Micelle directed synthesis of polyoxometalate nanoparticles and their improved catalytic activity for the aerobic oxidation of sulfides

Article abstract of DOI:10.1021/ja060696h

Micelle directed polyoxometalate nanoparticles were synthesized by depositing H_3+xPV_xMo_12-xO₄₀ (x = 0, 2) by precipitation on micelles prepared from cesium dodecyl sulfate. The cryo-TEM image showed particles of about ～10 nm roughly consistent with the particle size computed from an idealized model. HRTEM coupled with EELS imaging to map the distribution of the elements also supported the formation of micelle directed polyoxometalate nanoparticles. In the aerobic oxidation of various sulfides to sulfoxides and sulfones, the clustered polyoxometalate assemblies supported on hydrophilic silica showed significantly higher catalytic activity versus that of nonclustered assemblies. Copyright

Full text of DOI:10.1021/ja060696h

Published on Web 03/25/2006
Micelle Directed Synthesis of Polyoxometalate Nanoparticles and Their
Improved Catalytic Activity for the Aerobic Oxidation of Sulfides
Galia Maayan,† Ronit Popovitz-Biro,‡ and Ronny Neumann*,†
Department of Organic Chemistry and Department of Materials and Interfaces, Weizmann Institute of Science,
RehoVot, Israel 76100
Received January 30, 2006; E-mail: ronny.neumann@weizmann.ac.il
Nanoparticle-based materials are building blocks in many areas
of nanotechnology, being utilized in applications such as sensors,
information storage, and catalysis.1 These applications require the
ability to assemble molecules into a defined shape and size so that
the unique nanoscale properties can be effectively exploited.
Approaches for controlling nanoscale structures that are being used
for catalytic applications include microemulsion-mediated nano-
particle synthesis2 and the use of self-assembled polymer or
surfactant microstructures as templates for nanostructured materials.3
These methods, and others, are widely applied for the assembly of
inorganic nanoparticles, such as metal particles, metal oxide
particles, metal particles coated with silica, and silica nanospheres.4
Polyoxometalates are usually polyanions of discrete structure and
have been extensively investigated as acid and oxidation catalysts.5
An interesting challenge is to assemble such polyoxometalates,
forming defined structures that may have interesting, novel proper-
Figure 1. Cryo-TEM image of micelle directed polyoxometalate nano-
particles.
ties.6 In this research, we wished to show that the control of the
size, shape, and structure of polyoxometalate nanoparticles is
interesting in itself and might be important because of a possible
correlation between these parameters and their catalytic properties.
Since it is known that polyoxometalates of the Keggin-type with
cesium countercations are insoluble,7 we hypothesized that an
appropriate surfactant, cesium dodecyl sulfate (CDS), could be used
to prepare micelles in water upon which Keggin-type polyoxo-
metalates (POM), such as H3PMo12O40, could be deposited and
precipitated, yielding spherical polyoxometalate nanoparticles with
a surfactant core and polyoxometalate surface (Scheme 1).
Accordingly, cesium dodecyl sulfate was prepared from sodium
dodecyl sulfate (SDS) by dissolving 1 g of SDS in a minimum
amount of water (∼2 mL); the solution was acidified to pH ∼ 0
with 5% HCl (a few drops), and then a saturated aqueous solution
containing 750 mg of Cs2CO3 was added. The CDS precipitated
from the solution and was collected and dried. The micelle directed
synthesis of the polyoxometalate nanoparticles was carried out by
addition of 200 µL of a 0.01 M solution of H3PMo12O40 and 800
µL of a 0.1 M solution of poly(vinylpyrrolidone) (PVP) to 500 µL
of 0.01 M CDS. The molar ratio of CDS:H3PMo12O40:PVP was
2.5:1:40. The PVP enabled the formation of single micelle sized
particles by preventing particle aggregation during the precipitation
of H3PMo12O40 onto the micelle surface. After the nanoparticle
formation (5 min in a sonicator bath), PVP was extracted from the
micellular solution with chloroform (3 × 2 mL).
Scheme 1. Idealized Cartoon of the Cross Section of a CDS
Micelle-POM Particle
the expected cross section diameter of such particles as may be
computed (8 nm) from the idealized picture presented in Scheme
1. Some aggregation of particles may be observed in the solution.
In samples where the stabilizing polymer was not removed, no
aggregation was observed. Upon deposition of the solution onto
surfaces, the particles tend to aggregate (Figure S1).
Beyond the correlation of the observed particle size with the
idealized model, we were also interested in verifying the consistency
of the idealized model with the distribution of the elements in the
nanoparticles. A high-resolution transmission electron micrograph
(HRTEM) of the polyoxometalate nanoparticles together with a
mapping of the distribution of the oxygen, molybdenum, and cesium
atoms using electron energy loss spectrometry (EELS) is presented
in Figure 2. One may observe in the HRTEM particles of the size
seen in the cryo-TEM image (see middle of image) and larger
particles of up to 50 nm that are apparently a result of aggregation
of smaller particles when the sample is dried under high vacuum
needed for the HRTEM measurement. In all cases, the mapping of
the elements shows that molybdenum, oxygen, and cesium all
The polyoxometalate nanoparticles were directly observed by
cryo-transmission electron microscopy, as shown in Figure 1. From
the image, mostly single 10-13 nm particles may be observed. It
is noticeable that the particles have relatively low contrast because
of the large abundance of carbon and oxygen atoms in the
nanoparticle. Importantly, the particle size is roughly consistent with
† Department of Organic Chemistry.
‡ Department of Materials and Interfaces.
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10.1021/ja060696h CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
catalytic activity. Addition of PPG-750 to the control reaction (H5-
PV2Mo10O40 on hydrophilic silica) did result in some catalytic
activity, for example, 24 turnovers in the oxidation of dibutylsulfide
compared to 89 turnovers for the CDS/PV2Mo10O40/PPG. There
is, however, a caveat in this control experiment because the addition
of PPG to H5PV2Mo10O40 on hydrophilic silica may complex the
polyoxometalate since ethers are well-known to complex acidic
polyoxometalates, whereas no such complexation occurs for
insoluble cesium salts of polyoxometalates as are present in the
reactions catalyzed by CDS/PV2Mo10O40/PPG on hydrophilic silica.
Micelle directed polyoxometalate nanoparticles have been syn-
thesized and characterized by cryo-TEM and HRTEM techniques
including the use of EELS imaging to map the distribution of the
elements. In an aerobic oxidation of sulfides, the advantageous
catalytic activity of the clustered polyoxometalate assemblies versus
nonclustered assemblies was demonstrated.
Acknowledgment. This research was supported by the Minerva
Foundation and the Helen and Martin Kimmel Center for Molecular
Design. Eyal Shimoni and Sharon G. Wolf are thanked for their
help with the cryo-TEM measurements. R.N. is the Rebecca and
Israel Sieff Professor of Organic Chemistry.
Figure 2. HRTEM image of CDS-POM nanoparticles. Top left: zero
loss image. Top right: oxygen mapping image. Bottom left: molybdenum
mapping image. Bottom right: cesium mapping image.
Supporting Information Available: Details of the experimental
methods and additional transmission electron micrographs. This material
is available free of charge via the Internet at http://pubs.acs.org.
Table 1. Aerobic Oxidation of Sulfides Catalyzed by CDS/
PV2Mo10O40/PPG on Hydrophilic Silicaa
substrate
conversion, mol %
TONb
ratio SO/SO2
thioanisole
dibutylsulfide
diphenylsulfide
tetrahydrothiophene
91
100
82
81
89
53
95
22/78
26/74
70/30
0/100
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a Reaction conditions: 0.1 mmol sulfide, 2 µmol PV2Mo10O405- as 10%
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