Palladium supported on zinc ferrite: A highly active, magnetically separable catalyst for ligand free Suzuki and Heck coupling

Article abstract of DOI:10.1016/j.catcom.2013.02.003

An efficient superparamagnetic solid catalyst has been synthesized by loading Pd(0) species during synthesis of zinc ferrite nanoparticles by ultrasound assisted co-precipitation in the absence of surface stabilizer or capping agent. Various techniques were employed to characterize the as-synthesized Pd-ZnFe₂O₄ magnetic nanoparticles. The catalyst showed excellent performance for Suzuki and Heck coupling reactions under ligand free condition. The catalyst being super paramagnetic was separated by using an external magnet and reused for five cycles without any appreciable loss in the activity.

Full text of DOI:10.1016/j.catcom.2013.02.003

Catalysis Communications 35 (2013) 11–16
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Palladium supported on zinc ferrite: A highly active, magnetically
separable catalyst for ligand free Suzuki and Heck coupling
⁎
Abhilash S. Singh, Umakant B. Patil, Jayashree M. Nagarkar
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai, 400019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient superparamagnetic solid catalyst has been synthesized by loading Pd(0) species during synthesis of
zinc ferrite nanoparticles by ultrasound assisted co-precipitation in the absence of surface stabilizer or capping
agent. Various techniques were employed to characterize the as-synthesized Pd–ZnFe2O4 magnetic nanoparticles.
The catalyst showed excellent performance for Suzuki and Heck coupling reactions under ligand free condition.
The catalyst being super paramagnetic was separated by using an external magnet and reused for five cycles with-
out any appreciable loss in the activity.
Received 24 November 2012
Received in revised form 5 February 2013
Accepted 7 February 2013
Available online 15 February 2013
Keywords:
Magnetic nanoparticle
Pd–zinc ferrite
© 2013 Elsevier B.V. All rights reserved.
Heterogeneous catalyst
Co-precipitation
C\C coupling reaction
1. Introduction
most of the processes require tedious procedure to anchor the active pal-
ladium species on the magnetic support surfaces. Herein we report suc-
In recent years, for environmentally benign chemical methodologies
magnetic nano materials have emerged as alternatives to usual hetero-
geneous supports [1,2]. Metal nanoparticles dispersed on the surface of
an oxide works as an efficient catalyst for a wide variety of reactions as
compared to bulk particles [3,4]. Palladium catalyzed Suzuki and Heck
coupling reactions are widely used methods for the formation of C\C
bonds [5,6]. Generally these coupling reactions were studied by using
homogeneous catalyst of palladium complex with various ligands
such as phosphines [7], dibenzylideneacetone (dba) [8] and carbenes
[9]. The palladium compounds are also used in homogeneous catalysis
under ligand free condition [10,11]. However the high cost of palladium
complexes and difficulties in product separation makes the process
tedious [12]. A variety of materials such as carbon [13], clay [14],
metal oxide [15], silica [16], zeolite [17] and polymer resin [18] were
also developed as support for nanoparticles.
cessful loading of Pd(0) species on super paramagnetic zinc ferrite
without using organic capping agent or surface stabilizer under ultrasonic
irradiation. The catalytic activity and stability of the palladium nano
catalyst was tested by selecting Suzuki and Heck coupling as the model
reactions. The catalyst was found to be highly efficient, recyclable, and en-
vironmentally friendly.
2. Experimental section
2.1. Synthesis of palladium nanoparticles and its surface adsorption on
ZnFe2O4 magnetic nanoparticles
Pd NPs were synthesized by using poly(ethyleneglycol) (PEG) and
Pd(OAc)2 as precursors [28] (see supplementary data).
Magnetic nanoparticles (MNPs) have been effectively employed as
catalyst support for various organic transformations [19]. They serve as
support for the metal and as an alternative to filtration thereby preventing
loss of catalyst [20,21]. The thermal stability and the catalytic activity of
the palladium catalyst were profoundly increased by supporting it on
magnetic nanoparticles [22,23]. Magnetically separable Pd-based cata-
lysts have been well reported for coupling reactions [24–27]. Palladium
loaded on magnetic support is most essential heterogeneous catalyst as
they offered high surface area and easy catalyst separation. However,
2.2. Material characterization
The prepared catalyst was characterized by various techniques such
as SEM, TEM, XRD, FTIR, BET, DSC and TGA. Powder X-ray diffraction
(PXRD) pattern was recorded on a PANalytical MPD diffractometer
using Cu Kα radiation (λ=1.5406 A°) at 40 kV and 40 mA. Transmis-
sion electron microscopy (200 kV TEM, JEOL JEM2100) was used to ex-
amine morphology and size of the nano particles. Scanning electron
micrographs (SEM) and Energy dispersive X-ray Analysis (EDAX) anal-
ysis were recorded on a single instrument FEI Quanta 200 ESEM FEG.
The IR spectrum was recorded using FT-IR spectrophotometer (Perkin
Elmer Spectrum 100) as KBr pellets. The UV–vis analysis was performed
using a (Uv-1650 Pc Shimadzu). The BET analysis was performed by
⁎
Corresponding author. Tel.: +91 22 33611111/2222; fax: +91 22 33611020.
E-mail addresses: jm.nagarkar@ictmumbai.edu.in, jayashreenagarkar@yahoo.co.in
(J.M. Nagarkar).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.02.003

12
A.S. Singh et al. / Catalysis Communications 35 (2013) 11–16
(4.62 mol%). The reaction was carried under reflux condition for
3–9 h and monitored by Gas Chromatography (GC). The reaction mix-
ture was cooled to room temperature after completion and the catalyst
(MNPs) was separated using an external magnet. The catalyst was then
washed with ethanol, dried at 100 °C for 3 h and preserved for next run.
Similarly for Heck cross-coupling reactions 0.5 mmol aryl halide in
5 mL DMF, (1.0 mmol) methyl acrylate or styrene (1.5 mmol), Et3N
(1.5 mmol), and 4.62 mol% catalyst (0.12 mol% of Pd) were added to-
gether. The reaction mass was heated at 120 °C for specific time. The
separation and regeneration of catalyst were similar to Suzuki reaction.
3. Results and discussion
3.1. Characterization of the Pd incorporated ZnFe2O4 MNPs
The formation of Pd(0) species was observed from UV–vis spectro-
photometric measurement (300–600 nm) as shown in Fig. 1. The total
disappearance of peak at 400 nm in the UV–vis spectrum indicates
the complete conversion of Pd (II) to Pd(0) species.
The ZnFe2O4 MNPs were synthesized in high yield by using
sonochemical co-precipitation technique in aqueous medium without
additional surfactant or organic capping agent. The elemental compo-
sition of the dark brown particles was confirmed using EDAX analysis
as depicted in Fig. 2. The distribution of the elements in the Pd incor-
porated ZnFe2O4 was Zn=11.52%, Fe=23.62%, O=64.05% and Pd=
0.81%. The ratio of iron/zinc in ZnFe2O4 was found to be 2.05 which
are in very good agreement with the theoretical value of 2.0.
Fig. 3 depicts the powder X-ray diffraction pattern of the Pd- ZnFe2O4
MNPs catalyst and represents similarity with the characteristic reflec-
tions of cubic phase thereby confirming the structural purity of the sam-
ple. Since the concentration of palladium is very low, its characteristic
XRD peaks (200), (220) and (111) cannot be seen in the XRD pattern
of the catalyst. However broadened (111) diffraction peaks can be still
identified in PXRD pattern. The crystallite size for the nanoparticles
was based on X-ray diffraction line broadening and calculated by using
Scherrer equation [29]. It was found to be 10 5 nm which was in
good agreement with the particle size obtained from TEM micrographs.
Furthermore, the morphology of the Pd–ZnFe2O4 MNPs was inves-
tigated using SEM and TEM and the images are shown in Figs. 4 and 5,
respectively. The TEM image of the catalyst was also obtained after
the fifth cycles. There was no significant change in the morphology
Fig. 1. UV–vis spectra of (a) Pd(OAc)2 in PEG-600 (b) Pd(0) species in PEG-600.
nitrogen adsorption–desorption method at liquid N2 temperature
(Micromeritics-ASAP 2020). Differential scanning calorimetric and
thermogravimetric (DSC/TG) analysis were carried by using Mettler
Toledo instrument under N2 atmosphere. The yield of products has
been determined by GC (PerkinElmer Clarus 400). Purity of all com-
pounds was determined with the help of GC–MS analysis (Shimadzu
QP-2010). Magnetic measurements of the powder samples were deter-
mined by using vibrating-sample magnetometer (PPMS Quantum
Design Inc.).
2.3. General procedure for Suzuki and Heck reaction
The determined parameters for Suzuki cross-coupling reaction
consists of 0.5 mmol aryl halide in 5 mL of pure ethanol, 0.75 mmol
phenyl boronic acid, 1.5 mmol K2CO3 and 8 mg Pd–ZnFe2O4 catalyst
Fig. 2. EDX analysis of Pd–ZnFe2O4 MNPs.

A.S. Singh et al. / Catalysis Communications 35 (2013) 11–16
13
Fig. 3. PXRD pattern of Pd–ZnFe2O4 MNPs (* contribution from Pd NPs).
and dispersion of the particles. TEM images of the catalyst at different
3.2. Catalytic activity of the as-synthesized nano particles
magnifications were also recorded (Supporting Information Fig. 1).
From the N2 adsorption–desorption isotherm of Pd–ZnFe2O4
MNPs, the BET surface area of the particles was found to be 76 m2/g
(Supporting Information Fig. 5). The adsorption average pore width
is 241A°. The single point surface area at P/Po=0.2551 is 75 m2/g.
The surface of the catalyst was further examined with the help of
FT-IR spectra (Supporting Information Fig. 2). The IR spectrum shows
two absorption peaks at 3420 cm−1 and 1620 cm−1 due to the surface
adsorbed water with other characteristic peak of cubic ferrite. The mois-
ture content of the catalyst was determined by Karlfischer Titrator and
it was found to be 0.01%w/w. The magnetic properties of the as pre-
pared Pd–ZnFe2O4 MNPs were investigated by PPMS. The room temper-
ature magnetization curve of the catalyst represents very low coercive
force with no apparent hysteresis curve signifying a superparamagnetic
quality of the catalyst (Supporting Information Fig. 6). The magnetic
oxide nanoparticles with a size of 30–40 nm or less than that are
single-domain and illustrate superparamagnetic behavior [30].
The catalytic activity of the as-synthesized Pd–ZnFe2O4 MNPs was
evaluated for both Suzuki (Scheme 1) and Heck (Scheme 2) reactions.
Fig. 5. TEM images of the Pd–ZnFe2O4 (a) before reaction (b) after five cycles of Suzuki
Fig. 4. SEM image of Pd–ZnFe2O4 MNPs.
reactions.

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A.S. Singh et al. / Catalysis Communications 35 (2013) 11–16
X
Β(ΟΗ)2
Pd-ZnFe2O4, K2CO3
R
+
R1
Ethanol, Reflux
R
R1
R=CH3, OCH3, NO2, COCH3, OH, H
R1=CH3, OCH3, Cl, H
X=I, Br
Scheme 1. Suzuki cross coupling reaction of aryl halides with phenylboronic acid.
tested by using various electron-rich and electron-deficient substrates
(Table 1). No homocoupled product is formed when the yield is less.
The catalyst was separated by using external magnet. The reaction mix-
ture was then separated by decantation. Thereafter, the flask was
washed with ethanol and combined organic layer was dried over anhy-
drous sodium sulfate and evaporated in a rotary evaporator under re-
duced pressure followed by column chromatography to obtain the
pure product.
I
R1
Pd-ZnFe2O4, Et3N
DMF, 120 oc
R
R1
+
R
R=CH3, OCH3, CN, NH2, COCH3, OH, H
R1= CO2Bu, CO2Et, , Ph,
Scheme 2. Heck reaction of aryl halides and alkenes.
The application of Pd–ZnFe2O4 MNPs was also investigated for
Heck reaction. The reaction was optimized for the best conditions
(supporting information).
The scope of the catalyst for Heck reaction was studied for wide
variety of aryl halides with various alkenes (Scheme 2, Table 2). The cat-
alyst failed to catalyze the reaction of aryl chlorides. The catalyst separa-
tion and recyclability procedures are similar to Suzuki reactions. The
recyclability of the catalyst was estimated using a Suzuki reaction of
4-methoxyiodobenzene and phenylboronic acid. Heck reaction was car-
ried out between iodobenzene and ethylacrylate. The activity of Pd–
The reactions were carried out with different solvents, bases and at dif-
ferent temperatures to obtain the optimized reaction conditions
(supporting information). No product is formed when zinc ferrite is
used in the absence of palladium. The TON and TOF for Suzuki reaction
i.e. coupling of iodobenzene and phenylboronic acid was found to be
2168 and 542 h−1. Similarly the TON and TOF for Heck reaction
(coupling of iodobenzene and ethylacrylate) was 2168 and 1084 h−1
.
Suzuki reaction was studied for wide variety of aryl halides and bo-
ronic acids under optimized conditions. The scope of the catalyst was
Table 1
Suzuki reaction of various aryl halides with phenylboronic acid catalyzed by Pd–ZnFe2O4 MNP.a
Entry
1
Aryl halide
Boronic acid
Product(s)
Time (h)
4 h
Yield (%)b
92
2
3
9 h
9 h
85
89
4
9 h
91
5
6
7
8 h
90
91
90
3.5 h
3 h
8
9
3.5 h
3.5 h
94
91
10
4 h
95
11
12
3.5 h
4 h
91
93
13
14
3 h
4 h
92
92
15
3.5 h
92
a
Reaction condition: 0.5 mmol of aryl halide, 0.75 mmol of boronic acid, 1.5 mmol of K2CO3, 4.62 mol% of Pd–ZnFe2O4 MNPs and 5 mL ethanol, reflux.
Isolated yield by column chromatography.
b

A.S. Singh et al. / Catalysis Communications 35 (2013) 11–16
15
Table 2
Heck reaction of various aryl halides with alkenes catalyzed by Pd–ZnFe2O4 MNP.a
Entry
1
Aryl halide
Alkenes
Product(s)
Time (h)
2 h
Yield (%)b
96
2
6 h
88
3
4
6 h
5 h
90
92
5
6
2 h
3 h
94
90
7
8
2 h
4 h
96
89
9
6 h
12 h
12 h
12 h
6 h
94
65
80
90
95
90
82
80
85
10
11
12
13
14
15
16
17
6 h
12 h
14 h
10 h
a
Reaction condition: Iodobenzene (0.5 mmol), alkenes (1 mmol), Et3N (1.5 mmol), DMF (5 mL), 120 °C and 4.62 mol% of Pd–ZnFe2O4 MNPs.
Isolated yield based on column chromatography.
b
Table 3
ZnFe2O4 MNP catalyst is shown in Table 3. The amounts of Pd leaching
into solution for the Suzuki and Heck coupling reactions were deter-
mined through ICP-AES.
Activity of Pd–ZnFe2O4 MNP for Suzuki and Heck reactions.
c
Recycle runs
Conversion (%)
TOF (h−1
)
Suzuki reactiona
Heck reactionb
1st
100
100
99
99
98
100
100
99
98
97
542
542
528
528
516
1084
1084
1073
1062
1051
In Suzuki reaction, no deactivation of catalyst was found till third
cycle and after fifth cycle the Pd loss was found to be 0.5 wt.% of
total Pd content. The average Pd loss for the Heck reaction was
4.2 wt.% of total Pd content. If the Suzuki reaction is performed in a
condition similar to Heck reaction the leaching is almost similar,
due to high reaction temperature and solvent, but the product yield
is only 20%. The hot filtration test after 1 h gave 0% conversion for
Suzuki and 2% conversion for Heck after continuing the reaction for
24 h, indicating negligible palladium leaching and heterogeneity of
the catalyst. The ICP-MS elemental analysis of the colored solution
shows the palladium concentration of 17 ppb for Suzuki reaction
and 25 ppb for Heck reaction. This loss in Pd content explains the de-
crease in yield with increase in the number of recycle runs.
2nd
3rd
4th
5th
1st
2nd
3rd
4th
5th
a
Reaction conditions: 4-IPhOCH3, PhB(OH)2, K2CO3, Ethanol, catalyst 8 mg of Pd–
ZnFe2O4 and temp=reflux and time=3 h.
b
Reaction conditions: PhI, ethylacrylate, Et3N, DMF, catalyst 8 mg of Pd–ZnFe2O4,
Temp=120 °C and time=2 h.
c
TOF=mol product/mol catalyst (×h−1).

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Financial support from Green Tech. U.P.E. is gratefully acknowledged.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2013.02.003.
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