A mesoporous metal-organic framework constructed from a nanosized C 3-symmetric linker and [Cu24(isophthalate)24] cuboctahedra

Article abstract of DOI:10.1039/c1cc13170b

The mesoporous framework [Cu₃(L)(H₂O) ₃]·(DMF)₃₅·(H₂O)₃₅ (NOTT-119) shows on desolvation a BET surface area of 4118(200) m² g^-1, a pore volume of 2.35 cm³ g

Full text of DOI:10.1039/c1cc13170b

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A mesoporous metal–organic framework constructed from a nanosized
C3-symmetric linker and [Cu24(isophthalate)24] cuboctahedrawz
Yong Yan,a Sihai Yang,a Alexander J. Blake,a William Lewis,a Eric Poirier,b
c
a
a
¨
Sarah A. Barnett, Neil R. Champness and Martin Schroder*
Received 30th May 2011, Accepted 11th July 2011
DOI: 10.1039/c1cc13170b
The mesoporous framework [Cu3(L)(H2O)3]ꢀ(DMF)35ꢀ(H2O)35
(NOTT-119) shows on desolvation a BET surface area of
4118(200) m2 gꢁ1, a pore volume of 2.35 cm3 gꢁ1, a total H2
uptake of 101 mg gꢁ1 at 60 bar, 77 K and a total CH4 uptake of
327 mg gꢁ1 at 80 bar, 298 K.
N,N-dimethylformamide (DMF) in the presence of a
small amount of HCl at 90 1C afforded blue crystals of
[Cu3(L)(H2O)3]ꢀ(DMF)35ꢀ(H2O)35 (NOTT-119).
The single crystal X-ray structure of NOTT-119 confirms
the formation of a ubt-type13 network incorporating cubocta-
hedral cages constructed from 12 {Cu2(O2CR)4} paddlewheels
and 24 isophthalates from 24 different L6ꢁ linkers (Fig. 1). The
structure comprises fused cuboctahedral (Cage A), truncated
tetrahedral (Cage B) and truncated octahedral (Cage C) cages.
Significantly, despite the exceptionally long organic linker, the
structure of NOTT-119 is non-interpenetrating and the (3,24)-
connected structure affords a system in which the distance
between the carboxylates in adjacent arms of L6ꢁ defines the
spacing between adjacent cuboctahedra (Cage A; inner sphere
diameter ca. 13 A). This generates the mesocavities of Cage B
and C, which have, taking into account van der Waals radii,
inner sphere diameters of ca. 2.41 and 2.5 nm, respectively, the
longest cage length being ca. 4.3 nm for Cage C. The total
solvent-accessible volume of the desolvated framework was
estimated14 to be 84.2% (after removal of guest solvates and
coordinated water molecules) making it one of the most
porous MOF materials synthesised to date.12d,15–17 The largest
aperture in NOTT-119 is estimated to be 1.9 nm in diameter,
and the bulk crystal density for desolvated NOTT-119a was
Porous metal–organic frameworks (MOFs) are an important
class of crystalline materials with intriguing structural topologies
and tuneable pore properties. Those containing well-defined
mesopores (i.e., pore diameters in the range 2–50 nm) have
attracted increasing attention due to their potential applications
in energy/gas storage,1,2 CO2 capture,3 catalysis,4 sensing5 and
drug delivery.6 However, the search for structures with very
large pores (i.e., with diameters up to 3 nm) still presents great
challenges. The use of longer organic linkers to construct MOFs
incorporating giant pores/cavities is a promising strategy, but
one which also tends to lead to interpenetrated structures,7,8
and this drastically reduces the available surface area and pore
size. Moreover, linker expansion tends to dramatically reduce
the stability of the framework, leading to its partial or total
collapse upon removal of solvents from the porous host.8a,9
We report herein an extended nanosized hexacarboxylate
linker which has enabled the synthesis of a stable mesoporous
(3,24)-connected framework9–12 with Cu(II) (denoted as
NOTT-119) incorporating cages of up to 4.3 nm in length.
NOTT-119 is isostructural with the frameworks NOTT-112,10
NOTT-116,11 PCN-61,9 PCN-6109 and NU-100.12d,e
calculated to be 0.361 g cmꢁ3
.
TGA of solvated NOTT-119 under N2 showed loss of
solvent (DMF and water) below 135 1C, with the framework
stable up to 315 1C, beyond which the material decomposes
(see ESIw). The solvent molecules in the as-synthesised solids
were exchanged with acetone several times to ensure that
The new extended linker H6L (Scheme 1) was synthesised
via Pd(0)-catalyzed Suzuki coupling of the extended tri-iodo
precursor 1,3,5-tris(40-iodobiphenyl)benzene and 3,5-di(ethoxy-
carbonyl)phenylboronic acid in good yield (see ESIw).
Solvothermal reaction of H6L with Cu(NO3)2ꢀ3H2O in
a School of Chemistry, University of Nottingham, University Park,
Nottingham, NG7 2RD, UK. E-mail: M.Schroder@nottingham.ac.uk
b Optimal Inc, Plymouth, Michigan, 48170, USA
c Diamond Light Source, Diamond House, Harwell Science and
Innovation Campus, Didcot, Oxfordshire, OX11 0DE, UK
w This article is part of the ChemComm ‘Supramolecular chemistry’
web themed issue marking the International Year of Chemistry 2011.
z Electronic supplementary information (ESI) available: Details of
synthesis and characterisation of compounds, structural view of
NOTT-119, TGA, PXRD and gas adsorption data. CCDC 828163.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1cc13170b
Scheme 1 View of L6ꢁ, the organic linker in NOTT-119, and (L1)6ꢁ
,
the organic linker in NOTT-116.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9995–9997 9995

The Ar sorption isotherm for NOTT-119a reveals stepwise
adsorption and desorption with a significant desorption
hysteresis over the pressure range P/P0 0.08–1.0 (Fig. 2). In
the Ar adsorption branch, the first distinctive step is at P/P0 =
0.27, and there is a second step at P/P0 = 0.66. The Ar
desorption branch shows hysteresis due to condensation within
the mesopore,19 and three steps at P/P0 = 0.19, 0.1, and 0.08
reflecting the unique hierarchical assembly of polyhedral cages
in this material. NOTT-119a shows an extremely high Ar
uptake of ca. 2000 cm3 gꢁ1 at saturation, and the pore size
distribution was determined by analyzing the Ar adsorption
isotherm using non-local density functional theory (NLDFT)
(see ESIw). This revealed a distribution of mesopores (2.4–3.4 nm)
consistent with the crystallographically-determined diameters
of Cage B and Cage C. In addition, there is a narrow distribution
of mesopores (6.1–7.1 nm) corresponding to the mesoporous
channels formed by the windows of the extra-large cavities
in NOTT-119a. Compared to NOTT-116a,11 the pore volume
of NOTT-119a is higher (2.35 vs. 2.17 cm3 gꢁ1, respectively),
Fig. 1 View of the three types of cages in NOTT-119. Spheres
indicate the void space within the three types of cavities (Cage A,
violet; Cage B, turquoise; Cage C, yellow). Hydrogen atoms and
disordered solvent molecules are omitted for clarity. Carbon, black;
oxygen, red; copper, turquoise.
DMF was completely replaced by low-boiling point acetone,
and the desolvated framework NOTT-119a was generated by
heating the acetone-exchanged sample at 110 1C for 16 h under
dynamic vacuum. Significantly, the crystal lattice of this
activated sample remains intact on extended heating under
vacuum, as confirmed by powder diffraction analysis (see
ESIw). Thus, crystallinity and order of the (3,24)-connected
polyhedral framework architecture is retained in NOTT-119a.
However, when the size of the linker is extended beyond this
point, the network is no longer stable to thermal treatment and
suffers disruption of the structure due to surface tension effects.
Thus, the isostructural Cu(II)-based (3,24)-connected material
NU-10012d (also known as PCN-6109), which incorporates a
slightly longer linker than that in NOTT-119, undergoes complete
loss of crystallinity on thermal desolvation under vacuum.
However, this mesoporous network retains crystallinity on
desolvation using supercritical CO2.12d
but the surface area is lower (4118 vs. 4664 m2 ꢁ1). Thus,
g
NOTT-119a appears to be near a cusp where increasing pore
volume occurs at the expense of the overall surface area of the
porous host. Since gas storage capacities of porous MOFs are
controlled by the isosteric heat of adsorption of the substrate
at low pressures, by surface area of the host at moderate
pressures, and by pore volume at high pressures (close to
saturation),7,10,11,20 we investigated the high pressure uptake
of both H2 and CH4 by NOTT-119a.
High pressure H2 sorption isotherms for NOTT-119a up to
60 bar at 77 K were collected using a volumetric measurement
method, and this confirmed (Fig. 3) an excess H2 uptake of
59 mg gꢁ1, equivalent to 5.6 wt% [wt% = 100(weight of
adsorbed H2)/(weight of host material + weight of adsorbed
H2)] at 44 bar. This value, although high, is lower than those
of the structural analogues NOTT-112a (76 mg gꢁ1 at 35 bar;
The N2 isotherm for NOTT-119a shows (Fig. 2) typical Type
IV behaviour with a slight adsorption–desorption hysteresis in
the P/P0 range 0.05–0.25, indicative of the presence of mesopores.
The pore-filling step in NOTT-119a is shifted to a higher
BET surface area 3800 m2 gꢁ1, pore volume 1.62 cm3 gꢁ1 10
)
pressure (P/P0 = 0.25) compared to a value for P/P0
=
and NOTT-116a (68 mg gꢁ1 at 27 bar).11 Given the large pore
volume of NOTT-119a we anticipated that it would be in the
high pressure region that this material would be most effective.
This is indeed the case and NOTT-119a shows a high total H2
uptake of 101 mg gꢁ1, equivalent to 9.2 wt% at 77 K and 60 bar,
which is among the highest total H2 uptake capacities so
far observed for a MOF material.9–11,12d,16 NOTT-119a shows
a total volumetric H2 uptake of 37 g Lꢁ1 at 77 K and 60 bar
(see ESIw) reflecting its low density. The isosteric heat of H2
adsorption at zero coverage is 7.3 kJ molꢁ1 (ESIw) consistent
with the presence of exposed Cu(II) sites within the microporous
[Cu24(isophthalate)24] cuboctahedral cages.11
0.15 for NOTT-116a,11 which incorporates an alkyne moiety
in place of one of the phenyl groups in each arm of the ligand
(L1)6ꢁ (Scheme 1). This indicates that the accessible cages in
NOTT-119a have larger pore diameters. NOTT-119a exhibits
exceptionally high N2 uptake of ca. 1522 cm3 gꢁ1 at saturation
and a remarkably high total pore volume of 2.35 cm3 gꢁ1
,
among the highest reported for MOF materials. By applying
the Brunauer-Emmett-Teller (BET) model in the pressure
range P/P0 = 0.12 to 0.18, the specific surface area of
NOTT-119a was calculated to be 4118(200) m2 gꢁ1 (see ESIw),
making it one of the few MOFs with a BET surface area
exceeding 3800 m2 gꢁ1 18
.
Fig. 2 N2 and Ar isotherms for NOTT-119a at 77 and 87 K,
Fig. 3 H2 (left) and CH4 (right) sorption isotherms for NOTT-119a
respectively.
at 77 K and 298 K, respectively.
c
9996 Chem. Commun., 2011, 47, 9995–9997
This journal is The Royal Society of Chemistry 2011

CH4 uptake by NOTT-119a was measured at 298 K and at
pressures of up to 80 bar using the same sorption apparatus as
used for the H2 uptake experiments. While NOTT-119a shows
a respectable excess CH4 uptake of 154 mg gꢁ1 at 35 bar
(Fig. 3), this is lower than 186.1 mg gꢁ1 reported for the
isostructural PCN-689 (also known as NOTT-116) under the
same conditions. Due to its low crystallographic density,
NOTT-119a has an excess volumetric CH4 uptake (v/v) of
77.7 cm3 (STP) cmꢁ3 at 35 bar. The excess adsorption reꢁa3ches
saturation with an uptake of 194 mg gꢁ1 (97.8 cm3 cm ) at
66 bar, and the total uptake was calculated to be 327 mg gꢁ1
(165 cm3 cmꢁ3) at 80 bar. These values for gravimetric CH4
uptake in the high pressure region are comparable to other high
methane-storage MOFs such as MIL-1012b (excess 239 mg gꢁ1
at 80 bar), DUT-9-SCD17 (excess 219 mg gꢁ1 at 100 bar)
and DUT-61c/MOF-20516 (excess 230 mg gꢁ1 at 100 bar; total
394 mg gꢁ1 at 80 bar).
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In summary, the (3,24)-connected mesoporous framework
NOTT-119 has been synthesised by utilising a nanosized
C3-symmetric hexacarboxylate linker. NOTT-119 shows high
thermal stability due to the ubt-type network architecture,
exhibits a high BET surface area of 4118(200) m2 gꢁ1 and a
remarkably large total pore volume of 2.35 cm3 gꢁ1. Desolvated
NOTT-119a shows interesting Ar sorption behaviour with large
desorption hysteresis, consistent with the unique polyhedral
structure incorporating different sized cages, in addition to high
H2 and CH4 adsorption capacities in the high pressure region.
We thank the EPSRC, the U.K. Sustainable Hydrogen
Energy Consortium (http://www.uk shec.org.uk/) and the
University of Nottingham for funding. We are grateful to
Diamond Light Source for access to Beamline I19 and I11. MS
gratefully acknowledges receipt of an ERC Advanced Grant.
NRC gratefully acknowledges the receipt of a Royal Society
Leverhulme Trust Senior Research Fellowship. SY thanks
Shell-EPSRC for a DHPA Scholarship and the University of
Nottingham/EPSRC for a PhD+ Fellowship.
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This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9995–9997 9997