Immobilized Pd complexes over HMMS as catalysts for Heck cross-coupling and selective hydrogenation reactions

Article abstract of DOI:10.1039/c3nj01108a

High-performance Pd(0) on the surface of hollow magnetic mesoporous spheres (HMMS), with Fe₃O₄ nanoparticles embedded in the mesoporous shell was prepared. The catalyst was characterized by TEM, XRD, VSM and ICP. It showed high activity for selective hydrogenation of alkynes to alkenes and the Heck coupling reaction. The catalyst could be recovered in a facile manner from the reaction mixture and recycled six times without loss in activity.

Full text of DOI:10.1039/c3nj01108a

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Immobilized Pd complexes over HMMS as
catalysts for Heck cross-coupling and selective
hydrogenation reactions†
Cite this: New J. Chem., 2014,
38, 1138
Peng Wang, Hengzhi Liu, Mengmeng Liu, Rong Li* and Jiantai Ma*
Received (in Montpellier, France)
16th September 2013,
Accepted 4th November 2013
High-performance Pd(0) on the surface of hollow magnetic mesoporous spheres (HMMS), with Fe3O4
nanoparticles embedded in the mesoporous shell was prepared. The catalyst was characterized by TEM,
XRD, VSM and ICP. It showed high activity for selective hydrogenation of alkynes to alkenes and the
Heck coupling reaction. The catalyst could be recovered in a facile manner from the reaction mixture
and recycled six times without loss in activity.
DOI: 10.1039/c3nj01108a
www.rsc.org/njc
magnetic spheres (HMMS). Monodispersed hollow magnetic
spheres (HMMS) with well-defined structures have attracted
Introduction
Selective hydrogenation of alkynes to alkenes without further considerable interest in the past few decades, owing to their
reduction to alkanes is of significant importance in modern unique properties such as well defined morphology, uniform
chemical transformations. For effective polymerization, the size, low density, large surface, good compatibilities, and wide
trace alkynes should be removed from the alkenes feed because range of important applications.11 Compared to conventional
alkynes deactivate the polymerization catalyst and lower the mesoporous materials such as MCM-41 and SBA-15, the HMMS
degree of polymerization.1 Selective hydrogenation using Pd may be more suitable for drug delivery because of large internal
catalysts is one of the most effective methods to reduce alkynes volumes that could provide high drug loading and homo-
content to a low ppm level in the alkenes.2 Accordingly, a highly geneous spherical morphologies.12 As is well-known, magnetic
selective catalyst, which can readily convert alkynes to alkenes materials have been widely applied in catalytic system
but does not convert alkenes to alkanes, was prepared.
because they could be easily handled and recovered.13 Never-
On the other hand, much recent work has been directed theless, a key problem is to functionalize the magnetic nano-
toward the Heck reaction because it is the most powerful and particles surface such that it allows for the immobilization of
widely used method to couple alkenes with organic moieties the catalytically active metal on its surface whilst preventing
bearing suitable leaving groups, such as halide, triflate or their agglomeration. In particular, much attention has been
diazonium.3 The resulting products have found extensive applica- focused on amine functionalization, since amines are well
tions as intermediates in the preparation of materials,4 natural known to stabilize nanoparticles against aggregation without
products5 and bioactive compounds.6 The palladium-catalyzed disturbing their desirable properties and are also recognized
application of Heck reaction has drawn considerable attention. to increase their catalytic activity.14 For example, amine-
However, the separation and reuse of homogeneous Pd catalyst functionalized magnetic nanoparticles have been employed in
remains a scientific challenge.
a range of organic transformations, showing excellent catalytic
In the last few years, numerous methods have been developed activities in oxidation,15 hydrogenation16 and C–C coupling
to immobilize palladium on a large variety of solid supports, reactions.17 We reported an effective catalyst system composed
such as microporous polymers,7 mesoporous silica,8 amorphous of Pd on the surface of hollow magnetic mesoporous spheres
silica9 and carbon nanofibers,10 making it easy and simple to in ethanol for hydrogenation and Suzuki coupling reactions
separate the catalyst from the mixture. Here we prepared a using K2CO3 as a base. The catalyst was successfully applied
catalyst that Pd was immobilized on monodispersed hollow to hydrogenation and Suzuki coupling reaction with high
activity. Encouraged by these results, we have recently applied
the catalyst system to selective hydrogenation and the Heck
coupling reaction.
College of Chemistry and Chemical Engineering, Lanzhou University,
Lanzhou 730000, P. R. China. E-mail: majiantai@lzu.edu.cn;
Herein, we report an amine-functionalized magnetite
Fax: +86 931 8912311; Tel: +86 931 8912311
nanoparticle-supported stable Pd catalyst for application in
selective hydrogenation and the Heck coupling reaction.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj01108a
1138 | New J. Chem., 2014, 38, 1138--1143
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Experimental
toluene and acetone. The obtained solid material was dried in
vacuum at 50 1C for 24 h.
Styrene (St) and acrylic acid (AA) were purchased from Sinopharm
Chemical Reagent Company and distilled under reduced pressure Loading of Pd on aminopropyl-modified silica coated HMMS
before use to remove the inhibitor. Tetraethoxysilane (TEOS, 98%), (HMMS-NH2-Pd)
n-octadecyltrimethoxysilane (C18TMS, Aldrich), were used as
received without further purification.
500 mg of HMMS-NH2 samples were first dispersed in a 50 mL
ethanol solution under ultrasonication for 0.5 h. The formed
orange suspension was ultrasonically mixed with 3.0 mmol of a
PdCl2 solution for 1 h, and then an excess 0.01 M NaBH4
solution was slowly dropped into the above mixture with
vigorous stirring. After 2 h of reduction, the products were
obtained with the help of a magnet, washed thoroughly with
deionized water and then dried in a vacuum at room temperature
Synthesis of the hydrophobic magnetite nanoparticles
(Fe3O4 NPs)
The Fe3O4 NPs were prepared using a published method with a
slight modification.18 First, 4.8 g of FeCl3Á6H2O, 2.0 g FeCl2Á4H2O
were added to 40 mL of deionized water under vigorous stirring.
Second, the mixture solution was purged with nitrogen gas for
overnight. The content of Pd in the HMMS-NH2-Pd, as determined
30 min under nitrogen atmosphere. Third, the mixture solution
was heated to 90 1C. Finally, 10 mL of ammonium hydroxide
(28 wt%) was added rapidly to the solution, and it immediately
by inductive coupled plasma emission spectrometer (ICP) analysis
was 5.18 wt%.
Typical procedure for selective hydrogenation reaction
turned black. The reaction was kept at 90 1C for 2.5 h. The black
precipitate was obtained with the help of a magnet and dried at
323 K overnight.
Hollow magnetic mesoporous silica spherical magnetite
nanoparticles were synthesized via the versatile solvothermal
reaction reported by Rong.12
In a typical experiment, 1 mmol of the reagent was dissolved in
5 mL ethanol with 10 mg of catalyst under an atmosphere of
hydrogen (rubber balloon) at room temperature. The reaction
process was monitored by thin layer chromatography (TLC).
After the reaction, the catalyst was separated by a small magnet,
and the conversion was estimated by GC (P.E. AutoSystem XL)
or GC-MS (Agilent 6890N/5973N). The separated catalyst was
washed several times with ethanol and dried at room temperature
for use the next time.
Preparation of polystyrene latex with attached
Fe3O4 nanoparticles
Briefly, carboxylic polystyrene (PS) latex was prepared by soap-free
emulsion polymerization of St with acrylic acid (AA). 1.5 g Fe3O4
nanoparticles and 2.5 g negatively charged PS latex were dispersed
in 84 mL and 60 mL hydrochloric acid solution (pH = 2.3),
respectively. Then the latter suspension was added dropwise into
the former under vigorous stirring at room temperature. After 6 h,
the heteroaggregates, i.e. Fe3O4 nanoparticles attached on the
surface of PS latex particles, were separated from the solution by
an external magnet and washed several times with water until the
pH value of the solution was close to 7.
Typical procedure for the Heck coupling reaction
For Heck coupling reactions, 0.5 mmol of the aryl halides,
0.6 mmol of styrene or n-butyl acrylate, and 0.6 mmol of K2CO3
were taken into 5 mL of N-methyl-2-pyrrolidone (NMP). The
amount of catalyst used in each reaction was 20 mg, and the
reaction mixture was refluxed at 130 1C. The reaction process
was monitored by thin layer chromatography (TLC). After
completion of the reaction, the mixture was cooled to room
temperature, separated by magnetic decantation, the resultant
residual mixture diluted with 20 mL H2O, followed by extrac-
tion twice (2 Â 15 mL) with ethyl acetate. The organic fraction
was dried over MgSO4, the solvents evaporated under vacuum
and the residue redissolved in 5 mL of dichloromethane. An
aliquot was taken with a syringe and subjected to GC or GC-MS
analysis. Yields were calculated against the consumption of the
aryl halides. After the first cycle of the reaction, the catalyst was
recovered with the help of a magnet, successively rinsed with
NMP, distilled water (to remove excess of base) and ethanol,
and dried at room temperature ready for the next cycle.
Preparation of hollow magnetic mesoporous silica spheres
(HMMS)
First, 2.0 g heteroaggregate of the PS latex and Fe3O4 nano-
particles were dispersed in a solution composed of 75 mL of
ethanol, 5 mL of H2O, and 3.8 mL of NH3ÁH2O (28 wt%). After
stirring for 10 min, 0.4 mL TEOS and C18TMS mixture with a
molar ratio of 4.7 : 1 was added dropwise under vigorous
stirring. Then, the reaction proceeded for 8 h at room tempera-
ture under stirring. The resultant particles were separated by
centrifugation, purified by three cycles of magnetic separation/
washing in ethanol, and dried at room temperature for 12 h,
finally, the as-prepared products were dried at 313 K overnight
and calcined in air at 823 K for 7 h.
Characterization
XRD measurement was performed on a Rigaku D/max-2400
diffractometer using Cu-Ka radiation as the X-ray source in the
2y range of 10–701. The size and morphology of the magnetic
nanoparticles were observed by a Tecnai G2 F30 transmission
Synthesis of aminopropyl-modified silica coated HMMS
(HMMS-NH2)
The 1.0 g of HMMS was prepared by adding 0.5 mL of electron microscopy and samples were obtained by placing a
aminopropyl trimethoxysilane to 200 mL of dry toluene. The drop of a colloidal solution onto a copper grid and evaporated
resulting solution was refluxed for 24 h and then washed with in air at room temperature. Magnetic measurement of HMMS,
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HMMS-NH2 and HMMS-NH2-Pd was investigated with a quantum
design vibrating sample magnetometer (VSM) at room temperature
in an applied magnetic field sweeping from À15 to 15 kOe.
Results and discussions
Catalyst preparation and characterization
The preparation of the HMMS-NH2-Pd catalyst follows the steps
described in Scheme 1. Firstly, HMMS were prepared. Secondly, the
functionalization of the silica coated HMMS with 3-(2-aminoethyl-
amino) propyltrimethoxysilane. Thirdly, palladium chloride was
supported on the surface of HMMS, and the palladium chloride
reduced to palladium particles with sodium borohydride.
Fig. 2 EDX spectrum of HMMS-NH2-Pd.
The typical TEM image of HMMS, HMMS-NH2-Pd is showed
in Fig. 1. The average diameter of the HMMS was about 300 nm
in Fig. 1a. The resultant HMMS-NH2-Pd composites had good
dispersibility and spherical morphology. In the TEM image of
HMMS-NH2-Pd (Fig. 1b), the catalyst did not significantly
change, and the palladium nanoparticles deposited on the
surface of the magnetic properties of the hollow magnetic silica
sphere in Fig. 1c. It also can be observed from the picture that
the wall thickness was about 20 nm.
The elemental composition of the HMM-NH2-Pd samples
was determined by EDX analysis. The result shown in Fig. 2
reveals that the as-prepared products contain Fe, Si, Pd, Cu, C
and O. Among these elements, Cu, C and O are generally
influenced by the copper network support films and their
degree of oxidation, Si, O, Fe and Pd signals result from the
HMMS and Pd particles which form the products (Fig. 2).
The magnetic properties of the HMMS-NH2-Pd catalyst were
characterized using a vibrating sample magnetometer (VSM).
The magnetization curve of HMMS-NH2-Pd exhibited no remanence
effect (superparamagnetic property) with saturation magnetization
Fig. 3 Room temperature magnetization curves of HMMS-NH2-Pd.
of about 27.6 emu gÀ1 (Fig. 3). The figure also showed that the
catalyst can be efficiently separated from the solution by an
external magnetic field.
The XRD pattern of the samples HMMS, HMMS-NH2 and
HMMS-NH2-Pd are shown in Fig. 4, respectively. The reflection
peaks of iron oxide nanocrystals obviously indicate the existence of
magnetic components in the as-obtained products. Fig. 4c shows
that apart from the original peaks, the appearance of the new peaks
at 2y = 40.1 and 46.5 was attributed to the Pd species. The results
from XRD implied that the Pd nanoparticles had been successfully
immobilized on the surface of magnetic nanoparticles.
Scheme 1 Preparation of the catalysts.
Fig. 1 TEM images of (a) HMMS and (b, c) HMMS-NH2-Pd.
Fig. 4 XRD patterns of (a) HMMS, (b) HMMS-NH2 and (c) HMMS-NH2-Pd.
1140 | New J. Chem., 2014, 38, 1138--1143
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Table 1 Selective hydrogenation of phenylacetylene derivatives with
HMMS-NH2-Pda
Entry
R1
Time (min)
Conversionb (%)
Selectivity (%)
1
2
3
4
5
6
7
8
H
H
CH3
CH3
OCH3
OCH3
F
30
40
40
50
40
60
10
16
499
499
93
97.5
69.4
94
94.7
10.6
100
100
97.3
98
74.7
95
91.1
98
F
a
Fig. 5 XPS spectrum of the HMMS-NH2-Pd showing Pd 3d5/2 and Pd 3d3/2
binding energies.
Reaction conditions: phenylacetylene 0.5 mmol, catalyst 10 mg,
b
ethanol 5 mL, H2 balloon (about 1 atm). Determined by GC or GC-MS.
Since the good results from selective hydrogenation of
phenylacetylene derivatives encouraged us, we wanted to further
test the catalyst for selective hydrogenation properties of
heterocyclic alkynes. The reactions were carried out in ethanol
at room temperature and under 1 atm pressure. The conversion
and selectivity of 2-ethynyl pyridine and 2-ethynyl thiophene
are shown in Fig. 7 and 8. The HMMS-NH2-Pd catalyst was
highly active for the selective hydrogenation under such mild
conditions, since the amine groups are known to enhance the
catalytic activity of Pd.
Fig. 5 presented XPS elemental survey scans of the surface of
the HMMS-NH2-Pd catalyst. Peaks corresponding to oxygen,
carbon, nitrogen, palladium and iron are clearly observed. To
ascertain the oxidation state of the Pd, X-ray photoelectron
spectroscopy (XPS) studies were carried out. In Fig. 6, the Pd
binding energy of HMMS-NH2-Pd exhibits two strong peaks
centered at 340.9 and 335.5 eV, which were assigned to Pd 3d3/2
and Pd 3d5/2, respectively.
Catalyst testing for selective hydrogenation reactions
The efficacies of the HMMS-NH2-Pd catalyst for selective hydro-
genation were evaluated in a H2-filled flask. The products were
analyzed via gas chromatography. Remarkably, the HMMS-
NH2-Pd catalyst achieved almost complete conversion (499%)
within 30 min in the absence of any additives, with a high
selectivity of over 94% toward styrene (Table 1, entry 1). For
phenylacetylenes with substituents such as –CH3, –OCH3 and
–F, complete conversions were observed and the products in
70–98% yields were also achieved within 16–60 min (Table 1
entries 3–8). The phenylacetylenes bearing strong electron-
withdrawing groups needs a relatively longer time to obtain a
moderate yield.
Fig. 7 Selective hydrogenation of 2-acetylene pyridine (a) selectivity and
(b) conversion.
Fig. 6 XPS spectrum of the reused HMMS-NH2-Pd showing Pd 3d5/2 and
Fig. 8 Selective hydrogenation of 2-ethynyl-thiophene (a) conversion
Pd 3d3/2 binding energies.
and (b) selectivity.
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Catalyst testing for the Heck reaction
Recycling the HHMS-NH2-Pd catalyst was investigated, along
with the reaction of the 4-bromoacetophenone with styrene.
After each reaction, the catalyst could be easily separated
magnetically from the reaction mixture, washed three times
with ethanol and finally dried for the next run. As shown in
Table 3, the catalyst was recycled six times within 10 h under
each cycle and without a significant loss of activity. The results
further confirmed the high recyclability of HMMS-NH2-Pd.
After having proven the good activity of catalyst for selective
hydrogenation of acetylene compounds, we further tested the
activity of the catalyst for heck reaction. Table 2 shows the
results of the Heck reaction of aryl halides with styrene or
n-butyl acrylate. Excellent catalytic activity was established for
activated iodobenzene derivatives (Table 2, entries 1–10). The
HMMS-NH2-Pd catalyst showed a lower catalytic activity in the
bromobenzene and chlorobenzene systems, as normally
observed in the Heck reactions, using the identical conditions
to the study of iodobenzene. Additionally, there was a great
Conclusions
effect on increasing the reaction time from 8 h to 25 h. As We have established a simple method for preparing a novel
expected, for electron poor aryl bromides and chlorides recoverable catalyst that immobilized Pd on a hollow magnetic
(Table 2, entries 13, 14, 18, 19, 22 and 24) the reaction time mesoporous silica sphere under mild conditions. This novel
is short, and conversely for electron rich bromobenzene and palladium catalyst can be conveniently prepared by the general
chlorobenzene (Table 2, entries 12, 15, 17, 20) the reaction time method, it exhibits higher activity and selectivity for Heck
was relatively long.
coupling reaction and selective hydrogenation of arylacetylenes
and could be reused six times without significant loss in
catalytic activity and selectivity.
Table 2 Heck reaction in the presence of HMMS-NH2-Pda
Acknowledgements
The authors are grateful to the Key Laboratory of Nonferrous
Metals Chemistry and Resources Utilization, Gansu Province
for financial supports and references.
Entry
R1
X
R2
T (h)
Yieldb (%)
1
2
3
4
5
6
7
8
H
I
I
I
I
I
I
I
I
Ph
Ph
Ph
Ph
8
8
8
8
6
8
8
8
8
499
499
499
98
499
98
98
97
97
499
96
CH3
OCH3
OH
COCH3
H
CH3
OCH3
OH
COCH3
H
CH3
COCH3
NO2
NH2
H
CH3
COCH3
NO2
NH2
H
Notes and references
Ph
CO2nBu
CO2nBu
CO2nBu
CO2nBu
CO2nBu
Ph
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´
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I
I
´
´
´
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6
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
20
20
8
Ph
Ph
Ph
Ph
96
499
499
86
96
94
499
499
84
92
96
8
25
20
20
8
CO2nBu
CO2nBu
CO2nBu
CO2nBu
CO2nBu
Ph
8
25
26
18
26
18
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NO2
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NO2
Ph
CO2nBu
CO2nBu
90
95
a
The reaction was carried out with 0.5 mmol of aryl halides, 0.75 mmol
of styrene (or n-butyl acrylate), 0.75 mmol of K2CO3, 4 mol% Pd with
respect to the aryl halides and 5 mL of NMP under an N2 atmosphere.
b
Determined by GC or GC-MS.
´
´
´
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Entry
Yield (%)
Entry
Yield (%)
1
2
3
499
99
98
4
5
6
95
93
93
a
Reaction conditions: 4-bromoacetophenone (0.5 mmol), styrene
(0.75 mmol), recycled Pd catalyst, K2CO3 (1.0 mmol), NMP (5 mL) under
an N2 atmosphere.
1142 | New J. Chem., 2014, 38, 1138--1143
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