Hydrogen Storage in a Prototypical Zeolitic Imidazolate Framework-8
Hui Wu,†,‡Wei Zhou,†,§and Taner Yildirim*,†,§
NIST Center for Neutron Research, Gaithersburg, Maryland 20899-8562, Department of Materials Science and
Engineering, UniVersity of Maryland, College Park, Maryland 20742, and Department of Materials Science and
Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104
Received December 21, 2006; E-mail: email@example.com
Zeolitic imidazolate frameworks (ZIFs) are a new class of
nanoporous compounds which consist of tetrahedral clusters of MN4
(M ) Co, Cu, Zn, etc.) linked by simple imidazolate ligands.1,2As
a subfamily of metal-organic frameworks (MOFs), ZIFs exhibit
the tunable pore size and chemical functionality of classical MOFs.
At the same time, they possess the exceptional chemical stability
and rich structural diversity of zeolites.2Because of these combined
features, ZIFs show great promise for hydrogen storage applications.
However, in contrast to a large number of extensive studies for
other MOFs,3-5no experimental data concerning the nature of H2-
ZIF interactions and the manner in which hydrogen molecules are
adsorbed have been reported yet. Such fundamental studies hold
the key to optimizing this new class of ZIF materials for practical
hydrogen storage applications. In particular, the major adsorption
sites and their binding energies are the key features of a system
that determine its adsorption properties at a given temperature and
ZIF8 is a prototypical ZIF compound (Zn(MeIM)2, MeIM )
2-methylimidazolate) with a SOD (sodalite) zeolite-type structure,
exhibiting an interesting nanopore topology formed by four-ring
and six-ring ZnN4 clusters as shown in Figure 1. Since the
nanopores are only accessible through narrow six-ring funnel-like
channels (Figure 1a), one wonders how H2molecules are adsorbed,
where the major binding sites are, and what the binding energies
are. Herein, using the difference Fourier analysis of neutron powder
diffraction data along with first-principles calculations, we provide
answers to these questions for the first time. Surprisingly, the
strongest adsorption sites that we identified (see Figure 1b) are
directly associated with the organic linkers, instead of the triangular
faces of the ZnN4tetrahedra (i.e., metal sites), in strong contrast to
other MOFs, where the faces of the metal-oxide tetrahedra are
typically the primary adsorption sites. At high H2loading, the ZIF8
structure is capable of holding up to 28 H2molecules (i.e., 4.2 wt
%) in the form of highly symmetric novel three-dimensional (3D)
ZIF8 was synthesized using a solvothermal method as described
in ref 2. Neutron powder diffraction data were collected on the
high-resolution neutron powder diffractometer (BT-1) at NIST
Center for Neutron Research. Because of the large incoherent cross
section of H2, adsorption was studied as a function of D2
concentration per ZIF8 molecular formula (Zn6N24C48H60). Target
amounts of D2, that is, 3, 16, and 28 D2per 6 Zn, were loaded into
the ZIF8 sample at 70 K. One H2/6 Zn corresponds to ≈0.15 wt %
hydrogen uptake. The sample was then cooled to 30 K at which
point the D2 was completely adsorbed. Once the system was
equilibrated at 30 K, the sample was further cooled to 3.5 K before
the diffraction measurement. No evidence of solid deuterium was
observed on the structural refinement of the D2loaded samples,
indicating all D2was adsorbed into the ZIF8.
The top panel of Figure 2 shows the neutron diffraction data
from the ZIF8 bare material measured at 3.5 K. The refined lattice
parameters and atomic positions of Zn, N, and C agree well with
previously reported room-temperature X-ray diffraction results.2The
high-resolution neutron diffraction data also enabled us to unam-
biguously determine the orientation (i.e., the hydrogen positions)
of the methyl group, which was not possible in the X-ray
measurement. We note that the methyl group orientation and
associated tunnel-splitting is a very sensitive probe for determination
of the guest-host interactions.6
For comparison, the neutron diffraction patterns from ZIF8 with
the following D2concentrations: n D2) 3, 16, 28 per 6 Zn are
also shown in Figure 2 (also see Supporting Information). Using
the model of the refined ZIF8 host structure, we performed Rietveld
†NIST Center for Neutron Research.
‡University of Maryland.
§University of Pennsylvania.
Figure 1. (a) (001) view of refined crystal structure of ZIF8 host lattice
from neutron powder diffraction along with the available free space (pore
structure) for H2occupation, based on van der Waals interactions. (b) A
(111) view of the real-space Fourier-difference scattering-length density
superimposed with six-ring pore aperture of the ZIF8 structure, indicating
the location of the first adsorption sites (red-yellow regions).
Figure 2. Observed (dots), refined (line), and difference (noisy line) neutron
powder diffraction profiles (λ ) 2.079 Å, 3.5 K) for ZIF8 host lattice (space
group I43 hm) and ZIF8 with D2loading of 28 D2/6 Zn.
Published on Web 04/11/2007
5314 9 J. AM. CHEM. SOC. 2007, 129, 5314-5315
10.1021/ja0691932 CCC: $37.00 © 2007 American Chemical Society
structure refinements on the D2 adsorbed samples. The Fourier
difference method was used to find the scattering-length density
distribution for D atoms based on the refinements ignoring the
adsorbed D2molecules. Figure 1b is an example of such Fourier
difference plot, which clearly shows that the initial adsorption site
(i.e., D1) is on top of the MeIM organic linker and close to the
CdC bond, thus termed “IM site”.
Upon D2loading, the order of site filling in ZIF8 was determined
to be sequential from D1 to D6, shown in Figure 3. The already
mentioned D1 site and the second site (D2) are both determined
from the initial loading of 3 D2/6 Zn. The latter is located at the
center of the channel of the six-ring opening (termed “channel site
I”). The distances between the H2 center of mass and the
corresponding CdC bond are about 3 and 3.4 Å for D1 and D2
sites, respectively. For this particular D2 concentration, IM site
occupancy is ∼21% while it is ∼10% for the channel site, implying
slightly higher binding energy for D1 than for D2 sites.
For 16 D2/6 Zn loading, the D1 and D2 sites are nearly fully
occupied, and the third adsorption site (D3) starts to populate. The
third site is also at the center of the six-ring opening but located
on the other side of the complex, so we termed it “D3: channel
site II”. This site is less favorable than D2 channel site I owing to
the presence of the methyl group, mainly a geometry effect.
Interestingly, the first three adsorption sites discussed above form
a pseudocubic “nanocage” with the edges slightly bent toward the
ZIF8 framework as shown in Figure 3c. We also observed a small
amount (∼5%) of D2adsorbed at a fourth site (D4) located at the
face center of the H2-nanocage.
With further D2loading, the first three adsorption sites were
almost completely occupied. For the maximal loading of 28 D2/
Zn, the fourth site discussed above as well as two additional sites
(D5 and D6) close to the center of the void in ZIF8 (inside the
nanocage formed by the first three sites) are progressively occupied.
The nearest neighbor distances between these adsorption sites are
about 3.04 Å (see Supporting Information), significantly shorter
than the 3.6 Å found in solid H2. Self-assembled 3D interlinked
hydrogen nanostructures with short intermolecular distances were
previously observed3in MOF5 and seem to be a common novel
feature of these nanoporous metal-hybrid systems. Note that our
maximum loading of 4.2 wt % (absolute adsorption capacity)
obtained at 30 K is higher than the reported 3.1 wt % (excess
adsorption capacity) obtained at 77 K and 80 bar,2as expected for
adsorption at lower temperature.
To understand the hydrogen host-lattice interactions further, we
also performed total-energy calculations from density functional
theory (DFT) using the plane-wave implementation of the local-
density approximation to DFT7(see Supporting Information).
Consistent with experimental observation, our calculations suggested
that the IM and channel site I are the most energetically stable
adsorption centers. The calculated H2binding energies for these
two sites are 170 and 147 meV, respectively, in agreement with
the larger population of the IM site at low D2loading. We also
found about 20 meV variation in the binding energy with H2-
orientation, suggesting that interesting hindered-quantum rotational
dynamics could be present in the H2-ZIF8 system. Note that the
binding energies derived from DFT may be overestimated. How-
ever, the relative magnitude should be valid, and indeed a good
agreement between calculation and experiment was found in both
this work and previous work3on other types of MOFs, in terms of
correctly predicting the relative strength of various adsorptions sites.
In conclusion, we obtained detailed structural information such
as orientation of the methyl groups, H2adsorption sites, and binding
energies in the ZIF8 structure for the first time. The imidazolate
organic linker is primarily responsible for adsorption in contrast to
metal oxide-based MOFs. This suggests that modification of the
linkers rather than metal types in ZIFs is more important to optimize
these materials for higher storage capacity. At high concentration
of hydrogen loading, ZIF8 is able to hold hydrogen molecules up
to 4.2 wt % as self-assembled nanostructures with relatively short
intermolecular distances compared to solid hydrogen. This suggests
that ZIFs can be also used as an ideal template host-material to
generate molecular nanostructures with interesting properties.
Acknowledgment. This work was partially supported by DOE
BES Grant No. DE-FG02-98ER45701 (W.Z., T.Y.) and DOE EERE
Grant No. DE-AI-01-05EE11104 (H.W.).
Supporting Information Available: Detailed information about
the Rietveld refinement and the crystallographic data for ZIF8-nD2(n
) 0, 3, 16, and 28), and detailed information about the DFT
calculations. This material is available free of charge via the Internet
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Figure 3. The hydrogen adsorption sites obtained from Fourier difference
analysis: (a) top and side views of first three adsorption sites near a Zn-
hexagon opening; (b) pseudocubic nanocage formed by D1, D2, and D3
sites; (c) tetrahedron-like nanocage formed by D5 and D6 sites.
C O M M U N I C A T I O N S
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