A microporous metal-organic framework of a rare sty topology for high CH4 storage at room temperature

Article abstract of DOI:10.1039/c3cc38765h

A rare sty type microporous metal-organic framework, Cu₂(FDDI) (ZJU-25; H₄FDDI = tetramethyl 5,5′-(9H-fluorene-2,7-diyl) diisophthalate acid), was solvothermally synthesized and structurally characterized. With open metal sites and suitable pore space for their interactions with methane molecules, ZJU-25a exhibits absolute methane storage of 180 cm³(STP) cm^-3 at room temperature and 35 bar, enabling it to be one of the very few porous MOFs whose methane storage capacities have met and/or approached the DOE target of 180 cm³(STP) cm^-3 for material-based methane storage. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc38765h

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A microporous metal–organic framework of a rare sty
topology for high CH4 storage at room temperature†
Xing Duan,a Jiancan Yu,a Jianfeng Cai,a Yabing He,b Chuande Wu,c Wei Zhou,*de
Taner Yildirim,df Zhangjing Zhang,g Shengchang Xiang,g Michael O’Keeffe,h
Banglin Chen*ab and Guodong Qian*a
Cite this: Chem. Commun., 2013,
49, 2043
Received 6th December 2012,
Accepted 25th January 2013
DOI: 10.1039/c3cc38765h
www.rsc.org/chemcomm
A rare sty type microporous metal–organic framework, Cu2(FDDI) and low-cost, high energy density on-board storage for natural gas
(ZJU-25; H4FDDI = tetramethyl 5,50-(9H-fluorene-2,7-diyl)diisophthalate vehicles.1 Among the diverse materials for methane storage, porous
acid), was solvothermally synthesized and structurally characterized. metal–organic frameworks (MOFs) are very promising ones.2 This
With open metal sites and suitable pore space for their interactions is because (a) such porous framework materials can be readily self-
with methane molecules, ZJU-25a exhibits absolute methane assembled from metal ions/clusters with organic linkers with high
storage of 180 cm3(STP) cmꢀ3 at room temperature and 35 bar, porosities, (b) the pores can be systematically varied and/or tuned
enabling it to be one of the very few porous MOFs whose methane by the interplay between the metal containing secondary building
storage capacities have met and/or approached the DOE target of units and organic linkers of different lengths and geometries, and
180 cm3(STP) cmꢀ3 for material-based methane storage.
(c) the pore surfaces can be functionalized for their strong recogni-
tion of substrate molecules through the immobilization of different
Materials for high methane storage are highly in need because of functional sites, particularly open metal sites.3
the great promise and significant value of methane (the main
Porous MOFs for methane storage were pioneered by Kitagawa
component of natural gas) as an alternative clean fuel/energy, et al. and Yaghi et al. during 2000–2002.4,5 Although highly porous
particularly for natural gas vehicles. In fact, U.S. Department of MOFs can gravimetrically take a large amount of methane, their
Energy (DOE) recently initiated a new program called ‘‘MOVE’’ volumetric storage capacities are significantly limited because of
(Methane Opportunities For Vehicular Energy) to facilitate the their high porosities and low densities of the frameworks.6 The
development of transformational technologies that reduce the significant progress in porous MOFs for methane storage was
barriers to mass adoption of natural gas use in vehicles. Of realized during 2008–2009 in which two porous MOFs, namely
particular interest are technologies that enable at-home refueling PCN-14 by Ma et al.7 and Ni-MOF-74 by Wu et al.,8 were established
for their extraordinarily high volumetric methane storage (220 and
200 cm3(STP) cmꢀ3, respectively), surpassing the DOE target of
a State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials
180 cm3(STP) cmꢀ3 for methane storage at room temperature and
and Applications, Department of Materials Science & Engineering, Zhejiang
University, Hangzhou 310027, China. E-mail: gdqian@zju.edu.cn
b Department of Chemistry, University of Texas at San Antonio, One UTSA Circle,
San Antonio, Texas 78249-0698, USA. E-mail: banglin.chen@utsa.edu;
Fax: +1 210-458-7428
35 bar.9 The high methane storage capacities within these two
porous MOFs are apparently attributed to their high density of
open metal sites and suitable pore space for their efficient storage
of methane molecules. This very important concept (high density
of open metal sites and optimized pore space) has been further
confirmed by Guo et al. by demonstrating a MOF material with the
very high methane storage density of 0.22 g cmꢀ3 in the pores and
methane storage of 195 cm3(STP) cmꢀ3 (Scheme 1).10
Among diverse porous MOFs, those assembled from copper
paddle-wheel Cu2(COO)4 clusters and tetracarboxylates, typi-
cally of nbo topologies,11 have attracted extensive attention for
gas storage, particularly for methane and acetylene storage.11f,g,i
This series of MOFs have two unique features: (a) the readily
available open Cu2+ sites and (b) the systematically varied pore
spaces, making them very suitable to study the effects of both
open metal sites and variable pore spaces on gas storage.
c Department of Chemistry, Zhejiang University, Hangzhou 310027, China
d NIST Center for Neutron Research, Gaithersburg, Maryland 20899-6102, USA.
E-mail: wzhou@nist.gov
e Department of Materials Science and Engineering, University of Maryland,
College Park, Maryland 20742, USA
f Department of Materials Science and Engineering, University of Pennsylvania,
Philadelphia, Pennsylvania 19104-6272, USA
g Fujian Provincial Key Laboratory of Polymer Materials, Fujian Normal University,
3 Shangsan Road, Cangshang Region, Fuzhou 350007, China
h Department of Chemistry and Biochemistry, Arizona State University, Tempe,
AZ 85287, USA
† Electronic supplementary information (ESI) available: Synthesis and character-
ization of the organic linker H4FDDI and MOF ZJU-25, TGA, and PXRD. CCDC
913334. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc38765h
c
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one with RCSR symbol ssb,13 one of several ways of linking
vertices with planar 4-coordination with just one kind of link.14
The only previous example of this topology in a MOF of which
we are aware is that of PCN-12 although that MOF was not so
described.15 We prefer, for reasons given earlier,16 to consider
the linker as having two 3-coordinated branch points which act
as 3-coordinated vertices of the underlying net which is now the
(3,4)-coordinated net with RCSR symbol sty. Fig. 1d shows the
net in its augmented form, sty-a, in which the original vertices
are replaced by their vertex figures (squares and triangles in
this case).
Scheme 1 Schematic structure of the organic linker H4FDDI.
Herein we have developed a new tetracarboxylic acid, H4FDDI
(5,50-(9H-fluorene-2,7-diyl) diisophthalic acid), and its corres-
ponding microporous MOF, Cu2(FDDI) (ZJU-25; ZJU = Zhejiang
University). Interestingly, this new framework displays a topol-
ogy derived from ssb instead of nbo, and exhibits high methane
storage of 180 cm3(STP) cmꢀ3 at room temperature and 35 bar.
ZJU-25 was synthesized with Cu(NO3)2ꢁ2.5H2O and H4FDDI
in DMF/H2O with addition of a small amount of nitric acid at
60 1C for 3 days as small blue hexagonal flake-shaped crystals. The
structure was characterized by single-crystal X-ray diffraction
studies, and the phase purity of the bulk material was indepen-
dently confirmed by powder X-ray diffraction (PXRD, Fig. S1,
ESI†) and thermogravimetric analysis (TGA, Fig. S2, ESI†).
A single-crystal X-ray diffraction analysis revealed that ZJU-25
crystallizes in the hexagonal space group P63/mmc.‡ As expected,
the organic linkers are connected with paddle-wheel Cu2(COO)4
secondary building units (SBUs) (Fig. 1a) to form three-
dimensional (3D) framework structures (Fig. 1b and c). The 3D
framework of ZJU-25 has three different types of pore apertures.
One runs through the a axes of about 4.4 ꢂ 8.6 Å2 (Fig. 1b) and
the other two can be visualized along the c axes of about 3.6 and
5.4 Å in diameter, taking into account the van der Waals radii,
respectively (Fig. 1c). PLATON calculations indicate that the void
spaces are 68.3% for ZJU-25 (6753 Å3 out of the 9889.1 Å3 per unit
cell volume).12
The N2 sorption isotherm at 77 K showed that the suitably
activated ZJU-25a exhibits reversible Type-I sorption behaviour,
characteristic of microporous materials with an adsorbed
amount of N2 of 765 cm3 gꢀ1 (Fig. S3, ESI†). The Brunauer–
Emmett–Teller (BET) and Langmuir surface areas of ZJU-25a
are 2124 and 3304 m2
g
ꢀ1, respectively. As expected, these
values are higher than those of MOF-505 because of the longer
linker in ZJU-25; but slightly lower than those of NOTT-102
because of the bending of the FDDI linker in ZJU-25. ZJU-25a
has a pore volume of 1.183 cm3 gꢀ1
.
The moderately high porosity of ZJU-25a might enable it to
be a very promising material for methane storage; we thus
examine the methane sorption of ZJU-25a. As shown in Fig. 2
ZJU-25a takes up a large amount of methane. At 240 K, the
methane uptake of ZJU-25a can reach 302 cm3(STP) cmꢀ3 under
63 bar. At 300 K and 35 bar, the practical conditions for
methane storage, the absolute uptake of methane for ZJU-25a
reaches the DOE target of 180 cm3(STP) cmꢀ3. The methane
storage capacity of ZJU-25a is significantly higher than those of
most examined porous MOFs. In fact, ZJU-25a is among the
very few porous MOFs whose methane storage capacities have
met the DOE target at room temperature and 35 bar,8–10,17
indicating the promise of this new porous MOF for practical
methane storage applications. The methane storage is still not
saturated, and can be further increased to 227 cm3(STP) cmꢀ3
under 63 bar.
The most interesting structural feature of ZJU-25 is that its
framework topology is different from well-established nbo ones
assembled from Cu2(COO)4 SBUs and a number of tetracarboxy-
lates.9 If the linker is considered as one 4-coordinated vertex of
the underlying net, then in combination with the 4-coordinated
vertex corresponding to the metal paddle-wheels, the net is the
The initial Qst of CH4 adsorption in ZJU-25 is B15.1 kJ molꢀ1
,
comparable to those in other MOFs with copper paddlewheels.2
Computational grand canonical Monte Carlo (GCMC) simula-
tions indicate that all the open Cu sites are accessible and the
Fig. 1 X-ray single crystal structure of ZJU-25, indicating (a) each tetracarboxylate
ligand connects with four paddle-wheel Cu2(COO)4 clusters; (b) the structure
viewed along the a axes indicating irregular pores of about 4.4 ꢂ 8.6 Å2; (c) the
structure viewed along the c axes showing the small and the large pores of about
3.6 and 5.4 Å in diameter, respectively; (d) the framework topology of sty-a net.
Fig. 2 High-pressure CH4 absolute sorption isotherms of ZJU-25a at 240 K,
270 K and 300 K. Solid symbols: adsorption, open symbols: desorption. Inset:
isosteric heats of adsorption of methane, calculated using the virial method.
c
2044 Chem. Commun., 2013, 49, 2043--2045
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primary CH4 adsorption sites, which can take up B45 cm3 cmꢀ3
at RT and 35 bar. Other methane molecules are mainly located
within the small cages (Fig. S5, ESI†). From the crystal structure,
it can be observed that the pore sizes are in the range of 5 to 9 Å
which are favorable for methane storage, as pointed out by
Wilmer et al.17b It is interesting to note that these porous MOFs
exhibiting high methane storage at room temperature and
35 bar have volumetric surface areas in the range of 1000 to
1500 cm2 cmꢀ3 (Fig. S6, ESI†). Apparently, open metal sites,
pore/cage sizes, volumetric surface areas (a combined parameter
from gravimetric surface area and framework density) should be
collaboratively considered and enforced in order to secure high
methane uptake under such practical methane storage condi-
tions (room temperature and 35 bar), although MOF materials of
higher volumetric surface areas can take up more methane
under higher pressures such as 65 bar.
In summary, we have developed a new organic linker,
aromatic tetracarboxylic acid, and incorporated it into a
three-dimensional porous metal–organic framework. Interest-
ingly, ZJU-25 displays a very rare sty-a framework topology. The
activated ZJU-25a exhibits moderately high porosity with a BET
surface area of 2124 m2 gꢀ1. The open Cu2+ sites and suitable
pore spaces within ZJU-25 have enabled this new MOF to take
up a large amount of methane, reaching the DOE methane
storage target of 180 cm3(STP) cmꢀ3 at 300 K and 35 bar.
This work was supported by the National Natural Science
Foundation of China (Grants 51010002, 51272231 and
51229201), Grant CHE 0718281 from the National Science
Foundation, and Grant AX-1730 from the Welch Foundation
(B.C.). T. Y. acknowledges support from the DOE BES Grant No.
DE-FG02-08 ER 46522.
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