Metal-organic frameworks for oxygen storage

Article abstract of DOI:10.1002/anie.201408464

We present a systematic study of metal-organic frameworks (MOFs) for the storage of oxygen. The study starts with grand canonical Monte Carlo simulations on a suite of 10000 MOFs for the adsorption of oxygen. From these data, the MOFs were down selected to the prime candidates of HKUST-1 (Cu-BTC) and NU-125, both with coordinatively unsaturated Cu sites. Oxygen isotherms up to 30 bar were measured at multiple temperatures to determine the isosteric heat of adsorption for oxygen on each MOF by fitting to a Toth isotherm model. High pressure (up to 140 bar) oxygen isotherms were measured for HKUST-1 and NU-125 to determine the working capacity of each MOF. Compared to the zeolite NaX and Norit activated carbon, NU-125 has an increased excess capacity for oxygen of 237% and 98%, respectively. These materials could ultimately prove useful for oxygen storage in medical, military, and aerospace applications.

Full text of DOI:10.1002/anie.201408464

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Chemie
DOI: 10.1002/anie.201408464
Oxygen Storage
Hot Paper
Metal–Organic Frameworks for Oxygen Storage**
Jared B. DeCoste,* Mitchell H. Weston, Patrick E. Fuller, Trenton M. Tovar,
Gregory W. Peterson, M. Douglas LeVan,* and Omar K. Farha*
Abstract: We present a systematic study of metal–organic
frameworks (MOFs) for the storage of oxygen. The study starts
with grand canonical Monte Carlo simulations on a suite of
10000 MOFs for the adsorption of oxygen. From these data,
the MOFs were down selected to the prime candidates of
HKUST-1 (Cu-BTC) and NU-125, both with coordinatively
unsaturated Cu sites. Oxygen isotherms up to 30 bar were
measured at multiple temperatures to determine the isosteric
heat of adsorption for oxygen on each MOF by fitting to a Toth
isotherm model. High pressure (up to 140 bar) oxygen
isotherms were measured for HKUST-1 and NU-125 to
determine the working capacity of each MOF. Compared to
the zeolite NaX and Norit activated carbon, NU-125 has an
increased excess capacity for oxygen of 237% and 98%,
respectively. These materials could ultimately prove useful for
oxygen storage in medical, military, and aerospace applica-
tions.
pressure due to safety concerns. Porous carbons such as MSC-
7R and AX-21,[7] as well as the zeolite NaX,[8] have been
tested for oxygen storage. The biggest drawback of activated
carbons is the heterogeneity of the pore sizes which makes
their design difficult. However, the ability to tune the pore
geometry and size of MOFs makes them excellent candidates
for oxygen storage applications.[9]
Prior to performing experiments, 10000 hypothetical
MOF materials were simulated for oxygen capacities to
down select which MOFs were studied experimentally. The
database was created through the use of established high-
speed MOF generation techniques[10] and designed to max-
imize the breadth of material properties over a minimal set of
frameworks. Each material in the database was then run
through a charging algorithm and a grand canonical Monte
Carlo (GCMC) simulation, returning calculated room tem-
perature oxygen adsorption quantities as a function of
pressure, as depicted in Figure 1.[1f,11] The most promising
materials from this study were then curated, producing a final
M
etal–organic frameworks (MOFs) have been of broad
interest for their wide variety of applications such as gas
storage,[1] separations,[2] sensing,[3] catalysis,[4] light harvest-
ing,[5] and toxic gas removal.[6] However, to date there has
been minimal focus on oxygen storage. The storage of oxygen
has applications for use by first responders and military
personnel, as well as in the medical and aerospace industries.
In these applications there is a need to increase the amount of
oxygen stored per unit volume, or reduce the oxygen storage
[*] Dr. J. B. DeCoste
Leidos, Inc.
PO Box 68, Gunpowder, MD 21010 (USA)
E-mail: jared.b.decoste2.ctr@mail.mil
Dr. M. H. Weston, P. E. Fuller, Dr. O. K. Farha
NuMat Technologies
8025 Lamon Avenue, Skokie, IL 60077 (USA)
Dr. O. K. Farha
Department of Chemistry, Northwestern University
2145 Sheridan Rd., Evanston, IL 60208 (USA)
E-mail: o-farha@northwestern.edu
T. M. Tovar, Dr. M. D. LeVan
Department of Chemical and Biomolecular Engineering
Vanderbilt University
PMB 351604, Nashville, TN 37235 (USA)
E-mail: m.douglas.levan@vanderbilt.edu
G. W. Peterson
Edgewood Chemical Biological Center
5183 Blackhawk Rd., Aberdeen Proving Ground, MD 21010 (USA)
[**] J.B.D. and G.W.P. gratefully acknowledge the Defense Threat
Reduction Agency (DTRA) for financial support under
BA07PRO104, and M.D.L. acknowledges DTRA under W911SR-13-
C-0014.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408464.
Figure 1. Generation of MOFs and their computational screening for
sorption characteristics.
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list of materials for experimental validation. From the
information obtained in these simulations, it was determined
that HKUST-1[12] (Figure S1) and NU-125[13] (Figure S2) were
prime candidates for further studies, based on their superior
ability to adsorb oxygen and currently known synthesis
techniques. These MOFs, along with UiO-66 (Figure S3),
a zirconium based MOF known for its exceptional stabili-
ty,[9a,14] an activated carbon (Norit SX ultra, herein referred to
as N-AC), and zeolite NaX were examined by measuring
oxygen isotherms up to 30 bar at multiple temperatures and
high-pressure oxygen isotherms up to 140 bar at room
temperature. The 30 bar metric was chosen due to safety
concerns with higher pressure oxygen at elevated temper-
atures (up to 348 K), whereas the 140 bar metric at ambient
temperature was chosen based on the standard pressure used
in medical oxygen tanks. N-AC was used as a baseline
sorbent, as previous works have shown that activated carbons
can provide high oxygen storage capacities.[7]
temperature and pressure dependence of adsorption over
wide ranges and is described in detail in the Supporting
Information.[15] At high pressures, adsorption is not likely to
form a monolayer as assumed by a Langmuir model, and the
Toth equation allows an extra empirical parameter for fitting.
This work focuses on the adsorption of oxygen at high
pressures; therefore, data were not obtained below 1 bar,
where high-energy sorbent–sorbate interactions occur. It was
observed that the isosteric heat of adsorption for N-AC
decreases as a function of increased loading at 298 K,
indicative of adsorption with a heterogeneous material. On
the other hand, HKUST-1, NU-125, and NaX each show
a relatively stable isosteric heat of adsorption with increased
loading at 298 K, indicative of adsorption dominated mainly
by geometric phenomena on a more homogeneous surface. It
is important to have a high capacity while minimizing the
strong interactions of the material with the adsorbate,
because these interactions typically dominate at pressures
below the working pressure of gas cylinders. Both HKUST-
1 and NU-125 contain coordinatively unsaturated copper
sites; however, these sites do not seem to preferentially
adsorb oxygen at the high pressures studied here. The
adsorption of oxygen was observed to be geometric in the
GCMC calculations and oxygen did not cluster at the Cu sites.
However, it has been shown that coordinatively unsaturated
metal sites can enhance the adsorption of oxygen at low
pressures.[16] Furthermore, the isosteric heat of adsorption for
HKUST-1 is higher than that for NU-125, likely due to the
smaller pore size and a higher concentration of Cu sites in
HKUST-1.
Excess oxygen isotherms for HKUST-1, NU-125, and
UiO-66 were measured for pressures up to approximately
30 bar at 298 K and are compared to N-AC and NaX in
Figure 2. At pressures less than 2 bar, the inset shows that
NU-125 and N-AC have similar O2 capacities, whereas
Based on the isotherms studied up to 30 bar, we decided
to investigate HKUST-1, NU-125, and N-AC further. These
materials were studied up to pressures of 140 bar at room
temperature and compared to NaX from Wang et al.,[8] with
the excess oxygen isotherms shown in Figure 3 and Table 1.
The isotherm for each material compares well to the
predicted outcome from the Toth model and the oxygen
isotherm predicted from the GCMC calculations (HKUST-
1 and NU-125 only; Figures S9–S12). NU-125 and HKUST-
1 have excess capacities of 98 and 75% greater than that of
the N-AC at 140 bar, respectively. Based on volumetric
Figure 2. Excess oxygen isotherms measured at 298 K up to 30 bar.
HKUST-1 has a slightly lower capacity. The isotherm for N-
AC curves downward, while the HKUST-1 and NU-125
isotherms remain much more linear. At 30 bar NU-125,
HKUST-1, and N-AC have excess O2 capacities of 8.3, 6.0,
and 5.0 molkgÀ1, respectively. In contrast, UiO-66 and NaX
have significantly lower capacities of 3.5 and 2.0 molkgÀ1,
respectively. Ideal materials exhibit oxygen isotherms char-
acterized by gradual slopes at low pressures, indicating
minimal sorbent–sorbate interactions at low pressures, and
high overall capacities at high pressures, thereby maximizing
working deliverable capacity.
Excess oxygen isotherms were measured at 298, 323, and
348 K for each MOF and N-AC (Figures S4–S7). From this
data and the oxygen isotherms of NaX reported elsewhere,[8]
the isosteric heat of adsorption of oxygen was calculated for
each material at 298 K using the Toth model (Table S1,
Figure S8). The Toth equation can be used to describe the
Figure 3. Excess oxygen adsorption isotherms measured at room
temperature up to 140 bar.
2
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Table 1: Surface area, pore volume, excess and absolute oxygen capacity,
and deliverable oxygen capacity for NU-125, HKUST-1, activated carbon,
and NaX.
and 89%, respectively, over an empty cylinder. These MOFs
could also allow for one to obtain a similar capacity to an
empty cylinder at a decreased pressure, which could allow for
the use of less heavy-duty cylinder components, ultimately
reducing the cost and total mass of the oxygen storage device.
Increased capacities in oxygen cylinders can lead to longer
dives for divers, more efficient storage for fighter pilots and
astronauts, and a better quality of life for those who are
dependent on medical oxygen.
Material BET sur- Pore
Excess O2
Absolute O2 Deliverable
face area volume adsorbed[b] adsorbed[b]
O2
[c]
[m2 gÀ1
]
[ccgÀ1
]
[molkgÀ1
]
[molkgÀ1
]
[molkgÀ1
]
NU-125 2880
HKUST- 1880
1
1.29
0.77
10.1
8.9
17.4
13.2
15.7
11.9
N-AC
1040
560
0.76
0.27
5.1
3.0
9.4
7.2
8.0
6.6
NaX[a]
[a] From Ref. [7]. [b] Capacity at 140 bar. [c] Absolute capacity at 140 bar
minus the absolute capacity at 5 bar.
Experimental Section
The procedures for synthesizing HKUST-1, NU-125,[13] and UiO-66[17]
are outlined in detail in the Supporting Information. The activated
carbon Norit SX ultra (Aldrich, 53663) was purchased and used
without further modification.
parameters, HKUST-1 and NU-125 have capacities of 114 and
89% greater than an empty oxygen cylinder at 140 bar,
respectively.
A volumetric apparatus was used to collect oxygen isotherms at
298, 313, and 348 K over the range of 1–30 bar, as described in detail
in the Supporting Information and elsewhere.[8] This apparatus was
also used to collect oxygen isotherms at 298 K after repeated
adsorption/desorption cycles. High pressure oxygen isotherms (up
to 140 bar) were measured using a Micromeritics HPVA II-200
Volumetric Sorption Analyzer.
For HKUST-1 and NU-125, adsorption and desorption
cycles were performed to determine the structural integrity of
the MOFs upon exposure to high pressure oxygen. It can be
seen in Figure 4 that for each of the MOFs there is no
appreciable loss in capacity over 50 cycles run up to 30 bar,
and it can therefore be concluded that the structural integrity
is maintained in the presence of oxygen at the pressures
studied. This is an important feature when considering
recharging cycles.
Received: August 22, 2014
Published online: && &&, &&&&
Keywords: adsorption · metal–organic frameworks ·
.
microporous materials · oxygen
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Oxygen Storage
J. B. DeCoste,* M. H. Weston, P. E. Fuller,
T. M. Tovar, G. W. Peterson,
M. D. LeVan,*
O. K. Farha*
&&&& — &&&&
Metal–Organic Frameworks for Oxygen
Storage
MOFs store oxygen too: Two metal–
organic frameworks with open metal sites
store oxygen at capacities far greater than
that of an empty cylinder and are com-
parable to the state-of-the-art oxygen
storage materials. Self-assembled mate-
rials allow for rational design of materials
for adsorption of specific gases.
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