A NbO-type metal-organic framework derived from a polyyne-coupled di-isophthalate linker formed in situ

Article abstract of DOI:10.1039/c002767g

A NbO-type metal-organic framework, PCN-46, was constructed based on a polyyne-coupled di-isophthalate linker formed in situ. Its lasting porosity was confirmed by N₂ adsorption isotherm, and its H₂, CH ₄ and CO₂ adsorption capacity was examined at 77 K and 298 K over a wide pressure range (0-110 bar).

Full text of DOI:10.1039/c002767g

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A NbO-type metal–organic framework derived from a polyyne-coupled
di-isophthalate linker formed in situw
Dan Zhao, Daqiang Yuan, Andrey Yakovenko and Hong-Cai Zhou*
Received 9th February 2010, Accepted 1st April 2010
First published as an Advance Article on the web 6th May 2010
DOI: 10.1039/c002767g
A NbO-type metal–organic framework, PCN-46, was constructed
that by combining isophthalate ligand bearing terminal acetylene
with a slight excess of copper(II) salt, which can be readily
reduced to copper(I) under solvothermal reaction conditions,
a homocoupling product di-isophthalate can be generated.
Subsequently, the di-isophthalate linker reacts with the Cu(II)
salts giving rise to a NbO MOF.
based on a polyyne-coupled di-isophthalate linker formed in situ.
2
Its lasting porosity was confirmed by N adsorption isotherm, and
its H , CH and CO adsorption capacity was examined at 77 K
2
4
2
and 298 K over a wide pressure range (0–110 bar).
Metal–organic frameworks (MOFs) have emerged as a new
1
type of porous materials. Their large surface area, high pore
The ligand precursor used herein is 5-ethynylisophthalic acid
2
(H ei), which contains both isophthalic moieties and terminal
volume, uniform yet controllable pore size, and pore geometry
make them good adsorbents for gases, such as H , CH and
acetylene suitable for the coupling reaction. As expected, every
0
two of the H ei were coupled into one 5,5 -(buta-1,3-diyne-1,4-
2
2
4,
2
CO . Their gravimetric gas adsorption capacity, especially in
diyl)diisophthalic acid (H bdi) under the catalysis of Cu(I),
4
2
the high pressure range, is directly related to the specific
which came from the reduction of cupric acetate. In addition,
4ꢀ
2
d,3
surface area and pore volume.
which consist of 4-connected di-isophthalate ligands and
-connected secondary building units (SBU) (typically dicopper
paddlewheel), have shown excellent framework stability,
The NbO-type MOFs,
the in situ formed 4-connected ligand bdi was linked via the
4-connected dicopper paddlewheel SBU to form a NbO-type
MOF, PCN-46 (PCN stands for porous coordination network)
4
(Fig. 1) in 77% of yield (based on H
ei).z
2
2f,2g,4
porosity, and gas adsorption capacity.
On the other hand,
It must be pointed out that the exact chronological sequence
of events such as the coupling of the precursor molecules, the
deprotonation of the di-isophthalic acid, and the formation of
the MOF has not been deciphered. However, the high overall
yield of the procedure implies that the coupling reaction
benefits from the consumption of the tetracarboxylic acid by
MOF formation.
their hydrogen adsorption capacity at room temperature is
relatively low because of weak framework–hydrogen inter-
actions. In our continuing exploration of MOFs as a hydrogen
storage medium, aligned open metal sites have been shown to
5
have an impact on the heat of hydrogen adsorption. In addi-
2g
6
tion, double bonds and aromatic rings were also studied as
possible hydrogen-framework interacting sites. However, prior
to this work, carbon–carbon triple bonds, especially polyyne
chains, have not been systematically studied for gas adsorption
purposes in MOFs. In this contribution, we report an NbO-
type MOF that is constructed based on a polyyne-coupled
di-isophthalate ligand formed in situ. The ensuing MOF has
permanent porosity after the removal of guest molecules, and
exhibits excellent H , CH and CO adsorption capacity.
Calculated using the PLATON routine, PCN-46 has a
solvent accessible volume of 73.8%. This high porosity
prompts us to examine its gas adsorption property. The phase
purity of the bulk sample was confirmed by powder X-ray
diffraction (PXRD), and the framework retains its crystallinity
after the removal of guest molecules (Fig. S2, ESIw). Its
2
permanent porosity was confirmed by both N (77 K) and
Ar (77 and 87 K) sorption isotherms (Fig. 2 and Fig. S3,
2
4
2
The in situ ligand reactions under solvothermal conditions
7
are well documented in the literature. Herein our goal is to
ESIw). The isotherms show type-I sorption behavior, which is
2
typical for microporous materials. Based on the N sorption
2
isotherm, PCN-46 has a BET surface area of 2500 m g
ꢀ1
apply this versatile method to simplify the MOF synthesis by
combining the ligand preparation, purification, and MOF
synthesis into one simple step. It is hoped that the formation
of the MOF will drive the ligand formation to completion;
therefore, the purification of the ligand becomes unnecessary.
Copper(I)-catalyzed oxidative coupling of terminal acetylenes
was found as early as 1869 and has been developed into a
8
versatile synthetic tool for 1,3-diyne compounds. We reason
Department of Chemistry, Texas A&M University, PO Box 30012,
College Station, TX 77842, USA. E-mail: zhou@mail.chem.tamu.edu;
Fax: +1 979 845 4719; Tel: +1 979 845 4034
w Electronic supplementary information (ESI) available: Crystallo-
graphic data in CIF, experimental details, TGA curves, PXRD spectra,
Ar sorption isotherms at 77 K and 87 K, H isosteric adsorption
2
enthalpy. CCDC 747535. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c002767g
4ꢀ
Fig. 1 (a) In situ formed ligand bdi and the formation of NbO-type
framework; (b) atomic packing of PCN-46, viewed through [0 0 1]
direction; (c) atomic packing of PCN-46, viewed through [1 0 0]
direction.
4
196 | Chem. Commun., 2010, 46, 4196–4198
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Table 1 Ligand length, porosity and H
MOFs
2
uptake of selected NbO-type
BET
surface
Pore
volume/ mg g
2
H uptake/ Qst at low
1c
ꢀ
Ligand
length/A area/m g
coverage/
kJ mol
a
2
ꢀ1
3
cm g
ꢀ1b
ꢀ1
˚
MOFs
60bar
Fig. 2 The N
2
sorption isotherm at 77 K (solid symbols, adsorption;
NOTT-101 5.773
PCN-46 6.484
NOTT-102 10.098
2316
2500
2942
0.886
1.012
1.138
66.0
71.6
72.0
o5.5
7.20
o5.5
open symbols, desorption) and pore size distribution of PCN-46.
2
ꢀ1
(
Langmuir surface area: 2800 m g ) and a total pore volume of
a
b
Distance between 5-position of two isophthalate moieties. Calculated
3
ꢀ1
˚
.012 cm g . It has a uniform pore size around 6.8 A based on
1
c
2
from N isotherms at 77 K. Total uptake at 77 K.
the Horvath–Kawazoe model in the Micromeritics ASAP2020
9
software package (assuming cylinder pore geometry).
The high porosity and stable framework make PCN-46 a
good candidate for gas storage. In 2009, the U.S. Department
of Energy (DOE) reset the gravimetric and volumetric storage
targets for on-board hydrogen storage for 2010 (4.5 wt%,
adsorption in PCN-46, compared to those of other NbO type
MOFs shown in Table 1, can be attributed to the interaction
between dihydrogen molecules and the exposed and delocalized
4
ꢀ
p electrons in the polyyne unit in bdi , which is evidently
ꢀ
1
ꢀ1 10
4
ꢀ
4ꢀ
2
7
8 g L ) and 2015 (5.5 wt%, 40 g L ). At 77 K and
stronger than that for the phenyl rings in tpta and qpta . A
strong interaction between acetylene and a NbO-type MOF
containing alkyne unit was also discovered, in which the
high acetylene affinity towards the framework was particially
60 Torr, PCN-46 can reversibly adsorb 1.95 wt% of H2
(
Fig. 3). Under high pressure range, the saturated excess
ꢀ
1
gravimetric H uptake is 5.31 wt% (56.1 mg g ) at 32 bar.
2
4f
Taking the gaseous H
2
compressed within the void pore at
uptake can
reach as high as 6.88 wt% (73.9 mg g ) at 97 bar. Calculated
attributed to the p–p interaction. In addition, the replacement
of phenyl rings by polyyne chain leads to a boost of pore
7
7 K into consideration, the total gravimetric H
2
ꢀ
1
4
ꢀ
volume and hydrogen uptake. As Table 1 shows, the bdi
ꢀ
3
4ꢀ
from the crystal density of the activated form (0.6185 g cm ),
ligand is much shorter than qpta , but the pore volumes and
hydrogen uptakes of PCN-46 and NOTT-102 are very close.
Furthermore, in the absence of a catalyst, the hydrogen
addition reaction on the polyyne unit has not been observed
even at high pressure, as shown by the reversible hydrogen
sorption isotherms of PCN-46, validating the stability of the
MOF under hydrogen storage conditions.
ꢀ1
PCN-46 has an excess volumetric H uptake of 34.7 g L
2
ꢀ
1
(
32 bar) and a total volumetric uptake of 45.7 g L (97 bar).
Theoretical studies reveal that given the same MOF struc-
tural type, the longer the ligand, the higher the specific surface
area, and accordingly the higher the gravimetric H uptake
2
1
would be. This is further confirmed by the comparison of
1
4b
PCN-46 and the NOTT series NbO-type MOFs. As can be
seen from Table 1, when the length of ligand increases, the
surface area, pore volume, and high pressure hydrogen uptake
of the MOFs also increase, but not the heat of adsorption.
Based on a variant of the Clausium–Clapeyron equa-
Methane is another candidate as an on-board fuel. However,
it also suffers from a lack of reliable storage. The DOE
ꢀ
1
methane storage target has a volumetric basis: 180 v(STP) v
1
3
at 35 bar. With high surface area and high pore volume,
MOFs have proved reliable due to their high methane uptake
2
e–g
2
tion, the H isosteric adsorption enthalpy of PCN-46 reaches
capacity.
The high pressure methane adsorption in PCN-46
ꢀ
1
.20 kJ mol at low coverage, and decreased to 4.06 kJ mol
ꢀ1
7
at 298 K was also examined. As can be seen from Fig. 4, the
12
at medium coverage (Fig. S4). The increased heat of hydrogen
gravimetric excess CH
4
uptake in PCN-46 reaches saturation
ꢀ1
ꢀ1
at 12 mmol g (60 bar) (total: 17.2 mmol g , 110 bar). At
ꢀ1
3
5 bar, the volumetric excess CH uptake in PCN-46 is 150 v v
4
ꢀ
1
total: 172 v v ).
(
The capture and sequestration of CO
2
is considered to be an
effective way for the control of greenhouse gas emissions.
Most of the capture processes in large scale operation nowadays
are based on amine-based wet scrubbing systems, which have
1
4
high energy and resource consumption. MOFs have proven
2
to be good adsorbents for CO at ambient temperature. As
i,j
2
can be seen in Fig. 5, the saturation excess CO uptake in
2
ꢀ
1
PCN-46 is 21.0 mmol g (30 bar).
In summary, a NbO-type MOF, PCN-46, was synthesized
based on an in situ formed polyyne-coupled di-isophthalate
ligand by the copper(I)-catalyzed oxidative coupling of terminal
Fig. 3 Gravimetric and volumetric H
2
uptake in PCN-46 at 77 K
(solid symbols, adsorption; open symbols, desorption).
This journal is ꢁc The Royal Society of Chemistry 2010
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at 298 K; dashed line, density of
Solid line, density of compressed CO
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ꢀ1 15
liquid CO at 273 K (928 g L ).
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1
Notes and references
z X-Ray crystal data for PCN-46: C10
H
5
CuO
5
, M = 268.68, trigonal,
2205.
ꢀ
˚
space group R3m, a = b = 18.2386(8), c = 42.049(2) A, V =
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ꢀ3
˚
c
2113.4(9) A , Z = 18, D = 0.663 g cm , F000 = 2412, synchrotron
1
˚
radiation, l = 0.41328 A, T = 173(2) K, 2ymax = 30.01, 68 838
reflections collected, 3022 unique (Rint = 0.1069). Final GooF =
1 2
1.055, R = 0.0747, wR = 0.2042, 82 parameters. CCDC 747535.
4
198 | Chem. Commun., 2010, 46, 4196–4198
This journal is ꢁc The Royal Society of Chemistry 2010