Pd nanoparticles in silica hollow spheres with mesoporous walls: A nanoreactor with extremely high activity

Article abstract of DOI:10.1039/c0cc01786h

A true nanoreactor composed of mesoporous silica hollow spheres and Pd nanoparticles residing inside the spheres shows superior activity in Suzuki coupling reactions with 99.5% yield in 3 min.

Full text of DOI:10.1039/c0cc01786h

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Pd nanoparticles in silica hollow spheres with mesoporous walls:
a nanoreactor with extremely high activityw
Zhe Chen,a Zhi-Min Cui,b Fang Niu,a Lei Jiangb and Wei-Guo Song*a
Received 8th June 2010, Accepted 1st July 2010
DOI: 10.1039/c0cc01786h
A true nanoreactor composed of mesoporous silica hollow
spheres and Pd nanoparticles residing inside the spheres shows
superior activity in Suzuki coupling reactions with 99.5% yield
in 3 min.
All catalytic reactions occur in reactors, which accommodate
Scheme 1 Synthesis route to the composite nanoreactor.
the catalysts and reactants, and allow the chemicals to be
transported in and out of the reactor. In conventional
nanoparticles are all fully accessible. Both features enable the
catalysis, reactor design is typically an independent craft,
system to be very active in catalysis. In the Suzuki coupling
separate from catalyst design. However, some catalysts have
reaction a 99.5% yield is obtained in 3 min.
uniform internal structures that resemble a reactor on the
As illustrated in Scheme 1, we designed a three step
nano-/microscale. For example, the supercages inside Y zeolite
particles can be considered as nanoreactors.1 Another type of
procedure to produce the nanoreactor composite. Dividing
the procedure into three separate steps allows precise control
porous material, mesoporous silica, can draw the reactants
of the overall structure of the composite. Fig. 1 depicts the
into its pore channels, where the catalyst particles are located.
experimental realization of Scheme 1 (see ESIw for experi-
mental details). Fig. 1a shows the smooth surfaces of the
Thus the mesopores within the silica can be considered as
nanotubular reactors.2,3
carbon nanospheres with abundant hydroxyl groups,13 which
Fully functional nanoreactors are realized using hollow
help to anchor the Pd nanoparticles.14 Fig. 1b is a TEM image
spheres, especially mesoporous silica hollow spheres. Li et al.
of the Pd/C composite. The Pd nanoparticles with an average
have encapsulated several chiral catalysts into mesoporous
silica SBA-16 nanocages.3 The resulting catalytic system shows
size of about 5 nm are uniformly distributed on the surfaces of
the carbon nanospheres. Using TEOS as the silica source and
remarkable activity for asymmetric synthesis. Most recently,
CTAB as a soft template, a thin layer of mesoporous silica is
Yamada et al. reported TiO2 inside silica hollow spheres for
photoreactions.4 Lu et al. fabricated a series of yolk/shell
coated onto the Pd/C spheres. Calcination of the precursor
composite removes the carbon sphere and CTAB, leaving only
structured materials with movable cores and porous shells,
which have potential application in nanoreactors.5–7
Pd nanoparticles residing inside the hollow spheres.
Noble metal NPs tend to aggregate and sinter, leading to a
rapid decay in catalytic abilities.8,9 Restricting the noble metal
NPs in isolated nanoreactors can effectively protect them
against aggregation. Somorjai et al. designed a Pt@CoO
composite for the hydrogenation of ethylene.10 Song et al.
fabricated a Au@SiO2 yolk/shell structure for the catalytic
reduction of p-nitrophenol.11 Yin et al.’s multilayer core/shell
structures also resemble nanoreactors, in which Pd nano-
particles are protected by a silica coating.12
In this study, we produced a true nanoreactor that is
composed of mesoporous silica (meso-SiO2) hollow spheres,
and Pd nanoparticles residing inside the spheres. The fabrication
process is a three step procedure, and each step is well-
controlled (Scheme 1). The mesoporous walls of the silica
hollow spheres prompt facile mass transportation. The Pd
a Beijing National Laboratory for Molecular Sciences (BNLMS),
Institute of Chemistry, Chinese Academy of Sciences, Beijing,
100190, P. R. China. E-mail: wsong@iccas.ac.cn;
Fax: (+86) 10-62557908
b School of Chemistry and Environment, Beihang University, Beijing,
100191, P. R. China
Fig. 1 (a) TEM of carbon nanospheres; (b) TEM of Pd/C nano-
w Electronic supplementary information (ESI) available: Preparation
sphere; (c) TEM of final composite nanoreactor in low magnification.
of the composite nanoreactor; experimental details of Suzuki reaction;
The inset shows the mesopores of the SiO2 wall; (d) TEM of composite
characterization and measurement methods of catalyst; TEM of
catalyst post-use. See DOI: 10.1039/c0cc01786h
nanoreactor in high magnification.
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Apparently, the Pd nanoparticles are located at the inside
pore mouths of the mesopores. No Pd nanoparticles were
observed within the mesopore pores (inset of Fig. 1c). The
wormhole-like mesopores can be seen in the inset of Fig. 1c.
The thickness of the meso-SiO2 walls is about 40 nm, and can
be readily controlled by adjusting the concentration of TEOS.
Fig. 1d shows a TEM image of the composite in high
magnification, showing well-dispersed Pd nanoparticles on
the inside surface of the hollow spheres. EDX analysis (see
ESIw Fig. S1b) and elemental analysis results indicate 4.0 wt%
of Pd loading. We also produced composites with 20 wt% and
0.8 wt% Pd loadings. In these materials, the Sn content was
below 0.05 wt%, and the presence of Sn was not considered as
significant in the catalytic performance of the composites.
The small-angle XRD pattern depicted in Fig. 2a shows a
broad peak at 2.61, indicating a worm-like mesopore with a
uniform pore size. This is consistent with Fig. 1c and similar
mesoporous materials. Fig. 2b shows the nitrogen adsorption–
desorption isotherm of the composite with a narrow hysteresis
loop; both of which indicate the presence of mesopores. The
BET surface area of the composite is 1123 m2 gꢁ1, similar to
normal mesoporous silica. The inset of Fig. 2b shows the BJH
pore size distribution, showing a sharp peak at 2.1 nm.
Each of the composite spheres is a true nanoreactor with
outstanding features. The mesoporous channels on the sphere
walls provide entrances for the reactants and exits for the
products. The large surface of the composite enriches the
reactants around the composite. The 2.1 nm pore size also
keeps the Pd nanoparticles within the inner space of the hollow
space, as the Pd nanoparticle diameter is about 5 nm on
average. In addition, the hydrophilic nature of the mesoporous
silica can induce a wettability control over the catalytic reactions
in liquid media. Hydrophilic liquids may be preferentially
adsorbed into the composite nanoreactor.
Fig. 3 (a, b) Au NPs residing in meso-silica at different magnifications.
All bars are 100 nm.
platform to fabricate nanoreactors which can be used in a
broad range of noble metal catalyzed reactions.
We carried out Suzuki cross-coupling reactions (1) and a
Sonogashira reaction (2) as model reactions to evaluate the
catalytic ability of the composite nanoreactors. The Suzuki
cross-coupling reaction provides a simple yet versatile route to
form C–C and C–N bonds under relatively mild conditions.15
The results of Suzuki reactions using different reagents are
listed in Table 1. The R1 and R2 side groups on either
iodobenzene or phenylboronic acid are intentionally changed
to test the effects of side group substitution. The composite
catalyst showed outstanding activity in all these reactions.
ð1Þ
ð2Þ
In a typical Suzuki reaction in which neither iodobenzene
nor phenylboronic acid has side groups, the conversion of
iodobenzene reaches 99.5% within 3 min of reaction at 80 1C
under reflux conditions. The turn-over frequency (TOF) is
0.74 sꢁ1. Bromobenzene shows 32% conversion in 3 min, with
This synthesis route can also be applied for loading other
noble metal nanoparticles such as Au, Ag, and their bimetallic
alloy nanoparticles into the composite nanoreactors. As
shown in Fig. 3, Au NPs with a diameter of sub-10 nm are
dispersed uniformly inside silica. The system supplied a
Table 1 Suzuki reactions catalyzed by composite reactorsa
Run Reagent
1
Reagent
Time/min Yield (%)
3
3
3
99.5
32.4
67.2
2
3
4
5
3
86.3
10
3
80.0
50.6
6
a
Reaction conditions: 80 1C, ethanol (10 mL), iodobenzene
(0.5 mmol), phenylboronic acid (1 mmol), K2CO3 (2 mmol), catalyst
(10 mg, with Pd loading 4 wt%).
Fig. 2 (a) Small-angle XRD pattern of the composite nanoreactor.
(b) Nitrogen adsorption–desorption isotherms of the composite. Inset
is the BJH pore size distribution.
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a TOF 0.24 sꢁ1. Even at room temperature, the conversion of
iodobenzene reaches 52% at 10 min. The composite nanoreactor
is also very active for substituted iodobenzene and phenyl-
boronic acids, as shown in Table 1. In addition, the conversion
of iodobenzene reaches 55% in 30 min for the Sonogashira
reaction (reaction 2). These results prove that the composite
nanoreactors are extremely active for Pd-catalysed C–C
coupling reactions. For comparison, a reported Pd/TiO2
composite catalyst converts only 15% of iodobenzene in
3 min at 80 1C, and the iodobenzene conversion is nearly zero
at room temperature.
Table 2 Catalytic activity and Pd leaching of different catalysts
Catalysis
activitya
(%)
Pd in mother
liquid (ppb)
Mother liquor
activityb (%)
Catalyst
Composite
nanoreactor
Pd@Carbon
sphere
99.5
25.4
43
4.0
30
321
a
b
In iodobenzene conversion at 3 min. In iodobenzene conversion at
30 min. Conditions: 80 1C, ethanol (10 mL), iodobenzene (0.5 mmol),
phenylboronic acid (1 mmol), K2CO3 (2 mmol), catalyst (10 mg, with
Pd loading 4 wt%).
The catalyst can be easily recovered from the reaction
solution by centrifugation. The conversions of iodobenzene
reached 99.0% in 3 min in 4 consecutive runs, showing no sign
of deactivation. A TEM image of the used composite catalyst
(see ESIw Fig. S1c.d) shows no apparent change from the fresh
catalyst.
In summary, we have produced a composite nanoreactor
with mesoporous silica hollow spheres and Pd nanoparticles
inside. A multi-step and well-controlled route was developed
to synthesize this composite material. The nanoreactor-like
composite shows extremely high activity for Suzuki cross-
coupling reactions: near complete conversion was reached in
3 min. Pd leaching is substantially reduced on the composite,
which enables stable catalytic performance.
The superior catalytic performance of this nanoreactor
composite is due to the structural features of the composite
nanoreactor. The reagents (iodobenzene, phenylboronic acid
and K2CO3) can be adsorbed by high surface area meso-SiO2,
then diffuse through the mesoporous pores, being enriched
within the pore mouth. The concentration of reagents around
the Pd nanoparticles is relatively higher than that in the bulk
solution. The thin layer of mesoporous silica is only 40 nm,
indicating a very short diffusion course for both reactants and
products. Such a short diffusion course is beneficial for
heterogeneous catalysis.16 The 2 nm-pores may also pose a
certain space confinement effect to the reactants, leading to a
higher reaction rate.
We thank the National Natural Science Foundation of
China (NSFC 50725207, 20821003), National Basic Research
Program of China (2007CB936400, 2009CB930400) and the
Chinese Academy of Sciences for financial support.
Notes and references
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Improving the activity and stability of a catalyst is the core
of heterogeneous catalysis.17,18 In Pd and other noble metal
catalyzed reactions, leaching of the noble metal is a major
cause of catalyst deterioration. In this regard, the nanoreactor
feature of this composite prevents Pd nanoparticles from
leaching. In a series of controlled experiments, a composite
with Pd nanoparticles loaded on carbon nanospheres (as
shown in Fig. 1b, with no mesoporous SiO2 coating) was used
as catalyst. After the reactions, the catalyst was recovered, and
the Pd content in the mother liquor as well as the Pd content
on the catalyst was analyzed by ICP elemental analysis. After
removal of the solid catalyst by centrifugation, the mother
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activity of the composite nanoreactor. More significantly, the
Pd leaching from the Pd on carbon spheres composite is about
8 times that of the Pd in hollow meso-SiO2 composite. Note
that both catalysts have the same initial Pd loading of
4.0 wt%. This proves that the nanoreactor feature of
the composite indeed reduces Pd leaching. As a result, the
iodobenzene conversion using the mother liquor from the
composite nanoreactor as a catalyst is also much lower.
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This journal is The Royal Society of Chemistry 2010
6526 | Chem. Commun., 2010, 46, 6524–6526