A porous metal-organic framework with dynamic pyrimidine groups exhibiting record high methane storage working capacity

Article abstract of DOI:10.1021/ja501810r

We have realized a new porous metal-organic framework UTSA-76a with pyrimidine groups on the linker, exhibiting high volumetric methane uptake of ～260 cm³ (STP) cm^-3 at 298 K and 65 bar, and record high working capacity of ～200 cm³ (STP) cm^-3 (between 5 and 65 bar). Such exceptionally high working capacity is attributed to the central dynamic pyrimidine groups within UTSA-76a, which are capable of adjusting their orientations to optimize the methane packing at high pressure, as revealed by computational studies and neutron scattering experiments.

Full text of DOI:10.1021/ja501810r

Communication
pubs.acs.org/JACS
A Porous Metal−Organic Framework with Dynamic Pyrimidine
Groups Exhibiting Record High Methane Storage Working Capacity
Bin Li,† Hui-Min Wen,† Hailong Wang,† Hui Wu,‡,§ Madhusudan Tyagi,‡,§ Taner Yildirim,*,‡,∥
Wei Zhou,*,‡,§ and Banglin Chen*,†
†Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249, United States
‡NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
§Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States
∥Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
S
* Supporting Information
If we do not consider the adsorbent material packing loss (i.e.,
ABSTRACT: We have realized a new porous metal−
organic framework UTSA-76a with pyrimidine groups on
the linker, exhibiting high volumetric methane uptake of
∼260 cm3 (STP) cm−3 at 298 K and 65 bar, and record
high working capacity of ∼200 cm3 (STP) cm−3 (between
5 and 65 bar). Such exceptionally high working capacity is
attributed to the central “dynamic” pyrimidine groups
within UTSA-76a, which are capable of adjusting their
orientations to optimize the methane packing at high
pressure, as revealed by computational studies and neutron
scattering experiments.
assuming the gas tank filled with a large single crystal of
adsorbent), the new volumetric target corresponds to 263 cm3
(STP) cm−3, which is equivalent to that of CNG at 250 bar and
298 K.6
Although the potential of MOF materials for methane
storage has been conceptually established, it is still a grand
challenge to reach the new targets set by the DOE MOVE
program. Since the discoveries of the first several MOFs for
methane storage,7 significant progress has been made to
improve the methane storage capacities of MOF materials over
the past decade; however, their storage capacities are still quite
far away from the new DOE targets.8 Recently, some of us have
reported that HKUST-1 (a MOF first reported in 1999),9 upon
full activation, exhibits a volumetric storage capacity of 267 cm3
(STP) cm−3 at 65 bar and room temperature, the highest
reported so far.10a A recent work by another group also
reported similar results.10b These new studies are very
encouraging, because this is the first MOF material whose
volumetric methane storage capacity has potentially reached the
new DOE target if material packing loss is ignored.
As revealed, the saturated gravimetric methane storage
capacities of MOF materials are basically determined by their
porosities (pore volumes and/or BET surface areas).10a,11 In
order to optimize volumetric methane storage capacities, the
ideal MOFs should have balanced porosities and framework
densities, and high densities of functional sites/groups and pore
cages for the recognition of methane molecules.11a,12 In this
regard, the MOF-505 series of MOFs of NbO type structures
meet such criteria and are particularly of interest.13 On the basis
of these fundamental framework backbones, if additional
functional sites/groups can be introduced to enhance their
interactions with methane molecules, we should be able to
target some new porous MOFs with higher methane storage
capacities. With this in mind, we developed a new organic
linker containing pyrimidine group (Scheme 1, two linkers) and
synthesized and structurally characterized its copper MOF
[Cu2L(H2O)2]·5DMF·3H2O (we term as UTSA-76).
omestic natural gas production in the United States has
Drecently expanded dramatically because of a technology
known as “fracking” that makes retrieving natural gas from
shale formations more cost-effective.1 However, the widespread
use of natural gas as alternative fuel for transport applications
depends on the development of technologies to efficiently and
safely store and deliver it under ambient temperatures and
moderate pressures. While compressed natural gas (CNG),
stored as supercritical fluid at room temperature and 200−300
bar by using steel cylinder, might be still suitable for large-size
vehicles such as trucks, adsorbed natural gas (ANG) will be
better suited for small- or midsize cars in terms of both cost and
safety concerns.
Among the diverse porous adsorbents for methane storage
(methane being the main component of natural gas), porous
metal−organic frameworks (MOFs) are particularly promising
for such a purpose because of their high porosities, tunable
pores, and easily immobilized functional sites to optimize their
storage capacities.2,3 In fact, BASF has demonstrated model
vehicles fueled with natural gas by making use of MOF
materials.4 In order to fully implement natural gas fuel systems
for vehicles, it is very important to target adsorbents with high
methane storage capacities. As such, the Advanced Research
Projects Agency−Energy (ARPA-E) of the U.S. Department of
Energy (DOE) has recently set new methane storage targets to
guide the research on adsorbent based methane storage
systems, with the ambitious goal of a volumetric storage
capacity of 350 cm3 (STP) cm−3 and gravimetric storage
capacity of 0.5 g (CH4) g−1 (adsorbent) at room temperature.5
As well established, the organic linker H4L was readily
synthesized by Suzuki cross-coupling reaction, while MOF
Received: February 25, 2014
Published: April 14, 2014
© 2014 American Chemical Society
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Communication
cm3 (STP) cm−3 in NOTT-101a to 257 cm3 (STP) cm−3 in
UTSA-76a. This is really remarkable, featuring UTSA-76a as
the unique MOF having the second highest volumetric
methane storage capacity. Although UTSA-76a has slightly
lower methane storage capacity than HKUST-1 (267 cm3
(STP) cm−3), it has higher methane storage working capacity
of 197 cm3 (STP) cm−3 than HKUST-1 of 190 cm3 (STP)
cm−3. This is because HKUST-1 has much higher methane
uptake than UTSA-76a at 5 bar, attributed to the smaller pores
and stronger methane binding in HKUST-1, so the working
capacity of UTSA-76a, defined here as the difference of the
amount of methane adsorbed between 65 and 5 bar, is higher.
Furthermore, UTSA-76a has methane storage density of 0.263
g (CH4) g−1 (adsorbent), which is much higher than that of
0.216 g (CH4) g−1 (adsorbent) in HKUST-1 (Figure S12).
Given the fact that working capacity (also called deliverable
capacity, which determines the driving range of natural gas
vehicles) is more important than the total storage capacity
values, UTSA-76a is superior to HKUST-1 for practical
methane storage applications. Some detailed comparisons of
UTSA-76a, NOTT-101a, and HKUST-1 for methane storage
are given in Table 1.
Scheme 1. Schematic Structure of the Organic Ligands for
the Construction of UTSA-76 (a) and NOTT-101 (b)
UTSA-76 was straightforwardly constructed through solvother-
mal reaction (see Supporting Information). The single-crystal
X-ray diffraction analysis reveals that UTSA-76 has exactly same
structure with NOTT-101 (Figure 1a). Accordingly, the
The exceptionally high methane storage and working
capacity of UTSA-76a are rather surprising and very
encouraging. It indicates that immobilization of some functional
groups into MOF pore surfaces might have played some
important roles to enhance methane storage performance
significantly. In order to understand the origin of the enhanced
methane uptake by the pyrimidine groups within UTSA-76a,
we first examined both UTSA-76a and NOTT-101a in details
from structural perspective. Experimental X-ray diffraction data
clearly show that these two MOFs are isostructural with same
Figure 1. (a) X-ray crystal structure of UTSA-76 indicating two cages
of about 10.2 and 9.6 × 22.3 Å, respectively; (b) temperature-
dependent high-pressure methane sorption isotherms of UTSA-76a
(data of pure methane gas stored in a high pressure gas tank is
represented as dash black curve).
crystal symmetry (R3m). Lattice parameters of the fully
̅
activated samples are also very close, with a being nearly
identical and c differing by only ∼1.2% (Figures S13 and S14).
Introduction of pyrimidine groups into the organic linker does
not alter the overall MOF pore structure, so the difference in
methane adsorption of the two MOFs at room temperature is
not due to a change in pore geometry. Diffraction data and
experimental pore volumes also demonstrated that both MOF
samples are highly crystalline, consisting of pure single phase,
and fully activated, so it is not likely that the methane
adsorption capacity difference between these two MOFs is
because of different sample quality. The ∼6% difference of
gravimetric methane uptakes between these two MOFs
suggests that the improved methane storage performance of
UTSA-76a in comparison to NOTT-101a must have an
intrinsic origin related to the pyrimidine groups.
activated UTSA-76a takes up 698.2 cm3 g−1 N2 at 77 K
(Figure S7), and has the Brunauer−Emmett−Teller (BET)
surface area of 2820 m2 g−1, and the pore volume of 1.092 cm3
g−1, which are comparable to those of NOTT-101a (Table 1).
We examined methane storage capacity of UTSA-76a (Figure
1b), and compared with NOTT-101a (Figure S9). As expected,
the two MOFs take up similar saturated amounts of methane at
125 and 150 K because of their comparable porosities and pore
structures (Figure S10). The pore volume of UTSA-76a derived
from the saturated methane uptake at 125 K is 1.083 cm3 g−1, in
good agreement with the nitrogen pore volume. However, to
our big surprise, UTSA-76a exhibits much higher methane
uptake than NOT-101a at 65 bar and 298 K. The volumetric
methane storage capacity was significantly improved from 237
Table 1. Comparison of Some Microporous MOFs for the High-Pressure Methane Storage at Room Temperature and 65 and 35
bar
e
working capacity at 65 bar
d
total uptake at 65 bar (35 bar)
(35 bar)
a
b
c
MOFs
SBET m2 g−1 Vp cm3 g−1 Dc g cm−3
g g−1
cm3 cm−3
density
g g−1
cm3 cm−3 initial Qst kJ/mol
UTSA-76a
2820
1850
2805
1.09
0.78
1.08
0.699
0.883
0.688
0.263 (0.216)
0.216 (0.184)
0.247 (0.202)
257 (211)
267 (227)
237 (194)
0.184 (0.151)
0.191 (0.162)
0.169 (0.138)
0.201 (0.154)
0.154 (0.122)
0.189 (0.144)
197 (151)
190 (150)
181 (138)
15.44
17
HKUST-110a
NOTT-101a11a
15.49
a
b
c
BET surface areas calculated from N2 isotherms at 77 K. Pore volumes calculated from the maximum amounts of N2 adsorbed. Framework
d
e
densities without guest molecules and terminal waters. At 298 K and 65 (35) bar. Defined as the difference of the amount of methane adsorbed
between 65 (35) bar and 5 bar.
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Next, we explored whether the adsorption energy of CH4 in
the two MOFs could be different and responsible for the
difference in methane adsorption isotherms. We carried out
first-principles DFT-D (dispersion-corrected density-functional
theory) calculations, where van der Waals (vdW) interactions
were corrected by empirical r−6 terms.14 Structural optimiza-
tions were first performed on UTSA-76a and NOTT-101a
structures. The relaxed static structures of the two are quite
similar, as expected. We then introduced CH4 molecules into
the MOF structures. For other Cu-MOFs with similar crystal
structures, our previous combined neutron diffraction and
computational studies have shown that the open Cu sites and
cage window sites are the two primary CH4 adsorptions sites.15
For UTSA-76a and NOTT-101a, these major CH4 adsorptions
sites are the same. Adsorption of methane on the linker surfaces
are generally weaker secondary adsorption, where we expected
the two MOFs to exhibit some differences. To our surprise,
calculations show that CH4 molecules adsorbed next to the
pyrimidine sites of UTSA-76a exhibit rather similar binding
energies as those adsorbed on the central phenyl ring of
NOTT-101a. We found no new specific adsorption sites in
UTSA-76a introduced by the pyrimidine rings, compared to
NOTT-101a. Consequently, for static structures of the two
MOFs, the calculated adsorption affinities on the organic linker
pore surface, within the accuracy of DFT-D,16 are nearly
identical, and does not seem to be able to explain the different
methane storage performance.
This led us to consider the dynamic structures of the two
MOFs. Like many other nonflexible Cu-MOFs, NOTT-101
and UTSA-76 have relatively rigid frameworks, and the crystal
structures remain essentially unchanged upon solvent removal
and gas adsorption/desorption. We noticed that, however, the
central rings of the linkers have relatively large rotational
freedom, which may affect methane storage to certain extent.
We calculated the energy cost of such a rotational motion
around two equivalent orientations of the central rings.
Interestingly, we found that the UTSA-76a linker central ring
(pyrimidine) has a much shallower rotational barrier (∼8.2 kJ/
mol for pyrimidine in UTSA-76a vs ∼20.2 kJ/mol for benzene
in NOTT-101a, Figure 2), and thus is significantly more
“dynamic” than the NOTT-101a linker central ring (benzene).
We note that the calculated rotational barrier for the π-flipping
of the benzene group in the well-studied MOF-5 is ∼55 kJ/
mol,17 significantly higher than that in UTSA-76a. Con-
sequently, upon methane adsorption at room temperature
and high pressure, the central pyrimidine rings within UTSA-
76a can be more easily adjusted and oriented to optimize the
methane packing at high pressure than the central benzene
rings within NOTT-101a. Hence, at low pressures, both MOFs
show about the same uptake, but at higher loadings of methane,
UTSA-76a can adsorb more methane molecules, because of its
adjustable pyrimidine ring orientation, yielding higher working
capacity than NOTT-101a.
To experimentally confirm the central pyrimidine rings in
UTSA-76a indeed has higher rotational freedom, we performed
neutron scattering measurements. Neutron scattering is
dominated by incoherent scattering from hydrogen atoms.
For both UTSA-76a and NOTT-101a, the H atoms on the
phenyl rings of both ends of the linkers have very limited
freedom except thermal motion, because of the restriction of
the two carboxylate linkage to the metal centers. In contrast,
the H atoms on the central rings can exhibit additional motions,
including librations and two-site jumpings. Neutron scattering
is an ideal technique to probe these H motions on the central
rings of the MOF linkers. Therefore, we conducted elastic scans
of the neutron scattering intensity vs temperature for UTSA-
76a and NOTT-101a, from which the temperature-dependent
atomic mean-square displacements were derived (Figure
S15).18 We found that the rotational motion of the UTSA-
76a central ring enters the time scale window accessible by the
spectrometer (∼10−8 s) at much lower temperature than the
NOTT-101a central ring (∼70 vs ∼150 K). This proves that
UTSA-76a central ring indeed has higher mobility than NOTT-
101a, which agrees with our DFT calculations that suggest a
much lower rotational barrier for UTSA-76a. More detailed
quasielastic neutron scattering measurements are undergoing in
order to fully identify the rotational character of the H motion,
and will be published in the future.
Our observation on the higher central ring rotational
freedom within UTSA-76a and its possible effect on the
enhanced methane storage performance are also in line with the
fact that the experimental Qst of methane adsorption in the two
MOFs are overall quite close, with Qst(UTSA-76a) being
slightly higher than Qst(NOTT-101a) at high methane loading
(Figure S16).
In conclusion, we unexpectedly realize a novel NbO porous
MOF UTSA-76a with pyrimidine groups for exceptionally high
methane storage. Its volumetric methane uptake reaches 257
cm3 (STP) cm−3 at 298 K and 65 bar, which is the second
highest adsorbent for methane storage. The most important is
that UTSA-76a has higher methane storage working capacity of
197 cm3 (STP) cm−3 and storage density of 0.263 g/g than
HKUST-1 (the previous best methane storage material ever
reported), setting a new record material for methane/natural
gas storage for transport applications. Our discovery also
suggests a promising new route to optimize the methane
working capacity in MOFs, i.e., utilizing functionalized linkers
with adjustable internal orientational freedom. We are currently
looking for new MOFs with similar dynamic freedom of linkers,
targeting at even higher methane storage and working
capacities.
ASSOCIATED CONTENT
* Supporting Information
Crystallographic data (CIF), NMR, TGA, PXRDs, high-
pressure methane isotherms, neutron scattering experiments
■
Figure 2. The variation of the total energy as the central rings of the
UTSA-76a and NOTT-101a linker are rotated around the linker
backbone, derived from DFT calculations. The rotational barrier for
UTSA-76a is particularly small, less than half of that for NOTT-101a.
S
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A.; Veenstra, M.; Long, J. R. Chem. Sci. 2014, 5, 32.
and additional figures. This material is available free of charge
via the Internet at http://pubs.acs.org.
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2013, 6, 2735. (b) Kong, G.-Q.; Han, Z.-D.; He, Y.; Ou, S.; Zhou, W.;
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AUTHOR INFORMATION
■
Corresponding Author
banglin.chen@utsa.edu; wzhou@nist.gov; taner@seas.upenn.
edu
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by an Award AX-1730 from
WelchFoundation (BC). T.Y. acknowledges partial support
from the DOE BES Grant No. DE-FG02-08ER46522.
R.; Hubberstey, P.; Schroder, M. Angew. Chem., Int. Ed. 2006, 45, 7358.
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