Methane storage in metal-organic frameworks: Current records, surprise findings, and challenges

Article abstract of DOI:10.1021/ja4045289

We have examined the methane uptake properties of six of the most promising metal organic framework (MOF) materials: PCN-14, UTSA-20, HKUST-1, Ni-MOF-74 (Ni-CPO-27), NU-111, and NU-125. We discovered that HKUST-1, a material that is commercially available in gram scale, exhibits a room-temperature volumetric methane uptake that exceeds any value reported to date. The total uptake is about 230 cc(STP)/cc at 35 bar and 270 cc(STP)/cc at 65 bar, which meets the new volumetric target recently set by the Department of Energy (DOE) if the packing efficiency loss is ignored. We emphasize that MOFs with high surface areas and pore volumes perform better overall. NU-111, for example, reaches ～75% of both the gravimetric and the volumetric targets. We find that values for gravimetric uptake, pore volume, and inverse density of the MOFs we studied scale essentially linearly with surface area. From this linear dependence, we estimate that a MOF with surface area 7500 m²/g and pore volume 3.2 cc/g could reach the current DOE gravimetric target of 0.5 g/g while simultaneously exhibiting around ～200 cc/cc volumetric uptake. We note that while values for volumetric uptake are based on ideal single crystal densities, in reality the packing densities of MOFs are much lower. Finally, we show that compacting HKUST-1 into wafer shapes partially collapses the framework, decreasing both volumetric and gravimetric uptake significantly. Hence, one of the important challenges going forward is to find ways to pack MOFs efficiently without serious damage or to synthesize MOFs that can withstand substantial mechanical pressure.

Full text of DOI:10.1021/ja4045289

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Article
Methane Storage in Metal–Organic Frameworks:
Current Records, Surprise Findings, and Challenges
Yang Peng, Vaiva Krungleviciute, Ibrahim Eryazici, Joseph T. Hupp, Omar K. Farha, and Taner Yildirim
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4045289 • Publication Date (Web): 10 Jul 2013
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Methane Storage in Metal–Organic Frameworks:
Current Records, Surprise Findings, and Challenges
Yang Peng,†,‡ Vaiva Krungleviciute,†,‡ Ibrahim Eryazici,§ Joseph T. Hupp,§ Omar K. Farha,§,* and Taner
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Yildirim,†,‡,*
† NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899ꢀ6102
‡ Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104ꢀ6272
§ Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road,
Evanston, Illinois 60208
ABSTRACT: We have examined the methane uptake properties of six of the most promising metal organic framework (MOF)
materials: PCNꢀ14, UTSAꢀ20, HKUSTꢀ1, NiꢀMOFꢀ74 (NiꢀCPOꢀ27), NUꢀ111 and NUꢀ125. We discovered that HKUSTꢀ1, a mateꢀ
rial that is commercially available in gram scale, exhibits a roomꢀtemperature volumetric methane uptake that exceeds any value
reported to date. The total uptake is about 230 cc(STP)/cc at 35 bar and 270 cc(STP)/cc at 65 bar, which meets the new volumetric
target recently set by the Department of Energy (DOE) if the packing efficiency loss is ignored. We emphasize that MOFs with
high surface areas and pore volumes perform better overall. NUꢀ111, for example, reaches ~75% of both the gravimetric and voluꢀ
metric targets. We find that values for gravimetric uptake, pore volume and inverse density of the MOFs we studied scale essentialꢀ
ly linearly with surface area. From this linear dependence we estimate that a MOF with surface area 7,500 m2/g and pore volume
3.2 cc/g could reach the current DOE gravimetric target of 0.5 g/g while simultaneously exhibiting around ~200 cc/cc volumetric
uptake. We note that while values for volumetric uptake are based on ideal single crystal densities, in reality the packing densities
of MOFs are much lower. Finally, we show that compacting HKUSTꢀ1 into wafer shapes partially collapses the framework, deꢀ
creasing both volumetric and gravimetric uptake significantly. Hence, one of the important challenges going forward is to find ways
to pack MOFs efficiently without serious damage or to synthesize MOFs that can withstand substantial mechanical pressure.
Introduction
physisorption. While utilization of MOFs in methane storage
has not received nearly as much attention as utilization for
hydrogen storage3,4 or capture of carbon dioxide,5 a number of
researchers have investigated methane uptake6 by MOFs. Thus
far, several MOFs6ꢀ13 have been reported to exhibit sizable
volumetric capacities for methane uptake at room temperature
(see Ref.6a and 6r for a recent review of methane storage in
MOFs). However, very recently the U.S. Dept. of Energy has
initiated a new methane storage program14 with the following
ambitious targets: 0.5 g (CH4)/g(sorbent) for gravimetric
capacity and ρ = 0.188 g/cc (11.74 mmol/cc) for minimum
required volumetric capacity, which corresponds to the density
of compressed natural gas (CNG) at 250 bar and 298 K. The
new volumetric target is equivalent to 263 cc (STP: 273.15 K,
1 atm)/cc. If we consider 25% packing loss, the required
The demand for alternative fuels is greater now than perꢀ
haps ever before due to concerns over national and regional
energy security, groundꢀlevel air quality, and climate change.
While not a renewable fuel, natural gas (NG) – comprising
chiefly methane – has moved to the forefront as a potential
bridging fuel to a lowꢀcarbon energy future. Methane delivers
roughly twice the energy of coal in terms of the amount of
carbon dioxide released. In contrast to coal, methane does so
without dissipating mercury or producing uraniumꢀ and thoriꢀ
umꢀrich ash. In the United States, airꢀquality regulations are
driving electricity utilities toward gasꢀfired power plants as
replacements for coalꢀfired plants. Facilitating the transition
has been a spectacular recent drop in the price of natural gas1
due to deployment of inexpensive technologies for its recovery
from shale.
volumetric target becomes 330 cc(STP)/cc,
which is
significanly higher than the previous target of 180 cc(STP)/cc
at 35 bar.
Economic and environmental considerations have also
boosted interest in NG as a fuel for transportation, and espeꢀ
cially as a replacement for petrol (gasoline). A key challenge
is massꢀ and volumeꢀefficient, ambientꢀtemperature storage
and delivery. One potential solution is to store methane at very
high pressures (250 bar) as is done currently by most natural
gas powered vehicles today. While suitable for large vehicles
such as buses, this solution is less than satisfactory for cars.
An alternative solution is to use a porous material to store gas
at similarly high density, but substantially lower pressure.
Metalꢀorganic frameworks2 (MOFs) are nanoporous materials
that have great potential for highꢀdensity methane storage via
With these new targets, now is an opportune time to evaluꢀ
ate the most promising existing MOFs at pressures higher than
35 bar (the upper bound of most previous studies) and to asꢀ
sess gravimetric uptake, which has not usually been considꢀ
ered in earlier studies. A typical pipeline pressure is 35 bar,
but a more relevant storage pressure is 65 bar, as this correꢀ
sponds roughly to the upper limit achievable with comparaꢀ
tively inexpensive twoꢀstage compressors. Also important to
consider are distinctions between excess uptake, total uptake,
and working capacity (deliverable capacity). A representative
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lower pressure bound for NG delivery to a vehicle engine is 5
bar. Thus, an important design and assessment consideration is
how much methane is stored, but stranded, between 0 and 5
bar.
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Figure 1. The nano-cages and crystal structures of the MOFs under consideration. The empirical formulas for each MOF in de-
solvated form are also indicated. Some of the atoms are not shown for clarity. The gray, black, red, cyan, and blue spheres repre-
sent carbon, hydrogen, oxygen, copper, and nitrogen atoms, respectively.
Reꢀevaluation also provides an opportunity to: a) take adꢀ
vantage of advances in MOF synthesis and activation that may
yield larger surface areas and pore volumes than previously
obtained, b) benchmark experimental surface areas against
computationally estimated maximum values, and c) standardꢀ
ize comparisons to a specific experimental temperature
(298K). Using previously reported results obtained at temperaꢀ
tures lower than 298K and/or interpreted using densities of
nonꢀactivated (i.e. solvated) MOFs, would skew comparisons
– most notably by overꢀestimating volumetric capacities relaꢀ
tive to newly measured and analyzed data and materials.
Hence, to yield the most consistent and useful experimental
benchmarks for studies going forward, it is important to comꢀ
pare data obtained by the same experimentalists and on the
same volumetric Sievert apparatus.15 Herein we report such a
study. Perhaps our most surprising finding is that HKUSTꢀ18,
a MOF that is available commercially16 in gram scale, exhibits
the highest total, ambient temperature, volumetric uptake of
CH4 of any MOF reported thus far. The value for HKUSTꢀ1 at
65 bar and 298 K (and using the material’s singleꢀcrystal denꢀ
sity) is 267 cc(STP)/cc, on par with a CNG tank at 255 bar.
compression and retaining full sorption capacity. With the
most promising existing MOFs now uniformly experimentally
evaluated on an applicationsꢀrelevant basis, we anticipate that
the results of future investigations with new materials and/or
processing techniques will be straightforward to compare
against these benchmark findings.
Experimental Section
We chose to test the six MOFs shown in Figure 1. The first
five feature copper(II) paddleꢀwheel nodes. The last one is a
nickel(II) variant of the openꢀmetalꢀsite material, CPOꢀ27,
also often called NiꢀMOFꢀ74 or Ni(dobdc)29,10. These MOFs
were selected because they present high surface areas and a
broad range of pore sizes, and because previous studies indiꢀ
cated exceptionally high values for methane uptake. Recently
we reported the gas uptake characteristics of NUꢀ11113,17 and
NUꢀ12512. Consequently, here we include their only roomꢀ
temperature isotherms. For PCNꢀ147, HKUSTꢀ18, NiꢀMOFꢀ74
9, and UTSAꢀ2011, however, we include the results of new
sorption measurements made over a wide range of temperaꢀ
tures. In addition to yielding heats of adsorption for methane,
at the lowest temperature examined these measurements proꢀ
vide an independent experimental estimate of the maximum
amount of methane that can be taken up by a given MOF.
MOF samples were freshly prepared using methods reported
in the literature (see SI for details).7ꢀ11 All samples were synꢀ
thesized in several 100 mg quantities. For HKUSTꢀ1, PCNꢀ14,
and NUꢀ125, we have also repeated the room temperature
isotherm measurements using oneꢀgram samples.
With these results in mind, we additionally examined how
MOF packing density relates to experimentally obtainable
volumetric working capacity, and how intentional MOF comꢀ
paction and wafer formation influence both volumetric and
gravimetric working capacities. These considerations are likeꢀ
ly to lead to a reꢀordering of MOF performance rankings and
underscore the need to move toward materials that are effiꢀ
ciently packable, yet capable of withstanding mechanical
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Table 1. The BET surface areas, pore volumes (measured by N2 and CH4) and densities, ρ, of the MOFs that are studied.
The last column shows the volumetric metal-content in each MOF. The pore volumes and surface areas are calculated (Cal.)
using PLATON19 and nonorthoSA20, respectively. The pore volumes reported in the literature for these MOFs are as fol-
lows: HKUST-1: 0.758f, Ni-MO-74:0.449,10, PCN-14: 0.877, UTSA-20: 0.6311.
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BET (m2/g)
N2
Vpore (cc/g)
CH4
Metal
mmol/cc
4.38
ρ
MOFs
Cal.
2064
1240
2170
1960
3680
4650
N2
Cal.
0.78
0.49
0.76
0.69
1.32
2.03
(g/cc)
0.883
1.206
0.829
0.909
0.578
0.409
HKUSTꢀ1
NiꢀMOFꢀ74
PCNꢀ14
1850
1350
2000
1620
3120
4930
0.78
0.51
0.85
0.66
1.29
2.09
0.78
0.52
7.74
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0.78
2.59
UTSAꢀ20
NUꢀ125
0.66
3.61
1.23
1.82
NUꢀ111
2.12
1.36
All samples were thoroughly outgassed to remove residue
solvent; sample handling was done in a heliumꢀfilled glove
box. The samples were activated immediately before sorption
measurements. Gas sorption measurements were performed on
a carefully calibrated, high accuracy, Sieverts apparatus under
computer control. Instrument and measurementꢀprotocol deꢀ
tails have been published elsewhere;15 more details are given
in the supporting information (SI). All gases were of Research
or Scientific grade, with a minimum purity of 99.999%.
HKUSTꢀ1 used DMF and yielded some cupric oxide as a conꢀ
taminant, while the new synthesis uses water/ethanol and a
small amount of DMF, which is removable via thermal activaꢀ
tion.
Next we studied methane uptake over a wide range of presꢀ
sures and temperatures as shown in Figure 2. (See Figures
S15ꢀS25 for the excess and other isotherms.) The isotherms at
125 K were collected up to the saturation pressure of methane,
which is about 2.45 bar. For clarity, we have scaled the presꢀ
sure axis by 10 for isotherms at this temperature in Figure 2.
From the saturation values of the excess isotherms at 125 K,
we extracted the pore volumes listed in Table 1. In most of the
cases, the pore volumes from nitrogen and methane are in
excellent agreement, justifying the use of nitrogen pore volꢀ
ume in calculating the total methane uptake. Interestingly for
PCNꢀ14, we observed the largest deviation in pore volume
from nitrogen (0.85 cc/g) and methane (0.78 cc/g). The pore
volume for PCNꢀ14 was originally reported7 as 0.87 cc/g,
which is considerably greater than the PLATONꢀcalculated
value of 0.76. Similarly, in the original report, the saturation
methane uptake at 125 K was reported as 444 cc/cc, which
gives a pore volume of 0.94 cc/g. Our saturation value at 125
K is around 355 cc/cc, which gives a pore volume 0.77 cc/g, in
excellent agreement with the calculation but somewhat less
than the nitrogen pore volume of 0.85 cc/g. This suggests eiꢀ
ther that methane cannot access all the pores that nitrogen
reaches or that there are structural changes with gas loading,
yielding different pore volumes with different gases. It would
be interesting to perform inꢀsitu Xꢀray/neutron diffraction
studies to search for potential structural changes with gas loadꢀ
ing. Finally, it is worth mentioning, in calculating the total
methane uptake values, we used the pore volumes obtained
from nitrogen gas, as is usually done in the literature. Howevꢀ
er, the small differences in CH4 and N2 pore volumes do not
create noticeable error in values for total methane uptake, as
most of the gas adsorption is due to excess uptake.
Results and discussion
Shown in Figure 1 are HKUSTꢀ18, PCNꢀ147, UTSAꢀ2011,
NUꢀ11113,17, NUꢀ12512, and NiꢀMOFꢀ749, 10. HKUSTꢀ1 has a
structure with small cages of ~ 4, 10 and 11 Å diameter. PCNꢀ
14 contains two types of pores: one a relatively small spherical
cage of ~7 Å, and the other an elliptical cage extending along
the cꢀaxis. In UTSAꢀ20, two types of oneꢀdimensional chanꢀ
nels exist, each presenting unsaturated metal centers to potenꢀ
tial guest molecules. One type comprises rectangular pores of
about 3.4×4.8 Å, while the second comprises cylindrical pores
of ca. 8.5 Å diameter. NUꢀ125 and NUꢀ111 are representative
of secondꢀgeneration MOFs having surface areas exceeding
3,000 m2/g. NUꢀ125 has an rht topology with four distinct
cages of approximately 24, 16, 15, and 11 Å diameter. NUꢀ
11117 features a faceꢀcentered cubic (fcc) structure, with cages
located at the origin, tetrahedral and octahedral sites of an fcc
lattice. NiꢀMOFꢀ74 consists of a hexagonal array of 1D hexꢀ
agonal channels of 13.6 Å maximum diameter. The metal
nodes form stripes down the length of the channels, with each
node connecting three channels.
First, we studied the permanent porosity of the activated
MOFs by N2 adsorption measurements at 77 K (see Figures
S8ꢀS12). The BET surface areas, pore volumes, densities of
the deꢀsolvated structures, and the volumetric metalꢀ
concentrations are summarized in Table 1. The calculated
values utilizing PLATON19 are listed as well. The calculated
values are in excellent agreement with our measured values.
The pore volumes and surface areas reported in Table 1 are
mainly consistent with the values reported in the literature.
The pore volume of our HKUSTꢀ1 sample is 0.78 cc/g, which
is 4% higher than the previously report value of 0.75 cc/g8f.
The synthesis of HKUSTꢀ1 has been improved significantly as
evident from increasing pore volumes reported in the literaꢀ
ture: 0.33 cc/g by Chui8a, 0.4 cc/g by Lee8d, 0.68 cc/g by Morꢀ
ris8e and 0.75 cc/g by Rowsell8f. The original synthesis of
The most surprising finding in our study is the exceptionally
high volumetric uptake by HKUSTꢀ1 as shown in Figure 2. At
35 bar, HKUSTꢀ1 exhibit a total CH4 uptake of 227
cc(STP)/cc, a value midway between the old and new targets.
However, near 65 bar the total volumetric uptake by HKUSTꢀ
1 is 267 cc(STP)/cc, slightly higher than the new target, and
on par with compressed natural gas tank (CNG) at 255 bar.
Hence, having HKUSTꢀ1 in the storage tank will effectively
reduce the pressure fourꢀfold, simplifying and reducing the
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cost associated with the highꢀpressure technology. It is quite ation of 1ꢀ3%. Additionally, we examined a commercially
offered sample of HKUSTꢀ116 and obtained results similar to
those for our labꢀsynthesized samples (see Figure S22). The
commercial HKUSTꢀ1 sample shows about 5% lower uptake
at 65 bar than our samples. In view of the presumably large
scale of the commercial synthesis of HKUSTꢀ1, this outcome
is rather impressive and suggests that HKUSTꢀ1 may be a
good benchmarking compound for methane sorption by other
porous compounds.
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8
interesting that even though HKUSTꢀ1 is one of the earliest
MOFs to have been extensively studied,8 its exceptionally high
volumetric methane uptake properties were missed. In fact, a
few years ago, we measured the methane uptake of HKUSTꢀ1,
which showed low mmol/g uptake21; we did not recognize,
however, its high volumetric uptake.
400
HKUST-1
Ni-MOF-74
NiꢀMOFꢀ74 (Figure 2) exhibited remarkably high total volꢀ
umetric uptake, with values at the highest pressures approachꢀ
ing those seen with HKUSTꢀ1. The N2 isotherm of fresh Niꢀ
MOFꢀ74 gave a pore volume of 0.51 cc/g, i.e. somewhat highꢀ
er than the previously reported value of 0.44 cc/g but in good
agreement with the value expected based on PLATON analyꢀ
sis of the corresponding singleꢀcrystal Xꢀray strucꢀ
ture.10aSimilarly, the absolute methane uptake of 228 cc/cc at
298K and 35 bar is higher than the previously reported value
of 200 cc/cc. Given the differences, we carefully characterized
samples from multiple independent syntheses and in every
instance obtained larger pore volumes and larger values for
methane uptake than previously reported (see Figure S24).
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300
200
100
125 K
200 K
240 K
270 K
298 K
323 K
0
400
PCN-14
UTSA-20
10xP
10xP
As pointed out in Ref. 9, the most exceptional compositionꢀ
al feature of MꢀMOFꢀ74 (M=Ni, Mg, etc) is that it has a very
high volumetric density of metal centers (see Table 1). Hence,
by adsorbing one methane molecule at each metal site, the
volumetric uptake reaches 173 cc/cc (¾ of the total uptake at
35 bar and 298 K). The downside of NiꢀMOFꢀ74 is its high
density and therefore the gravimetric uptake is low. Additionꢀ
ally, relying heavily on open metal sites may prove problematꢀ
ic due to competition from water for these sites.23
300
200
100
0
0
20 40 60
Pressure (bar)
0
20 40 60
Pressure (bar)
The third best value for volumetric uptake comes from
PCNꢀ14. At 35 bar and 298 K, and employing the density of
the fully activated structure, we obtain 195 cc(STP)/cc at
298K. We tested the reproducibility of the PCNꢀ14 results by
synthesizing several samples ranging from 250 mg to 1 g and
obtained the same isotherms within 1% (See Figure S24). The
originally reported value of 230 cc(STP)/cc was measured at
290K and the density of the structure with coordinated water
was used. Hence, our measurements agree reasonably well
with earlier findings and clearly support the earlier claim of
exceptionally high volumetric uptake of PCNꢀ14 near room
temperature.
Figure 2. Total volumetric uptake isotherms at various tem-
peratures for HKUST-1, PCN-14, UTSA-20, and Ni-MOF-74.
The gray horizontal lines indicate the DOE’s old and new
volumetric targets, respectively. The pressure axes for the 125
K isotherms are scaled by 10 for clarity. The pore volumes
obtained from the saturation uptake at 125 K are summarized
in Table 1. The orange lines in the background are isotherms
obtained using He-cold volumes with sample in, while the
other lines are isotherms obtained using cold-volumes of emp-
ty cell measured separately (see SI for details). Ni-MOF-74
shows significant He-adsorption at high pressures at 240 K
and below. The temperature dependent isotherms for NU-125
and NU-111 are provided in Figures S19-S20 in the SI (and
were recently published in Refs. 12 and 13).
HKUST
25
Ni-MOF-74
PCN-14
UTSA-20
Since the results of HKUSTꢀ1 were quite unexpected, we
sought to confirm them and, in particular, to eliminate any
possible measurement error attributable to sample size. Thus,
we repeated the measurement of room temperature isotherms
using oneꢀgram samples of HKUSTꢀ1 and NUꢀ12512. In Figꢀ
ure S21 we show that the isotherms from 100 mg sample and
oneꢀgram sample are almost identical, giving confidence that
the reported isotherms here are accurate. We also emphasize
that we are reporting two isotherms; one obtained from Heꢀ
cold volumes with sample in and the other from emptyꢀcell
cold volumes, which are measured separately. The isotherms
are in excellent agreement,22 supporting the accuracy of the
measurements. We tested the reproducibility of the HKUSTꢀ1
results by synthesizing several samples ranging from 100 mg
to 2 g and obtained the same high uptake values within a deviꢀ
NU-111
NU-125
20
15
10
0
1
2
Amount adsorbed per metal-site
3
0.5
1.5
2.5
Figure 3. The isosteric heat of adsorptions as a function of
methane uptake per metal-site for each MOF studied.
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Table 2. The methane uptake characteristics of the MOFs under consideration at 298 K. The working capacity is defined as
the difference in total uptake between 65 bar and 5 bar. The density is given in g/cc and corresponds to the density of me-
thane gas in a compressed tank which has the same amount of methane stored in the pores of MOF.
1
2
3
4
5
6
7
8
9
Excess (35 bar)
Total (35 bar)
Excess (65 bar)
Total (65 bar)
Working Capacity
Qst
kJ/mol
ꢀꢀ
g/g
cc/cc
ꢀꢀ
g/g
cc/cc
263
227
228
195
184
182
138
g/g
cc/cc
ꢀꢀ
g/g cc/cc density
0.5 263 0.188
0.216 267 0.191
0.148 251 0.180
0.197 230 0.164
0.181 230 0.164
0.287 232 0.166
0.360 206 0.147
g/g
cc/cc
DOE Target
HKUSTꢀ1
NiꢀMOFꢀ74
PCNꢀ14
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
0.5
0.165
0.122
0.146
0.131
0.192
0.191
204
206
171
167
155
109
0.184
0.135
0.169
0.145
0.225
0.241
0.178
0.125
0.157
0.150
0.223
0.262
220
210
183
191
181
150
0.154
0.077
0.136
0.134
0.227
0.313
190a
129a
157
170
183
179
17.0
21.4
18.7
18.2
15.1
14.2
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UTSAꢀ20
NUꢀ125
NUꢀ111
aThe working capacities are 216 cc/cc and 165 cc/cc for HKUSTꢀ1 and NiꢀMOFꢀ74, if the end point is taken at 323 K and 5 bar.
Finally, UTSAꢀ20 comes as the fourth best MOF in terms of
total volumetric uptake. Our measured value of 184
cc(STP)/cc at 35 bar and 298 K is slightly lower than the origꢀ
inally reported value of 195 cc(STP)/cc.
The next best MOF is NUꢀ125, which can be easily syntheꢀ
sized in gram scale.12
300
250
200
In order to gain better insight into the nature of the adsorpꢀ
tion sites and CH4ꢀMOF interactions, we extracted isosteric
heats of adsorption (Qst) from the temperatureꢀdependent isoꢀ
therms shown in Figure 2 using the ClausiusꢀClapeyron equaꢀ
tion (see Figures S27ꢀS33). The results are summarized in Fig.
3. The initial values of Qst are roughly proportional to the metꢀ
al content in each MOF (see Table 1), suggesting that the metꢀ
al center is the initial adsorption site. NiꢀMOFꢀ74 shows alꢀ
most constant Qst and then drops sharply near a loading of one
methane molecule per Niꢀsite. With increasing gas loading, the
CH4ꢀCH4 interaction starts to play an important role, causing
an increase in Qst at high loading. We also note that the Qst at
zero coverage limit decreases in MOFs with larger pore volꢀ
ume and surface area. This suggests that the observed initial
Qst for copperꢀpaddle wheel MOFs are not purely gasꢀCu inꢀ
teraction but there is significant contribution from pore conꢀ
finement. As the MOF pores get larger as in the case of NUꢀ
125 and NUꢀ111, the initial Qst values are very low and repreꢀ
sent mainly the copperꢀCH4 interaction. We also note that
high Qst results rapid increase in the isotherm, yielding lower
working capacity. Finally, the values of Qst reported in Table 1
are in good agreement with other studies 6a,6r,6s. The biggest
discrepancy is the initial Qst for PCNꢀ14 which was reported as
30 kJ/mol while we obtain a number around 19 kJ/mol.
HKUST-1
Ni-MOF-74
PCN-14
UTSA-20
NU-125
NU-111
150
100
50
0
0.4
0.3
0.2
0.1
0
0
10
20
30 40
Pressure (bar)
50
60
70
Figure 4. Total volumetric and gravimetric uptakes for six
MOFs studied. The gray horizontal lines show the old and
new DOE targets for volumetric methane storage. The gravi-
metric target is 0.5 grams of methane per gram of sorbent.
We note that the total volumetric and gravimetric uptake
values are not what determine a material’s performance. Ultiꢀ
mately the working capacity, defined here as the difference in
uptake at two pressures (taken as 65 bar and 5 bar) determines
how far a car powered by methane could go. From Table 2,
one can conclude that NiꢀMOFꢀ74, despite its high total capacꢀ
ity, has a comparatively poor working capacity – indeed, by
far the lowest of the six (admittedly highꢀperforming) MOFs
examined. The problem with the Niꢀcontaining MOF is that
nearly half the total capacity at 65 bar is already reached at 5
bar, leaving a comparatively small difference between the two.
MOFs presenting fewer open metal sites and engaging in
weaker interactions with methane are the better choice for
achieving high working capacities. NUꢀ111 and NUꢀ125 fall
at the opposite end of the range of MOF densities and metal
In Figure 4 and Table 2, we compare the room temperature
values for total volumetric (top) and gravimetric (bottom) meꢀ
thane uptake for the six MOFs investigated. From the figure, if
the volumetric uptake is the main target, then clearly HKUSTꢀ
1 and NiꢀMOF74 are the top two materials. They both exhibit
rather high total uptake – around 230 cc/cc at 35 bar and 250ꢀ
270 cc/cc at 65 bar, meeting DOE’s new volumetric target (if
the packing loss is ignored). However we note that in addition
to volumetric uptake, the gravimetric uptake is important and
in fact DOE’s new target of 0.5 g/g is challenging to reach as
shown in the bottom panel of Figure 4. In terms of gravimetric
uptake, NUꢀ111, a MOF with a surface area of 4,930 m2/g and
a pore volume of 2.09 cc/g, exhibits the best result, reaching
almost 75% of both the volumetric and gravimetric targets.
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content, so should (and do) exhibit better working capacities the case for the MOFs that we have studied here. We obtain a
1
2
3
4
5
6
7
8
than NiꢀMOFꢀ74. Indeed the working capacities of these
MOFs are comparable to HKUSTꢀ1 and PCNꢀ14 even though
values for total volumetric uptake by NUꢀ125 and NUꢀ111 are
not as high. Interestingly, HKUSTꢀ1 seems to be rather optiꢀ
mum for both the total uptake and the working capacity. It
shows an impressive 190 cc/cc working capacity – roughly 5%
greater than NUꢀ125 and NUꢀ111. However, the mass per unit
volume of HKUSTꢀ1 is twice that for NUꢀ111. For onꢀboard
applications, therefore, NUꢀ111 might well prove superior
since it does well in terms of both gravimetric and volumetric
capacities. Working capacities can be also optimized by comꢀ
bining temperature and pressure swing processes. For example
if the end point is taken at 323 K (instead of 298 K) and 5 bar,
the working capacities of HKUSTꢀ1 and NiꢀMOFꢀ74 become
216 cc/cc and 165 cc/cc, respectively.
nearly perfectly linear variation of Vpore with BET surface
area, with the bestꢀfit line passing through the origin (i.e. no
surface area ꢀ no pore volume). From this plot, we estimate
that a MOF surface area of about 7,500 m2/g would be needed
to reach 0.5 g/g uptake of methane. Among existing MOFs,
NUꢀ109 and NUꢀ110 at ca. 7,000 and 7,100 m2/g, respectiveꢀ
ly, are the two that most closely approaching this benchmark.24
In Figure 5 we also show that the total volume Vtotal (i.e.
1/ρ) is also proportional to surface area. Hence the 7,500 m2/g
surface area suggests a density of 0.28 g/cc, which means that
our hypothetical MOF with surface area 7500 m2/g and pore
volume Vp=3.2 cc/g should exhibit a volumetric uptake of ca.
195 cc/cc.25 The skeletal density (1/(VtotalꢀVpore)) of our extrapꢀ
olated MOF is 2.5 g/cc, which is less than HKUSTꢀ1 (2.8
g/cc) and, interestingly, comparable to PCNꢀ14 (2.57 g/cc),
further giving confidence that such an extrapolated MOF is
quite physical and could be within reach experimentally.
Needless to say, there are other factors that can affect the gas
uptake properties of MOFs and it is quite possible that we may
discover a new MOF which can go beyond the 200 cc(STP)/cc
limit that we seem to have from our simple linear correlation
shown in Figure 5.
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4
VTotal=3.6 cc/g (ρ=0.28 g/cc)
Vpore=3.2 cc/g
3
Vol. CH4 Uptake
S=7500 m2/g
195 cc(STP)/cc
2
1
0
VTotal = 1/ρ (cc/g)
Vpore (cc/g)
Total Gravimetric uptake (g/g)
0
2000
4000
6000
8000
BET Surface Area S (m2/g)
Figure 5. Total gravimetric uptake, pore volume Vpore, and
total volume VTotal (1/ρ) as a function of BET surface area,
showing a nice linear dependence for each case. The horizon-
tal grey line indicates the DOE’s gravimetric target 0.5 g/g,
which suggests a MOF with surface area S = 7,500 m2/g and
pore volume 3.2 g and density 0.28 g/cc can exhibit 0.5 g/g
gravimetric uptake and ~195 cc/cc volumetric uptake
Figure 6. Total volumetric methane uptake by HKUST-1 for
different packing densities. The insets show a picture of
HKUST-1 powder packed in a 1 ml syringe by hand press
(top) and a pressed wafer (bottom).
Finally, we address the importance of the density value used
in determining the total volumetric uptake. So far, we have
used the ideal singleꢀcrystal MOF densities to obtain the volꢀ
umetric uptake values, as this is done traditionally in the literaꢀ
ture. However, here we show that the density of a packed polꢀ
ycrystalline powder sample can differ considerably from the
ideal singleꢀcrystal density, resulting in experimental volumetꢀ
ric methane capacities that are much lower than we have sugꢀ
gested. In Figure 6, we compare the ideal volumetric uptake
versus the actual volumetric uptake by an HKUSTꢀ1 powder
sample packed by hand. As shown in the inset to Figure 6, we
filled a 1 ml syringe with MOF powder and pressed hard (by
hand) while loading. We could pack about 215 mg into a 0.5
ml volume (Figure 6), yielding a so called “tapped density” of
0.43 g/cc, i.e. roughly half of the ideal density of 0.883 g/cc.
Hence the excess volumetric uptake is reduced from its ideal
value by half. We tried increasing the packing density by
pressing the MOFs into wafers as shown in Figure 6. Nitrogen
Next we address the question of whether there is a MOF
that can meet the DOE’s new gravimetric target of 0.5 g/g. A
simplistic way of addressing this issue is by looking at the
correlation between gas uptake values of the MOFs examined
here and their surface areas. As shown in Figure 5, these mateꢀ
rials (admittedly a small number) show a good linear correlaꢀ
tion between the gravimetric uptake and BET surface area.
This is somewhat surprising since the total uptake has two
contributions. One is from the excess uptake, which should
increase roughly linearly with increasing surface area. For the
materials studied, this contribution accounts for between 71
and 79% of the total uptake at 65 bar. The second contribution
is from the methane that is present solely because of encapsuꢀ
lation by the material’s pores. This contribution is estimated as
ρ(CH4, P, T)xVpore. The linear dependence of the total uptake
therefore suggests that Vpore is also roughly linearly proporꢀ
tional to the surface area. Indeed, as shown in Figure 5, this is
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Journal of the American Chemical Society
isotherms for the compacted wafer samples show that overall
Supporting Information
1
2
3
4
5
6
7
8
micropore volumes are significantly diminished (see Figure
S34). Powder Xꢀray diffraction measurements are characterꢀ
ized by broadened peaks and reduced intensities gone down
significantly, suggesting partial collapse of the HKUSTꢀ1
framework with pressure. Indeed, for a sample subjected to 5
tons of mechanical pressure, we obtained a density of 1.1 g/cc,
significantly larger than the ideal crystal density of 0.883 g/cc.
Even though the density is higher, the total volumetric uptake
value is very low as shown in Figure 6 due to loss of the poꢀ
rosity. Clearly needed going forward are either nonꢀdestructive
ways of compacting highꢀporosity MOFs, or alternative MOF
formulations and structures that are little affected by mechaniꢀ
cal compaction.
Details of sample synthesis, individual absolute and excess
adsorption isotherms , the 77 K N2 isotherms, consistency and
BET plots, xꢀray data of press pellets.This material is available
free of charge via the Internet at http://pubs.acs.org.
Author Information
Corresponding Author
*To whom correspondence should be addressed.
Email: taner@seas.upenn.edu or oꢀfarha@northwestern.edu
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Notes
The authors declare no competing financial interest.
Conclusions
Acknowledgements
We have synthesized and studied methane uptake by six of
the most promising existing MOFs, using a single highꢀ
pressure Sievert apparatus and a standardized measurement
protocol intended to yield to consistent results. We discovered
that both HKUSTꢀ1 and NiꢀMOFꢀ74 exhibit exceptionally
high volumetric methane uptake, with both reaching DOE’s
new volumetric target at 65 bar if the packing loss is ignored.
When working capacities (i.e. deliverable capacities) are exꢀ
amined, however, NiꢀMOFꢀ74 fares poorly, as its comparaꢀ
tively high affinity for CH4 results in substantial uptake beꢀ
tween 0 and 5 bar (where 5 bar constitutes the lowest deliverꢀ
able methane pressure for vehicle applications). Thus, altꢀ
hough the targets posed by DOE are for total amounts of meꢀ
thane that can be stored, it is clear that progress at a practical
level will require that we and others focus primarily on delivꢀ
erable capacities.
TY acknowledges the support by the U. S. Department of Energy
through BES Grant No. DEꢀFG02ꢀ08ER46522. OKF and JTH
gratefully acknowledge the U. S. DOE’s ARPAꢀE program.
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1
2
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4
5
6
7
8
9
http://www.sigmaaldrich.com/catalog/product/aldrich/688614?la
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other reported work. Such identification does not imply recomꢀ
mendation or endorsement us, nor does it imply that the materiꢀ
als or equipment identified are necessarily the best available for
the purpose.
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both of which were based on gravimetric adsorption measureꢀ
ments. Unlike volumetric adsorption measurements as done in
this work, the gravimetric measurements are complicated due to
buoyancy effects. The reported excess gravimetric uptake in
Ref. 8b and 8g is around 0.16 g/g, which is 10% lower than our
measurement of 0.18 g/g. The difference is probably due to
sample quality as the pore volume in in Ref. 8b was 0.71 cc/g ,
10% lower than ours, 0.78 cc/g.
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