A NbO-type metal-organic framework exhibiting high deliverable capacity for methane storage

Article abstract of DOI:10.1039/c5cc01055a

A copper-based NbO-type metal-organic framework ZJNU-50 constructed from a tetracarboxylate incorporating phenylethyne as a spacer exhibited an exceptionally high methane working capacity of 184 cm³ (STP) cm^-3 for methane storage. The value is among the highest reported for MOF materials. This journal is

Full text of DOI:10.1039/c5cc01055a

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A NbO-type metal-organic framework exhibiting
high deliverable capacity for methane storage
Cite this: DOI: 10.1039/x0xx00000x
Chengling Song,a Yajing Ling,a Yunlong Feng,a Wei Zhou,b,c Taner
Yildirim,b,d Yabing He*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
A copper-based NbO-type metal-organic framework ZJNU- tetracarboxylate have been increasingly explored for methane
50 constructed from tetracarboxylate incorporating storage due to their salient merits of high surface area, tuneable
a
phenylethyne as spacer exhibited exceptionally high methane pore size and open copper sites suitable for gas adsorption. For
working capacity of 184 cm3 (STP) cm-3 for methane storage. example, Zhou group reported several NbO-type MOFs
The value is among the highest reported for MOF materials.
incorporating anthracene derivatives, double bond or triple bond
moieties exhibiting good methane adsorption properties.3 Snurr
et al. used molecule simulations to discover some very useful
structure–property relationship regarding methane storage and to
identify a NbO-type MOF with an exceptionally high methane
adsorption capacity at room temperature and 35 bar.4 We
recently investigated a series of NbO-type MOFs for methane
storage and derived an empirical equation for predicting methane
storage capacity.5 Undoubtedly, these results demonstrate that
NbO-type MOFs are promising in terms of methane storage, and
encourage us to further synthesize and explore more NbO-type
MOFs for methane storage.
Growing concerns on depletion of conventional energy sources
and environmental issue associated with CO2 emission have
triggered tremendous studies to develop new cleaner energy
carries. Natural gas, whose main component is methane, is
considered as an attractive fuel as compared with other
conventional energy sources due to its increasing supply and
lower emission of greenhouse gases and other pollutants.
However, its comparatively low volumetric energy density under
standard conditions presents a giant challenge for its wide
utilization, especially as a transportation fuel in vehicular
application. Realization of safe and efficient methane storage is
thus highly desired. The physisorption based processes,
involving adsorbing methane within the micropores of porous
materials to achieve a density competitive with compressed
natural gas (CNG) but at a lower storage pressure, offer an
attractive technique due to its mild operating condition and high
energy efficiency. The key lies in finding an efficient adsorbent
material. The past decade has seen great progress in methane
storage material development. One of the key development is
the use of metal-organic frameworks (MOFs), a new class of
highly porous materials assembled thorough the modular
combination of inorganic and organic building blocks, for storing
methane providing improvement in the methane storage
performance.1 In fact, several MOFs exhibit methane adsorption
capacities exceeding those of the conventional activated
carbons.2 Also, BASF has demonstrated model vehicles fuelled
by natural gas using BASF MOF materials.
For vehicular application, an ideal MOF material should not
only have high methane storage capacity, but more importantly
should have high methane deliverable capacity. It is well known
that the methane storage and deliverable capacities of MOF
materials depend on a variety of factors such as pore space,
pore size distribution, framework densities, and open metal sites.
To optimize these factors to enhance the methane deliverable
capacity, we developed a novel tetracarboxylate derivative H4L,
5,5’-(benzene-1,4-diyl-ethyne-1,2-diyl)
diisophthalic
aicd
(Scheme 1), and used it to construct the corresponding NbO-
type MOF termed as ZJNU-50, which is based on the following
HOOC
COOH
H3COOC
COOCH3
COOCH3
COOCH3
Br
O
O
B
Br
I
COOCH3
COOCH3
1) NaOH
2) HCl
Pd(PPh3)4, K3PO4, dioxane
PdCl2(PPh3)2, CuI, Et3N
H3COOC
COOCH3
H3COOC
HOOC
COOH
COOCH3
Of various MOFs, NbO-type MOFs constructed from dicopper
paddlewheel secondary building units (SBUs) and
H4L
2
1
Scheme 1. Synthesis of tetracarboxylic acid H4L used to construct ZJNU-50
.
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DOI: 10.1039/C5CC01055A
considerations. 1) Under the premise of ensuring structural
stability and no significant reduction of the framework density, we
elongate the organic linker, 1,4-benzenediisophthalate, in NOTT-
500
400
300
200
100
0
250
200
150
1015-6 by inserting a slim C
≡C triple bond, expecting to achieve a
low methane uptake at a low pressure of say 5 bar and thus a
high methane working capacity; 2) To our knowledge, no study
based on this unsymmetrical organic linker has been reported.
Our studies show the activated ZJNU-50a (thereafter, the letter
“a” indicates activated MOF materials) exhibits an exceptionally
high methane storage capacity of 178 and 229 cm3 (STP) cm-3 at
298 K under 35 and 65 bar, respectively. Suppose 65 and 5 bar
as the upper and lower limiting working pressures, respectively,
up to 80% of the methane storage amount, namely, 184 cm3
(STP) cm-3 can be delivered at 298 K, which is among the
100
273 K, excess
298 K, excess
273 K, total
298 K, total
50
0
0
10
20
30
40
50
60
70
P (bar)
Fig. 2 High-pressure excess (square) and total (circle) methane sorption
isotherms of ZJNU-50a at 273 K (red) and 298 K (blue). Solid and open symbols
represent adsorption and desorption, respectively.
highest reported for porous MOFs. As compared to NOTT-101a
,
ZJNU-50a has a higher gravimetric methane deliverable capacity,
despite a similar volumetric one.
Single-crystal X-ray diffraction studies reveal that ZJNU-50
crystallizes in the space group R-3m. As expected, the Cu2+ ions
form dicopper paddlewheel Cu2(COO)4 clusters serving as
square-planar SBUs that are linked with the rectangular organic
building blocks to give rise to a three-dimensional (3D) NbO-
based network containing two different types of nanocages (Fig.
1). One consists of 12 ligands and 6 SBUs, and the diameter is
about 14 Å, taking into account the van der Waals radii of the
atoms; while the other is constructed from 6 ligands and 12
SBUs with dimensions of ca. 11 × 24 Å. Due to the elongation of
the linker, the sizes of the two cages are systematically larger
than those in NOTT-101. It is worth noting that the central phenyl
ring was twisted out of the plane defined by the two terminal
isophthalates with the dihedral angle smaller than the one
observed in NOTT-101, suggesting the rotation of the central
phenyl ring around the molecular long axis is relatively free. Such
a low rotation barrier might play a role in optimizing gas
adsorption at high pressure.7
Scheme 1 outlines the synthesis of the ligand, 5,5’-(benzene-
1,4-diyl-ethyne-1,2-diyl) diisophthalic acid. We began with the
synthesis of dimethyl 5-(2-(4-bromophenyl)ethynyl)isophthalate
by Sonogashira coupling reaction of 1-bromo-4-iodobenzene
with dimethyl 5-ethynylisophthalate. Subsequently, Suzuki
coupling of dimethyl 5-(2-(4-bromophenyl)ethynyl)isophthalate
with dimethyl 5-(pinacolboryl)isophthalate afforded the
corresponding tetramethyl ester precursor which was
1
2
,
hydrolyzed and acidified to give the desired tetracarboxylic acid.
1
The chemical structure was characterized unambiguously by H
NMR and 13C NMR spectroscopy.
A solvothermal reaction of the tetracarboxylic acid and
Cu(NO3)2·3H2O in a mixed solvent of N,N-dimethy formamide
(DMF), methanol and H2O under acidic conditions afforded
ZJNU-50 as blue rhombic crystals in a good yield. The single-
crystal X-ray structure was characterized and the phase purity
was confirmed by powder X-ray diffraction studies (PXRD, Fig.
Before gas adsorption measurement, the as-synthesized
ZJNU-50 was solvent-exchanged with dry acetone, and then
S1).
ZJNU-50
can
be
best
formulated
as
[Cu2(L)(H2O)2]·6DMF·7H2O based on single-crystal X-ray
diffraction studies, thermogravimetric analysis (TGA, Fig. S2),
and microanalysis.
o
evacuated at 100 C under high vacuum, affording the activated
ZJNU-50a. The integrity of the framework was confirmed by
powder X-ray diffraction (Fig. S3). The textural feature was
characterized by N2 adsorption isotherm at 77 K. As shown in Fig.
S4, the N2 sorption isotherm shows
a type-I adsorption
behaviour, characteristic of microporous materials. The
Brunauer-Emmett-Teller (BET) surface area and the total pore
volume are estimated to be 3308 m2 g-1 and 1.184 cm3 g-1 (Fig.
S5). These values are systematically higher than those of NOTT-
101a, and are among the highest reported for NbO-type MOFs
(Table S1).
Given that the gravimetric surface area falls in the optimal
range for methane storage,4 we evaluate the performance of
ZJNU-50a for methane storage in detail. High-pressure methane
sorption measurements were performed at the Center for
Neutron Research, National Institute of Standards and
Technology (NIST) using
a
computer-controlled Sieverts
apparatus (Fig. S6). As shown in Fig. 2, the adsorption and
desorption branches well overlapped with each other, suggesting
Fig. 1 Single-crystal X-ray diffraction structure showing that each organic ligand
connects four dicopper paddlewheel SBUs to form a 3D network (b) containing
two different types of cages (c). The hydrogen atoms are omitted for clarity.
2 | Chem. Commun., 2015, 00, 1-3
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that the adsorption process is reversible and the adsorbed
methane can be fully recovered during the desorption process.
When evaluating the performance of an adsorbent for
vehicular application, the deliverable capacity of methane gas is
more important than the total storage capacity because it
determines the driving range of natural gas vehicles. Taking 5
bar and 65 bar as specific lower and upper pressure limits, the
deliverable capacity of ZJNU-50a is 184 cm3 (STP) cm-3 at 298
K; that is, a tank filled with ZJNU-50a can deliver 71.4% as much
fuel as the CNG tank operating on the same lower pressure limit
and 248 bar as the upper pressure limit, indicating ZJNU-50a as
a potential material for methane delivery. As compared to NOTT-
Table 1 Comparison of some MOFs for highꢀpressure methane storage
Total uptake
Working capacity
at 65 bara
at 65 bar (35 bar)
MOFs
Ref.
cm3 cmꢀ3
g gꢀ1
cm3 cmꢀ3
g gꢀ1
This
ZJNUꢀ50
229 (178) 0.274 (0.213)
184
0.220
work
2a
HKUSTꢀ1
UTSAꢀ76
NOTTꢀ102
UTSAꢀ80
NUꢀ125
PCNꢀ14
UTSAꢀ20
ZJUꢀ25
267 (227) 0.216 (0.184)
257 (211) 0.263 (0.216)
237 (181) 0.288 (0.220)
233 (192) 0.240 (0.198)
232 (182) 0.287 (0.225)
230 (195) 0.197 (0.169)
230 (184) 0.181 (0.145)
229 (180) 0.263 (0.206)
228 (190) 0.240 (0.200)
190
197
192
174
183
157
170
181
168
163
174
182
166
179
175
166
0.154
0.201
0.233
0.178
0.227
0.136
0.134
0.208
0.177
0.149
0.222
0.209
0.191
0.313
0.252
0.206
7
5
101
,
ZJNU-50a has
a
comparable volumetric methane
8
2a
2a
2a
9
deliverable amount, but a higher gravimetric methane deliverable
capacity. The volumetric methane deliverable amount is
comparable to or even better than those of MOFs that show the
most promise for deliverable methane storage: PCN-14 2a
61 12 PCN-80 14 ZJU-5 10 ZJU-25 9 ZJU-36 13 UTSA-20 2a,
UTSA-80 8 NU-111 2a MOF-52b (Table 1).
,
PCN-
10
11
12
2b
3b
2a
13
14
15
ZJUꢀ5
17
,
,
,
,
,
,
CuꢀTDPAT 222 (181) 0.202 (0.165)
,
,
PCNꢀ61
MOFꢀ5
PCNꢀ46
NUꢀ111
ZJUꢀ36
PCNꢀ80
SDUꢀ6
219 (171) 0.279 (0.218)
214 (150) 0.246 (0.172)
206 (172) 0.237 (0.198)
206 (138) 0.360 (0.241)
203 (142) 0.292 (0.204)
196 (147) 0.244 (0.183)
To understand the gas-framework interaction, the adsorption
isotherm at 273 K is also measured. The isosteric heat of
methane adsorption was extracted from the temperature-
dependent isotherms using clausius-clapeyron equation. As
shown in Fig. S7, the isosteric heat of adsorption for methane
does not decrease with the methane loading. The initial value is
calculated to be 15.0 kJ mol-1, which is slightly lower than that of
most reported copper-based frameworks (Table S1), thus
responsible for a low methane uptake at low pressure and a high
methane working capacity.
(172)
(0.201)
The working capacity is defined as the difference in total uptakes between 65
and 5 bar.
At 298 K and 35 bar, the excess gravimetric methane adsorption
capacity is 256 cm3 (STP) g-1. By using the N2-derived pore
volume and the bulk phase density of methane, the total
gravimetric CH4 uptake is calculated to be 296 cm3 (STP) g-1. In
order to calculate the volumetric methane uptake, which is the
most appropriate one to quantify the adsorption capacity of the
adsorbents, especially for ANG vehicular application, the crystal
density of 0.5978 g cm-3 was used. ZJNU-50a shows a total
volumetric methane storage capacity of 178 cm3 (STP) cm-3 at
298 K and 35 bar. This capacity approaches to the DOE’s
previous target of 180 cm3 (STP) cm-3 for methane storage when
the packing loss is ignored, and is comparable with those of NU-
In summary, we developed a new organic linker and used it to
construct a NbO-type MOF ZJNU-50a exhibiting exceptionally
high volumetric methane storage and deliverable capacities. At
298 K and 65 bar, the volumetric methane storage and
deliverable capacities reach 229 and 184 cm3 (STP) cm-3,
respectively, which are among the highest reported for all MOFs.
Such high storage and deliverable capacities are attributed to the
balanced porosity and framework density, suitable pore/cage
size, and the moderate density of open metal sites within ZJNU-
50a. It is expected that this work will stimulate more investigation
of porous MOFs for enhanced methane storage by tuning or
optimizing the pore structure of framework.
125 16 NOTT-102 5 Cu-TDPAT 11 ZJU-25 9 PCN-46 3b SDU-615
, , , ,
,
(Table 1). When the pressure increases to 65 bar, the total
volumetric methane uptake reaches 229 cm3 (STP) cm-3, which
is 87% of the DOE’s new target of 263 cm3 (STP) cm-3 assuming
no packing loss. The value is among the highest reported for all
MOFs, and among the copper-based MOFs, is only lower than
the adsorption capacities of the two best MOFs, HKUST-1 (267
cm3 (STP) cm-3)2a and UTSA-76 (257 cm3 (STP) cm-3),7 at the
same conditions, making ZJNU-50a as a promising MOF
material for volumetric methane storage (Table 1). The methane
storage capacity corresponds to 87% of the methane stored in a
CNG tank at 248 bar and 298 K, and is equivalent to the amount
of methane stored in a CNG tank at 210 bar and 298 K. Although
ZJNU-50a has a lower volumetric methane storage capacity than
HKUST-1 and UTSA-76, it has a higher gravimetric methane
storage capacity of 0.274 g g-1 than HKUST-1 of 0.216 g g-1 and
UTSA-76 of 0.263 g g-1 due to a larger pore volume and a lower
framework density.
This work was supported by the National Natural Science
Foundation of China (No. 21301156), and Open Research Fund
of Top Key Discipline of Chemistry in Zhejiang Provincial
Colleges and Key Laboratory of the Ministry of Education for
Advanced Catalysis Materials (ZJHX201313).
Notes and references
a
College of Chemistry and Life Sciences, Zhejiang Normal University,
Jinhua 321004, China. Eꢀmail: heyabing@zjnu.cn.
b
NIST Center for Neutron Research, Gaithersburg, Maryland 20899ꢀ
6102, USA.
c
Department of Materials Science and Engineering, University of
Maryland, College Park, Maryland 20742, USA.
d
Department of Materials Science and Engineering, University of
Pennsylvania, Philadelphia, Pennsylvania 19104ꢀ6272, USA.
†
Electronic Supplementary Information (ESI) available: PXRD, TGA,
N2 sorption isotherms, FTIR, NMR, highꢀpressure methane adsorption
properties in reported NbOꢀtype MOFs, and CCDC 1058417, See
DOI: 10.1039/c000000x/
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