Polyoxometalate nanocone nanoreactors: Magnetic manipulation and enhanced catalytic performance

Article abstract of DOI:10.1002/anie.201006155

Magnetic personality: Nanocone nanoreactors consisting of polyoxometalates functionalized with surfactant alkyl chains and magnetite nanocrystals (NCs) provide enhanced catalytic performance for the oxidation of sulfides to sulfones by a trap-release mechanism and advanced catalyst recovery under an external magnetic field. Copyright

Full text of DOI:10.1002/anie.201006155

DOI: 10.1002/anie.201006155
Heterogeneous Catalysis
Polyoxometalate Nanocone Nanoreactors: Magnetic Manipulation and
Enhanced Catalytic Performance**
Amjad Nisar, Yao Lu, Jing Zhuang, and Xun Wang*
Rationally designed assembly structures are of high scientific
and technological importance for the development of multi-
nanocrystals and their controlled manipulation in the reaction
system. For example, we applied these nanocones for the
catalytic oxidation of sulfides in which they act as nano-
reactors to provide enhanced efficiency, selectivity, and easier
recovery under an external magnetic field.
Figure 1 illustrates the concept of the nanocone nano-
reactors. According to the accepted mechanism for the
increased efficiency of the POM hybrid building block
[
1]
functional materials and devices. Among various types of
nanobuilding blocks, polyoxometalates (POMs) are a class of
metal oxide nanoclusters that offer rich structural versatility
[2]
and enormous applications. The recent progresses in the
synthesis of POM-based amphiphilic units have revolution-
ized POM chemistry, and consequently a variety of robust and
well-defined assembly architectures with tunable properties
(DODA) PW O
(DODA = dimethyldioctadecylammo-
3
12 40
[
3]
have been developed, including one-dimensional wires and
[
4]
[5]
[6]
fibers, two-dimensional thin-films and disks, and three-
[
7]
[8]
[6,9]
[10]
dimensional vesicles, spheres, tubes,
and flowers.
Although, these assembly structures hold great promise for
the design of new functional materials, in reality they have
been less explored for their potential use in various scientific
fields.
POMs have been extensively applied as catalysts for the
oxidation of a variety of compounds such as alkenes, alcohols,
[
11]
and sulfides.
sulfones, which is environmentally highly important, yet
Particularly, the oxidation of sulfides to
[
12]
brings up a key challenge. To meet this challenge, many
strategies in both homogeneous and heterogeneous catalytic
reaction systems have been developed. The homogeneous
approaches utilize the POM catalyst in bare form or in
combination with a phase-transfer reagent, which generally
have drawbacks associated with difficult catalyst separation
[
13]
and recovery.
The heterogeneous approaches involve
encapsulation with specific cations, immobilization of POM
into silica or polymer matrices, or microemulsion forma-
[
14]
tion.
Although, heterogeneous systems provide easier
catalyst recovery, generally it is based on filtration, which
may be highly cumbersome on an industrial scale. In addition,
the immobilization of POMs in supporting matrices involves
[
14f,15]
Figure 1. a) Schematic illustration of a POM nanocone nanoreactor.
b) Oxidation of sulfides to sulfones in the presence of the cones as a
catalyst.
complicated and lengthy catalyst preparation processes.
Therefore, alternative multifunctional and superior
approaches are needed. Recently, we reported well-defined,
robust Keggin ion based nanocones, obtained by a simple and
[
6]
fast synthesis technique at room temperature. Herein, we
report the functionalization of nanocones with magnetite
nium) relative to that of bare H PW O , the highly hydro-
3
12 40
3
À
phobic alkyl chains attached on the PW O
40
surface
1
2
encapsulate the relatively nonpolar substrate (sulfide) mole-
cules as a result of hydrophobic interactions, but easily release
[
*] A. Nisar, Y. Lu, J. Zhuang, X. Wang
Department of Chemistry, Tsinghua University
Beijing 100084 (P.R. China)
the product molecules (sulfone), which have much higher
[14f]
polarity.
Thus, the alkyl chains act as a dynamic trap to
enhance the probability of an interaction between the
E-mail: wangxun@mail.tsinghua.edu.cn
3
À
substrate and the catalytic center (PW O ). From this
1
2
40
[
**] This work was supported by NSFC (20971078, 20725102,
fact, we may hypothesize that the hybrid building blocks in
the nanocone assemblies may provide more-enhanced cata-
lytic efficiency due to self-organization of the building blocks
in compact lamellar patterns (XRD; see Figures S4 and S5 in
the Supporting Information) and thus increase in alkyl-chain
2
0921001), the Fok Ying Tung Education Foundation (111012), and
the State Key Project of Fundamental Research for Nanoscience and
Nanotechnology (2011CB932402).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006155.
Angew. Chem. Int. Ed. 2011, 50, 3187 –3192
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density around the catalytic center. While being functional-
that the external surface of the cone assemblies comprises
alkyl chains of the highly hydrophobic surfactant (Figure 2a,
inset). It is important to note that the preparation of the cones
is very fast (15–20 min at room temperature) and involves
only a single-step synthesis. We think this promising fact is
significant for their easier application in several fields.
Nanoscale incorporation of the ferromagnetic NCs into
the assembly structures is an effective technique to develop
functional nanomaterials with advanced applications in
ized with magnetite nanocrystals (NCs), the nanocones
provide an advanced and swift method of catalyst recovery
from the reaction system under an external magnetic field.
The
supramolecular
nanobuilding
block
(
DODA) PW O was prepared by ion exchange reaction
3
12 40
[6]
between DODABr and phosphotungstic acid (H PW O ),
3
12 40
and characterized by fundamental analytical techniques (see
the Supporting Information). Figure 2a shows a scanning
electron microscopy (SEM) image of nanocones obtained
from the optimized mixed solvent system of chloroform and
n-butanol (2:1.15, v/v) after about 20 min of ageing. It is
evident that the cones are highly pure in morphology and can
be obtain in bulk scale (Figure S8 in the Supporting Informa-
tion). Further, static contact angle measurements revealed
[16]
various areas.
We selected magnetite (Fe O ) NCs (6–
3 4
7 nm) for their excellent ferromagnetic properties (Fig-
ure 2 f). Composite nanocones with incorporated magnetite
NCs were obtained by a simple and fast synthesis approach
(Figure 1). Figure 2b shows a low-resolution transmission
electron microscope (TEM) image of the corresponding
composite cones. The high-resolution TEM measure-
ments of different regions of the cones clearly indicate
the presence of magnetite NCs in the multilamellar
cone assemblies (Figure 2c–e). Energy dispersive spec-
troscopy coupled with TEM further confirmed the
existence of magnetite NCs and verified the TEM
results (Figure S6 in the Supporting Information). The
incorporation of magnetite NCs does not influence the
self-organization of the nanobuilding blocks into a
lamellar pattern nor the resultant assemblyꢀs conical
shape. This fact may be attributed to the relatively small
quantity of magnetite NCs relative to the number of
POM building blocks. We observed that magnetite NCs
exhibit strong affinity for the cone assemblies and
rapidly adsorb on their surface when they were
dispersed (in powder form) into the nanocone solution.
The nanocones containing magnetite NCs were
found to be sufficiently robust, stable, and physically
manipulatable under an external magnetic field in
petroleum fuel solvents such as n-hexane, cyclohexane,
and n-dodecane. The real-time-controlled manipulation
of the nanocones was examined by optical microscopy.
A small magnet bar was used as external manipulator.
Figure 3 shows a series of optical microscope images in
the presence and absence of the manipulating force in
n-hexane. In the absence of an external magnetic field
the cone assemblies exhibit Brownian movement,
whereas with the application of magnetic field all
assemblies are transported quickly toward the magnet.
The transportation direction and speed were found to
be highly controllable and adjustable depending on the
magnet position and strength. It is interesting to note
that in the presence of a magnetic field the cones orient
and transport in such a way that their open end leads
the sharp tip (Figure 3b–d,f–i). We suppose this behav-
ior may result from the relatively greater number of
magnetite NCs at the larger open end. Moreover,
continuous application of a magnetic field self-aligns
the cones in rows (Figure 3d,i). Notably, however, the
cones become separated and start Brownian motion
with removal of the external magnetic field (Figure 3e),
which we think eliminates any possibility of permanent
self-aggregation and will be vital for their practical
applications, for example, in catalysis.
Figure 2. a) SEM image of (DODA) PW O cones. Inset: static contact angle
3
12 40
of a film of cones (1468). b) TEM image of the cones functionalized with
magnetite NCs. c,d) Magnified TEM images of different areas of a cone,
clearly indicating the magnetite NCs. The inset in (c) is the respective cone,
highlighting the magnified areas. The inset in (d) is the magnetization curve
of such cones. e) Tip of a cone at higher magnification. f) TEM image of
magnetite NCs. The insets are a high-resolution TEM image of a magnetite
NC and the corresponding fast Fourier transform (FFT) analysis.
3
188
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Angew. Chem. Int. Ed. 2011, 50, 3187 –3192

Figure 3. Optical microscopy study of the manipulation of (DODA) PW O cones functionalized with magnetite NCs. The arrows indicate the
3
12 40
direction of movement of the cones and the position of the manipulator (magnet). a) Cones in the state of Brownian motion. b,c) Nanocones
controlled movement in the marked directions. d) Cones self-alignment in rows and movement in the direction of the applied magnetic field.
e) Cones after removal of the magnetic field in state of Brownian motion. f–i) Manipulation of cones in various directions.
Sufficient magnetization ability of magnetic materials is
an essential requirement for their remote and controlled
manipulation in a reaction system. To further explore and
estimate the magnetic properties of the nanocone assemblies
carrying magnetite NCs, the magnetization curve at room
temperature was plotted (inset in Figure 2d) revealing super-
paramagnetic behavior of the composite assemblies without
the formation of a hysteresis loop, which we think is the key
factor of self-alignment and redispersion of the cones in the
presence and absence of an external magnetic field, respec-
Owing to its facile, reversible electron-transfer ability,
high thermal and chemical stability, and photochemical
3
À
activity, Keggin ion PW O
40
is a promising choice as a
1
2
[
2j,11b,14f]
catalyst for oxidation reactions.
To evaluate the
catalytic properties of the nanocones, we selected sulfur-
containing compounds such as dibenzothiophene (DBT),
diphenyl sulfide (DPS), and dimethyl sulfide (DMS), for their
selective conversion into sulfones. Hydrogen peroxide (30%)
was selected as an oxidant, and n-hexane was chosen as
solvent because of its low boiling point and thus ease of
sample preparation for high performance liquid chromatog-
raphy (HPLC) analysis.
tively. The saturation magnetization of the composite cones
À1
(
0.29 emug ) was found to be considerably below that of
À1
pure magnetite NCs (15 emug ) of similar size, which may be
attributed to the fairly small amount of magnetite NCs
incorporated in the cones. However, the saturation mag-
netization value also provides a good estimate of the amount
of magnetite NCs in the cones.
Figure 4b demonstrates the progress of the DBT oxida-
tion reaction with time. From the chromatograms, it is
[17]
apparent that the DBT peak area (at t = 5.15 min) rapidly
R
decreases with increase in reaction time and reaches almost to
zero in about 38 min, whereas the peak area of dibenzothio-
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3189

Communications
Figure 4. a) FTIR spectra of dibenzothiophene sulfone (DBTO ) and diphenyl sulfone (DPSO ). b) HPLC analysis of the oxidation of DBT catalyzed
2
2
by (DODA) PW O nanocones at various time intervals at 508C. c) DBT oxidation conversion and ln(C /C ) versus the reaction time. d) HPLC
3
12 40
t
0
analysis of the oxidation of DPS catalyzed by nanocones at various time intervals at 508C. e) DPS oxidation conversion and ln(C /C ) versus the
t
0
reaction time. f) MS spectra of the ketonization reaction product cyclohexanone. The inset shows the corresponding GC chromatogram, revealing
the cyclohexanol and cyclohexanone signals.
phene sulfone (t = 2.58 min) rapidly increases and reaches a
maximum value in about 38 min. The identification of the
monomers may result from the lower density of hydrophobic
alkyl chains around the POM catalytic centers relative to the
cone assemblies.
R
reaction product DBTO2 was further confirmed by the
À1
presence of two strong bands (1285 cm SO asymmetric,
Further, to understand the kinetics of the reaction, DBT
conversion and ln(C /C ) were plotted against reaction time
2
À1
1
162 cm SO symmetric) in the Fourier transform (FT) IR
2
t
0
spectrum and by mass spectrometry (MS) measurements
Figure 4a and Figure S9 in the Supporting Information). To
(Figure 4c; C and C symbolize the initial concentration and
0 t
(
the concentration at time t of DBT). The linear fit to the
experimental data demonstrates that the nanocone-based
catalytic reaction agrees well with the reported homogeneous
and heterogeneous catalytic reaction systems and follows
compare the catalytic efficiency of the nanocones with the
that of the building block monomers, we also performed the
DBT oxidation reaction by replacing the nanocones with the
building block monomers. The catalytic efficiency of the
monomers was found to be notably lower with reaction times
of more than 2 h. The lower efficiency of the building block
[
13a,14f]
pseudo-first-order reaction kinetics.
The corresponding
equations are given in the Supporting Information. From the
linear fit, the reaction rate constant k was calculated to be
3
190
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Angew. Chem. Int. Ed. 2011, 50, 3187 –3192

À1
0
.1066 min , which indicates that the nanocones exhibit high
allows these cones to act as nanovehicles to transport foreign
materials to target spaces.
efficiency for the oxidation of DBT. Moreover, as the k value
lies between those for bare POM (homogeneous)
immobilized POM (heterogeneous)
[
13a]
and
[
14f]
oxidative catalytic
Received: October 1, 2010
Revised: January 28, 2011
Published online: March 2, 2011
reaction systems, we suspect that the nanocones form a
quasi-homogeneous catalytic reaction system. The oxidation
of DPS to diphenyl sulfone also obeys the pseudo-first-order
reaction kinetics (Figure 4d,e), and the reaction is complete
with 100% selectivity in about 20 min, which further reveals
the high efficiency of the nanocone assemblies. The nanocone
assemblies were not only found to be efficient and highly
selective catalysts for the conversion of DBT and DPS but
also for that of DMS. The reaction time for 100% conversion
was estimated to be about 20 min by MS measurements
Keywords: heterogeneous catalysis · magnetic properties ·
.
nanostructures · polyoxometalates
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