A mesoporous "shell-in-shell" structured nanocatalyst with large surface area, enhanced synergy, and improved catalytic performance for Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1039/c4cc04527k

A novel mesoporous "shell-in-shell" structured nanocatalyst (@Pd/meso-TiO₂/Pd@meso-SiO₂) with large surface area, enhanced synergy, and improved catalytic performance is created for catalyzing Suzuki-Miyaura coupling and 4-nitrophenol reduction reactions. This journal is

Full text of DOI:10.1039/c4cc04527k

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A mesoporous ‘‘shell-in-shell’’ structured
nanocatalyst with large surface area, enhanced
synergy, and improved catalytic performance for
Suzuki–Miyaura coupling reaction†
Cite this: Chem. Commun., 2014,
50, 12356
Received 14th June 2014,
Accepted 9th August 2014
Baocang Liu,abc Yuefang Niu,a Yan Li,a Fan Yang,a Jiamin Guo,a Qin Wang,ac
DOI: 10.1039/c4cc04527k
Peng Jing,a Jun Zhang*abc and Guohong Yunc
www.rsc.org/chemcomm
A
novel mesoporous ‘‘shell-in-shell’’ structured nanocatalyst
Mesoporous SiO2 (meso-SiO2) coating is an effective route to
(@Pd/meso-TiO2/Pd@meso-SiO2) with large surface area, enhanced improve the stability of nanocatalysts.18,19 The meso-SiO2 shell
synergy, and improved catalytic performance is created for catalyz- can resist the aggregation of active species, reduce the leaching
ing Suzuki–Miyaura coupling and 4-nitrophenol reduction reactions. of noble metal, and improve the accessibility of active species.20
Meanwhile, the selectivity of reactants can also be tuned via
Heterogeneous catalysis, a promising route to obtain desired changing the pore size of meso-SiO2 and hydrophilicity or
targets, has aroused great research interest because it is widely hydrophobicity of the meso-SiO2 surface.21,22 Furthermore, a
applicable in various catalytic reactions with good recovery resorcinol-formaldehyde (RF)-resin polymer, a common easily
capacity and recyclability.1 Noble metal nanocatalysts have removable hard template, has been widely used to synthesize
been widely investigated in heterogeneous catalysis.2,3 How- carbon spheres23 and various hollow sphere materials.24,25
ever, the noble metal nanocatalysts tend to aggregate, leading
Herein, we design a novel mesoporous ‘‘shell-in-shell’’
to the loss of catalytic activity under high temperature treatment structured nanocatalyst (@Pd/meso-TiO2/Pd@meso-SiO2). The
or reaction conditions.4 Loading noble metal nanoparticles on nanocatalyst is composed of mesoporous double TiO2 and SiO2
surface-modified supports is an ideal approach to solve this shells with the ultrafine Pd nanoparticles (PNPs) uniformly
problem,5,6 because diverse supports can limit the migration of distributed on the external and internal surfaces of the meso-
noble metal nanoparticles and prevent aggregation. In addition, TiO2 shell. Such a structural configuration endows the nano-
the synergetic effects between the noble metal nanoparticles and catalyst with large surface area due to the mesoporous shells,
the supports are in favor of improving catalytic performance.7
enhanced synergy between PNPs and the TiO2 shell, and
Among the investigated metal oxide supports, TiO2 is the independent chambers formed by two shells. Ultimately, the
most promising one due to its excellent redox ability and nanocatalyst, which is used as an effective nanoreactor, exhibits
chemical and thermal stability.8 In order to pursue the optimum improved catalytic activity and selectivity for Suzuki–Miyaura
catalytic performance, TiO2 with diverse morphologies has been coupling reaction and high catalytic activity and stability for
widely studied.9,10 Among these studies, the hollow mesoporous 4-nitrophenol reduction reaction. Depending on the restriction
TiO2 nanocatalyst has been extensively investigated in recent of mesopores of the outer meso-SiO2 shell on reactant molecules,
years, due to its superior catalytic properties.11–13 Furthermore, the catalytic selectivity for Suzuki–Miyaura coupling is success-
the TiO2 nanocatalyst can be further integrated by loading the fully realized.
noble metal nanoparticles in the cavity14,15 on the internal16
or external surfaces of the hollow TiO2 sphere17 to enhance its TiO2/Pd@meso-SiO2 nanocatalyst follows the procedures illu-
The synthesis of the ‘‘shell-in-shell’’ structured @Pd/meso-
catalytic performance.
strated in Scheme 1. Initially, a layer of TiO2 is coated onto SiO2
spheres via a sol–gel process to obtain SiO2@TiO2 spheres.
Then, the SiO2 spheres are removed by etching with NaOH
solution to obtain @TiO2 spheres. After modifying the @TiO2
spheres with –NH2 groups using (3-aminopropyl)triethoxysilane
(APTES) to render a hydrophilic surface, a Pd2+ ion-diffusion
process is carried out. With this step, Pd2+ ions can be diffused
easily into the central cavity of the @TiO2 spheres through meso-
porous TiO2 shells. Then, the @TiO2 spheres with Pd2+ ions
loaded on the internal and external surfaces are subsequently
a College of Chemistry and Chemical Engineering, Inner Mongolia University,
Hohhot 010021, P. R. China. E-mail: cejzhang@imu.edu.cn;
Fax: +86 471 4992278; Tel: +86 471 4992175
b College of Life Science, Inner Mongolia University, Hohhot 010021, P. R. China
c Inner Mongolia Key Lab of Nanoscience and Nanotechnology,
Inner Mongolia University, Hohhot 010021, P. R. China
† Electronic supplementary information (ESI) available: Detailed synthetic pro-
cedures, catalytic performance, SEM, TEM, EDX, XRD, FT-IR, and XPS. See DOI:
10.1039/c4cc04527k
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with an interlayer chamber existing between TiO2 and SiO2
shells (Fig. 1c–e). In addition, light spots on both sides of TiO2
shells further suggest the good distribution of PNPs on the
internal and external surfaces of the TiO2 shell. The highly
dispersed PNPs on the TiO2 support may enhance the catalytic
activity and stability.16 EDX elemental mapping clearly shows
that the @Pd/meso-TiO2/Pd@meso-SiO2 nanocatalyst is composed
of inner TiO2 and outer SiO2 shells with PNPs uniformly
distributed on the internal and external surfaces of TiO2 shells
(Fig. 1f). In particular, it can be confirmed from the EDX
elemental mapping image that a large quantity of PNPs with
very small size (B5 nm) exists in the nanocatalyst, which may
be advantageous for improving the catalytic performance.26
The SEM images further indicate that the ‘‘shell-in-shell’’
structured @Pd/meso-TiO2/Pd@meso-SiO2 nanocatalyst has a
relatively smooth surface, better monodispersity, and good
integrity (Fig. 1g and h, and Fig. S3, ESI†).
Scheme 1 Schematic illustration showing the synthesis of the ‘‘shell-in-shell’’
@Pd/meso-TiO2/Pd@meso-SiO2 nanocatalyst.
coated with the layers of resorcinol formaldehyde (RF) and
mesoporous SiO2. Following the calcination and reduction
under a hydrogen atmosphere, the @Pd/meso-TiO2/Pd@meso-
SiO2 nanocatalyst is obtained.
The enlarged image of a crack sphere further demonstrates
the double-shell structure of the @Pd/meso-TiO2/Pd@meso-SiO2
nanocatalyst. The XRD pattern of the nanocatalyst displays a
diffraction peak attributed to SiO2 (Fig. 1i and Fig. S5, ESI†). The
diffraction peaks of TiO2 and PNPs are not detectable mainly
because of the monodispersity of PNPS with small size and the
formation of the amorphous TiO2 shell. This may suggest that
the RF and m-SiO2 shell can prevent the heat from conducting to
the inner shell during the calcination process, and affect the
crystallization of the TiO2 shell (Fig. S6, ESI†). Such a meso-
porous ‘‘shell-in-shell’’ structured nanocatalyst may exhibit
greatly improved catalytic performance for various reactions.
Fig. 2 shows the nitrogen adsorption–desorption isotherm and
BJH pore size distribution of the @Pd/meso-TiO2/Pd@meso-SiO2
nanocatalyst. It displays a typical IV isotherm with H1-type hyster-
esis loops (P/P0 4 0.5), indicating the presence of mesopores in
TiO2 and SiO2 shells. The nanocatalyst has a relatively high surface
area (B700 m2 gÀ1). The BJH pore size distribution shows that the
nanocatalyst has two obvious pore sizes (B2.2 and B3.8 nm)
attributed to the mesoporous SiO2 and TiO2 shells12,27 consistent
with the SAXRD result (Fig. S7, ESI†), which is beneficial for
improving its catalytic activity.
The resultant @Pd/meso-TiO2/Pd@meso-SiO2 nanocatalyst
shows good monodispersibility and possesses a well-defined
‘‘shell-in-shell’’ structural configuration consisting of meso-
porous TiO2 and SiO2 shells with ultrafine PNPs loaded on
both internal and external surfaces of the meso-TiO2 shell
(Fig. 1a and b, Fig. S3 and S4, ESI†). The diameters of hollow
TiO2 and SiO2 spheres are estimated to be B170 and B300 nm,
respectively. The thickness of TiO2 and SiO2 shells is around
B35 and B24 nm, respectively. The mesopores on TiO2 and
SiO2 shells can be clearly observed from the enlarged image
(inset in Fig. 1a). Due to the ‘‘shell-in-shell’’ structural feature,
an interlayer chamber of B50 nm is created. The STEM images
further confirm the ‘‘shell-in-shell’’ structural configuration
In order to verify the superiority of such a featured nano-
catalyst in catalysis, the Suzuki–Miyaura coupling and 4-nitrophenol
reductions are employed as model reactions to evaluate the catalytic
capability of the @Pd/meso-TiO2/Pd@meso-SiO2 nanocatalyst. In
contrast, the @meso-TiO2/Pd@meso-SiO2, @meso-TiO2/Pd@SiO2,
Fig. 1 TEM (a and b), STEM (c, d, and e), SEM images (g and h), and the
XRD pattern (i) of the ‘‘shell-in-shell’’ @Pd/meso-TiO2/Pd@meso-SiO2
nanocatalyst; (f) EDX elemental mapping of Ti, Si, O, and Pd in the
‘‘shell-in-shell’’ @Pd/meso-TiO2/Pd@meso-SiO2 nanocatalyst shown in (e).
The inset image of (a) is the partially enlarged image of (b) showing the
existence of mesoporosity in TiO2 and SiO2 shells.
Fig. 2 Nitrogen adsorption–desorption isotherm (A) and BJH pore size
distribution (B) of (a) @meso-TiO2/Pd@meso-SiO2, (b) @Pd/meso-TiO2/
Pd@meso-SiO2, and (c) @meso-TiO2/Pd2+@RF@SiO2 nanocatalysts.
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Table 1 Suzuki–Miyaura coupling reactions of aryl halides on the @Pd/
synergy between PNPs and TiO2. In addition, the ‘‘shell-in-shell’’
@Pd/meso-SiO2/Pd@meso-SiO2 catalyst shows less leaching of Pd
in comparison with the traditional catalyst (Table S5, ESI†). This
suggests that the double-shell architecture of the @Pd/meso-TiO2/
Pd@meso-SiO2 catalyst may be effective in preventing the leaching
of Pd from the internal and external surfaces of inner TiO2 walls,
which may be beneficial for the improvement of catalytic perfor-
mance.20 XPS results show that the PNPs in a double-shell
nanoreactor exist in the valence state of Pd0 (Fig. S10, ESI†), which
may possess superior catalytic activity. Third, the –OH group on
the surface of the amorphous TiO2 shell, as proven by FT-IR results
(Fig. S11 and S12, ESI†), is in favor of catalyzing Suzuki–Miyaura
coupling reaction.30 Fourth, the mesoporous TiO2 and SiO2 shells
facilitate the fast diffusion of reactants and products from the
active sites on the TiO2 shell, which is beneficial for heterogeneous
catalysis according to the tests shown in Table S6 (ESI†).27 Last, the
large specific surface area can increase the exposure of the active
component in the nanocatalysts, thus promoting the catalytic
performance. Thus, the double-shell architecture can be expected
to be a unique catalyst system for tuning the catalytic performance
by manipulating its shell porosity, interstitial spaces, and configu-
ration on micro- and nano-scales.31 In addition, various aryl
meso-TiO2/Pd@meso-SiO2 nanocatalysta
Entry
X
I
R1
R2
Base
T/min Con./%b TOF/hÀ1 c
1
2
3
4
5
6
7
8
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
Cs2CO3 10
Cs2CO3 60
Cs2CO3 10
Cs2CO3 10
Cs2CO3 10
Cs2CO3 10
Cs2CO3 10
Cs2CO3 10
Cs2CO3 10
Cs2CO3 10
99
99
100
99
99
97
96
95
88
53
15 390
3961
Br –H
I
I
I
I
I
I
I
I
I
I
–NO3
–F
–OH
–COOH
–OCH3
–CH3
–NH2
–COCH3 –H
H
H
15 546
15 390
15 390
15 078
14 925
14 769
13 680
8220
9
10
11
12
–nBu Cs2CO3 10
–tBu Cs2CO3 10
90
Trace
13 992
—
a
Reaction conditions: 80 1C, ethanol (10 mL), iodobenzene or bromobenzene
(0.5 mmol), phenylboronic acid (1 mmol), K2CO3 or Cs2CO3 (1 mmol),
catalyst (10 mg or 25 mg), and reaction time (10 min or 60 min). b Deter-
mined by HPLC using pentamethylbenzene as internal standard. c TOF is
defined as the moles of reacted aryl halide molecules per mole of the Pd atom
in the catalyst per hour.
@Pd/meso-TiO2/Pd, and @meso-TiO2/Pd nanocatalysts were also iodides with different substituents are used to investigate the
synthesized according to the synthetic procedures illustrated in reaction scope of the @Pd/meso-TiO2/Pd@meso-SiO2 nanocatalyst
Scheme S1 (Fig. S8, ESI†). The catalytic performance of different (Table 1, entries 2–10). The results show that the @Pd/meso-TiO2/
nanocatalysts in Suzuki–Miyaura coupling reaction under var- Pd@meso-SiO2 nanocatalyst possesses outstanding catalytic per-
ious base, solvent, and temperature conditions (Tables S1–S4, formance for iodobenzene containing –CH3, –OCH3, –NH2, –NO3,
ESI†) was investigated. @Pd/meso-TiO2/Pd@meso-SiO2 displays –COOH, –F, –COCH3 and –OH groups. The biaryl yields can reach
outstanding catalytic performance in the Suzuki–Miyaura cou- 53–99% within 10 min. Meanwhile, according to the catalytic
pling reaction of halogeno benzene and phenylboronic acid results shown in Table 1 (entries 11 and 12), we can infer that
(Table 1, entries 1 and 2).3,28 99% conversion of iodobenzene the mesopores in the SiO2 shell may selectively make the reactants
can be realized within 10 min, and the turnover frequency (TOF) diffuse into the nanoreactor owing to the pore size and shape to
can reach as high as 15 390 hÀ1, higher than most of the achieve catalytic selectivity for different reactants depending on the
previously reported heterogeneous catalysts.16 It also exhibits reactant molecules.21,22 Due to the unique ‘‘shell-in-shell’’ architec-
high bromobenzene conversion with a conversion of 99% and a tural configuration, the ‘‘shell-in-shell’’ @Pd/TiO2/Pd@meso-SiO2
TOF of 3961 hÀ1 within 60 min.
nanocatalyst also shows super performance with high catalytic
For comparison, the catalytic performance of the contrast- activity and stability for the reduction of 4-nitrophenol to
ing @meso-TiO2/Pd@meso-SiO2, @meso-TiO2/Pd@SiO2, @Pd/ 4-aminophenol (Fig. S13, ESI†).
meso-TiO2/Pd, and @meso-TiO2/Pd nanocatalysts is also
We presented a well-controlled strategy to construct a novel
evaluated (Table S1, ESI†). It can be clearly seen that the ‘‘shell mesoporous ‘‘shell-in-shell’’ @Pd/meso-TiO2/Pd@meso-SiO2
in shell’’ structured @Pd/meso-TiO2/Pd@meso-SiO2 nanocatalyst nanocatalyst composed of mesoporous double TiO2 and SiO2
exhibits higher catalytic activity than @meso-TiO2/Pd@meso-SiO2, shells with PNPs uniformly distributed on the external and
@meso-TiO2/Pd@SiO2, @Pd/meso-TiO2/Pd, and @meso-TiO2/Pd internal TiO2 shell. The unique structural configuration
nanocatalysts. Combining with the Pd content in different cata- endows the nanocatalyst with unusual features of large specific
lysts, we deduce that the outstanding catalytic performance of the surface area, mesoporous TiO2 and SiO2 shells, two indepen-
@Pd/meso-TiO2/Pd@meso-SiO2 nanocatalyst may be mainly dent nanoreactors, good dispersity and decreased leaching of
ascribed to its unique structural configuration.29 First, two inde- PNPs, and enhanced synergistic effects. These features can
pendent chambers composed of double shells of TiO2 and SiO2 in largely improve the catalytic activity and stability of the nano-
the nanocatalyst can act as a nanoreactor to gather the reactants catalyst, leading to extremely high catalytic activity for Suzuki
and further accelerate the catalytic reaction rate due to the Miyaura coupling and 4-nitrophenol reduction reactions with
confinement effect of microenvironments, which is proven by TOFs of 15 390 hÀ1 and 760 hÀ1, respectively for the meso-
the absorption experiments (Fig. S9, ESI†). Second, the ‘‘shell-in- porous ‘‘shell-in-shell’’ @Pd/meso-TiO2/Pd@meso-SiO2 nano-
shell’’ architecture also allows the Pd nanoparticles to be uni- catalyst. The catalytic stability is also superior with the
formly dispersed on the internal and external surfaces of the TiO2 conversion remaining over 89% even after ten cycles of catalytic
shell, which may enrich the active sites of PNPs and increase reaction for 4-nitrophenol reduction. The synthetic methodo-
the contact area between reactants and catalysts to enhance the logy may provide an effective route to create highly efficient
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