Designing Higher Surface Area Metal−Organic Frameworks: Are
Triple Bonds Better Than Phenyls?
Omar K. Farha,*
,†,∥
Christopher E. Wilmer,
‡,∥
Ibrahim Eryazici,
†,∥
Brad G. Hauser,
†
Philip A. Parilla,
§
Kevin O’Neill,
§
Amy A. Sarjeant,
†
SonBinh T. Nguyen,
†
Randall Q. Snurr,*
,‡
and Joseph T. Hupp*
,†
†
Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208-3113, United States
‡
Department of Chemical & Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3120,
United States
§
National Renewable Energy Laboratory, Golden, Colorado 80401, United States
*
S
Supporting Information
ABSTRACT: We have synthesized, characterized, and
computationally validated the high Brunauer−Emmett−
Teller surface area and hydrogen uptake of a new,
noncatenating metal−organic framework (MOF) material,
NU-111. Our results imply that replacing the phenyl
spacers of organic linkers with triple-bond spacers is an
effective strategy for boosting molecule-accessible gravi-
metric surface areas of MOFs and related high-porosity
materials.
T
he chemical and structural diversity of metal−organic
frameworks (MOFs) is one of the most notable
characteristics of these materials. MOFs are hybrid materials
composed of inorganic nodes and organic struts.
1−3
The most
intriguing examples exhibit large internal surface areas; ultralow
densities; uniform channels, cavities, and voids; and permanent
porosity. Because of these exceptional properties, MOFs are
being investigated for many potential applications, including gas
storage,
4 −8
gas and chemical separations,
9 −12
chemical
catalysis,
13,14
sensing,
15
ion exchange,
16
drug delivery,
17
and
light harvesting.
18,19
Furthermore, the availability of single-
crystal structures of MOFs allows the use of computational
modeling to calculate guest adsorption capabilities and other
properties, which can help in screening MOFs for particular
applications and improving our understanding of their
performance.
20
The fact that these computational methods
can be usefully applied gives MOFs a significant advantage over
their amorphous counterparts.
Rising concerns about climate change have intensified the
search for environmentally friendly and renewable fuels such as
water-derived H
2
, cellulosic ethanol, and photo- or electro-
chemically generated methane. Although molecular hydrogen is
a compelling alternative to gasoline in many respects, high-
density storage is a significant challenge for the viability of
hydrogen-powered vehicles. In order to drive 300 miles, 5 to 13
kg of H
2
are needed. Therefore, technologies that can efficiently
concentrate gases at lower pressures, such as adsorption on
porous materials, are desirable. The U.S. Department of Energy
(DOE) has set targets for on-board H
2
storage systems for the
year 2017: 5.5 wt % in gravimetric capacity and 40 g/L of
volumetric capacity at an operating temperature in the range
−40 to 60 °C under a maximum delivery pressure of 100 atm.
21
Recently, automobile manufacturer Mercedes-Benz has an-
nounced its intention to use MOFs for mobile hydrogen
storage at cryogenic temperatures.
22
Required are materials with
surface areas of ∼24 million square feet of surface area per
pound (4900 m
2
/g) and the ability to store substantial
hydrogen at 435 psi (30 bar). MOFs are powerful contenders
relative to other porous materials in meeting these conditions.
We set out to make a MOF that satisfies both of the
aforementioned requirements (∼4900 m
2
/g and high hydrogen
uptake at 30 bar). We turned our attention to (3,24)-
paddlewheel-connected MOF networks (rht topology),
23
for
which catenation (interpenetration or interweaving of multiple
frameworks) is impossible. The use of this topology was
pioneered by Eddaoudi
23
and has been used by us
24
and the
Zhou
25
and Schro
der
26
groups. Recently, a MOF denoted as
either PCN-69
27
or NOTT-119
28
with the rht topology was
synthesized by both the Zhou and Schroder groups. PCN-69/
NOTT-119 has a Brunauer −Emmett−Teller (BET) surface
area of ∼4000 m
2
/g and is made from the phenyl-based strut
LH
6
(1) (Scheme 1). Even though LH
6
(1) is longer than
LH
6
(2), the strut used to make NOTT-112
26
(3800 m
2
/g), the
surface area of PCN-69/NOT T-119 did not increase
significantly. To date, the best known strategy for obtaining
Received: March 17, 2012
Published: June 7, 2012
Scheme 1. Hexacarboxylic Acid Struts Used To Construct
PCN-69/NOTT-119 [LH
6
(1)], NOTT-112 [LH
6
(2)], and
NU-111 [LH
6
(3)]
Communication
pubs.acs.org/JACS
© 2012 American Chemical Society 9860 dx.doi.org/10.1021/ja302623w | J. Am. Chem. Soc. 2012, 134, 9860−9863
higher-surface-area MOFs has been to use longer linkers.
24,29
However, as the case of PCN-69/NOTT-119 shows, it is not
only the length of the linker but also the shape and density that
matter for high surface area.
With the above considerations in mind, we decided to
replace the two phenyl spacers in each arm of the linker in
PCN-69/NOTT-119 with two triple-bond spacers. Our initial
notions were based on hypothetically exposing the edges of
phenyl spacers by dividing each linker arm into three individual
but chemically linked pieces, each consisting of two carbon
atoms linked by a triple bond. These pieces are equivalent to
acetylenes, but with carbon−carbon single bonds to neighbor-
ing pieces replacing the terminal hydrogen atoms. Thus, six
linked alkynes would replace two linked phenyl groups. Given
the synthetic challenge presented by the corresponding trigonal
linker, however, we chose instead to start by replacing each
phenyl spacer in LH
6
(1) by one carbon−carbon triple bond.
In this report, we describe the synthesis and properties of a
new MOF, NU-111 (NU = Northwestern University), formed
from the new hexacarboxylic acid linker LH
6
(3) that contains
three pairs of triple-bond spacers. The synthesis and character-
ization of LH
6
(3) is described in the Supporting Information
(SI). Briefly, LH
6
(3) was obtained via saponification of the
corresponding hexaester precursor, which in turn was obtained
via Sonogashira coupling of 1,3,5-triiodobenzene with 1,3-
diethoxycarbonyl-5-(butadiynyl)benzene.
Solvothermal reactions of LH
6
(3) and Cu(NO
3
)
2
·2.5H
2
Oin
DMF/EtOH/HCl (DMF = dimethylformamide) at 80 °C gave
MOFs with the framework formula [Cu
3
(L(3))(H
2
O)
3
]
n
in
75% yield after 48 h. X-ray analysis of single crystals of NU-111
revealed a noncatenated structure with a cubic space group,
Fm3
m, in which the framework nodes consist of Cu
II
2
units
coordinated by the carboxylates of L(3)
6−
in paddlewheel
fashion (Figure 1A,B). The axial sites of the Cu
II
2
units are
coordinated by water molecules that were well-resolved in the
X-rayanalysis.Theexperimentalstructurehasunit-cell
dimensions of a = b = c = 48.9 Å at 225 K. NU-111 can be
represented using three types of space-filling polyhedra (also
termed cages) that are derived from drawing straight lines
between copper paddlewheels (Figure 2B−D column 1) and
are fused in such a way that they form continuous channels
(Figure 2E). The smallest cage is cuboctahedral and is formed
from 24 isophthalate groups from L(3)
6−
units and 12 pairs of
copper ions (Figure 2B). The second cage is a truncated
tetrahedron and is formed from isophthalate groups from four
L(3)
6−
linkers and 12 pairs of copper ions (Figure 2C). The
largest cage can be described as a truncated cuboctahedron and
is formed by 24 Cu
2
II
paddlewheel nodes and portions of eight
distinct L(3)
6−
units (Figure 2D). However, there is no unique
representation of a MOF in the form of space-filling polyhedra,
and the scheme described above, though both common and
useful for rht topologies, neglects an important feature of NU-
111: a fourth cavity that can be detected experimentally (Figure
1C, blue). An alternate choice of space-filling polyhedra can
lead to greater physical insight; in this case, taking the curvature
of the ligand into account gives four types of polyhedral cages,
each corresponding to one of the four experimentally
discovered cavities (Figure 2A−D column 2 and Figure 2F).
Therefore, there are two ways of describing the topological
features of NU-111 (see Figures S21 and S22 in the SI). The
phase purity of a bulk sample of NU-111 was established via
powder X-ray diffraction (PXRD) measurements since the
simulated PXRD pattern was found to be in excellent
agreement with the experimental one (Figure S11). Addition-
ally, thermogravimetric analysis (TGA) of NU-111 revealed a
mass loss at ∼135 °C assigned to solvent (DMF), and no
further mass loss occurred until ∼300 °C (Figure S12).
After the guest solvent molecules were removed using
supercritical CO
2
29−33
(see the SI for activation details), the
porosity was examined by nitrogen adsorption at 77 K. The
experimental and simulated N
2
isotherms were in excellent
agreement, as shown in Figure 3A. The experimental BET
surface area of NU-111 was found to be 5000 ± 80 m
2
/g,
which is higher than that of PCN-69/NOTT-119 (3989/4118
m
2
/g). It is worth mentioning that the experimental BET
surface area is in excellent agreement with that determined
from the computationally simulated isotherm (4915 m
2
/g).
The total experimental pore volume of NU-111 is 2.38 cm
3
/g.
The pore size distribution of NU-111, calculated from Ar
Figure 1. (A, B) Structurally derived views of NU-111. (C) Calculated
pore size distribution of NU-111 based on experimental Ar adsorption
at 87 K. DFT kernel used: Ar at 87 K_zeolites/silica (cylindrical pore,
NLDFT equilibrium model).
Journal of the American Chemical Society Communication
dx.doi.org/10.1021/ja302623w | J. Am. Chem. Soc. 2012, 134, 9860−98639861
adsorption experiments at 87 K using nonlocal density
functional theory (NLDFT), shows peaks at 14, 17, 19, and
24 Å (Figure 1C), in excellent agreement with a geometric
calculation based on the crystal structure (Figure S19).
ThelargesurfaceareaandporevolumeofNU-111
prompted us to measure its high-pressure hydrogen capacity.
These mea surements were done at the DOE Hydrogen
Sorption Center of Excellence at the National Renewable
Energy Laboratory (NREL). Sorption data for H
2
were
collected up to 110 bar at 77 K. It should be noted that only
the “excess” gas adsorption is directly accessible experimentally.
Excess gas adsorption is the amount adsorbed as a result of the
presence of the adsorbent. The total adsorption is the sum of
the excess adsorption and the amount that would be found
within the pore volume, on the basis of the finite bulk-phase
density of the gas, if the adsorbent were not present. For gas
storage and delivery purposes, the total amount adsorbed is the
more relevant quantity. The excess hydrogen uptake of NU-
111 was 21 mg/g at 1 bar and 69 mg/g at 32 bar (Figure 3B
and Figure S23). From the N
2
-derived pore volume (2.38 cm
3
/
g) and the bulk phase density of H
2
, the total H
2
uptake at 110
bar and 77 K was calculated to be 135 mg/g (Figure 3C). The
uptake of NU-111 is within range of the DOE’s revised long-
term systems target for onboard H
2
storage, 5.5 wt % (58 mg/
g), albeit at cryogenic rather than ambient temperature. The
simulated H
2
isotherm of NU-111 was in only qualitative
agreement with the experimental H
2
measurements using the
classical force field model
34
(Figure 3B). Incorporating the
Feynman−Hibbs (FH) corrections for quantum diffraction
effects resulted in excellent agreement with the experimental
isotherm.
35
The stability of NU-111 was examined by running multiple
cycles of high-pressure hydrogen adsorption at room temper-
ature (see the SI), and it showed no loss of capacity. In
addition, the N
2
isotherms for NU-111 were measured before
sending the sample to NREL and upon receiving the sample
back from NREL (see the SI). The N
2
isotherm showed no loss
of porosity as a result of the shipping and measurements.
In conclusion, we have synthesized and characterized NU-
111, a stable and promising material for hydrogen storage at
cryogenic temperatures. NU-111 was constructed from a linker
whose arms each contain two triple-bond spacers in place of the
one or two phenyl spacers in previously used linkers. Our
results suggest that replacing the phenyl spacers with triple-
bond spacers may be a better strategy than using longer linkers
to obtain higher-surface-area MOFs. The experimental and
simulated data for NU-111 are in excellent agreement, which
leads us to believe that a high-throughput computational
screening approach, as recently described by Wilmer et al.
36
for
the related problem of high-pressure methane storage, may be
sufficiently accurate to identify excellent candidate MOFs for
Figure 2. (A−D) Polyhedra in NU-111 obtained by drawing straight
lines between the copper paddlewheels (column 1) and by taking into
account the curvature of the ligands (column 2). (E, F) Packing of the
polyhedra shown in (E) column 1 and (F) column 2. Color code: Cu,
teal; C, gray; O, red; H, white.
Figure 3. Adsorption isotherms of NU-111 at 77 K: (A) N
2
isotherms;
(B) excess H
2
isotherms; (C) excess and absolute (total) H
2
isotherms.
Journal of the American Chemical Society Communication
dx.doi.org/10.1021/ja302623w | J. Am. Chem. Soc. 2012, 134, 9860−98639862
high-pressure hydrogen storage. This may save experimentalists
enormous amounts of time and effort as well as enable the
efficient discovery and assessment of more promising MOFs
such as NU-111.
■
ASSOCIATED CONTENT
*
S
Supporting Information
General procedures, materials, and instrumentation; synthesis
and characterization (
1
H and
13
C NMR) of LH
6
(3); synthesis
and X-ray cr ystallographic data (CIF) for NU-111; and
sorption isotherms, BET analysis, and theoretical pore size
distribution for NU-111. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
o-farha@northwestern.edu; snurr@northwestern.edu; j-hupp@
northwestern.edu
Author Contributions
∥
O.K.F., C.E.W., and I.E. contributed equally.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
J.T.H., S.T.N., and R.Q.S. gratefully acknowledge the DOE
Office of Energy Efficiency and Renewable Energy for primary
financial support. O.K.F. acknowledges the Northweste rn
NSEC, which provided additional general support on MOF
design and synthesis.
■
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