Hydrogen storage in a highly interpenetrated and partially fluorinated metal-organic framework

Article abstract of DOI:10.1021/ic101153c

A partially fluorinated metal-organic framework, Zn(bpe)(tftpa) A·cyclohexanone [bpe = 1,2-bis(4-pyridyl)ethane; tftpa = tetrafluoroterephthalate], has been synthesized, and its H₂ storage properties are reported. The structure is highly interpenetrated yet contains templating cyclohexanone molecules, which can be easily removed to give a porous material with fluorine atoms exposed to the pore surface. The material adsorbs 1.04 wt % H₂ at 77 K and 1 atm with an adsorption enthalpy (Q _st) of 6.2 kJ/mol, which represents a slight enhancement in the binding strength due to the presence of fluorine atoms over comparable metal-organic frameworks, which bind through purely physisorptive methods.

Full text of DOI:10.1021/ic101153c

Inorg. Chem. 2011, 50, 403–405 403
DOI: 10.1021/ic101153c
Hydrogen Storage in a Highly Interpenetrated and Partially Fluorinated
Metal-Organic Framework
Zeric Hulvey,*,† Dorina A. Sava,‡ Juergen Eckert,†,‡ and Anthony K. Cheetham*,§
†Materials Research Laboratory, University of California, Santa Barbara, California 93106-5121,
United States, ‡Department of Chemistry, University of South Florida, 4202 East Fowler Avenue,
Tampa, Florida 33620, United States, and §Department of Materials Science and Metallurgy,
University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom
Received June 8, 2010
A partially fluorinated metal-organic framework, Zn(bpe)(tftpa)
(Qst) of 8.0 kJ/mol at low coverages,7 a significant enhance-
3
cyclohexanone [bpe = 1,2-bis(4-pyridyl)ethane; tftpa = tetra-
fluoroterephthalate], has been synthesized, and its H2 storage
properties are reported. The structure is highly interpenetrated yet
contains templating cyclohexanone molecules, which can be easily
removed to give a porous material with fluorine atoms exposed to
the pore surface. The material adsorbs 1.04 wt % H2 at 77 K and
1 atm with an adsorption enthalpy (Qst) of 6.2 kJ/mol, which
represents a slight enhancement in the binding strength due to the
presence of fluorine atoms over comparable metal-organic frame-
works, which bind through purely physisorptive methods.
ment over materials such as MOF-5, which contain large
pores and adsorb through purely physisorptive methods
(typically 4-5 kJ/mol). This enhancement was attributed to
a combination of the fluorine atoms exposed to the pore
˚
˚
surface and the small size of the pore (3.7 A Â 6.7 A). The
only other fluorinated hybrid structure for which a Qst value
has been reported is Zn(SiF6)(pyrazine)2, which displays a
similar value of 8.2 kJ/mol and a comparably sized pore
8
˚
˚
(4.5 A Â 4.5 A). It is therefore unclear to what degree this
enhancement can be attributed to the fluorine atoms, if at all.
Herein we present another partially fluorinated material that
contains a much larger pore than these previous examples.
The gas storage results for this structure suggest that the
enhancement seen in these previous cases is more attributable
to the small pore size than to the presence of fluorine atoms.
We have previously shown that perfluorinated dicarboxy-
lates are easily incorporated into hybrid structures when a
second nitrogen-donor ligand is present.9-13 For this work,
1,2-bis(4-pyridyl)ethane (bpe) was chosen in combination
with tftpa. The hydrothermal reaction of zinc acetate, bpe,
and H2tftpa at 100 °C for 24 h yields colorless crystals of
Zn(bpe)(tftpa) (1),12 a three-dimensional coordination poly-
mer comprising two interpenetrating sublattices. The sub-
lattice contains trimerlike units, composed of a ZnO4N2
octahedron and two identical ZnO3N2 trigonal bipyramids.
These trimerlike units are then connected in one direction
through bpe ligands and in the other two directions through
bridging tftpa ligands. The large pore present in the sublattice
is of the correct size to accommodate the second sublattice
with no empty space (see the Supporting Information).
Recent activity in the field of metal-organic frameworks
(MOFs) has been dominated by reports of porous structures
that adsorb hydrogen and other gases.1 The physisorptive
binding of H2 to the surfaces of these materials is weak, and
therefore increasing the strength of this interaction has
become important. The incorporation of coordinatively un-
saturated metal sites within the structure has been one suc-
cessful method of achieving this, as has the engineering of
materials with smaller pore sizes that approach the kinetic
2-4
˚
diameter of the H2 molecule (2.89 A).
We and other
groups have reported interesting H2 adsorption properties
in materials containing fluorinated ligands.5-8 Our previous
example, Zn5(trz)6(tftpa)2(H2O)2 (trz = triazolate; tftpa =
tetrafluoroterephthalate), displays a H2 adsorption enthalpy
*To whom correspondence should be addressed. E-mail: zhulvey@
gmail.com (Z.H.), akc30@cam.ac.uk (A.K.C).
ꢀ
(1) Murray, L. J.; Dinca, M.; Long, J. R. Chem. Soc. Rev. 2009, 38, 1294.
(2) Forster, P. M.; Eckert, J.; Heiken, B. D.; Parise, J. B.; Yoon, J. W.;
Jhung, S. H.; Chang, J.-S.; Cheetham, A. K. J. Am. Chem. Soc. 2006, 128,
16846.
(9) Hulvey, Z.; Wragg, D. S.; Lin, Z.; Morris, R. E.; Cheetham, A. K.
Dalton Trans. 2009, 1131.
(10) Hulvey, Z.; Ayala, E.; Furman, J. D.; Forster, P. M.; Cheetham,
A. K. Cryst. Growth Des. 2009, 9, 4759.
(11) Hulvey, Z.; Ayala, E.; Cheetham, A. K. Z. Anorg. Allg. Chem. 2009,
635, 1753.
ꢀ
(3) Dinca, M.; Long, J. R. Angew. Chem., Int. Ed. 2008, 47, 6766.
(4) Collins, D. J.; Zhou, H.-C. J. Mater. Chem. 2007, 17, 3154.
(5) Lee, J. Y.; Li, J.; Jagiello, J. J. Solid State Chem. 2005, 178, 2527.
(6) Yang, C.; Wang, X.; Omary, M. A. J. Am. Chem. Soc. 2007, 129,
15454.
(7) Hulvey, Z.; Falcao, E. H. L.; Eckert, J.; Cheetham, A. K. J. Mater.
Chem. 2009, 19, 4307.
(8) Uemura, K.; Maeda, A.; Maji, T. K.; Kanoo, P.; Kita, H. Eur. J.
Inorg. Chem. 2009, 2329.
(12) Hulvey, Z.; Furman, J. D.; Turner, S. A.; Tang, M.; Cheetham, A. K.
Cryst. Growth Des. 2010, 10, 2041.
(13) Hulvey, Z.; Melot, B. C.; Cheetham, A. K. Inorg. Chem. 2010, 49,
4594.
r
2010 American Chemical Society
Published on Web 12/17/2010
pubs.acs.org/IC

404 Inorganic Chemistry, Vol. 50, No. 2, 2011
Hulvey et al.
this channel with a closest intermolecular H-H distance of
˚
2.32 A.
A number of MOFs have been reported that contain high
degrees of interpenetration and permanent porosity.16-20
The use of small organic molecules as templates in the
synthesis of MOFs has been a successful method of reduc-
ing interpenetration, incorporating porosity, and introducing
chirality into structures.16,17,21-23 However, this is the first
time, to our knowledge, that cyclohexanone has been used for
this purpose and the first example of a material for which
organic templating simultaneously increases interpenetration
and porosity. The permanent porosity of 2 was confirmed by
thermogravimetric analysis data, which indicate that the
cyclohexanone molecules are completely removed in one step
between125 and 170°C andthat theresulting structure (2a) is
stable to 230 °C (see the Supporting Information). The
structure of 2 can be completely desolvated by heating the
material in a vacuum oven at 150 °C for 16 h. Crystals of 2a
degrade slowly when exposed to the atmosphere; however,
we successfully obtained unit cell information from single-
crystal X-ray diffraction data for the desolvated structure
(though structure solution attempts were unsuccessful).24
The unit cell volume of 2a indicates a 5.6% increase over
that of 2, which indicates that the framework structure is
fairly flexible upon removal of the cyclohexanone molecules.
Because of the permanent porosity of 2a, gas sorption
experiments were performed. A Brunauer-Emmett-Teller
surface area for 2a of 512 m2/g and a pore volume of 0.203
mL/g were determined based on the Ar adsorption isotherm
at 77 K. H2 adsorption isotherms were carried out on 2a
at both 77 and 87 K from 0 to 1.0 atm (Figure 3). The
isotherm at77 K displays a maximum uptakeof1.04 wt % H2
at 1.0 atm, which is comparable to other hybrid frameworks
under similar conditions with similar pore sizes. In accor-
dance with previous reports of H2 adsorption in fluorinated
hybrids, the desorption curve displays a slight hysteresis.6,8
The Qst value, obtained by applying the Clausius-Clayperon
equation to the isotherms obtained at 77 and 87 K, is
approximately 6.2 kJ/mol at low coverages.25 This value
can only be viewed as a slight enhancement over any
physisorptive binding to nonfluorinated structures and does
not approach the two previous fluorinated examples men-
tioned above, which achieved Qst values of around 8 kJ/mol.
Because the pore in 2a is large enough to discount any
additional binding enhancement due to confinement effects,
it is apparent that the fluorine atoms in this structure are only
slightly enhancing the Qst value.
Figure 1. Structure of 2. Left: View of one sublattice down the b axis.
Right: View of all four sublattices down the channel axis (b direction),
with one templating cyclohexanone molecule shown in the center (others
omitted).
Figure 2. Simplified view of the 4-fold interpenetration of 2 with the
different sublattices shown in red, yellow, green, and blue.
We attempted to synthesize the noninterpenetrated version
of 1 by a variety of methods, one of which was the use of
templating organic molecules to effectively block the inter-
penetration. Surprisingly, this strategy was both successful
and unsuccessful when a slight excess of cyclohexanone was
added to the reaction under the same conditions.14 The
resulting structure, Zn(bpe)(tftpa) cyclohexanone (2),15 in-
3
corporates cyclohexanone molecules as a templating agent
but is simultaneously 4-fold-interpenetrated. The structure
contains diamond-like sublattices built up from ZnO2N2
tetrahedra bound to two bpe ligands and two tftpa ligands
(Figure 1). Remarkably, the cyclohexanone molecules cause
an alignment of the four sublattices such that they occupy
a large one-dimensional channel down the b axis with
(16) Choi, E.-Y.; Park, K.; Yang, C.-M.; Kim, H.; Son, J.-H.; Lee, S. W.;
Lee, Y. H.; Min, D.; Kwon, Y.-U. Chem.;Eur. J. 2004, 10, 5535.
(17) Ma, S.; Sun, D.; Ambrogio, M.; Fillinger, J. A.; Parkin, S.; Zhou,
H.-C. J. Am. Chem. Soc. 2007, 129, 1858.
(18) Chen, B.; Ma, S.; Hurtado, E. J.; Lobkovsky, E. B.; Zhou, H.-C.
Inorg. Chem. 2007, 46, 8490.
(19) Xue, M.; Ma, S.; Jin, Z.; Schaffino, R. M.; Zhu, G.-S.; Lobkovsky,
E. B.; Qiu, S.-L.; Chen, B. Inorg. Chem. 2008, 47, 6825.
(20) Zhang, Z.; Xiang, S.; Chen, Y.-S.; Ma, S.; Lee, Y.; Phely-Bobin, T.;
Chen, B. Inorg. Chem. 2010, 49, 8444.
˚
a diameter of approximately 6.5 A (Figure 2). The cyclo-
hexanone molecules are effectively close-packed down
(14) A mixture of Zn(CH3CO2)2 2H2O (0.022 g, 1.00 mmol), bpe (0.018 g,
3
0.10 mmol), H2tftpa (0.024 g, 0.10 mmol), and cyclohexanone (0.028 g, 0.30
mmol) was added to 3 mL of water and heated at 100 °C for 18 h in a 23-mL
Teflon-lined stainless steel autoclave. Colorless block-shaped crystals of 2
were isolated by filtration and washed with water and acetone. Elem anal.
Calcd for 2 (ZnC26H22F4N2O5): C, 53.5; H, 3.80; N, 4.80. Found: C, 53.3; H,
3.82; N, 4.92.
(21) Tanaka, D.; Kitagawa, S. Chem. Mater. 2008, 20, 922.
(22) Biradha, K.; Fujita, M. J. Chem. Soc., Dalton Trans. 2000, 3805.
(23) Lin, Z.; Slawin, A. M. Z.; Morris, R. E. J. Am. Chem. Soc. 2007, 129,
4880.
˚
(24) Unit cell data for 2a at 150 K: monoclinic, P21/n, a = 12.908(4) A,
3
˚
˚
˚
b = 12.219(4) A, c = 16.313(5) A, β = 90.080(8)°, V = 2572.9(13) A .
(25) Rouquerol, J.; Rouquerol, F.; Sing, K. Adsorption by Powders and
Solids: Principles, Methodology, and Applications; Academic Press: London,
1998.
˚
(15) Crystal data for 2 at 150 K: monoclinic, P21/n, a = 11.7527(13) A,
3
˚
˚
˚
b = 12.4829(13) A, c = 16.6089(18) A, β = 90.477(3)°, V = 2436.6(5) A ,
Z = 4, F = 1.592 g/cm3, μ = 1.080 mm-1, R1 = 0.0656, wR2 = 0.1184.

Communication
Inorganic Chemistry, Vol. 50, No. 2, 2011 405
The inability to synthesize analogous structures obviously
makes direct comparisons between fluorinated and nonfluori-
nated structures difficult. A second issue is that both 2a and the
previous examples mentioned above do not contain comple-
tely fluorinated pores; rather, they contain fluorine atoms in
one of the two ligands exposed to the pore surface. There has
yet to be a Qst value reported for a MOF with totally
perfluorinated pores, and it is therefore still unclear what
effect this would have on H2 binding interactions. It is likely
that multiple structure characteristics, such as pore surface
alteration, pore size effects, and coordinatively unsaturated
metal sites, may need to be combined in one material to
provide the substantial increase in H2 binding strengths that
are needed to meet the U.S. Department of Energy targets.
Acknowledgment. This work was funded by the U.S.
Department of Energy (Grant DE-FC36-50GO15004)
and made use of the MRL Central Facilities supported
by the National Science Foundation (Grant DMR05-
20415). A.K.C. thanks the European Research Council
for an Advanced Investigator Award. We thank Guang
Wu for assistance with X-ray diffraction experiments.
Figure 3. H2 adsorption and desorption curves for 2a at 77 K (red) and
87 K (blue).
These results underscore some key problems in quantifying
the extent to which fluorination affects the H2 binding
strength. First, perfluorinated ligands show significant con-
formational differences relative to their nonfluorinated ana-
logues, so that the synthesis of analogous structures is not
possible.12 Our recent work with perfluorinated benzenedi-
carboxylates has illustrated that the extent to which the
carboxylate group is twisted out of the plane of the benzene
ring when fluorine atoms are present is so significant that
the manner in which metals bind to these ligands is altered.
Supporting Information Available: Description of the synthe-
sis and structure of 1, thermogravimetric analysis data and
powder X-ray diffraction patterns for 2, description of the
structure solution, and crystallographic data in CIF format
for 1 and 2. This material is available free of charge via the
Internet at http://pubs.acs.org.