Adsorption and catalytic properties of the inner nanospace of a gigantic ring-shaped polyoxometalate cluster

Article abstract of DOI:10.1002/anie.200903142

Functional inner space: A gigantic ringshaped {Mo154} polyoxometalate cluster anion can be stabilized by encapsulation in dimethyldioctadecylammonium (DODA) cations. Its inner nanospace enables adsorption of gases and vapors, and it acts as a water-tolerant solid acid catalyst (see scheme).

Full text of DOI:10.1002/anie.200903142

Angewandte
Chemie
DOI: 10.1002/anie.200903142
Polyoxometalates
Adsorption and Catalytic Properties of the Inner Nanospace of a
Gigantic Ring-Shaped Polyoxometalate Cluster**
Shin-ichiro Noro,* Ryo Tsunashima, Yuichi Kamiya, Kazuhiro Uemura, Hidetoshi Kita,
Leroy Cronin,* Tomoyuki Akutagawa, and Takayoshi Nakamura*
Porous materials with structurally well-defined nanoscale
cavities (e.g., carbon nanostructures, zeolites, mesoporous
[
1–3]
silica, and coordination compounds)
are of great interest
because of their unique properties such as adsorption,
separation, exchange, and catalysis, which are all associated
with the presence of a functional nanospace. In this respect,
polyoxometalate clusters, which are nanosized metal-oxide
macroanions built from molecular precursors with a range of
unique redox and acidic properties, are ideally suited for the
development of functional nanospaces but have not yet been
Scheme 1. Protection of {Mo154} ring by DODA cations.
[
4–6]
thus explored.
Indeed, ring-shaped POMs have a great
deal of potential for allowing creation of an exceptionally well
defined and accessible nanosurfaces and nanospaces. These
ring clusters date back to 1995, when the synthesis and
structural analysis of gigantic mixed-valent ring-shaped POM
cluster (NH ) [Mo (NO) O H (H O) ] ({Mo154} ring)
been greatly under-utilized and -investigated, probably due to
poor stability.
[8]
Because of this fundamental limitation, we opted to
examine the stability and functionality of a {Mo154} ring
encapsulated by dimethyldioctadecylammonium (DODA)
cations (Scheme 1), because the DODA cation, with a
length of about 2.5 nm, introduces amphiphilic-like properties
into the POM–cation hybrid, and POM–DODA hybrids have
shown some interesting properties, for example, for formation
4
28
154
14 448 14
2
5a]
70
[
was first reported by Mꢀller et al.,
and later this was
followed by a much more facile synthesis of the analogous
compound
Scheme 1;
Na [Mo154O462H (H O) ]·400H O
for further information on ring properties,
(1,
1
4
14
2
70
2
[
5b–d]
[
9,10]
see Ref. [5f–j]) The diameter of the POM ring and inner void
space are about 3.9 and 1.7 nm, respectively, while the height
of Langmuir–Blodgett films.
Further, we hypothesized
that encapsulation by DODA will also help to increase the
stability of the nanoscale wheel cluster. Not only will nano-
scale molecular assembly of the hybrid material be controlled
by electrostatic interactions between the {Mo154}-ring polyan-
ion and DODA cations, hydrophobic interactions between
the alkyl chains should contribute to stabilization of the
[7]
of ring is about 1.8 nm. Despite the great potential of ring-
shaped POMs to form porous materials, POM clusters have
[*] Dr. S.-i. Noro, Dr. R. Tsunashima, Prof. T. Akutagawa,
[9]
Prof. T. Nakamura
hybrid material in the solid state. As reported previously,
Research Institute for Electronic Science, Hokkaido University
Sapporo 001-0020 (Japan)
Fax: (+81)11-706-9420
E-mail: snoro@es.hokudai.ac.jp
tnaka@es.hokudai.ac.jp
the POM–DODA hybrid shows high stability in CHCl3
solution, while the ring structure of 1 is not robust in aqueous
solution and the solid state (see Figures S1–S4, Supporting
Information).
Here we report on the inner nanospace functionality of
Prof. L. Cronin
[11]
POM–DODA hybrid (DODA) [Mo154O462H ]·nH O (2).
2
3
5
2
WestCHEM, Department of Chemistry, University of Glasgow
Glasgow G128QQ, Scotland (UK)
Fax: (+44)141-330-4888
E-mail: l.cronin@chem.gla.ac.uk
Homepage: http://www.croninlab.com
Initially, dynamic light scattering (DLS) experiments were
conducted to confirm the presence of the {Mo154} ring of 2 in
solution and showed that the particle diameter in a solution in
THF is about (4.0 Æ 0.6) nm, similar to the size of the {Mo154}
ring (Figure S5, Supporting Information). Hence, these data
are consistent with the hypothesis that the {Mo154} ring is
stabilized after reaction of 1 with DODA, whereby the
DODA cations effectively encapsulate the outer part of the
ring, as shown in Scheme 1. Thermogravimetric analysis of 2
shows a weight loss of 4.7% in the temperature range from
Prof. Y. Kamiya
Research Faculty of Environmental Earth Science
Hokkaido University
Sapporo 060-0812 (Japan)
Dr. K. Uemura, Prof. H. Kita
Graduate School of Science and Engineering, Yamaguchi University
Yamaguchi 755-8611 (Japan)
2
98 to 440 K (Figure S6, Supporting Information), which
[
**] This work was partly supported by a Grant-in-Aid for Science
Research from the Ministries of Education, Culture, Sports, Science
and Technology of Japan.
corresponds to about 95H O molecules. To confirm removal
2
of all H O molecules, the temperature dependence of the IR
2
spectrum was measured. The OH stretching and OH bending
bands at 3400 and 1620 cm , respectively, are observed at
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903142.
À1
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room temperature. As the temperature increases, the inten-
sity of these bands decreases (Figures S7 and S8, Supporting
Information). At 433 K, these bands almost disappear, that is,
all H O molecules are removed from 2. On the other hand,
2
bands due to the DODA cation and the POM ring scarcely
change. The UV spectra of 2 before and after removal of all
H O in CHCl solution are similar to each other (Figure S9,
2
3
Supporting Information). These results suggest that removal
of all H O molecules has little influence on the ring structure
2
of 2. The irregular assembly of the ring-shaped POMs, as
shown in Scheme 1, was confirmed by XRD patterns, in which
only some of the broad peaks of 2 were identifiable
(
Figure S10, Supporting Information), and this gives a very
Figure 1. Adsorption isotherms of N and CO in dehydrated 1 (N :
interesting perspective for future design of these architec-
tures, since a regular assembly of POMs with designed (e.g.,
rigid) cations has potential to give access to a more prominent
nanospace.
2
2
2
circles, CO : squares) and 2 (N : triangles, CO : diamonds) at 77 and
2
2
2
195 K (adsorption: empty symbols, desorption: filled symbols).
In the case of hybrid 2, it is important whether the DODA
cations are located inside or outside the {Mo154} ring. The
1
[9]
H NMR spectra of 2 in CDCl3 indicate that the electrostatic
interaction between the negatively charged {Mo154} ring and
positively charged nitrogen atom of DODA reduces the
motional freedom of the head group of the DODA cation,
and the relatively large thermal motions of the two octadecyl
chains of DODA cations are maintained. In addition, the
observation that 2 is highly soluble in nonpolar solvents such
as toluene, CH Cl , and CHCl , as well as the fact that the
2
2
3
DODA cation is considerably larger than the inner space of
the wheel, indicates that the DODA cations are predom-
inately located on the outside of the {Mo154} ring. Further-
more, the maximum adsorbed amount of H O molecules also
2
supports our suggestion. This idea can be expanded further by
considering that the inner volume of the {Mo154} ring
Figure 2. Adsorption isotherms of H O (circles), MeCN (squares), and
benzene (triangles) in dehydrated 2 at 298 K (adsorption: empty
2
calculated from the inner diameter and the height is about
symbols, desorption: filled symbols).
3
4
.1 nm , while up to 150H O molecules can be adsorbed per
2
{
Mo154} ring. Considering the liquid density of H O, the
2
3
adsorbed H O molecules occupy a volume of about 4.5 nm ,
All isotherms for dehydrated 2 show a gradual increase in the
amount of gas adsorbed. In addition, the higher the mea-
surement temperature is, the more the adsorbed amount
increases. These results support that the gases and vapors
diffuse into densely aggregated alkyl chains of DODA cations
with structural transformations, that is, positional displace-
2
which is similar to the inner volume of the {Mo154} ring.
Further, the amount of H O adsorbed by 2 is significantly
2
lower than the amount in as-synthesized 1 (ca. 470). In the
case of as-synthesized 1, a large amount of H O is located on
2
the outside of the {Mo154} ring, while the outer space of 2 is
occupied by alkyl chains of DODA cations, which decrease
ments of alkyl chains. The fact that the hydrophilic H O,
2
the amount of H O adsorbed by 2. Considering the high
hydrophobic benzene, and medium-polarity MeCN are all
adsorbed by dehydrated 2 indicates coexistence of a hydro-
philic nanospace formed inside the {Mo154} ring and hydro-
phobic nanospace existing inside the aggregated alkyl chains.
Infrared spectra of pyridine-loaded 2 (Figure 3) were
measured to explore the presence of Lewis and Brønsted
acidic sites inside the {Mo154} ring. The positions of in-plane
vibration bands of a pyridine ring strongly depend on its
2
hydrophilicity of H O, it can be concluded that the inner space
2
is largely filled with H O molecules, and the outer surface is
2
covered by the DODA cations.
To confirm the presence of the nanospace after removal of
all H O molecules, gas and vapor adsorption isotherms were
2
measured on samples that had been dried at 353 K under
vacuum. Figure 1 shows the adsorption isotherms of N and
2
[12]
À1
CO at 77 and 195 K. Dehydrated “unprotected” 1 shows no
binding state.
The bands at 1539 and 1456 cm are
2
adsorption of N and CO over the entire pressure range, and
attributed to pyridine adsorbed on Brønsted and Lewis
acidic site, respectively, and a band at 1442 cm , derived
2
2
À1
this indicates that dehydrated 1 has no accessible space for
these gases. In contrast, the adsorption isotherms of desol-
vated “protected” 2 show gradual uptake, that is, dehydrated
from hydrogen-bonded pyridine, is also observed. Because of
the high affinity for H O molecules, H O molecules are easily
2
2
2
has a permanent and accessible nanospace. The adsorption
adsorbed by 2 during the handling of dehydrated 2 in the
atmosphere, which was confirmed by TG and IR measure-
ments (see Figures S6 and S11, Supporting Information).
isotherms of H O, MeCN, and benzene for dehydrated 2 at
2
2
98 K (Figure 2) also show gradual uptake by the nanospace.
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ration of organic solvents, the residue was recrystallized from CHCl /
3
À
EtOH (15/15 mL). The absence of Cl anions was confirmed by
elemental analysis. Elemental analysis of 2 ((DODA) [Mo154O462H5]·
2
3
7
0H O) calcd (%) for C874H1985Mo154N O532: C 29.07, H 5.54, N 0.89,
2
2
3
Cl 0; found: C 29.28, H 5.43, N 1.00, Cl 0. The quantity of adsorbed
H O molecules in 2 was checked by thermogravimetric analysis.
2
Physical measurements: Dynamic light scattering was measured
with an Otsuka Electronics FPAR-1000 in THF solution at 298 K.
Thermogravimetric analyses and differential thermal analyses were
performed with a Rigaku Thermo plus TG8120 apparatus in the
temperature range between 298 and 773 K in a N atmosphere and at
2
À1
a heating rate of 10 Kmin . XRD data of powder samples were
collected on a Rigaku RINT-Ultima III diffractometer with CuKa
radiation. The adsorption isotherms for N and CO , and benzene
were measured with BELSORP-max volumetric adsorption equip-
2
2
Figure 3. IR spectra of pyridine-loaded 2 (solid line) and 2 (dashed
line).
ment. The adsorption isotherms for H O and MeCN were measured
2
with BELSORP-18 volumetric adsorption equipment. The IR spectra
were measured on a Perkin-Elmer Spectrum 2000 with samples
prepared as KBr pellets. A high-temperature transmission cell, HT-32
Therefore, it can be concluded that dehydrated 2 has both
Lewis and Brønsted acid sites on the inner surface of the
(Thermo Spectra-Tech), was used to measure the temperature
dependence of the IR spectra.
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M
o
}
r
i
n
g
.
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d
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o
r
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t
i
o
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o
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y
r
i
d
i
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e
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t
a
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d
s
i
t
e
s
o
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h
e
{
M
o
1
5
4
1
5
4
Pyridine-loaded 2 was prepared as follows: After heating 2 at
353 K under vacuum for 4 h, the dehydrated 2 obtained was exposed
to anhydrous pyridine (Wako Pure Chemicals) for 1 h and then dried
at room temperature under vacuum for 2 h.
ring clearly supports that the inner nanospace is accessible for
guests. To our knowledge, this is the first time that a ring-
shaped POM has been demonstrated to have an open, stable,
Hydrolysis of ethyl acetate was carried out in an ampoule
and accessible nanospace after removal of all the H O
2
3
(
(
10 cm ) at 333 K with a 5 wt% aqueous solution of ethyl acetate
molecules.
3
3 cm , 1.7 mmol of ethyl acetate). The weight of catalyst used was
Protection by DODA cations makes 2 insoluble in H O, so
2
80 mg. The reactants were commercial-grade reagents (Wako Pure
we hypothesized that 2 may be a potential water-tolerant acid
catalyst that is environmentally friendly with respect to
corrosiveness, safety, waste generation, ease of separation,
Chemicals) and were used as received. All solutions were prepared
from Milli-Q ultrapure water (Millipore). Products were analyzed
with a gas chromatograph (Shimazu GC-14B, FID) equipped with a
ZB-WAX column.
[
13]
and recovery. To investigate this, hydrolysis of ethyl acetate
in the presence of excess water, which is a useful standard
reaction for investigating water-tolerant solid acid catalysts,
was preliminarily performed with 2 as catalyst. The conver-
sion after 1 h was about 6.4%, comparable with those of
Cs H PW O (ca. 6%) and H-ZSM-5 zeolite (ca. 5%),
Received: June 11, 2009
Revised: July 30, 2009
Published online: October 12, 2009
2
.5 0.5
12 40
Keywords: adsorption · heterogeneous catalysis ·
[
13d]
.
which are typical water-tolerant solid acids.
No by-product
microporous materials · organic–inorganic hybrid composites ·
polyoxometalates
was observed and these results indicate that 2 has high
potential as a water-tolerant acid catalyst.
In conclusion, we have succeeded in stabilizing ring-
shaped POM–DODA hybrid 2 in the solid state. Our studies
have demonstrated, for the first time, that it is possible to
utilize the {Mo154} big-wheel species as a functional nanospace
and stabilize the dehydrated form; lack of stability has been a
major stumbling block for wider application of these species
until now. Further, we showed that hybrid 2 not only has
adsorption properties for gases and vapors but also high-
performance water-tolerant catalytic properties, comparable
to those of Cs H PW O and H-ZSM-5 zeolite, which are
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À1
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[
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[
[
7] The size was measured by considering van der Waals radii for
constituting atoms.
8] The replacement of inner H O guests by other organic molecules
2
[5e,i,j]
in ring-shaped POMs have been reported.
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Angew. Chem. Int. Ed. 2009, 48, 8703 –8706