PdCl2(py)2 encaged in monodispersed zeolitic hollow spheres: A highly efficient and reusable catalyst for Suzuki-Miyaura cross-coupling reaction in aqueous media

Article abstract of DOI:10.1039/c2gc35302d

By encaging the PdCl₂(py)₂ complex (py = pyridine) in the interior space of silicalite-1 hollow spheres (SHS), a novel solid palladium catalyst, PdCl₂(py)₂@SHS, was successfully prepared. The structure and composition of the solid catalyst was characterized by SEM, TEM, XRD, N₂ sorption, FT-IR and XPS. This catalyst afforded fast conversions for the Suzuki-Miyaura cross-coupling reactions of various aryl halides and arylboronic acids even at Pd loadings of 0.0188 mol% in aqueous media. The turnover frequency (TOF) could be up to 63210 h^-1 under mild conditions in air. In particular, because of the antileaching effect of the surrounding zeolitic shell toward the entrapped Pd species, PdCl ₂(py)₂@SHS showed outstanding stability and reusability, which could be reused at least 10 times without appreciable loss of its activity. The developed solid catalyst combined with the mild conditions represented one of the most efficient heterogeneous systems for the Suzuki-Miyaura cross-coupling reactions of aryl halides. The Royal Society of Chemistry 2012.

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PAPER
PdCl2(py)2 encaged in monodispersed zeolitic hollow spheres: a highly
efficient and reusable catalyst for Suzuki–Miyaura cross-coupling reaction
in aqueous media†
Zhenhong Guan,a Jianglin Hu,a Yanlong Gu,a,b Haojun Zhang,a Guangxing Lia,b and Tao Li*a,b
Received 29th February 2012, Accepted 24th April 2012
DOI: 10.1039/c2gc35302d
By encaging the PdCl2(py)2 complex (py = pyridine) in the interior space of silicalite-1 hollow spheres
(SHS), a novel solid palladium catalyst, PdCl2(py)2@SHS, was successfully prepared. The structure and
composition of the solid catalyst was characterized by SEM, TEM, XRD, N2 sorption, FT-IR and XPS.
This catalyst afforded fast conversions for the Suzuki–Miyaura cross-coupling reactions of various aryl
halides and arylboronic acids even at Pd loadings of 0.0188 mol% in aqueous media. The turnover
frequency (TOF) could be up to 63 210 h−1 under mild conditions in air. In particular, because of the
antileaching effect of the surrounding zeolitic shell toward the entrapped Pd species, PdCl2(py)2@SHS
showed outstanding stability and reusability, which could be reused at least 10 times without appreciable
loss of its activity. The developed solid catalyst combined with the mild conditions represented one of the
most efficient heterogeneous systems for the Suzuki–Miyaura cross-coupling reactions of aryl halides.
homogeneous and the heterogeneous processes is always the
Introduction
focus in catalysis.1k,11
The Suzuki–Miyaura cross-coupling reaction, which is unique,
requiring mild reaction conditions and showing a high degree of
tolerance for a variety of functional groups, has become unargu-
ably one of the most powerful synthetic methods for preparing
biaryl compounds, such as natural products, pharmaceuticals,
and polymers, etc.1 Because of the great significance of this reac-
tion, it was recognized by awarding the 2010 Nobel Prize in
Chemistry.2 Most applications of the reaction involve homo-
geneous catalysts consisting of palladium complexes with a
variety of ligands.3 These catalytic systems generally exhibit
better activity and selectivity than heterogeneous systems,
however, they suffer from the practical problems such as catalyst
separation and recycling. To solve these problems and make the
process more green, researchers have immobilized palladium
complexes on various supports such as carbon,4 magnetic
materials,5 silica,6 hydroxyapatite,7 zeolites,8 MOFs9 and
organic polymers10 to create heterogeneous catalysts. Conse-
quently, achieving a combination of the advantages of both the
Among various shell materials, zeolitic shell hollow spheres
have been demonstrated extraordinarily to be a nontoxic, highly
biocompatible, and hydrothermal/mechanically stable material,
so they have many practical applications in the areas as diverse
as biological chemistry, synthesis, photonics, and catalysis.12
These spheres act as bi-functional systems for catalysis appli-
cations where the access to the interior of the particles having
the active components is controlled by the zeolitic micropores on
the shell. However, over the past decade, most of the research
efforts have been focused only on hollow spheres as the support
of noble metal nanoparticle catalysts for heterogeneous cata-
lysis.13 So far, there are few reports on hollow spheres as
“containers” for homogeneous catalysts, especially noble metal
complexes.
Another concern in such reactions, is the complete or at least
partial replacement of organic solvents by environmental benign
reaction media.3d,14 We wish to report herein our preliminary
result to develop a more active and reusable heterogeneous palla-
dium complex catalyst for Suzuki–Miyaura cross-coupling reac-
tions under mild and environmental friendly conditions. A novel
and non-phosphine catalyst, PdCl2(py)2 (py = pyridine) encaged
in the interior space of silicalite-1 hollow spheres (SHS),
PdCl2(py)2@SHS, was successfully prepared through a “ship-in-
bottle” method.15 This microcapsular reactor afforded fast con-
versions for the Suzuki–Miyaura cross-coupling reactions of
various aryl halides and arylboronic acids even at catalyst load-
ings of 0.0188 mol%. The turnover frequency (TOF) could be
up to 63 210 h−1 under mild conditions in air. Investigations on
the reusability and stability of the heterogeneous palladium
aSchool of Chemistry and Chemical Engineering, Huazhong University
of Science and Technology, Luoyu Road 1037, Wuhan, Hubei 430074,
PR China. E-mail: taoli@mail.hust.edu.cn; Fax: +86-27-87543632;
Tel: +86-27-87557048
bKey Laboratory for Large-Format Battery Materials and System,
Ministry of Education, Huazhong University of Science and Technology,
Wuhan 430074, PR China
†Electronic supplementary information (ESI) available: Experimental
details and instrumentation of the synthesis of SHS, SEM images of
MSS, silicalite-1 seeds, silicalite-1 coated MSS and shell-crushed
PdCl2(py)2@SHS, 1H NMR data. See DOI: 10.1039/c2gc35302d
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Scheme 1 Preparation of the microcapsular reactor, PdCl2(py)2@SHS.
Fig. 2 XRD patterns: (a) silicalite-1; (b) SHS; (c) PdCl2(py)2@SHS.
Fig. 1 (a) SEM and (b) TEM images of PdCl2(py)2@SHS. The inset
in (a) is the SEM image at high magnification to depict the zeolite lattice
fringes.
catalyst were also discussed herein. To the best of our knowl-
edge, this is the first example of employing silicalite-1 hollow
spheres as “containers” for palladium complexes in Suzuki–
Miyaura cross-coupling reactions.
Results and discussion
Catalyst preparation and characterization
As illustrated in Scheme 1, silicalite-1 hollow spheres (SHS) are
fabricated via a layer-by-layer technique and a two-step hydro-
thermal treatment.16 By means of in situ synthesis, PdCl2 and
pyridine are introduced into the interior space of SHS by batch
equilibrium technique successively. The white powder turns into
brown firstly, then light yellow. After washing with solvent
thoroughly and drying, the solid remains white. The Pd content
of the PdCl2(py)2@SHS catalyst is 0.10 wt% measured by
atomic absorption spectroscopy (AAS). The N and C contents of
the PdCl2(py)2@SHS catalyst determined by combustion chemi-
cal analysis are 0.03 wt% and 0.11 wt%, respectively. The atom
ratios of Pd, N and C (1 : 2.1 : 9.5) on the solid catalyst fall into
a good agreement with the ratios expected (PdCl2(py)2,
C10H10N2PdCl2).
Fig. 1a depicts the scanning electron microscopy (SEM)
images of the PdCl2(py)2@SHS catalyst, the catalyst presents
with a monodispersed, uniform spherical morphology with the
coarse outer surface composed of closely packed silicalite-1
nanocrystals. The particle diameter is ranging from 1.0 to
1.2 μm. The zeolite lattice fringes observed in the image (inset
of Fig. 1a) at high magnification extend through the entire crys-
tals, also manifesting that the capsular shells are, indeed, com-
posed of silicalite-1 nanocrystals. The hollow structures of the
spheres are proved by transmission electron microscopy (TEM)
(Fig. 1b). The hollow interior diameter is nearly 1.0 μm and the
thickness of shell is about 200 nm.
Fig. 3 N2 sorption isotherm of PdCl2(py)2@SHS. The inset is the cor-
responding pore size distribution.
the silicalite-1 zeolite consistently, confirming the MFI structure
of the shell,17 which gives no evidence for another crystalline
phase. The N2 sorption isotherm of PdCl2(py)2@SHS (Fig. 3)
displays a type I like behavior of microporous materials. The
BJH pore size distribution of PdCl2(py)2@SHS (inset of Fig. 3)
shows a sharp peak at 0.56 nm which is in agreement with the
microporous dimension of silicalite-1 zeolite.18 These results
further confirm the shells of PdCl2(py)2@SHS are composed of
silicalite-1.
The FT-IR spectra of PdCl2(py)2, SHS and PdCl2(py)2@SHS
catalyst are shown in Fig. 4 in the 400–4000 cm−1 region. As
can be seen, both the SHS and PdCl2(py)2@SHS exhibit the
band typical of 550 cm−1, which confirms that the sample pos-
sesses a MFI type framework, and the intensity ratio of 550 to
450 cm−1 is indicative of the high crystallinity of the product.19
Compared with pure SHS, PdCl2(py)2 and PdCl2(py)2@SHS
show characteristic adsorptions at 1602.6 cm−1 and 1448.3 cm−1
in both cases attributed to the stretching vibrations of the CvN
and CvC double bonds. Furthermore, the characteristic adsorp-
tion peaks at 694.6 cm−1 is assigned to the deformation
vibrations of thevC–H bonds. These results indicate the exist-
ence of the palladium complex, PdCl2(py)2 in the catalyst.
The XRD patterns of silicalite-1, SHS and PdCl2(py)2@SHS
are shown in Fig. 2. The characteristic diffraction peaks assign to
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Table 1 The effect of solvents and basesa
Entry Solvent
Base
Time (min) Yield (%)b
1
2
3
4
5
6
7
8
MeOH
K2CO3
60
60
60
60
60
60
60
60
60
60
60
5
9
24
23
40
56
78
69
DMF–H2O (10 : 1) K2CO3
H2O
EtOH
K2CO3
K2CO3
EtOH–H2O (1 : 2) K2CO3
EtOH–H2O (1 : 1) K2CO3
EtOH–H2O (3 : 2) K2CO3
EtOH–H2O (3 : 2) Na2CO3
EtOH–H2O (3 : 2) NaHCO3
EtOH–H2O (3 : 2) NaOAc·3H2O
EtOH–H2O (3 : 2) KOAc
EtOH–H2O (3 : 2) Na3PO4·12H2O 60
EtOH–H2O (3 : 2) K3PO4·3H2O
H2O K3PO4·3H2O
9
62
70
71
10
11
12
13
14
15
16
85
60
60
10
20
99 (97)c
53
EtOH–H2O (3 : 2) K3PO4·3H2O
EtOH–H2O (3 : 2) K3PO4·3H2O
99 (96)c
90d
Fig. 4 FT-IR spectra: (a) PdCl2(py)2; (b) SHS; (c) PdCl2(py)2@SHS.
a Reaction conditions: 10 mg of PdCl2(py)2@SHS (0.0188 mol% of
Pd), 0.5 mmol of bromobenzene, 0.75 mmol of phenylboronic acid,
1 mmol of base, 2 ml of solvent (v/v), 60 °C in air. b GC yield
(naphthaline as internal standard). c Isolated yield in parentheses.
d 10 mmol scale.
The Suzuki–Miyaura cross-coupling reaction
We first tested the catalytic activity of PdCl2(py)2@SHS in the
Suzuki–Miyaura cross-coupling reaction between bromobenzene
and phenylboronic acid, and the results are summarized in
Table 1. Previous studies have showed that solvents and bases
had remarkable influences on the reactivity of the Suzuki reac-
tion.3d,6d,21 Our interest is using water to replace organic solvents
in such reaction because water solvent endows the reaction with
green and safe properties.14 In addition, the hydrophilic nature of
the microporous silica can induce a wettability control over the
catalytic reactions in aqueous media. Hydrophilic liquids may be
preferentially adsorbed into the composite microcapsular reactor.
Our preliminary screening of solvent revealed that the presence
of ethanol could efficiently promote the Suzuki–Miyaura reac-
tion in water. And the optimal condition when using K2CO3 as
base, 60% aqueous ethanol was the best choice for the reaction
medium (Table 1, entries 1–7). Under the same conditions,
Na2CO3, NaHCO3, NaOAc·3H2O and KOAc gave moderate
biphenyl yields (Table 1, entries 8–11). Remarkably, when
Na3PO4·12H2O and K3PO4·3H2O were used (Table 1, entries 12
and 13), good to excellent biphenyl yields were obtained, while
K3PO4·3H2O in pure water only gave 53% of biphenyl (Table 1,
entry 14). Indeed, K3PO4·3H2O in 60% aqueous ethanol was
found to be the best choice in view of almost quantitative bi-
pehnyl yield obtained within a considerably short reaction time
(10 min) (Table 1, entry 15). A reaction in 10 mmol scale was
also carried out, and it was found that the reaction proceeded
uneventfully, indicating the effectiveness of this catalyst for prac-
tical synthesis (Table 1, entry 16).
Fig. 5 XPS Pd 3d spectra: (a) PdCl2(py)2; (b) PdCl2(py)2@SHS; (c)
10th recycled PdCl2(py)2@SHS. All the samples were heavily ground to
expose the Pd species before XPS characterization (see ESI, Fig. S5†).
In order to obtain an insight into the oxidation state and
coordination environment of palladium in the PdCl2(py)2@SHS
catalyst, X-ray photoelectron spectroscopy (XPS) experiments
were performed. Fig. 5 displays the Pd binding energy of
PdCl2(py)2 and PdCl2(py)2@SHS. PdCl2(py)2@SHS exhibits
two observable peaks centered at 343.3 and 338.0 eV, which are
assigned to Pd 3d3/2 and Pd 3d5/2, respectively.20 This result is
the same with PdCl2(py)2 complex and implies that the oxidation
state of Pd is +2 and also a coordination interaction between Pd
and pyridine might be involved. The above characterization
experiments confirm that the Pd complex, PdCl2(py)2 is success-
fully encaged in the SHS, the schematic model of the
PdCl2(py)2@SHS catalyst proposed in Scheme 1 should be
reasonable.
With the optimized solvent and base in hand, we investigated
the activity of PdCl2(py)2@SHS for various substrates. All the
reactions were carried out at 60 °C in air using K3PO4·3H2O as
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Table 2 Suzuki–Miyaura
cross-coupling
catalyzed
by
PdCl2(py)2@SHSa
Time
(min)
Yield
(%)b
TOF
(h−1 c
Entry ArX
ArB(OH)2
)
1
2
10
15
97
97
30 970
20 650
3
20
5
88
99
99
96
92
99
7
14 050
63 210
9120
4
5
35
10
10
15
60
5
6
30 650
29 370
21 070
275
7
Fig. 6 Recycling test of PdCl2(py)2@SHS before and after shell-
crushed by heavily grinding.
8
9
reaction time (60 min) (Table 2, entry 9). PdCl2(py)2@SHS also
showed a high activity for the Suzuki–Miyaura cross-couplings
of 4-methylphenylboronic acid, satisfactory yields could be also
obtained within 5 min–35 min (Table 2, entries 10–17). For
4-cyanobromobenzene and iodobenzene, the TOF also reached
ca. 63 210 h−1. We further tested the catalytic performances for
the couplings of various arylboronic acids with bromobenzene
(Table 2, entry 18–20). For arylboronic acids bearing electron-
donating group –OCH3 and electron-withdrawing groups –CF3
and –Cl, 86%, 83% and 88% yields were obtained, respectively.
In order to determine whether the catalysis was derived from
the PdCl2(py)2@SHS or from leaching palladium, which is
released from the SHS hollow spheres during the reaction and
then re-absorbed to the SHS at the end, we performed a hot fil-
tration test.22 We focused on the Suzuki–Miyaura cross-coupling
reaction of 4-bromobenzaldehyde with phenylboronic acid.
Rapid separation of the PdCl2(py)2@SHS catalyst by centrifu-
gation after a 15 min reaction time (the conversion of 4-bromo-
benzaldehyde reached 63%) and allowed the hot liquid mixture
to react further. We found that no further reaction was observed
and no palladium could be detected in the hot liquid mixture by
AAS. This result demonstrates that the palladium catalyst
remains inside the SHS during the reaction.
10
11
96
98
61 300
20 860
15
12
13
14
15
16
17
18
19
20
15
5
94
99
98
99
99
94
86
83
88
20 010
63 210
8940
35
5
63 210
31 610
20 010
18 300
13 250
18 730
10
15
15
20
15
a Reaction conditions: 10 mg of PdCl2(py)2@SHS (0.0188 mol% of
Pd), 0.5 mmol of aryl halide, 0.75 mmol of arylboronic acid, 1 mmol of
K3PO4·3H2O, 2 ml of EtOH : H2O (3 : 2 v/v), 60 °C in air. b Isolated
yield. c mol biaryl/mol Pd h.
The recyclability of PdCl2(py)2@SHS was also investigated
with consecutive Suzuki–Miyaura cross-coupling reactions of
iodobenzene with phenylboronic acid (Fig. 6). This catalyst
could be recycled 10 times without any loss of activity. The
average yield of biphenyl in consecutive reactions promoted by
the 1st through to the 10th recycled catalyst was up to 97% and
the total TON (turnover numbers) for 10 cycles was up to
25 800. Undoubtedly, this result is very promising and encoura-
ging from a practical point of view.1j,k For comparison, shell-
crushed PdCl2(py)2@SHS was also investigated. As expected, it
deactivated quickly in two or three cycles. Furthermore, accord-
ing to the SEM and TEM images shown in Fig. 7, the used
PdCl2(py)2@SHS spheres can retain their dense zeolitic shell
base and 60% aqueous ethanol solution as reaction medium. The
results are summarized in Table 2. At the Pd loading of
0.0188 mol%, PdCl2(py)2@SHS afforded satisfactory biaryl
yields (88–99%) for aryl bromides containing –t-Bu, –OCH3,
–CN and –CHO groups within 5 min–35 min (Table 2, entries
2–5). It is worth noting that the TOF reached ca. 63 210 h−1 in
the case of 4-cyanobromobenzene. For iodobenzene and other
aryl iodides with substituents such as –CH3 and –OCH3, com-
plete conversions were observed and the corresponding biaryl
products in 92–99% yields were also achieved within 10 min–
15 min (Table 2, entries 6–8). Further study indicated that
chlorobenzene can also react with phenylboronic acid. However,
the yield of biphenyl was very poor (7%) even prolonged the
even after 10 recycles, indicating
a good stability of
PdCl2(py)2@SHS in liquid reaction. The palladium content of
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the catalyst was determined by AAS after ten consecutive runs
and no appreciable change was observed. XPS analysis revealed
the binding energy of Pd 3d5/2 in the 10th recycled catalyst
was 335.9 eV (Fig. 5c). The remarkable decrease (2.1 eV) in
Pd binding energy was observed comparing with the fresh cata-
lyst. This result indicated that the reduction of the starting
palladium(II) complex to the lower valent state Pd(0) had taken
place during the Suzuki–Miyaura cross-coupling reaction.23
To further confirm the Pd species were effectively entrapped
inside the intact zeolitic shell in our PdCl2(py)2@SHS catalyst,
we employed a mixture of iodobenzene and sterically hindered
5-iodo-m-xylene as the substrates, 98% of biphenyl, and 3% of
3,5-dimethylbiphenyl were obtained by coupling with phenyl-
boronic acid (Scheme 2). The size of 5-iodo-m-xylene is too
large to permeate into the zeolitic shell and can not be success-
fully converted on the encaged Pd species consequently. On the
contrary, after the zeolitic shell was destroyed by heavy grinding
(see ESI, Fig. S5†), the shell-crushed PdCl2(py)2@SHS dis-
played a very high product yields of both biphenyl and 3,5-
dimethylbiphenyl within 10 min (97% and 95%, respectively).
Similarly, for an isomeric mixture system of methyl 4-iodo-
benzoate and methyl 2-iodobenzoate, the PdCl2(py)2@SHS cata-
lyst gave 92% of 4-methoxycarbonylbiphenyl and only 5% of
2-methoxycarbonylbiphenyl, while the shell-crushed catalyst
gave 93% and 89%, respectively. Extraordinarily, the superior
substrate selectivity of the PdCl2(py)2@SHS catalyst in both
sterically hindered and isomeric systems originated from the
shape-selectivity of the zeolitic shell, allowing it to find practical
applications in some mixture separation-inextricable organic
synthesis.
channels on the shell have a dimension of about 0.56 × 0.53 nm
(i.e., the microporous dimension of silicalite-1 zeolite)18 and
provide entrances for the reactants, exits for the products as well
as protection for PdCl2(py)2 complex from leaching. Further-
more, the interior space (diameter 1 μm) is large enough to
accommodate a desired number of metal complexes in a
confined space; and the catalytic complexes can wander freely
since there are no covalent linkages or other strong interactions
between the complexes and the internal surface of silicalite-
1 hollow spheres (SHS). Consequently, the PdCl2(py)2 can be
trapped within the micrometer-sized hollow interior space to
form an unique encaged microenvironment for the reaction as
the homogeneous catalyst. On the other hand, the whole catalyst
apparently behaves just like a heterogeneous catalyst, which can
be recovered and reused easily in virtue of the high thermal/
chemical stability of the zeolitic shell.
Conclusions
By encaging the PdCl2(py)2 complex in the interior space of sili-
calite-1 hollow spheres (SHS), a novel and non-phosphine solid
palladium catalyst, PdCl2(py)2@SHS, was successfully pre-
pared. This catalyst afforded fast conversions for the Suzuki–
Miyaura cross-coupling reactions of various aryl halides and
arylboronic acids even at catalyst loadings of 0.0188 mol%. The
turnover frequency (TOF) could be up to 63 210 h−1 under mild
conditions in air. In particular, because of the antileaching effect
of the surrounding zeolitic shell toward the entrapped Pd
species, PdCl2(py)2@SHS shows outstanding stability and re-
usability, can be reused at least 10 times without any activity
decrease. In addition, the developed solid catalyst exhibits
superior substrate selectivity in a mixed system for practical
synthesis.
Indeed, each of the spheres is a true micrometer-sized reactor
with active ingredients entrapped in the hollow space surrounded
by a zeolitic shell of ca. 200 nm. The uniform microporous
Experimental
Reagents and materials
n-Hexadecylamine, tetrapropylammonium hydroxide (TPAOH,
1 M in water), aryl halides were purchased from Alfa Aeser, pal-
ladium chloride (PdCl2), pyridine (py), naphthalene, K3PO4·
3H2O and other bases, polydiallyldimethyl ammonium chloride
(PDDA, Mw = 160 000, 40 wt% in water), poly(sodium
4-styrene sulfonate) (PSS, Mw = 200 000, 20 wt% in water),
tetraethoxyorthosilane (TEOS) were commercially available. All
Fig. 7 (a) SEM; (b) TEM images of 10th recycled PdCl2(py)2@SHS.
The inset in (b) is the TEM image at high magnification.
Scheme 2 PdCl2(py)2@SHS for mixed system.
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Characterization and analysis
chemicals were used without further purification. The syntheses
of mesoporous silica spheres (MSS) and silicalite-1 seeds were
expatiated in the ESI.†
N2 physical adsorption was carried out on a Micrometrics ASAP
2020 M volumetric adsorption analyzer (before the measure-
ments, samples were degassed at 383 K for 8 h under 10−5 bar
vacuum). The Brunauer–Emmett–Teller (BET) surface area was
evaluated from data in the relative pressure range from 0.05 to
0.25. The total pore volume of each sample was estimated from
the amount adsorbed at the highest P/P0 (above 0.99). Pore
diameters were determined from the adsorption branch using
Barrett–Joyner–Halenda (BJH) method. Scanning electron
microscopy (SEM) images were obtained on a FEI Sirion 200
scanning electron microscope at 10.0 kV. Transmission electron
microscopy (TEM) was carried out on a FEI Tecnai G2 20 elec-
tron microscope running at 200 kV. The XRD patterns were
obtained on a Bruker Advanced D8 diffractometer over a 2θ
range of 5°–70° with Cu Kα radiation. Pd content was analyzed
by atomic absorption spectrometry (AAS) on a Perkin Elmer
AA-300. The C, N elemental analyses were carried out on a
Vario Micro cube Elemental Analyzer. FT-IR spectra in KBr
pellet were collected on Bruker Equinox 55 FT-IR spectropho-
tometer in the range 4000–400 cm−1. The XPS analysis was
carried out with a VG Multilab 2000 spectrometer using Al Kα
radiation at a power of 300 W. The pass energy was set at
100 eV, and C1s line at 284.6 eV was used as a reference. All the
samples were washed with acetone, dried and heavily ground to
expose the Pd species before XPS characterization. The products
of the coupling reaction were identified by 1H NMR spectra
using a 400 MHz Bruker AV400 instrument in CDCl3. Chemical
shifts are reported in parts per million (ppm) downfield from
Synthesis of silicalite-1 hollow spheres (SHS)
Silicalite-1 hollow spheres were fabricated according to the lit-
erature,16 with some modifications. The silicalite-1 coated MSS
were treated through a two-step hydrothermal approach: 0.1 g of
silicalite-1 coated MSS were hydrothermally treated in 15 ml of
1.5 wt% TPAOH aqueous solution at 100 °C for 1 h, and then
continuously treated in 15 ml aqueous solution with a molar
composition of TPAOH : TEOS : H2O = 3 : 10 : 2000 at 100 °C
for 8 h. The solid product was separated by centrifugation and
washed with distilled water three times. After drying at 60 °C for
2 h, templates in the samples were removed by calcination in air
at 550 °C for 4 h.
Loading of PdCl2(py)2 complex into SHS
The introduction of PdCl2(py)2 complex into hollow spheres
was performed by using batch equilibrium technique. Firstly,
0.1 g of hollow spheres was immersed into 2 ml of Na2PdCl4
aqueous solution with initial concentrations of 0.5 M. Then they
were left for 24 h at room temperature in order to reach the con-
centration equilibrium. Upon equilibration, the solid products
were recovered by centrifugation. After drying in vacuum over
night, a light brown solid was obtained, and then 2 ml pyridine
in acetone was added. The mixture was left for 24 h at room
temperature. Finally, the light yellow solid was separated by cen-
trifugation, washed with water, DMF and acetone thoroughly in
sequence, then Soxhlet extraction with acetone for 48 h was also
employed to make sure no Pd2+ and Pd complex adhered to the
outside of the hollow spheres. A white solid was obtained after
drying at 60 °C under vacuum overnight and denoted as
PdCl2(py)2@SHS.
1
TMS. The H NMR spectra of biaryls and the melting points of
solid biaryls were in accordance with literature reports. GC ana-
lyses were performed in a Fuli GC9790, equiped with a FFAP
(30 m × 0.25 mm × 0.25 μm) capillary column and a flame ion-
ization detector (FID).
Acknowledgements
We acknowledge the National Natural Science Foundation of
China (20873047/20973068) for financial support. We are also
grateful for Opening Foundation of Key Laboratory of Catalysis
and Materials Science of the State Ethnic Affairs Commission &
Ministry of Education, South-Central University for Nationalities
and the spectroscopic analysis conducted at the Analytical and
Testing Center, Huazhong University of Science and Technology,
Wuhan, China. We also thank Prof. Yi Tang, Dr Nan Ren in
Fudan University and Prof. Hengquan Yang in Shanxi University
for invaluable discussions regarding preparation and activity
tests of the catalyst, respectively.
Catalytic activity test and recycling
All reactions were conducted in a 10 mL tubular flask equipped
with a magnetic stirring bar. In a typical procedure, 10 mg
PdCl2(py)2@SHS, 0.5 mmol bromobenzene, 0.75 mmol phenyl-
boronic acid and 1 mmol K3PO4·3H2O were added to 2 ml
solvent (EtOH : H2O = 3 : 2 v/v). The mixture was stirred at
60 °C in air for 10 min. The product was obtained by preparative
TLC using petroleum ether (bp 60–90 °C) as the eluting solvent.
Particularly, for the 4-cyanbiphenyl, a mixed solution of ethyl
acetate and petroleum ether (1 : 10, v/v) was used. The product
was confirmed by 1H NMR.
For the recycling experiments, 20 mg PdCl2(py)2@SHS,
0.5 mmol iodobenzene, 0.75 mmol phenylboronic acid, 1 mmol
K3PO4·3H2O, and 50 mg naphthaline (as internal standard
for GC analysis) were added to 2 ml solvent (EtOH : H2O = 3 : 2
v/v). The reaction was carried out at 60 °C in air for 10 min.
Then the mixture was separated quickly by centrifugation. The
clear supernatant was analyzed by GC. The obtained catalyst
was washed by distilled water and ethanol, dried under vacuum
and used in the next run.
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