Pd colloid grafted mesoporous silica and its extraordinarily high catalytic activity for Mizoroki-Heck reactions

Article abstract of DOI:10.1016/j.molcata.2010.02.014

A Pd colloid grafted heterogeneous catalyst, with as low as 0.1 mol% Pd loading in mesoporous silica, was presented. In this composite, the palladium was evenly reduced by Si-H group to form metal colloids existing as isolated islands both on the inner and outer surfaces. The evenly dispersed catalyst species showed high catalytic activity with extremely low amounts of Pd catalysts for Mizoroki-Heck reactions. The large and open pore network, ultrahigh specific surface area and highly dispersed catalyst species made it one of the most active heterogeneous catalysts. The palladium colloid showed very high stability against leaching from the support and can be recycled for repeated use.

Full text of DOI:10.1016/j.molcata.2010.02.014

Journal of Molecular Catalysis A: Chemical 322 (2010) 50–54
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Pd colloid grafted mesoporous silica and its extraordinarily high catalytic
activity for Mizoroki–Heck reactions
Yingjun Fenga, Liang Lia,∗, Yongsheng Lia, Wenru Zhaoa, Jinlou Gua, Jianlin Shia,b,∗∗
a Laboratory of Advanced Functional Materials, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road,
Shanghai 200237, People’s Republic of China
b State Laboratory of High Performance Ceramic and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050,
People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A Pd colloid grafted heterogeneous catalyst, with as low as 0.1 mol% Pd loading in mesoporous silica, was
presented. In this composite, the palladium was evenly reduced by Si–H group to form metal colloids
existing as isolated islands both on the inner and outer surfaces. The evenly dispersed catalyst species
showed high catalytic activity with extremely low amounts of Pd catalysts for Mizoroki–Heck reactions.
The large and open pore network, ultrahigh specific surface area and highly dispersed catalyst species
made it one of the most active heterogeneous catalysts. The palladium colloid showed very high stability
against leaching from the support and can be recycled for repeated use.
Received 14 October 2009
Received in revised form 8 February 2010
Accepted 9 February 2010
Available online 13 February 2010
Keywords:
Palladium
Heck reaction
Heterogeneous catalysis
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The palladium catalyzed Mizoroki–Heck reaction is among
the most important and widely used reactions for the formation
The application of metal nano-particles in catalysis has been
an important frontier of researches in recent years, as they have
a characteristic high surface-to-volume ratio and consequently
more active sites, which make them promising catalysts in chemi-
cal synthesis [1,2]. However, the direct application of noble metal
nano-particles in catalysis is often difficult due to their ultra-small
size and high tendency toward aggregation. Generally, these metal
nano-particles are often supported on solid matrix and employed as
heterogeneous catalysts [3–8]. The selectivity and activity of these
systems are often found to be strongly dependent on surface struc-
ture, particle morphology, and size of the supports, and also on
the presence of promoters and poisons. Thus, recent protocols in
developing new metal catalysts mostly concern the high dispersion
of metal species on the high surface area porous support, such as
silica, alumina, titania, and zirconia, as well as the increased sta-
bility of the active species [9–15]. Although extensive efforts have
been made and significant progress has been achieved, the develop-
ment of more simple methodology for preparing highly dispersed
metal catalysts with high catalytic activities still remains a great
challenge.
of carbon–carbon bonds, which allows the arylation or vinyla-
tion of various alkenes through their reaction with aryl, vinyl,
benzyl or allyl halides, acetates, or triflates in the presence of
suitable base in a single step under mild conditions [16–19]. It
constitutes extremely useful tools in organic synthesis, and its
applications have significantly broadened when new variants of
the reaction with heteroatom nucleophiles are developed [20].
Academic and industrial interest in this reaction has increased
in recent years and much progress has been made in prepara-
tion of new supported palladium catalyst to catalyze these kind
of reactions. Among them, the majority of the novel heteroge-
nized catalysts are based on ordered mesoporous silica supports
[14–16,20–23], primarily not only because of their excellent sta-
bility (chemical and thermal), good accessibility and the fact
that organic groups can be robustly anchored to the surface to
provide catalytic centers but also their well defined, nanosized
and size-tunable porosity and large surface area. Metallic pal-
ladium has been incorporated into various ordered mesoporous
silica such as MCM-41 [14,21], HMS [22], SAB-15 [15] and ETS-
10 [23]. These heterogeneous systems have been effectively used
as catalysts in hydrogenation and in Mizoroki–Heck coupling reac-
tions.
In our previous studies [15a], we reported an in situ reduc-
tion method for the controlled deposition of metals onto the inner
surface of mesoporous silica materials. It was shown that the immo-
bilized Si–H group on the pore channel surface was able to in situ
reduce metal ions resulting in the formation of a uniform thin metal
∗
Corresponding author.
Corresponding author at: Laboratory of Advanced Functional Materials, School
∗∗
of Materials Science and Engineering, East China University of Science and Technol-
ogy, 130 Meilong Road, Shanghai 200237, People’s Republic of China.
E-mail addresses: liliang@ecust.edu.cn (L. Li), jlshi@sunm.shcnc.ac.cn (J. Shi).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.02.014

Y. Feng et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 50–54
51
colloid layer on the pore channels [15a]. Recent studies indicated
2.3. Characterization of Pd–SBA
that even if this kind of colloid layer was also present on the outer
surface of the matrix, the catalyst species still showed very high
stability against leaching from the support in the reaction [15b].
In both of the previous work, the metal loading levels and used
amounts in catalytic reaction, though already very low as compared
with the literature, was still as high as 5 wt% (for both Pd and Rh)
and 0.02 mol%, respectively.
Considering the special metal colloid coating structure, we
believed it would be necessary to further lower the loading amount
of metal colloids in the mesoporous matrix and investigate its cat-
alytic performance with used amount of the catalyst minimized.
Therefore, in the present work, we report the synthesis and cat-
alytic properties of palladium colloids grafted mesoporous silica
materials with extremely low loading amount of 0.1 mol% Pd in
mesoporous matrix and also very low amount of 0.002 mol% Pd
in Mizoroki–Heck reactions. The metal colloids were evenly dis-
persed on both the outer and inner surfaces of the mesoporous
silica materials. The catalytic experimental results show that the
synthesized composite catalysis materials still have high catalytic
activity and high TOF value in Mizoroki–Heck reactions compared
with the previous reports [14,15a].
Transmission electron microscopy (TEM) observations and
energy dispersive spectrum (EDS) detections were performed on
a field emission JEM-3000F (JEOL) electron microscope operated
at 300 kV and equipped with a Gatan-666 electron energy loss
spectrometer and energy dispersive X-ray spectrometer. X-ray
photoelectron spectra (XPS) were recorded on a Physical Elec-
tronics XPS-5700 spectrometer with Al K␣ X-ray line (1486.6 eV).
X-ray diffraction (XRD) data were collected on Bruk D8 Focus
diffractometer with a graphite-monochromatized Cu K␣ radiation
(ꢀ = 0.15405 nm). FT-IR spectra were obtained on Nicolet 7000-C
with 4 cm−1 resolution. Powder samples were dispersed in KBr pel-
lets for IR analysis. N2 adsorption and desorption isotherms were
measured at 77 K on a Micromeritics ASAP2020 system. The specific
surface area and the pore size distribution were calculated by using
the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda
(BJH) methods, respectively. Content of Pd in the solution was
determined by inductively coupled plasma (ICP) atomic emission
spectroscopy (Seiko SPS1700HVR).
3. Results and discussion
3.1. Structure and textural characterization
2. Experiment
The transmission electron microscopy (TEM) investigation
presents a direct evidence for the mesoporous structure and dis-
tribution of Pd in Pd/SBA. Fig. 1a and b shows the TEM images of a
Pd/SBA sample with the electron beam parallel and perpendicular
to the pore channel, respectively. The mesoporous channels were
well ordered with a characteristic hexagonal structure. Although
Pd2+ ions were reduced both on the outer and inner surface of the
matrix, the structure and morphology of matrix were kept intact
and the Pd colloidal could be scarcely perceived even in a high mag-
nification image. No metal bulk aggregates or nano-particles could
be found on the mesoporous particle surface or in pore channels,
even after the catalytic reaction (Fig. 1c), and the pore channels are
still open. The only noticeable difference was the scattering con-
trast between walls and pores, compared with the original SBA-15.
The distribution of the metal colloids was, therefore, uniform not
only on the inner but also on the outer surface. The existence of Pd
colloids can be further proved by energy dispersive spectroscopy
analysis of X-ray (EDAX), which revealed the Pd content of about
0.1 mol% on average.
X-ray photoelectron spectrum (XPS) analysis was used here to
identify the status of palladium in Pd/SBA. As shown in Fig. 2, two
peaks centered at a binding energy of 340 and 335 eV, matched
well with Pd0 3d3/2 and Pd0 3d5/2, respectively. For Pd2+, these two
peaks are expected at 342.2 and 336.8 eV, which were not observed
in the XPS survey spectrum. Accordingly, almost all of the Pd species
in Pd/SBA had been reduced in situ to Pd0 by Si–H groups on the
surface of parent materials.
The mesoporous silica SBA-15 was synthesized according to the
literature by using tri-block poly(ethylene oxide)–poly(propylene
oxide)–poly(ethylene oxide), EO20PO70EO20, as a template in acidic
conditions [14]. The mesoporous silica SBA-15 was directly used as
support without any pretreatment.
2.1. Preparation of Pd/SBA catalyst
SBA-15 (1.5 g) was dispersed in dry CHCl3 (50 mL) and
(CH3O)3SiH (10 mL) was added dropwise under stirring. The mix-
ture was filtered after being heated at 80 ◦C under N2 atmosphere
for 24 h. The solid obtained after repeated washing with CHCl3 was
named H/SBA.
H/SBA (1.5 g) was dispersed in dry THF (20 mL); 0.05 M palla-
dium acetate THF solution (30 mL) was added slowly afterwards.
After stirring for 4 h, the solid was filtered, washed with THF and
dried in vacuum at room temperature.
2.2. Catalytic studies
The 20 mmol aryl halide, 30 mmol vinyl substrate, 22 mmol
triethylamine, 10 mmol dodecane (as internal standard for GC anal-
ysis) and 20 mL NMP were introduced into a three-necked flask.
Catalyst (Pd/SBA) was added through a funnel and the reaction
mixture was heated to reaction temperature (mol% catalyst used,
reaction time and temperature are shown in Table 1).
Table 1
Catalytic properties of the prepared Pd/SBA for Mizoroki–Heck reactionsa.
Vinyl
Aryl halide substrate
Reaction condition
Conversion %b (selectivity %)c
Pd in solution (ppm)
TOF (mol/mol h)
Styrene
C6H5I
C6H5Br
4-Br-C6H4OCH3
140 ◦C; 2 h
140 ◦C; 2 h
120 ◦C; 2 h
100 (100)
57 (94)
60 (91)
0.010
0.036
0.017
2.5 × 104
1.3 × 104
1.4 × 104
C6H5I
C6H5Br
4-Br-C6H4OCH3
140 ◦C; 2 h
140 ◦C; 2 h
120 ◦C; 2 h
100 (100)
54 (94)
62 (98)
0.078
0.036
<0.01
2.5 × 104
1.3 × 104
1.5 × 104
Methyl acrylate
a
All reactions were carried out in air with 0.002 mol% Pd as catalyst in form of Pd/SBA.
Conversion of reactant was determined by GC–MS.
Selectivity = mole of coupling product/mole of reactant converted.
b
c

52
Y. Feng et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 50–54
Fig. 3 depicts the small-angle XRD pattern of SBA-15 and pal-
ladium colloid grafted mesoporous silica composite (Pd/SBA). In
agreement with the above TEM observations, both samples show
well-resolved reflections corresponding to hexagonal mesoporous
structure. The location of diffraction peaks had no great change
after metal ions was reduced on the surface of the matrix. Three
peaks at a 2ꢁ value of about 0.95◦, 1.58◦ and 1.82◦ can be indexed as
the (1 0 0), (1 1 0) and (2 0 0) reflections. These diffraction features
can be ascribed to the basal series arising from p6mm hexagonal
symmetry. The only difference between them is the peak inten-
sity due to the guest loading in Pd/SBA. In general, palladium metal
Fig. 2. XPS spectrum of Pd/SBA sample.
has major diffraction peaks at 2ꢁ = 40.1◦ (1 1 1) and 46.7◦ (2 0 0),
which were not found in the wide-angle XRD pattern of Pd/SBA
(Pd content: 0.1 mol%) even after the catalytic reaction (Fig. 4),
further proving that reduced palladium was highly dispersed as
amorphous and extremely small nano-colloids in the mesoporous
silica material.
FT-IR data shown in Fig. 5 clearly support the chemical
modifications of the matrix and the Pd reductive process. The
original SBA-15 showed adsorption bands at 960 cm−1, which is
attributable to Si–O stretching of the Si–OH groups. On the other
hand, when it was modified with trimethoxysilicane (CH3O)3SiH,
this peak almost disappeared and two peaks at about 2240 and
880 cm−1 evolved. These additional peaks can be assigned as Si–H
stretching and bending vibrations, respectively. This proved that,
during the treatment of SBA-15 with trimethoxysilane, a reaction
between (CH3O)3SiH and SiOH groups occurred, resulting in the
creation of Si–H on the inner and outer surfaces of the mesoporous
material. It was this Si–H group that reduced the Pd2+ ions to form
the metal colloids on the surface of the silica matrix.
As a primary measurement for the physical properties, nitro-
gen physisorption was conducted for system analysis before and
after Pd loading. Fig. 4 shows the nitrogen adsorption/desorption
Fig. 3. Small-angle XRD patterns of (a) SBA-15 and (b) Pd/SBA.
Fig. 1. TEM images of Pd-SBA catalyst before (a and b) and after (c) catalytic reaction.

Y. Feng et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 50–54
53
Fig. 6. Nitrogen adsorption/desorption isotherms of (a) SBA-15, (b) H/SBA and (c)
Pd/SBA.
Fig. 4. Wide-angle XRD patterns of Pd/SBA before (a) and after (b) catalysis.
in/on SBA-15. In just similar manner to our previous studies [15a],
under the in situ reduction condition (Si–H groups as reductant),
the BET surface area and BJH pore volume caused only limited
decrease after the metal incorporation. In this case, BET surface area
decreased from 608 to 514 m2/g and BJH pore volume from 0.971
to 0.679 cm3/g. This suggests that the incorporated Pd did not block
the pore and only occupied a very limited space. Accordingly, all the
pore channels of the host silica remained open. This property is very
important for catalytic applications. Only under that condition, the
guest molecules can be easy to diffuse into the host mesoporous sil-
ica and access catalyst nano-particles for reactions to be catalyzed
(Fig. 6).
The mesoporous Pd/SBA material catalyzed the Mizoroki–Heck
carbon–carbon coupling reaction (Scheme 1). The catalytic activity
of this material was investigated with activated and non-activated
aryl halides, with styrene and methyl acrylate as the vinyl sub-
strate. All the reactions were conveniently carried out in a reactor
in air. The yields for the aryl halides with respect to reaction time,
amount of catalyst used and TOF values all showed that the Pd/SBA
catalyst has an excellent activity for Mizoroki–Heck carbon–carbon
coupling reactions. Compared with other palladium catalysts sup-
ported on mesoporous silicas [14,15], this material showed very
efficient catalytic activities, and only about 1/10 to 1/20 amount
of Pd catalyst was needed to reach the same conversion under the
same reaction conditions (Table 1). This property may be related to
the dispersing behavior of the palladium in the matrix. The aggre-
gation of Pd particles, and also the continuous/dense coating of Pd
colloids on supports should be responsible for the relatively low
TOF values for the reactions in the previous reports [14,15]. In the
present synthesis, the surfactant template of the silica matrix was
directly eliminated by calcination resulting in significantly lower
amount of Si–OH groups on the surface, compared with that by sol-
vent extraction [15a]. Thus, after modified with Si–H group, much
less palladium ions were reduced to Pd colloids on the pore or outer
surfaces than those in mesoporous silica with template removed by
Fig. 5. FT-IR spectra for samples of (a) SBA-15 and (b) H/SBA.
isotherms for SBA-15, H/SBA and Pd/SBA. All of the samples exhib-
ited type IV isotherms with a H1-type broad hysteresis loop,
corresponding to typical large-pore mesoporous materials with
1D cylindrical pore channels. Capillary condensation of nitrogen
caused a sudden step increasing in nitrogen uptake in the typical
relative pressure (P/P0) range of 0.6–0.8 for all samples studied,
suggesting well-defined mesoporous structure with uniform pore
diameters. The pore parameters of samples are summarized in
Table 2. The decrease in BET surface area, BJH pore volume and
average pore size, together with the increased thickness of pore
wall, can be attributed to the incorporation of Pd nano-colloids
Table 2
Pore structure parameters of SBA-15, H/SBA and Pd/SBA composite.
SBA-15
H/SBA
Pd/SBA
Surface areas (m2/g)
Pore diameter (nm)
645
608
515
7.65
1.06
2.46
7.01
0.97
2.76
6.48
0.68
3.27.
Pore volume (cm3/g)
Thickness of pore wall (nm)
Scheme 1. Heck coupling.

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Y. Feng et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 50–54
over six times, the activity of Pd/SBA shows little decrease during
reuse. Meanwhile, it still retained relatively high activity and could
be used more times. The relatively stable conversion showed that
immobilized catalyst could be repeatedly used without apparent
decrease in its catalytic activity.
4. Conclusions
In conclusion, a heterogeneous catalyst system Pd/SBA, with
extremely low Pd loading amount of 0.1 mol%, had been success-
fully prepared by directly in situ reduction method. The palladium
ions were evenly reduced to form metal colloids which existed
as isolated islands on both the inner and outer surfaces. With
extremely low used amount (0.002 mol%) of Pd as the cata-
lyst, extraordinarily high catalytic activity and TOF values for
Mizoroki–Heck reactions were achieved, and during the reactions,
the catalyst kept high surface-to-volume ratios. The large and open
pore network, ultrahigh surface area and highly dispersed catalyst
species made it one of the most active heterogeneous catalysts for
Mizoroki–Heck reactions. The catalyst species showed very high
stability against leaching from the support and can be recycled for
repeated use.
Fig. 7. Time conversion plot for the Mizoroki–Heck reaction between methyl acry-
late and bromobenzene under the presence of 0.002 mol% Pd as catalyst in form of
Pd/SBA.
Table 3
Reuse properties of Pd/SBA for the reaction between styrene with bromobenzene.
Acknowledgments
State
Conversion (%)
Selectivity (%)a
1st
57.2
56.9
56.6
54.6
55.1
54.1
95
94
95
96
95
91
This work was supported by Shanghai Pujiang Program (grant
no. 08PJ14035) and National Nature Foundation of China (grant no.
20633090).
2nd
3rd
4th
5th
6th
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