Third Day Poster Session, 17 June 2010 - NanoTR-VI

Poster Session, Thursday, June 17
Theme F686 - N1123
Hydrogen Storage and Release Mechanisms in MOF-5
M. MANI-BISWAS 1 , T. CAGIN 1,2
1 Materials Science and Engineering, Texas A&M University, College Station, TX 77843, USA
2 Department of Chemical Engineering, Texas A&M University, Texas, TX 77843-3122, USA
Abstract- Metal organic framework MOF-5 is a hybrid porous crystalline material. It has high porosity and large
surface area and hence potential application in gas storage, catalysis, drug delivery, etc. For applications as a gas
storage material, it is important to find out a suitable gas delivery mechanism. Here we propose such a
mechanism by taking advantage of near shear instability of MOF-5. Using molecular simulation we show that at
high pressure MOF-5 gets deformed to 55% of its original volume. We also show that during this deformation
process; MOF-5 passes through certain stages from where, by decreasing the pressure, 100% reversibility can be
achieved. Based on this behavior, a purely mechanical process is proposed for gas (H 2 ) storage and release.
Keywords: Hydrogen storage, Metal organic frameworks, Molecular Dynamics, sorption simulation, mechanical instability.
Metal organic frameworks (MOF) are hybrid
porous crystalline materials. They have the highest
pore size, low density and large surface area of any
crystalline material 1-4 . In general, MOFs are made
up of metal oxide clusters positioned at the vertices
and connected by organic linkers. For example, the
simplest structure MOF-5 (IRMOF-1) is made up
of Zn 4 O clusters are positioned at the corners of the
cubic cell and connected by benzene dicarboxylate
(BDC) linkers. The framework molecules take up
only a small fraction of the available space in the
crystal and about 80 % of the volume is free to
accommodate any guest molecule 1 . MOFs can be
easily prepared in the laboratory and have good
thermal stability (till 300-400 0 C) 3 . All these
properties make MOFs suitable for applications
such as gas storage/separation, catalysis, molecular
recognition, etc. 5, 6 MOFs have potential to adsorb
gases like H 2 , CH 4 , CO 2 , N 2 , Ar, etc. and the
adsorption capacity may be improved by changing
the functionality of the linker and thus increasing
MOF-guest interaction energy, incorporating open
metal sites, catenation of framework, etc 4 . MOF
filled containers have demonstrated enhanced
storage capacity (44% more hydrogen, 4 times
more Xenon and 3 times more propane) compared
to empty containers 5 , further strengthening the
potential of MOFs as gas storage medium.
Studies on the mechanical property have revealed
that MOF-5 is a soft material and it is nearly
unstable 7-8 , implying that the crystal is flexible
enough to transform to a new structure in the
presence of an external stimulus. Single-crystal-tosingle-crystal
transformations by exchange of guest
molecule or by varying temperature condition have
been reported for some MOFs 9 and these
transformations have been implicated in controlled
delivery of the guest molecules. Here we show by
theoretical methods, that at high pressure MOF-5
undergoes reversible structural transformation i,e
volume compression/decompression stages which
may be continued for number of cycles. Taking
advantage of the cyclic nature of MOF-5
deformation under pressure, a purely mechanical
gas storage and delivery system has been proposed.
We considered hydrogen as a representative gas
and performed simulations with hydrogen filled
MOF-5. Given the pore size of MOF-5 (available
volume ~ 11267 Å 3 ), at 100 MPa and at room
temperature, ~167 molecules of hydrogen can be
entrapped inside the crystal (considering density of
hydrogen at this condition is 49.25 kg/m 3 ). This
amounts to 7wt % H 2 per gm of MOF-5. Under
pressure as the crystal deforms the entrapped gas
will be released, which may be used further. In the
proposed process, using pressure induced
mechanical gas delivery system, efficiency as high
as 90% may be achieved.
*Corresponding author: cagin@che.tamu.edu
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6th Nanoscience and Nanotechnology Conference, zmir, 2010 765