Nanoscale magnetic stirring bars for heterogeneous catalysis in microscopic systems

Article abstract of DOI:10.1002/anie.201410360

Nanometer-sized magnetic stirring bars containing Pd nanoparticles (denoted as Fe₃O₄-NC-PZS-Pd) for heterogeneous catalysis in microscopic system were prepared through a facile two-step process. In the hydrogenation of styrene, Fe₃O₄-NC-PZS-Pd showed an activity similar to that of the commercial Pd/C catalyst, but much better stability. In microscopic catalytic systems, Fe₃O₄-NC-PZS-Pd can effectively stir the reaction solution within microdrops to accelerate mass transfer, and displays far better catalytic activity than the commercial Pd/C for the hydrogenation of methylene blue in an array of microdroplets. These results suggested that the Fe₃O₄-NC-PZS-Pd could be used as nanoscale stirring bars in nanoreactors.

Full text of DOI:10.1002/anie.201410360

Angewandte
Chemie
DOI: 10.1002/anie.201410360
Heterogeneous Catalysis
Nanoscale Magnetic Stirring Bars for Heterogeneous Catalysis in
Microscopic Systems**
Shuliang Yang, Changyan Cao,* Yongbin Sun, Peipei Huang, Fangfang Wei, and Weiguo Song*
Abstract: Nanometer-sized magnetic stirring bars containing
Pd nanoparticles (denoted as Fe3O4-NC-PZS-Pd) for hetero-
geneous catalysis in microscopic system were prepared through
a facile two-step process. In the hydrogenation of styrene,
Fe3O4-NC-PZS-Pd showed an activity similar to that of the
commercial Pd/C catalyst, but much better stability. In micro-
scopic catalytic systems, Fe3O4-NC-PZS-Pd can effectively stir
the reaction solution within microdrops to accelerate mass
transfer, and displays far better catalytic activity than the
commercial Pd/C for the hydrogenation of methylene blue in
an array of microdroplets. These results suggested that the
Fe3O4-NC-PZS-Pd could be used as nanoscale stirring bars in
nanoreactors.
nanochains with the help of surfactants.[5a] Chenꢀs group
prepared a nanometer-sized magnetic stirring bar,[4b] by first
assembling Fe3O4 nanoparticles to form a 1D chain, which
was then preserved in silica shells. The SiO2 shell endowed the
Fe3O4 magnetic chains with rigidity and quick response to
a common magnetic stirring plate, and the nanostirrer could
rotate quickly inside small droplets. However, so far there is
no report about the use of nanometer-sized magnetic stirring
bars in heterogeneous catalysis.
Herein, we report a facile two-step synthesis method to
produce nanometer-sized magnetic stirring bars that contain
Pd nanoparticles for heterogeneous catalysis in microscopic
systems. The preparation process is illustrated in Scheme 1.
E
ffective mixing of reactants is beneficial for mass transport
in chemical reactions, especially in heterogeneous catalysis, to
enhance the reaction rate and reduce energy consumption.[1]
For optimal mixing, magnetic stirring bars or mechanical
stirrers are usually used in reactors of various sizes. However,
conventional stirring methods cannot be used in nanoscale
reactors such as microdroplets or micelles, which are of great
importance for lab-on-chip applications[2] and microliter
bioassay,[3] because conventional stirring bars are much
larger than the nanoscale reactor. Therefore, it is necessary
to design nanometer-sized stirring bars that are sufficiently
small but still able to rotate under external influence. In this
study, we produced such nanoscale magnetic stirring bars.
Magnetic stirring is the most convenient option for
stirring. For microscopic systems, the size of the magnetic
stirrer must be within nanoscale. Thus, magnetic nanoparti-
cles are usually assembled into rigid nanochains. Previous
reports showed that magnetic induction[4] and external coat-
ing by induced self-assembly[5] are two effective methods for
forming magnetic nanochains with nanoparticles. Some nano-
meter-sized magnetic stirring bars based on nanochains have
been reported. For example, Pyun et al. reported polystyrene-
coated cobalt nanoparticles, which were assembled into
Scheme 1. Preparation of Fe3O4-NC-PZS-Pd catalytically active mag-
netic stirring bars.
Fe3O4 nanoparticles were prepared as building blocks for 1D
nanochains (denoted as Fe3O4-NC).[6] With the help of
ultrasound and the paramagnetic response of Fe3O4 nano-
particles, a highly cross-linked polymer poly(cyclotriphospha-
zene-co-4,4’-sulfonyldiphenol) (PZS, please see Figure S1 for
the polymer structure) was coated as the shell to stabilize the
Fe3O4 nanochains (denoted as Fe3O4-NC-PZS).[7] The PZS
coating also provided functional sites to anchor the Pd
nanoparticles onto the surface of Fe3O4-NC-PZS[8] to produce
the catalytically active stirring bars (denoted as Fe3O4-NC-
PZS-Pd). In tiny microdrops, in which a conventional stirring
bar could not be used, the synthesized Fe3O4-NC-PZS-Pd
could stir the microdrops to accelerate mass transfer, and
displayed far better catalytic activity than the commercial Pd/
C during the hydrogenation of arrays of microdroplets
containing methylene blue (MB).
As shown in Figure 1a, the synthesized Fe3O4 nano-
particles exhibited uniform morphology with diameters of
about 250 Æ 20 nm. Under ultrasonic irradiation, the Fe3O4
nanoparticles were assembled into nanochains (Figure 1b),
and PZS shells were formed and coated the Fe3O4 nanochains
through in situ polymerization.[7,9] As shown in Figure 1c, the
amorphous polymer layer was quite even and thin (red
dashed line in Figure 1c). The presence of nitrogen atoms in
the PZS coating could act as anchor sites to stabilize Pd
nanoparticles due to the strong affinity of N for Pd.[10] Based
on this design, Pd nanoparticles were loaded onto the surface
[*] S. Yang, Dr. C. Cao, Y. Sun, P. Huang, F. Wei, Prof. W. Song
Beijing National Laboratory for Molecular Sciences
Laboratory of Molecular Nanostructures and Nanotechnology
Institute of Chemistry, Chinese Academy of Sciences
100190, Beijing (China)
E-mail: cycao@iccas.ac.cn
wsong@iccas.ac.cn
[**] We thank the National Basic Research Program of China
(2011CB933700), the National Natural Science Foundation of China
(NSFC 21333009, 21273244), and the Chinese Academy of Sciences
(KJCX2-YW-N41) for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410360.
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stirring and no ultrasonic irradiation was employed, the PZS
could not be coated uniformly on the surface of Fe3O4
nanochains (the sample was denoted as SP-2). Instead, the
polymer aggregated by itself, thereby forming irregular
particles, and the Pd nanoparticles were located on the
surface of PZS but not on the surface of Fe3O4 (Figure S7). In
another experiment, in which no PZS coating was employed,
the Pd nanoparticles tended to aggregate and could not be
loaded onto the surface of Fe3O4 uniformly (denoted as SP-3,
Figure S8).
Due to the presence of Fe3O4 nanochains, the prepared
Fe3O4-NC-PZS-Pd stirring bars displayed good magnetic
response. They could be used as stirrer as well as catalyst in
the solution (see Video S1 in the Supporting Information, SI).
The hydrogenation of styrene was chosen as a model reaction
to test the heterogeneous catalysis properties of Fe3O4-NC-
PZS-Pd.[11] No commercial magnetic stirring bar was used.
Compared to the other two samples (SP-2, SP-3), Fe3O4-NC-
PZS-Pd exhibited the best catalytic activity (Figure S9). We
ascribed this result to the following two reasons: one is the
better dispersivity of Pd nanoparticles on Fe3O4-NC-PZS-Pd;
the other is the better mixing effects due to the Fe3O4
nanochains in the Fe3O4-NC-PZS-Pd.
The catalytic activity of Fe3O4-NC-PZS-Pd was compara-
ble to that of commercial Pd/C at the same weight percent of
Pd (Figure 2a). However, Fe3O4-NC-PZS-Pd showed much
Figure 1. TEM images of the Fe3O4 nanocrystal clusters (a) and Fe3O4
nanochains with PZS coating (b, c) and the Fe3O4-NC-PZS-Pd (d, e).
Inset of Figure 1d: Distribution of Pd nanoparticles in Fe3O4-NC-PZS-
Pd.
of Fe3O4-NC-PZS.[8] After Pd loading, the morphology of
Fe3O4-NC-PZS was maintained. The distribution of Pd nano-
particles was shown in the inset of Figure 1d. The average
diameter of Pd nanoparticles was 3.5 Æ 1.5 nm. When the
edge of the Fe3O4-NC-PZS-Pd was magnified, Pd nano-
particles could be observed (Figure 1d). PZS layer and lattice
fringes of Pd and Fe3O4 can be seen in the high-resolution
TEM image (Figures 1e and S2).
Figure S3 shows the XRD pattern of Fe3O4-NC-PZS-Pd.
Besides the characteristic peaks of Fe3O4, relatively weak and
broad peaks of Pd could also be observed. Due to the
amorphous nature of the PZS layer, no diffraction peak from
the PZS layer was observed. The results from XPS analysis
confirmed that Fe3O4-NC-PZS-Pd contained Fe, Pd, C, O, N,
P, Cl, and S (Figure S4). High-resolution XPS of the Pd 3d
peak indicated that most of the palladium was metallic Pd
(inset of Figure S4a). Comparison of the Pd 3d peak of Fe3O4-
NC-PZS-Pd with the peak of metallic Pd, showed that the
binding energy of Fe3O4-NC-PZS-Pd shifted to a lower
position, indicating that the N atoms of PZS coordinate
with Pd to stabilize it (Figures S4b and S5). Elemental
mapping of Fe3O4-NC-PZS-Pd also showed that the stirring
bars were composed of the desired segments and Pd nano-
particles were distributed uniformly (Figure S6).
Figure 2. a) Conversion versus reaction time curve for styrene hydro-
genation with Fe3O4-NC-PZS-Pd (green curves) and Pd/C (red curves)
as catalysts, respectively. b) Recycling of Fe3O4-NC-PZS-Pd (green
columns) and commercial Pd/C (red columns) catalyst using styrene
hydrogenation as model reaction.
better stability during cycling tests. There was no obvious
decrease of activity after six cycles. In contrast, an obvious
activity decrease was observed for the commercial Pd/C after
three runs (Figure 2b). After the reaction, the supernatant of
the Fe3O4-NC-PZS-Pd reaction system was analyzed by
inductively coupled plasma atom emission spectroscopy
(ICP-AES), and no Pd was detected. TEM images showed
that after multiple catalytic cycles, Fe3O4-NC-PZS-Pd main-
tained its morphology and the Pd nanoparticles were still
loaded on the surface of the PZS layer (Figure S10). Substrate
expansion and TOF values are presented in Table S1. The
results showed the excellent catalytic activity of Fe3O4-NC-
PZS-Pd for different substrates.
Ultrasonic irradiation played a vital role during the PZS
coating. In a control experiment, in which only magnetic
Such an excellent stability could be ascribed to the
following reasons: 1) the N atoms in the PZS act as anchor
2
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sites to immobilize Pd nanoparticles, which efficiently pre-
vents the aggregation and leaching of Pd nanoparticle. 2) The
PZS layer protects the Fe3O4 internal core from the corrosion.
This protection is advantageous for the integrity of the
catalyst. 3) When the reaction was finished, Fe3O4-NC-PZS-
Pd can be attracted by an external magnet and recovered
easily with essentially no loss. However, the Pd/C catalyst was
still dispersed in solution (Figure S11) and has to be recovered
by centrifugation. The facile recovery is beneficial for
recycling of the catalyst and decreases the catalyst loss.
During a strong-acid corrosion test, the Fe3O4 internal
core of Fe3O4-NC-PZS-Pd was maintained after 48 h in
concentrated nitric acid, whereas the SP-3 sample, which
has no PZS protection layer, was completely dissolved
(Figure S12). This test confirmed that due to the protection
of the PZS layer Fe3O4-NC-PZS-Pd can be used in corrosive
solutions.
In summary, we produced nanoscale catalytically active
magnetic stirring bars for microscopic catalytic system. In the
hydrogenation of styrene, the Fe3O4-NC-PZS-Pd catalyst
showed catalytic activity comparable to that of the commer-
cial Pd/C catalyst and far better stability. When the hydro-
genation of MB was run in microdroplets, in which conven-
tional stirring bars cannot be used, the Fe3O4-NC-PZS-Pd
catalyst showed remarkable ability to mix the reactants inside
small liquid droplets and achieved superior catalytic activity.
Received: October 22, 2014
Published online: && &&, &&&&
Keywords: heterogeneous catalysis · magnetic stirring bars ·
microscopic systems · nanoparticles
.
In microscopic catalytic systems, commercial magnetic
stirring bars cannot be used.[12] We envisioned that Fe3O4-NC-
PZS-Pd might be especially useful in microscopic system, in
which the mixing of reactants must be achieved by nano-
stirrers. To test the catalytic activity of Fe3O4-NC-PZS-Pd in
a microscopic system, the hydrogenation of methylene blue
(MB) in arrays of microdroplets was chosen as a model
reaction (the color change of the blue MB solution is a direct
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Figure 3. Hydrogenation of methylene blue (MB) in arrays of micro-
droplets with Fe3O4-NC-PZS-Pd as magnetic stirring bars and catalyst.
the blue microdroplets became colorless in 75 s after adding
Fe3O4-NC-PZS-Pd. In Video S2 (SI), the rotation of Fe3O4-
NC-PZS-Pd during the reaction process can be observed
when the magnetic stirring plate was turned on. In compar-
ison, the reaction rate with Pd/C as catalyst was much lower
than that of Fe3O4-NC-PZS-Pd (video S3).This was due to the
far better mixing in the microscopic system containing Fe3O4-
NC-PZS-Pd. This assumption was confirmed by the result
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Communications
Heterogeneous Catalysis
S. Yang, C. Cao,* Y. Sun, P. Huang, F. Wei,
W. Song*
&&& — &&&
Nanoscale Magnetic Stirring Bars for
Heterogeneous Catalysis in Microscopic
Systems
Nanostirrers: Nanometer-sized magnetic
stirring bars containing Pd nanoparticles
for heterogeneous catalysis in micro-
scopic system were prepared. They dis-
played far better catalytic activity than the
commercial Pd/C for the hydrogenation
of methylene blue in an array of micro-
droplets.
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!