Incorporation of palladium nanoparticles and 10-molybdovanado phosphoric acid in FeCo layered double hydroxide structure: electrochemical and catalytic investigation for Mizoroki–Heck coupling reactions

Article abstract of DOI:10.1007/s11164-017-3013-5

Palladium (Pd) has been immobilized on FeCo layered double hydroxide (LDH) intercalated 10-molybdovanado phosphate (FeCo/Mo₁₀V₂-Pd) for the first time. The structure was characterized using various characterization techniques. The electrocatalytic behavior of FeCo/Mo₁₀V₂-Pd was investigated using cyclic voltammetry and linear sweep voltammetry techniques. The presence of Mo₁₀V₂ enhanced the catalytic activity of FeCo-Pd in the Heck coupling reaction. Also, the collaboration of Pd and Mo₁₀V₂ with LDH as a mesoporous support was studied. These reusable solid catalysts exhibited excellent activity, and the methodology is applicable to diverse substrates providing good to excellent yields of the desired products. This method has advantages of high yields, low reaction times, elimination of ligand and base, heterogeneous catalysts and simple methodology. It is notable that FeCo/Mo₁₀V₂-Pd can easily separate from the reaction mixture using an external magnet and reused for at least four successive runs without any considerable decrease in its catalytic activity.

Full text of DOI:10.1007/s11164-017-3013-5

Res Chem Intermed
DOI 10.1007/s11164-017-3013-5
Incorporation of palladium nanoparticles and
10-molybdovanado phosphoric acid in FeCo layered
double hydroxide structure: electrochemical
and catalytic investigation for Mizoroki–Heck coupling
reactions
Ezzat Rafiee1,2 Masoud Kahrizi1
•
Received: 17 April 2017 / Accepted: 16 June 2017
Ó Springer Science+Business Media B.V. 2017
Abstract Palladium (Pd) has been immobilized on FeCo layered double hydroxide
(LDH) intercalated 10-molybdovanado phosphate (FeCo/Mo10V2-Pd) for the first
time. The structure was characterized using various characterization techniques. The
electrocatalytic behavior of FeCo/Mo10V2-Pd was investigated using cyclic
voltammetry and linear sweep voltammetry techniques. The presence of Mo10V2
enhanced the catalytic activity of FeCo-Pd in the Heck coupling reaction. Also, the
collaboration of Pd and Mo10V2 with LDH as a mesoporous support was studied.
These reusable solid catalysts exhibited excellent activity, and the methodology is
applicable to diverse substrates providing good to excellent yields of the desired
products. This method has advantages of high yields, low reaction times, elimina-
tion of ligand and base, heterogeneous catalysts and simple methodology. It is
notable that FeCo/Mo10V2-Pd can easily separate from the reaction mixture using an
external magnet and reused for at least four successive runs without any consid-
erable decrease in its catalytic activity.
Keywords Layered double hydroxide Á Mizoroki–Heck reaction Á Heteropoly
acid Á Palladium Á Cobalt-iron
Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-3013-5)
contains supplementary material, which is available to authorized users.
&
Ezzat Rafiee
e.rafiei@razi.ac.ir; Ezzat_rafiee@yahoo.com
1
2
Faculty of Chemistry, Razi University, Kermanshah 67149, Iran
Institute of Nano Science and Nano Technology, Razi University, Kermanshah 67149, Iran
123

E. Rafiee, M. Kahrizi
Introduction
The Mizoroki–Heck coupling reaction plays an important role in carbon–carbon
bond formation [1–3]. The palladium (Pd)-catalyzed Mizoroki–Heck reaction
finds a lot of prominent positions in synthesis due to its exceptional versatility
[4–6]. Nowadays, numerous kinds of Pd-based catalysts have been considered
which have enabled this transformation to be applied with a broad substrate scope,
a wide functional group tolerance and low catalyst loading. This reaction is
generally considered to be a homogeneous reaction most commonly catalyzed by
soluble Pd(II) species with a variety of ligands, such as palladium–phosphine
complexes [7], oxime palladacycles [8], palladium–salen complexes [9], and
palladium–N-heterocyclic carbine complexes [10]. Separation of the expensive
catalyst from the product for reuse is often problematic in these homogeneous
systems. Moreover, aggregation and precipitation of Pd metal in the homogeneous
systems always leads to lost activity of the catalysts. Thus, heterogeneous
catalysts are highly desirable, especially in large-scale synthesis, from both
environmental and economic aspects. Much effort has been paid to developing
heterogeneous catalytic systems that can be efficiently reused [11, 12]. Original
ways have been found in the elaboration of magnetic catalysts that can be easily
recovered using external magnets, and in the use of supported forms of catalytic
systems without practical loss.
Immobilization of Pd nanoparticles on proper solid bases such as layered double
hydroxides (LDHs) can helps in the heterogenization of Pd species and provide
many benefits in the catalytic properties of these catalysts [13–15].
LDH, namely hydrotalcite-like compounds, is a class of clay materials with
positively charged metal-oxide layers, with the layer surface charges being balanced
3?
by interlayer anions [16–21]. The general formula for LDH is [M2?
M (-
x
(1-x)
OH)2][Am- (x/m)].nH2O, where M2? and M3? are divalent and trivalent metal
cations and Am- is a simple or complex anion such as Cl-, NO3-, CO3 or
2-
heteropoly acids (HPAs) [22–25]. The most attractive characteristic of LDHs is their
anionic exchange ability where a wide range of organic or inorganic guests can be
placed in the LDHs structures [26–28]. LDHs can provide ‘‘flexible’’ confining
spaces that can be adjusted by changing the size and arrangement of the guest
molecules. The flexible interlayer space can accommodate HPAs [29–31]. As the
properties of the LDH can be altered based on the metal cations and interlayer
anions, the applications of LDHs include catalysts, biomaterials, polymer additives,
environmental materials (such as water treatment to ‘trap’ anionic pollutants), thin
film applications, fire retardant additives, anticorrosion coatings, and controlled
drug release in medications (with pharmaceutical compounds used as the interlayer
anion) [32–39].
HPAs constitute a great family of inorganic oxo-metal clusters which are often
composed of early transition metals [40–43]. The fascinating properties of HPAs
make them attractive for application as catalysts [44]. Recently, HPAs have been
widely used as nanomaterial catalyst in coupling reactions [45–48].
123

Incorporation of palladium nanoparticles and…
In continuation of our investigations into carbon–carbon coupling reactions
[45–47], Pd immobilized on FeCo LDH intercalated 10-molybdovanado phosphate
(FeCo/Mo10V2-Pd) was developed for the Mizoroki–Heck reaction.
Experimental
Material information
All reagents and solvents were commercially purchased from Fluka, Merck, and
Aldrich. X-ray diffraction (XRD) patterns were recorded on an Inel French,
EQUINOX 3000 model X-ray diffractometer using Cu-K radiation. Transmission
electron microscopy (TEM) analysis was investigated using a TEM microscope
(Philips CM 120 kV; Netherlands). Scanning electron microscopy (SEM) was
studied using an AIS2300C microscope with a scanning range from 0 to 20 keV.
Electron dispersive X-ray spectroscopy (EDX) measurements were made with an
IXRF model 550i attached to the SEM. SEM samples were prepared by coating
solid particles into a conductive layer. The magnetic properties of FeCo/Mo10V2-Pd
were measured using a BHV-55 (Riken, Japan) vibrating sample magnetometer
(VSM). Fourier transform infrared spectroscopy (FT-IR) spectra were recorded with
KBr pellets using a Bruker ALPHA FT-IR spectrometer. The Pd and Mo content
were evaluated using inductively coupled plasma atomic emission spectroscopy
(ICP-AES) on a Spectro Ciros CCD spectrometer. The electrochemical behavior of
the catalyst was measured using a computer-controlled m-Autolab modular
electrochemical system (SAMA500 ElectroAnalyzer system). Thin layer chro-
matography (TLC) was carried out on pre-coated silica gel. Fluorescence of 254 nm
(0.2 mm) on aluminum plates was used for monitoring the reactions. The cross-
1
coupling products were characterized by their H NMR spectra.
Preparation of FeCo LDH
FeCo LDH was prepared as follows: an aqueous alkaline solution (100 ml, 0.32 g
Na2CO3, 0.48 g NaOH) was added to a mixed Fe3? and Co2? metal nitrate aqueous
solution (Co2?/Fe3? = 3.0 mol mol-1) at 70 °C under vigorous stirring until the
solution pH was adjusted to 10.0. The obtained slurry was then aged at 90 °C for
12 h. The resulting was cooled to room temperature and separated using external
magnet, then frequently washed with distilled water and dried under vacuum.
Preparation of FeCo-Pd
FeCo-Pd was prepared by a deposition–precipitation procedure as follows: 0.5 g
FeCo LDH was added to 255 ml PdCl2 aqueous solution (0.008 M) and its pH was
adjusted to 10.0 by the addition of 0.1 M NaOH and then positioned in a 80 °C
water bath for 24 h. The resulting black slurry was recovered by a magnet, followed
by washing with deionized water until no Cl- remained, and then dried at 60 °C for
24 h to produce FeCo-Pd.
123

E. Rafiee, M. Kahrizi
Preparation of FeCo/Mo10V2-Pd
Anion exchange reaction with the [PMo10V2O40]5- (Mo10V2) anion was begun with
the preparation of 50 ml of Mo10V2 (0.3 g) under N2 atmosphere. Then, 1.00 g of
FeCo-Pd was added to the previous solution. The mixture was kept under N2
atmosphere and refluxed in the temperature range of 90–95 °C for 10 h. The
resulting solid was separated by an external magnet and washed with boiling water
three times, and then dried in vacuum.
General procedure for the Heck coupling reaction
For the Heck coupling reaction, a mixture of aryl halide (4 mmol), styrene
(4 mmol), tetrabutylammonium bromide, TBAB (4 mmol), FeCo/Mo10V2-Pd
(0.02 g) and 5 mL dimethyl formamide (DMF) was charged to a reaction tube.
The resulting mixture was refluxed at 120 °C under a dry N2 atmosphere for an
appropriate time. The progress of the reaction was monitored by TLC. Upon
completion of the reaction, the mixture was then cooled to room temperature and
FeCo/Mo10V2-Pd was separated by a magnet, washed with diethyl ether
(2 9 10 mL), and then dried under vacuum for reuse. The residual mixture was
extracted by CH2Cl2 (3 9 20 mL), and the combined organic layers were dried over
MgSO4. The solvent was evaporated and the crude products were characterized by
1H NMR spectroscopy [49].
Results and discussion
FeCo/Mo10V2-Pd was synthesized for the first time and used as a nanocatalyst for
the Mizoroki–Heck reaction under base- and ligand-free conditions.
Scheme 1 shows the synthetic route for producing of the designed heterogeneous
catalyst. The FeCo/Mo10V2-Pd was obtained by using a method that involves the
separate synthesis of FeCo LDHs, immobilization of Pd nanoparticles on FeCo
LDHs and intercalation of Mo10V2 in the FeCo-Pd structure.
In order to investigate the morphology of FeCo/Mo10V2-Pd, the catalyst samples
have been studied by SEM (Fig. S1). It can be observed clearly from the SEM
images that the fresh FeCo/Mo10V2-Pd presents well- developed, thin, flat crystals
Scheme 1 Synthetic route of FeCo/Mo10V2-Pd
123

Incorporation of palladium nanoparticles and…
with obvious edges indicating the layered structure (Fig. S1a, c). The SEM image of
the catalyst shows the immobilization of Pd nanoparticles on layers of the catalyst.
The mapping data of the catalyst provide a clear observation of the existence of the
Pd nanoparticles, FeCo LDH and Mo10V2 in the structure of the char residue of
FeCo/Mo10V2-Pd. Elementary analysis of the surface was studied by using EDX
(Fig. S1i). This pattern of the catalyst shows major peaks of Co, Fe, Mo, V and Pd.
The results obtained from the EDX analysis show the presence of FeCo LDH,
Mo10V2 and Pd nanoparticles on the surface of the catalyst.
TEM images of FeCo/Mo10V2-Pd are shown in Fig. 1. Both images reveal the
presence of small Pd nanoparticles deposited on the larger LDH flakes. The
supported nanoparticles, covering the lower nanometer size range up to ca 25 nm,
are highly dispersed.
Figure 2 shows the XRD patterns of FeCo LDH, Mo10V2, FeCo/Mo10V2-Pd and
the recovered FeCo/Mo10V2-Pd powders collected after fabrication. In the XRD
pattern of FeCo LDH, strong diffraction peaks at 2h of 11.06, 21.4°, 34.1°, 38.2°,
44.9°, 58.02° and 59.2° can be indexed to the (003), (006), (009), (012), (015), (110)
and (113) facets of the LDHs, respectively, which are characteristic of a lamellar
material (curve a) [50]. In order to show the presence of Mo10V2 in the catalyst, the
XRD pattern of the Mo10V2 (curve b) is presented. After intercalation of Mo10V2
into the LDH layer, the diffraction pattern of the resulting material (curve c) shows a
reflection characteristic of the LDH layer and Mo10V2 reflections. Curve c, after the
Pd immobilization processes, shows the presence of Pd nanoparticles on the surface
of the LDH. Pd(0) crystals in the catalyst have a crystalline structure with a peak
appearing at 68.04° corresponding to the (220) plane of the Pd nanoparticles
{JCPDS-05-0681}.
The presence of Mo10V2 in the LDH interlayers was identified by FT-IR. In
natural LDHs before anion exchange processes, these interlayer anions are
carbonate and water molecules [51]. In the FT-IR spectrum of FeCo LDH in
addition to characteristic peaks of carbonate at about 1351 cm-1, the absorption
bands at 483 and 755 cm-1 which are attributed to the M–O vibration modes of the
Fig. 1 TEM images of FeCo/Mo10V2-Pd
123

E. Rafiee, M. Kahrizi
Fig. 2 XRD patterns of a FeCo LDH, b Mo10V2, c FeCo/Mo10V2-Pd, d recovered FeCo/Mo10V2-Pd
LDH structure were observed (Fig. S2a). Moreover, the band at 1630 cm-1 is
attributed to the -OH stretching vibration related to the surface hydroxyl and H2O.
The FT-IR spectra of FeCo/Mo10V2-Pd, in addition to the FeCo LDH peaks at 492
and 768 cm-1, display the features of a Keggin-type structure at 1057, 1006, 984
and 891 cm-1 which is in agreement with the results reported previously (curve
Fig. S2b) [45]. Peaks at about 1351 cm-1 due to the carbonate band disappeared in
the FeCo/Mo10V2-Pd sample, suggesting that the anion exchange was complete.
In order to evaluate the magnetic property of FeCo/Mo10V2-Pd, the VSM
measurement was carried out at 298 K. Magnetization was saturated up to 30 emu/g
at an applied field of 8500 Oe and reveals the superparamagnetic behaviors at room
temperature (Fig. S3). The high magnetic property of FeCo/Mo10V2-Pd facilitates
the recovery of the catalyst from the reaction media.
The electrocatalytic properties of FeCo LDH and FeCo/Mo10V2-Pd were studied
in 0.1 M pH 4.0 phosphate buffers (PBS) by cyclic voltammetry (CV). Comparison
of cyclic curves of FeCo LDH, FeCo-Pd and FeCo/Mo10V2-Pd (Fig. S4) shows the
presence of Mo10V2. Enhancing the oxidation peak current of FeCo LDH decreased
its oxidation peak potential from 0.41 and 0.34 in FeCo LDH and FeCo-Pd to
0.09 V in FeCo/Mo10V2-Pd, suggesting that the oxidation strength of FeCo LDH
was facilitated by the presence of Mo10V2 after the anion exchange process. Also, as
depicted in Fig. S4, the placement of Pd nano-particles in the LDH structure
decreases the oxidation peak potential and increases the oxidation peak current of
the nano-particles. These results show that collaboration of Pd nano-particles and
LDH layers increases the oxidation strength of the nano-particles.
123

Incorporation of palladium nanoparticles and…
Figure S5 shows the linear sweep voltammetry (LSV) curves of FeCo LDH,
FeCo-Pd and FeCo/Mo10V2-Pd. The onset potentials were evaluated to be 0.392,
0.298 and 0.088 V for FeCo LDH, FeCo-Pd and FeCo/Mo10V2-Pd, respectively. The
comparison of these LSV curves clearly reveals that the FeCo/Mo10V2-Pd shows a
lower onset potential and higher negative limiting current density of -0.35 mA cm-2
.
On the basis of the obtained results from the electrocatalytic experiments, we
speculate that Mo10V2, by facilitating the electron transfer by Pd species, intensively
helps to enhance the electrocatalytic activity of FeCo/Mo10V2-Pd in the Heck
reaction.
In order to probe the catalytic activity of FeCo/Mo10V2-Pd and find the best
reaction conditions, the reaction of bromobenzene (4 mmol) and styrene (4 mmol)
at 120 °C under ligand- and base-free conditions was chosen as a model reaction
(Scheme 2).
To establish the best reaction conditions, the amount of FeCo/Mo10V2-Pd was
tested, and it seems that 0.02 g of FeCo/Mo10V2-Pd was sufficient to drive the
reaction in 90% yield. We found that using a smaller amount of FeCo/Mo10V2-Pd
led to la ower yield even with extended reaction time, and increasing in the catalyst
quantity showed no significant effect on the productivity of the reaction. Therefore,
0.02 g of the catalyst was chosen as the best amount of FeCo/Mo10V2-Pd for further
reactions (Table 1, entries 1–3). Next, the effects of various solvents, such as H2O,
dimethyl acetamid (DMAc), DMF and mixture of DMF and H2O, have been studied
(entries 1, 4–6). DMF at 120 °C is the best chosen in terms of yield and reaction
time (entry 1).The amount of Pd loading on the surface of FeCo/Mo10V2-Pd as a
main catalyst was 6.4 wt%, which was determined using ICP-AES measurements.
According to the obtained results from the ICP-AES measurements, the content of
Mo was 22.51 wt%, which indicated that the amount of Mo10V2 in FeCo/Mo10V2-
Pd structure was 40.76 wt%.
To demonstrate the efficiency and generality of FeCo/Mo10V2-Pd and this
process, reaction of a series of aryl halides with styrene was performed under the
optimized experimental conditions (Table 2). Each aryl halide containing electron-
withdrawing or electron-releasing groups reacts efficiently with styrene for the
eneration of the corresponding products. Substituent effects on the aromatic ring of
the aryl halides had a significant influence on the yield of the reaction. The aryl
halides containing electron-withdrawing groups gave corresponding products in
higher yields (entries 2 and 5), but aryl halides bearing electron-releasing groups
gave the desired products in lower yields (entries 3, 4 and 6). To our surprise,
condensation of chlorobenzene with styrene afforded the respective product in a
moderate yield (entry 9). It should be pointed out that, when iodobenzene was
Br
FeCo/Mo10V2-Pd
+
Solvent, TBAB, 120 oC
Scheme 2 The reaction of styrene and bromobenzene as a model reaction
123

E. Rafiee, M. Kahrizi
Table 1 Optimization of Mizoroki–Heck coupling reaction conditions
Entry
Amount of the catalyst
(mol% of Pd)
Solvent
Time (h)
Yield (%)a
1
2
3
4
5
6
0.02 g (12 9 10-4
0.01 g (6 9 10-4
0.03 g (18 9 10-4
0.02 g (12 9 10-4
0.02 g (12 9 10-4
0.02 g (12 9 10-4
)
DMF
8
12
8
89
75
94
41
73
80
)
DMF
)
)
)
)
DMF
H2O
24
12
12
DMAc
DMF/H2O (5:1)
Reaction conditions: styrene (4 mmol), bromobenzene (4 mmol), TBAB (4 mmol) under N2 atmosphere
at 120 °C
a
Isolated yield
Table 2 Mizoroki-Heck
reaction of various aryl halides
with styrene
Entry
1
Aryl halide
Time (h)
8
Yield (%)a
89
2
3
8
93
71
12
4
5
6
7
12
8
69
96
62
79
12
12
8
9
12
24
73
61
Reaction conditions: Styrene
(4 mmol), aryl halide (4 mmol),
TBAB (4 mmol) and FeCo/
Mo10V2-Pd (0.02 g) at 120 °C
10
6
8
94
92
11b
a
Isolated yield
b
Butylacrylate was used as
olefin species
123

Incorporation of palladium nanoparticles and…
selected as aryl halide, the reaction time was reduced and maximum yield of the
desired product was reached (entry 10). The observed higher yield and lower
reaction time for the reaction of iodobenzene is due to the strength of the C–I bond
which causes the rate of oxidative addition C–I to Pd(0) species to be fast. Also,
butylacrylate reacted with bromobenzene efficiently to provide the desired products
in maximum yield (entry 11).
The reaction data, along with some literature data for comparison, are given in
Table 3. These catalysts showed good reactivity. However, the use of base, ligand
and non-recyclable catalysts in these reported systems is not beneficial to industrial
and synthetic applications.
For practical application in the Mizoroki–Heck reaction, the lifetime of the
heterogeneous catalysts and their reusability are very important factors. After the
completion of the reaction, the catalyst was recovered by an external magnet,
washed with diethyl ether and water, and reused in a model reaction (Fig. S6). The
catalyst revealed a remarkable activity and was reused up to at least four
consecutive cycles without any significant decrease in catalytic activity. After the
fourth reaction run, the yield of the product decreased from 89 to 77%. According to
the obtained results from the XRD and FT-IR techniques, after the completion of the
reaction and recovering process, the structure of the FeCo/Mo10V2-Pd was observed
to be unchanged (Figs. 2, S1b, S2c)). It is notable that, according to the obtained
results from the ICP-AES measurements, after the fourth reaction run, the Pd
content in the FeCo/Mo10V2-Pd structure was evaluated with no significant
difference with fresh catalyst.
Table 3 Comparison of the reaction data with other reported methods
Entry Catalyst
Solvent
Temperature
(°C)
Time
(h)
Yield
(%)
References
1
2
3
Pd-Fe3O4
TiO2@Pd
DMF
DMF
H2O
110
140
100
24
24
15
76
91
71
[52]
[53]
[54]
Benzimidazolium salt/
Pd(OAc)2
4
Pd(II)/[BMIM][TPPMS]
Pd(dba)2/P(OPh)3
Pd(0) MCM-41
[BMIM][OAc] 140
3
20
24
12
40
20
8
40
43
90
71
94
90
89
[55]
5
DMF
100
[56]
6
DMF
100
[57]
7
Pd/(SiO2/Fe3O4)
NHC-Pd/SBA-16-IL
Pd/SiO2
CH3CN/H2O
NMP
80–90
140
[58]
8
[59]
9
DMA
135
[60]
10
FeCo/Mo10V2-Pd
DMF
120
This work
123

E. Rafiee, M. Kahrizi
Scheme 3 Proposed mechanism for Heck coupling reaction in the presence of FeCo/Mo10V2-Pd
A possible mechanism for the Mizoroki–Heck reaction in the presence of FeCo/
Mo10V2-Pd is presented in Scheme 3. In the first step, the C–X bond of an aryl
halide is oxidatively added to the Pd atom. The electron transfer ability of Mo10V2
facilitates the oxidation of Pd(0) to Pd(II). Pd then forms a p complex with the
alkene and, next, the alkene inserts itself into the Pd–carbon bond (Pd–Ar). In the
next step, b-hydride elimination causes the formation of a new Pd-alkene p
complex. Finally, this complex is destroyed and the desired product is released. The
Pd(0) compound is regenerated by reductive elimination of the Pd(II) compound
using Mo10V2, which could facilitate the reduction of Pd(II) to Pd(0) and eliminate
HX by electron transfer and proton donation, respectively.
Conclusion
Here, we have reported the first example of Pd deposited on a FeCo LDH structure
with Mo10V2 anions as the interlayer for using as a catalyst for the Mizoroki–Heck
reaction. Characterization of FeCo/Mo10V2-Pd was performed by XRD, FT-IR,
123

Incorporation of palladium nanoparticles and…
VSM, ICP-AES, SEM and TEM techniques. The CV and LSV investigations show
that the presence of Mo10V2 is essential because it can probably activate precursors
of the reaction and may transfer the electrons with Pd species which facilitates the
oxidation of Pd(0) in the oxidative addition and regeneration of Pd(II) species in the
reductive elimination steps. FeCo/Mo10V2-Pd can easily be recovered using an
external magnet. The study of the catalytic properties of the sample exhibits high
activity for the Mizoroki–Heck reaction.
Acknowledgements The authors thank the Razi University Research Council and Iran National Science
Foundation (INSF) for support of this work.
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