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COMMUNICATION
A novel MOF with mesoporous cages for kinetic trapping of hydrogenw
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Published on 18 November 2011 on http://pubs.rsc.org | doi:10.1039/C1CC15687J
Qian-Rong Fang, Da-Qiang Yuan, Julian Sculley, Wei-Gang Lu and Hong-Cai Zhou*
Received 14th September 2011, Accepted 29th October 2011
DOI: 10.1039/c1cc15687j
A stable MOF, assigned PCN-105, with two types of mesoporous
cages, has been prepared by using a new multidentate flexible
ligand with amine functional groups, and PCN-105 exhibits a
marked N2, O2, Ar and H2 hysteretic behaviour.
Over the past decade, numerous studies of porous metal–organic
frameworks (MOFs) have been reported because of their unique
architectures and potential applications in gas separation,
molecular storage, and heterogeneous catalysis.1,2 In particular,
flexible MOFs have attracted extensive attention owing to their
unusual hysteretic and stepwise gas adsorption behavior.3 For
instance, Schröder and coworkers prepared a new anionic MOF
constructed from In(III) ions and tetracarboxylic acid linkers.3a
In this structure, the negative charge of the framework is
balanced by divalent piperazinium cations that partially block
the channels. Notably, the example reported exhibits cationdependent kinetic trapping of H2, and this represents a new
class of flexible, modifiable MOF capable of a marked hysteresis.
Hysteretic H2 adsorption is of great interest because it could
provide a kinetics-based trapping mechanism for storing hydrogen
at modest pressures.3b To date, however, such MOFs involving
hysteretic H2 adsorption behavior still remain largely unexplored.
On the other hand, some mesoporous MOFs with the sizes
of channels or cages between 2–50 nm have recently been
prepared.4 Compared with the large number of microporous
MOFs published each year, the preparation of mesoporous
MOFs still is a great challenge. Two principal strategies have
been devised to push MOFs into the mesoporous arena: ligand
extension and construction of mesoporous cages from multinuclear secondary building units (SBUs). The first approach
presents two clear problems, one of which being interpenetration
and the other being framework collapse upon solvent removal.
The second tactic for growing mesoporous frameworks, growing
multinuclear SBUs has been successfully employed in the
construction of MIL-100,4c MIL-1014d and UMCM-2.4i It is
worth noting that, in such MOFs with mesoporous cages, it is
possible to confine H2 if the access to the porosity is controlled
dynamically by the openings.
Department of Chemistry, Texas A&M University, PO Box 30012,
College Station, TX 77842-3012, USA.
E-mail: zhou@mail.chem.tamu.edu; Fax: +1 513 529 0452;
Tel: +1 979 845 4034
w Electronic supplementary information (ESI) available: Experimental
details, IR, TGA and PXRD. CCDC 772889 (PCN-105). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc15687j
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Chem. Commun., 2012, 48, 254–256
In this context, we reported for the first time the H2 trapping
based on mesoporous cages with narrow windows. Our strategy
for preparing MOFs with mesoporous cages is to design multidentate ligands which tend towards forming multinuclear SBUs. A
new ligand, HTDBD (H2HTDBD=4,40 -(6-hydroxy-1,3,5-triazine-2,4-diyl)bis(azanediyl)dibenzoic acid), which possesses amine
functional groups and multiple coordination sites, was designed to
build mesoporous structures. As a result, we herein present a
mesoporous MOF, Cd4Na(H2O)2(HTDBD)3(TDBD)10(DMF)
6(EtOH)3(H2O) (designated PCN-105; PCN = porous
coordination network; DMF = N,N-dimethylformamide;
EtOH = ethanol) with two types of mesoporous cages (referred
to as A and B cages). The A cage has an internal free diameter
of 20 Å and triangular windows of 4 Å; the B cage possesses an
internal free diameter of 21 Å and square channels of 9 Å.
Interestingly, because of the miniscule windows of the A cages,
the adsorption and desorption isotherms of N2, O2, Ar and H2
exhibit a marked hysteresis.
Crystals of PCN-105 were prepared by the method of
solvent diffusion. H2HTDBD, Cd(NO3)24(H2O) and NaOH
were dissolved in DMF/ethanol/H2O. Subsequently, 1-amino2-propanol was slowly diffused to the mixture at 60 1C for
4 days to produce colorless block crystals (see ESIw for details).
X-Ray crystallography reveals that PCN-105 crystallizes in
the space group Fm3% c (Fig. 1a).z In this structure, each
multidentate HTDBD ligand coordinates to four different
Cd atoms through two carboxylates and two nitrogen atoms
of the triazine group and one Na atom through its hydroxyl
group. Four crystallographically identical cadmium atoms
adopt distorted octahedral coordination geometries with four
oxygen atoms from two carboxylate groups and two nitrogen
atoms from two triazine groups, which were square-arranged
with Cd Cd of 5.7 Å. Simultaneously, an octahedral sodium
atom is located in the center of four cadmium atoms by
bonding to four nitrogen atoms from four triazine groups
and two terminal aqua, thereby forming a rare pentanuclear
SBU. As shown in Fig. 1b, six such pentanuclear SBUs are
linked by eight HTDBD ligands to generate a highly symmetrical
cage structure with an internal diameter of 20 Å and triangular
windows of 4 Å (assigned as A cage; van der Waals radii of the
atoms have been taken into account). These A cages are further
interconnected through HTDBD ligands to generate a 3-D
extended open network. Remarkably, at the center of eight
A cages there is a second mesoporous cage (assigned as B cage)
with an internal diameter of 21 Å and square windows of 9 Å
(Fig. 1c). Removal of the guest molecules reveals that the
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Fig. 2 The simplified structure of PCN-105 with ReO3 (reo) topology.
Pentanuclear SBU, green ball; HTDBD ligand, pink rod.
Fig. 1 The crystal structure of PCN-105: (a) the 3-D framework
structure, (b) A cage and (c) B cage. C, yellow; N, blue; O, red; Cd,
green; Na, purple. All hydrogen atoms have been omitted for clarity.
effective free volume of PCN-105, calculated by PLATON
analysis,5 is 63.5% of the crystal volume (59 042.0 Å3 of the
92 960.0 Å3 unit cell volume).
A clearer view can be obtained by reducing the framework of
PCN-105 to a node-and-linker model.6 In this framework, each
pentanuclear SBU can be defined as an 8-connected node. Since
the HTDBD ligand only acts as a bridging ligand, it will not be
considered in the topological analysis. Based on this simplification,
PCN-105 can be described as an 8-connected 3-D network
with the Schläfli symbol (38485864), which corresponds to the
uncommon ReO3 (reo) topology in MOFs (Fig. 2).7
The permanent porosity of PCN-105 is confirmed by N2, O2
and Ar sorption isotherms at 77 K or 87 K, as shown in
Fig. 3a. The marked hysteresis, that is, the amount of gas
molecules in PCN-105 on the desorption branch is higher than
on the adsorption branch is apparent in all of the sorption
isotherms. This also means that gas adsorbed at high pressure
is only released when the pressure is lowered sufficiently. It is
evident that the hysteresis was caused by the large pore volume
and the small windows of the A cages (Scheme 1). For gas
molecules to enter or exit the A cages, they must funnel
through the narrow 4 Å windows, which should be easier at
higher pressures. In this case, the pressure is used as the
stimulus for creating or reversing the trapping. Although there
have been a few previous reports of hysteretic gas sorption in
MOFs driven by framework flexibility,3 the gas trapping based
on mesoporous cages with micro windows has never been
presented. As the result of the N2 sorption isotherm, the
Brunauer–Emmett–Teller (BET) surface area, calculated within
the pressure range P/P0 o 0.10, and Langmuir surface area
were determined to be 1067 and 1317 m2 g1, respectively.
Furthermore, low-pressure hydrogen sorption measurements
of PCN-105 were also performed at 77 K and 87 K to
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Fig. 3 Gas sorption behavior of PCN-105. (a) N2, Ar and O2
sorption isotherms for PCN-105, and (b) H2 sorption isotherms
at 77 K and 87 K, respectively (the saturation pressure of O2
at 77 K is B156 torr).
investigate the H2 uptake (Fig. 3b). Similar to the above
results, H2 adsorption and desorption isotherms of PCN-105
also showed a hysteresis, although less pronounced because of
the smaller kinetic diameter of the H2 molecule (2.89 Å). The
total H2 uptakes of PCN-105 at 1.0 bar are 1.51 wt% at 77 K
Chem. Commun., 2012, 48, 254–256
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2
3
Scheme 1 Simplified schematic representation of gas hysteretic
behavior in PCN-105.
and 1.06 wt% at 87 K. Virial analysis8 of the H2 adsorption
isotherms measured at 77 K and 87 K revealed that the
isosteric heat of adsorption of PCN-105 at zero surface coverage
is approximately 7.33 kJ mol1. Although the H2 adsorption
capacity of PCN-105 is still low, it represents a new class of
flexible mesoporous MOF that could provide a kinetics-based
trapping mechanism for storing hydrogen.
In summary, a novel mesoporous MOF, PCN-105, with two
types of mesoporous cages (A and B cages), has been obtained
by extending pentanuclear SBUs with a multidentate flexible
ligand, HTDBD. PCN-105 exhibits a marked N2, Ar, O2 and
H2 hysteretic behaviour. It is anticipated that such MOFs,
based on mesoporous cages with narrow windows, can lead to
the efficient design and discovery of new H2 storage materials
with hysteretic adsorption properties.
4
Notes and references
z Crystal data for PCN-105: C68H40Cd4N20NaO22, M = 1961.79, cubic,
space group Fm3% c (No. 226), a = 45.300(5) Å, V = 92960(18) Å3,
Z = 24, Dc = 0.841 g cm3, F000 = 23208, T = 208(2) K, 2ymax =
50.01, 114 738 reflections collected, 3557 unique (Rint = 0.0676). Final
GooF = 1.079, R1 = 0.0712, wR2 = 0.2136. CCDC 772889.
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