based on the ambient
pressure route for Cu (BTC) (BTC = 1,3,5-benzenetricarb-
Since lithium alkoxide groups are well-known functional
groups in organic chemistry, this particular modification
appears to be the most practicable from our point of view.
Our approach to establish lithium-doping in the above
mentioned manner involves the treatment of a MOF which
incorporates pendant hydroxy groups (that is, hydroxy groups
which are not coordinating), and exhibits permanent porosity
with a suitable lithium base. But among the MOFs that fulfill
3
2
[
12]
oxylate) from our previous work. Owing to the very low
solubility of H BDC-OH in pure water even under reflux, a
2
1:1 mixture of N,N’-dimethylformamide (DMF) and water
was used. Pure DMF is not suitable because the MIL-53(Al)
network incorporates bridging hydroxy ions that link the
[
5]
the first requirement, there are only two examples for
materials that were sufficiently characterized to be named a
AlO octahedra into infinite chains. Thus, water must be
6
[5a–c]
microporous coordination polymer.
However, they were
present in the synthesis mixture to provide the hydroxy ions
for the formation of the MIL-53 framework. This approach
resulted in compound 1, exhibiting almost the same powder
X-ray diffraction (XRD) pattern as the DMF form of MIL-53
discussed in ref. [8b] (Figure S2, Supporting Information)
showing that 1 has a framework analogous to the MIL-53
structure. To remove impurities and to exchange the DMF
guests for a solvent which is easier to remove (Figure S3,
Supporting Information) a soxhlet extraction with ethanol
was performed. The resulting compound, 2 (MIL-53(Al)-
OH), shows exactly the same powder XRD pattern as the
low-temperature form (lt form) of MIL-53(Al) (Figure 1;
after degassing and rehydration).
[*] D. Himsl, Prof. Dr. M. Hartmann
Institut fꢀr Physik, Advanced Materials Science
Universitꢁt Augsburg
Universitꢁtsstrasse 1, 86159 Augsburg (Germany)
Fax: (+49)821-598-3227
E-mail: martin.hartmann@physik.uni-augsburg.de
Dr. D. Wallacher
Helmholtz Zentrum fꢀr Materialien und Energie GmbH
Glienicker Strasse 100, 14109 Berlin (Germany)
[
**] This project was supported by General Motors Fuel Cell Activities in
addition to the basic funding provided by the Universitꢁt Augsburg.
The authors are very grateful to Dr. Ulrich Eberle (GM Fuel Cell
Activities) for interesting discussions and perspectives on hydrogen
and fuel cell technologies for automotive applications.
Compound 2 shows significant microporosity as evident
from the nitrogen-adsorption isotherm (Figure 2) yielding
2
À1
2
À1
specific surface areas of 1566 m g and 1631 m g according
to the BET- and Langmuir model, respectively. Thus, a
Angew. Chem. Int. Ed. 2009, 48, 4639 –4642
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4639