In situ stabilization of Pd0-nanoparticles into the nanopores of modified Montmorillonite: Efficient heterogeneous catalysts for Heck and Sonogashira coupling reactions

Article abstract of DOI:10.1016/j.molcata.2012.09.024

In situ generation of Pd⁰-nanoparticles into the nanopores of modified Montmorillonite and their catalytic performance in carboncarbon bond formation namely Heck and Sonogashira reactions, have been carried out. The modification of Montmorillonite was carried out by activating with H ₂SO₄ under controlled conditions for generating nanopores on the surface, which act as a 'Host' for Pd⁰-nanoparticles and was prepared by loading of K₂PdCl₄ metal precursor through incipient wetness impregnation technique followed by reduction with hydrazine hydrate. Powder-XRD, SEM-EDX, TEM, N₂ adsorption, XPS, etc. analyses were carried out to characterize the stabilized nanoparticles as well as the supports. TEM study reveals that Pd⁰-nanoparticles of size below 10 nm were evenly distributed on the support and exhibit face centered cubic (fcc) lattice. The supported metal nanoparticles serve as efficient heterogeneous catalyst for the Heck coupling reaction in which the vinylation of aryl halides with olefins result cross-coupling products with maximum 96% isolated yield and >99% trans selectivity while in the alkynylation of aryl halides with terminal alkynes, i.e. in Sonogashira coupling reaction, a maximum of 94% isolated yield with 100% selectively cross-coupling products were observed. The nanocatalysts could be recycled and reused several times without significant loss of their catalytic activities.

Full text of DOI:10.1016/j.molcata.2012.09.024

Journal of Molecular Catalysis A: Chemical 366 (2013) 202–209
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
In situ stabilization of Pd0-nanoparticles into the nanopores of modified
Montmorillonite: Efficient heterogeneous catalysts for Heck and Sonogashira
coupling reactions
Bibek Jyoti Borah, Dipak Kumar Dutta∗
Materials Science Division, CSIR-North East Institute of Science and Technology, Jorhat 785 006, Assam, India
a r t i c l e i n f o
a b s t r a c t
In situ generation of Pd0-nanoparticles into the nanopores of modified Montmorillonite and their cat-
alytic performance in carbon carbon bond formation namely Heck and Sonogashira reactions, have been
carried out. The modification of Montmorillonite was carried out by activating with H2SO4 under con-
trolled conditions for generating nanopores on the surface, which act as a ‘Host’ for Pd0-nanoparticles and
was prepared by loading of K2PdCl4 metal precursor through incipient wetness impregnation technique
followed by reduction with hydrazine hydrate. Powder-XRD, SEM-EDX, TEM, N2 adsorption, XPS, etc.
analyses were carried out to characterize the stabilized nanoparticles as well as the supports. TEM study
reveals that Pd0-nanoparticles of size below 10 nm were evenly distributed on the support and exhibit
face centered cubic (fcc) lattice. The supported metal nanoparticles serve as efficient heterogeneous
catalyst for the Heck coupling reaction in which the vinylation of aryl halides with olefins result cross-
coupling products with maximum 96% isolated yield and >99% trans selectivity while in the alkynylation
of aryl halides with terminal alkynes, i.e. in Sonogashira coupling reaction, a maximum of 94% isolated
yield with 100% selectively cross-coupling products were observed. The nanocatalysts could be recycled
and reused several times without significant loss of their catalytic activities.
Article history:
Received 16 July 2012
Received in revised form
20 September 2012
Accepted 23 September 2012
Available online 29 September 2012
Keywords:
Modified Montmorillonite
Pd0-nanoparticles
C
C coupling
Heck reaction
Sonogashira reaction
Face centered cubic
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
controllable size and large pore volume [28,29]. In addition, due
to stringent and growing environmental regulations, there has also
Metal nanoparticles exhibit a wide range of applications in
recent time intrigued by distinguishable differences compared
to their corresponding bulk metals [1–6]. The development of
novel metal and metal oxide nanomaterials as catalyst precur-
sors for organic synthesis is a challenge for researchers [7–16].
The nano-catalyst contains highly reactive surfaces and their cat-
alyzed reactions are likely to provide the advantages of high
atom efficiency, simplified isolation of products and easy recovery
and recyclability of the catalysts [1–21]. The overall performance
of nanomaterials is dependent on the size, shape and textural
parameters of the particles as well as the supporting materials
on which these nanoparticles are stabilized [7–16]. Different types
of materials have been employed to support nanoparticles includ-
ing mesoporous materials, activated carbon, polymers, dendrimers,
organic ligands, metal oxides [14,22–27]. Among the materials
used, the fabrication of the nanoparticles on the surfaces of porous
supports with high surface area, in particular mesoporous matrixes
is quite attractive as these systems offer well-ordered pores with
been increasing interest to employ ‘green’ procedure for synthe-
sis of nanoparticles. Recently, our group have reported [8,9] very
efficient Cu0- and Ru0-nanocatalysts stabilized on nanopores of
modified Montmorillonite clay for synthesizing important organic
molecules by following adequate ‘greener’ procedure.
Palladium catalyzed cross-coupling reactions is a versatile tool
for the generation of carbon carbon bonds [30–33]. Therefore,
tremendous attention has been given recently to develop new
and advantageous catalytic system for Pd catalyzed cross-coupling
reactions [34–40]. Among the different coupling reactions, Heck
reaction involving the coupling between aryl halides and olefinic
compounds is an excellent method for the synthesis of organic
compounds [41–48]. There are many chemical and pharmaceutical
intermediates with alkene structural units, which are synthe-
sized using Heck reaction in the presence of Pd catalysts [41–48].
Alkynylation of aryl halides, frequently called Sonogashira reac-
tion is another synthetically useful coupling reaction catalyzed
by Pd-catalyst [41–48]. The carbon carbon triple bonds contain-
ing compounds, especially aryl alkynes and conjugated enzymes
are frequently used in materials science and medicinal chemistry
due to their potential application as antibiotics, drug intermediate,
agrochemical, etc. [41–48]. These two coupling reactions have been
∗
Corresponding author. Tel.: +91 376 2370 081; fax: +91 376 2370 011.
E-mail address: dipakkrdutta@yahoo.com (D.K. Dutta).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.09.024

B.J. Borah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 366 (2013) 202–209
203
studied with different homogenous Pd catalysts involving ligands
such as phosphines, amines, carbenes and dibenzylidineacetone
(dba) [30–48]. Recently, considering the inherent problem of
homogeneous catalyst, there are widespread interest to design het-
erogeneous Pd–catalyst system to increase the efficiency of Heck
and Sonogashira reactions [30–53]. The use of heterogeneous cat-
alyst in organic synthesis also complies with the requirement of
‘green’ chemistry as it offers easy separation, recovery of the cata-
lyst from the reaction products and its recyclability. In the present
work, we have reported a ‘greener’ synthetic procedure for syn-
thesis of Pd0-nanoparticles using environmentally benign, cheap
and robust Montmorillonite clay as a micro-mesoporous support
and their application as a heterogeneous catalyst precursor for
various carbon carbon bond formation cross-coupling reactions.
The Montmorillonite clay is collected from the natural deposition
of western part of India, processed and activated with mineral
acid under controlled conditions to generate a matrix having high
surface area and contain micro- and mesopores with diameter in
the range 0–10 nm on the surface. These nano range pores act as
‘host’ for in situ generation of Pd0-nanoparticles and also restrict
the growth of the particles up to desired range [8–10,12,13]. The
Pd0-nanoparticles act as efficient heterogeneous catalysts for Heck
and Sonogashira reactions under mild reaction condition. These
nanoparticles maintain their catalytic efficiency for several cycles
and serve as potential reusable catalyst.
Fig. 1. N2 adsorption/desorption isotherms of AT-Mont., Pd0-Mont. and recovered
catalysts.
from the mixture by filtered through sintered funnel (G-3). The
recovered catalyst was washed with acetone, dried in a desiccator
and stored for another consecutive reaction run. The product was
extracted with ethyl acetate followed by evaporation of the solvent
to yield the crude product which was finally purified by silica gel
column chromatography using ethyl acetate and hexane as eluents.
The products were identified by 1H NMR, mass spectrometry and
melting point determination followed by their comparison with the
standard literature data [45].
2. Experimental
2.1. Preparation of support
Purified Na+-Montmorillonite (10 g) was taken into a 250 ml
three necked round bottom flask and 200 ml 4 M sulfuric acid was
added to it. The resulting dispersion was refluxed for 1 h. After
cooling, the supernatant liquid was discarded and the activated
Montmorillonite was repeatedly washed with deionized water and
finally dried in a hot air oven at 50 5 ◦C overnight to obtain the
solid product. The activated Montmorillonite was designated as AT-
Mont. The surface characterizations of activated Montmorillonite
were presented in Table 1.
3. Results and discussion
3.1. Characterization of support
3.1.1. X-ray diffraction
The parent Montmorillonite (Parent Mont.) exhibited an intense
basal reflection at 7.06◦ 2Â corresponding to a basal spacing of
2.2. Preparation of supported Pd0-nanoparticles
˚
12.5 A (supporting information). The intensity of basal reflection
decreases considerably after 1 h acid activation, which implies that
the layered structure of AT-Mont. disrupted and also a low intense
broad reflection in the range 20–30◦ 2Â confirmed the formation of
amorphorous silica [54].
One gram of AT-Mont. was taken into a 100 ml beaker and 16 ml
(0.5 mmol) aqueous solution of K2PdCl4 was added slowly under
vigorous stirring condition. The stirring was continued for another
6 h followed by evaporation to dryness in a rotary evaporator. 0.5 g
of dry clay-[PdCl4]2− composite was dispersed in 20 ml dry ethanol
in a 100 ml three necked round bottom flask under nitrogen envi-
ronment and then 10 ml of hydrazine hydrate was added slowly
over 15 min under constant stirring condition. The reaction started
immediately and the color changed from yellow to black, due to
conversion of Pd(II) into Pd0-nanoparticles. The black solid mass
was recovered and washed with distilled water for several times
and then dried in a desiccator for 12 h. The sample thus prepared
was designated as Pd0-Mont. and stored in airtight bottle for further
use.
3.1.2. Specific surface area and pore size distribution
The AT-Mont. contained micro- (<2 nm) and mesopores
(2–50 nm) with average pore diameters ∼4.28 nm, a high specific
surface area up to 422 m2/g and a large specific pore volume of
∼0.6 cm3/g (Table 1). The adsorption–desorption isotherm (Fig. 1)
was of the type-IV with a H3 hysteresis loop at P/Po ∼0.4–0.9, indi-
cating mesoporous solid. The increase of surface area as well as pore
volume is due to the leaching of Al from the octahedral sites of the
clay matrix during acid activation, which also introduced perma-
nent porosity on the clay surface. The differential volumes versus
pore diameter plot (Fig. 2) indicated relatively narrow pore size
distributions and these nano range pores can be advantageously
utilized for the in situ synthesis of various metal nanoparticles as
reported by us recently [8–10,12,13].
2.3. General procedure for Heck and Sonogashira coupling
reactions
Aryl halide (1 mmol), alkene or alkyne (1.2 mmol), triethylamine
(3 mmol) and 25 mg (0.07 mol%) Pd0-Mont. were added to 10 ml
acetonitrile solution and the reaction mixture was stirred at 82 ◦C
for stipulated time period. The progress of the reactions was mon-
itored by TLC. After completion of the reaction, the mixture was
cooled to room temperature and the solid catalyst was separated
3.1.3. FTIR spectra
The Parent Mont. exhibited an intense absorption band at
∼1034 cm−1 for Si O stretching vibrations of tetrahedral sheet.
The bands at 522 and 460 cm−1 were due to Si
O Al and Si O Si

204
B.J. Borah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 366 (2013) 202–209
Table 1
Surface properties of different Montmorillonite clay based support/catalysts.
Surface properties of support/catalysts
Specific Average pore BJH pore size Specific pore
Samples
surface area diameter
distribution
volume
(m2/g)
422
(nm)
4.28
(nm)
3.77
(cm3/g)
0.58
AT-Mont.
Catalyst
After run Fresh
1
6.47
7.28
315
220
3.81
3.84
0.43
0.41
Pdo-Mont.
2
3
189
172
8.02
3.87
3.90
0.36
0.30
12.46
bending vibrations (supporting information). Acid activation
shifted the band from ∼1034 cm−1 to ∼1045 cm−1. The appearance
of a band near 795 cm−1 indicated amorphous silica [55]. Mont-
morillonite also showed absorption bands at 3634 cm−1 due to
stretching vibrations of OH groups of Al OH. The bands at 916, 875
and 792 cm−1 were related to AlAl OH, AlFe OH and AlMg OH
vibrations [55]. The disappearance of these bands during acid acti-
vation indicated the removal of Al, Fe and Mg ions from the clay
mineral structure.
both the layered and non-layered structures of clay mineral are
clearly seen.
3.2. Characterization of supported Pd0-nanoparticles
The evidence of formation of Pd0-nanoparticles (Pd0-Mont.) into
the nanopores of AT-Mont. was obtained from powder XRD anal-
ysis. Fig. 4(A) shows broad peaks of 2Â values 40.1◦, 46.6◦, 68.1◦,
82.2◦ and 86.5◦ which are assigned to the corresponding (1 1 1),
(2 0 0), (2 2 0), (3 1 1) and (2 2 2) indices of fcc lattice of metal-
lic Pd [46,53]. Among the peaks observed, the intensity of (1 1 1)
peak is the strongest, indicating that this plane was predominant
crystal facet in Pd0-Mont. The size of Pd0-nanoparticles was also
determined from X-ray line broadening applying Debye–Scherrer
formula using the half width of the intense (1 1 1) plane reflection
and the average size is found to be below 10 nm.
3.1.4. SEM-EDX investigation
The typical SEM image of AT-Mont. (supporting information)
showed the formation of pores on the clay surface. The surface
probe elemental analysis at the surface (supporting information)
revealed that predominant amounts of Si compared to Al were
present on the surface. At the pores (supporting information), only
Si was present and no other element was detected.
The particle size distribution of nanoparticles as observed in
the TEM image (Fig. 5(A)) confirmed that the dispersed Pd0-
nanoparticles were formed in the micro- and mesopores of
AT-Mont. The supported Pd0-nanoparticles were spherical in
shape, uniformly distributed and anchored along the heteroge-
neous mesoporous surface of AT-Mont. with sizes of below 10 nm,
which is quite consistent with data obtained from the XRD anal-
ysis. TEM study reveals that few Pd0-nanoparticles were found to
be larger than the pore size of the support which may be due to
the presence of Pd0-nanoparticles on the outer surface of the sup-
port rather than inside the pores. A typical HRTEM image (Fig. 3(B))
shows that Pd0-nanoparticles are stabilized in the layers as well as
in the well-tuned nanopores on the surface of modified clay matrix.
This observation suggested that the meso-structure of the sup-
port is stable even after the formation of Pd0-nanoparticles. The
HRTEM image (Fig. 5(B)) of Pd0-nanoparticles showed the reticular
lattice planes inside the nanoparticles and these planes are contin-
uously extended to the whole particle without any stacking faults
or twins, indicating the single crystalline nature. The selected area
electron diffraction (SAED) pattern, shown in the inset of Fig. 5(B),
exhibited three diffused rings, which could be assigned to (1 1 1),
(2 2 0) and (3 1 1) reflections of fcc lattice of Pd.
3.1.5. HRTEM investigation
The typical HRTEM image of AT-Mont. (Fig. 3(A)) shows the lat-
tice fringe image of layered structure of clay minerals [56]. During
controlled acid activation for 1 h, most of the layered structure of
clay minerals is disrupted and only a few are retained. In Fig. 3(A),
In order to ascertain the chemical oxidation state as well as to
ensure the complete reduction, the samples were characterized by
XPS analysis (Fig. 6) and show two peaks with binding energies
335.2 and 340.3 ev, which are characteristic of 3d5/2 and 3d3/2 peaks
of Pd(0) [57,58]. It indicates that the alumino-silicate constituent
of clay matrix efficiently stabilized Pd0-nanoparticles in their well
developed nanopores and prevents from oxidation during the syn-
thesis and stabilizing process.
For the better performance of heterogeneous catalyst, the high
surface to volume ratio is an important factor. The acid acti-
vated Montmorillonite clay which is used in the present study
has high specific surface area, uniformly distributed pores, and
large specific pore volume (Table 1). On the other hand, the
Fig. 2. BJH pore size distribution curves of AT-Mont., Pd0-Mont. and recovered cat-
alysts.

B.J. Borah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 366 (2013) 202–209
205
Fig. 3. Typical HRTEM images of AT-Mont. (A) before and (B) after supporting Pd0-nanoparticles.
Fig. 4. The powder XRD pattern of Pd0-Mont. (A) fresh and (B) recovered after 3rd cycle.
is maintained even after the encapsulation of Pd0-nanoparticles. A
decrease of the specific surface area and the specific pore volume
is observed after supporting Pd0-nanoparticles (Table 1) might be
due to clogging of some pores by Pd0-nanoparticles. The absence
of an abrupt change in the pore volume and surface area of the
support after Pd0 encapsulation further confirms that pores of the
support are not blocked by Pd0 particles larger than the pore size of
the support, revealing that the well-tuned nanopores on the sup-
port materials do not allow the agglomeration or migration of the
nanoparticles but instead stabilize or anchor them on the pore wall
structure. In addition, the presence of Pd0-nanoparticles may cause
complexities in porosity measurement with N2 sorption, because
the electrostatic forces between an adsorbate (i.e. N2) and metallic
purified non-activated Montmorillonite clay contains low surface
area (∼101 m2/g) and low pore volume (∼0.15 cm3/g). An attempt
was made to prepare Pd0-nanoparticles on non-activated Montmo-
rillonite clay but we obtained agglomerated Pd0-nanoparticles and
also observed negligible catalytic activities on different C C bond
formation coupling reactions.
The encapsulation of the Pd0-nanoparticles into the nanopores
of the modified clay has made a change in the textural parameters of
the materials. The N2 adsorption–desorption study of the different
clay matrix (Fig. 2) before and after the encapsulation of the Pd0-
nanoparticles, shows type IV isotherm with a H3 hysteresis loop at
P/Po ∼0.4–0.9. The identical shape of the isotherms and the hys-
teresis loop reveal that the highly ordered porous structure of clay
Fig. 5. (A) TEM image and (B) HRTEM image and corresponding SAED pattern (inset) of Pd0-nanoparticles.

206
B.J. Borah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 366 (2013) 202–209
proceed efficiently in the presence of Et3N as a base and acetonitrile
as a solvent at reflux condition (i.e. 82 ◦C).
The conditions for Sonogashira reaction were optimized by
using iodobenzene and phenylacetylene as model substrates under
copper free condition. In this reaction, we also adopted same condi-
tions as Heck reaction and we obtained 90% isolated yield with 100%
selectivity. Therefore, we have chosen the same reaction conditions
for Sonogashira reaction as that of Heck reaction.
3.3.1. Heck coupling reaction
Heck coupling reactions is the Pd catalyzed vinylation of aryl
halides and considered as one of the most important C C bond
formation coupling reactions. Therefore, tremendous attention has
been given to develop new and improve protocols for the Heck
coupling reaction after the pivotal discovery by Heck and Mizoroki
in the early 1970s [63–65]. Many Pd complexes have been used
as catalyst in these reactions [41–43], but most of them suffer
from the problems of transformation into Pd(0), which is formed
in situ during reaction, into inactive ‘Palladium black’. Thus, it is
necessary to develop supported catalysts which can easily be recov-
ered from the reaction system and reused. With these precedents
in mind and encouraged by our earlier works [8,9,12,13,37], we
believed it of interest to explore the versatility and limitations of
various substrates as well as the efficiency of our catalysts (i.e. Pd0-
Mont.) for various substituted aryl halides and alkenes as substrates
and the results are illustrated in Table 3. The catalytic activity of
Pd0-Mont. is dependent on the type of aryl halides and alkenes
used in the reactions and in all the cases, products are obtained
with very good to excellent yields and >99% trans selectivity. The
model reaction, i.e. reaction between iodobenzene and methylacry-
late (entry H1) gives trans-methylcinnamate with 92% yield. The
iodobenzenes bearing electron donating functional groups (entries
H2–H4) give slightly higher isolated yield than electron withdraw-
ing group (entry H5). So, our catalyst is efficient enough to active
the iodobenzene having electron withdrawing group to give desire
products with excellent yield and selectivity. The less reactive
styrene (entries H6–H8) also proceeded smoothly to furnish the
desired products with high yield and selectivity (i.e. >99% trans
isomer). In case of iodobenzene with electron withdrawing group
(entry H5) and also for coupling with styrene (entries H6–H8), the
reactions needed comparatively longer time, i.e. 5 h for completion.
Fig. 6. XPS spectrum of Pd0-Mont.
surface may affect the measured values to some extent. However,
increase of average pore diameter may be due to rupture of some
smaller pores to generate bigger ones during the formation of Pd0-
nanoparticles into the pores [8,10].
The Pd contents in Pd0-Mont. as analyzed by ICP-AES, reveals
the presence of 0.07 mol% of Pd.
3.3. Catalytic activity
In order to introduce a cheap, robust and ecofriendly het-
erogeneous catalyst to be acted under mild reaction conditions,
the synthesized Pd0-Mont. was tested as a catalyst precursor for
organic transformations involving the aforementioned C C cou-
pling reactions. Particular attention has been paid to the coupling
reaction of aryl halides with alkenes and alkynes, commonly called
Heck and Sonogashira reactions respectively. Different types of
mesoporous molecular sieves and amorphous silica materials like
MCM-41, SBA-15, etc. have been used as supports for Pd and their
catalytic studies for various C C bond formation reactions are well
documented in literature [59–62]. The reported catalysts although
exhibited comparable activities with the present catalysts, but
show several disadvantages. Most of the catalyst supports are syn-
thetic and required laborious effort for their synthesis as well as
surface functionalization, moreover, molecular sieves or zeolites
in general exhibit low or restricted pore sizes. On the other hand,
our catalyst supports are naturally occurring clay mineral, required
no surface functionalization and also can be tailor-made their pore
sizes by applying controlled acid activation [10].
The condition for Heck reaction were optimized by using
iodobenzene and methylacrylate as model substrates to evaluate
the effects of different solvents, type of bases and required opti-
mum temperature for the reaction and the results are summarized
in Table 2. In the initial selection, we used dimethylformamide
(DMF) as solvent and K2CO3 as base for the reaction in the presence
of Pd0-Mont. at 120 ◦C, where methyl cinnamate was obtained in
60% isolated yield after 6 h. The solvent mixture, DMF:H2O (1:1) at
100 ◦C also does not increase the yield of desired product (Table 2,
entry 2). Other solvents like toluene, methanol, ethanol, THF, and
acetonitrile were also evaluated and acetonitrile as a solvent and
triethylamine (Et3N) as base at 82 ◦C yield surprisingly the highest
conversion (92%) among all the solvents and bases tested (Table 2,
entry 4).
3.3.2. Copper-free Sonogashira coupling reaction
Alkynylation of aryl halides, frequently called Sonogashira reac-
tion is another synthetically useful coupling reaction catalyzed
by Pd based catalysts. Although Cu(I) has been initially used as
co-catalyst with an amine as solvent, a few Cu-free procedures
were subsequently developed but are usually required ultrasound
and microwave activation [65–67]. Our catalytic system is unique
and operationally simple and does not associate Cu(I) salt which
requires inert atmosphere and also form unwanted homocoupling
products. Under the optimized reaction conditions, various substi-
tuted aryl iodides were subjected to couple with aromatic/aliphatic
terminal alkynes in presence of catalyst Pd0-Mont. All the sub-
strates irrespective of substituent give good to excellent conversion
and selectivity (Table 4). The model reaction, i.e. reaction between
iodobenzene and phenylacetylene (entry S1) gives selectively
cross-coupling product with about 90% yield. The iodobenzenes
bearing electron donating functional groups (entries S2–S4) give
marginally higher conversion than electron withdrawing group
(entry S5) when coupled with phenylacetylene. In case of aliphatic
terminal alkyne, the iodobenzene with electron donating group
(entry S7) gives slightly higher conversion than other two iodoben-
zenes (entries S6 and S8).
During our optimization studies, various temperatures were
examined and it was found that the temperature plays a signifi-
cant role in terms of reaction rate and isolated yield (Table 2). It,
therefore, appears that for Heck reaction, our catalyst (Pd0-Mont.)

B.J. Borah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 366 (2013) 202–209
207
Table 2
Pd0-nanoparticles catalyzed Heck reaction between iodobenzene and methylacrylate.
O
Catalyst
O
I
+
OCH3
OCH3
Entry
Solvent
DMF
DMF:Water (1:1)
Toluene
Base
Temp. (◦C)
Time (h)
Yield (%)
1
2
3
4
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
Et3N
Et3N
Et3N
120
100
110
82
82
65
6
6
6
3
3
3
3
3
60
55
60
65
92
75
70
65
Acetonitrile
5
6
7
Methanol
Ethanol
THF
78
66
3.3.3. Recyclability of the catalyst
was recovered by simple filtration technique after each experi-
ment. The recovered catalyst was washed with acetone, dried in
a desiccator and reused directly with fresh reaction mixture with-
out further purification for the desired coupling products synthesis
up to 3rd run. The results (Table 3, entries H1, H2 and H6) showed
that in each case the catalyst remain active for several run without
For practical applications of heterogeneous catalysts, the level of
reusability is a very important factor. The recyclability of our cata-
lyst for the Heck reaction was investigated (Table 3) in the coupling
of methylacrylate with iodobenzene (entry H1) and 4-iodoanisole
(entry H2) and iodobenzene with styrene (entry H6). The catalyst
Table 3
Heck coupling reaction with aryl halides and alkenes catalyzed by Pd0-nanoparticles.
Pdo catalyst(0.07 mol%)
CH3CN,Et 3N, 82oC
R/Ar
R/Ar
Ar
X
+
Ar
Alkenes
O
Entry
Aryl halides
Products
Time (h)
Yield (%)
O
I
OCH3
OCH3
O
H1
H2
92a
89b
86c
3
O
H3CO
I
I
H3CO
OCH3
OCH3
96a
91b
87c
3
O
H3C
H2N
O2N
O
O
O
H3C
H2N
O2N
OCH3
OCH3
OCH3
OCH3
H3
H4
3
3
93
90
O
I
I
OCH3
O
OCH3
H5
H6
5
5
86
I
87a
83b
80c
H3CO
O2N
I
H3CO
O2N
H7
H8
5
5
90
85
I
Yields are given for isolated products. Selectivity: 100%.
a
1st run.
2nd run.
3rd run.
b
c

208
B.J. Borah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 366 (2013) 202–209
Table 4
Sonogashira coupling reaction with aryl halides and alkynes catalyzed by Pd0-nanoparticles.
Pdo catalyst (0.07 mol %)
Ar
X
+
R
Ar
R
CH3CN, Et N, 82oC
3
Entry
S1
Aryl halides
Alkynes
Products
Time (h)
Yield (%)
I
90a
86b
83c
3
I
H3CO
H3CO
S2
94a
90b
86c
3
I
H3C
H2N
O2N
H3C
H2N
O2N
S3
S4
S5
3
3
5
93
92
85
I
I
I
86a
82b
80c
3
3
S6
3
H3CO
O2N
I
H3CO
O2N
3
3
3
S7
S8
3
5
90
85
I
3
Yields are given for isolated products. Selectivity: 100%.
a
1st run.
2nd run.
3rd run.
b
c
significantly loss in efficiency. In the case of Sonogashira reaction
(Table 4), the recyclability of the catalyst was tested in the coupling
of phenylacetylene with iodobenzene (entry S1) and 4-iodoanisole
(entry S2) and 1-heptyne with iodobenzene (entry S6) and found
the catalyst remain active for several run without significant loss
in efficiency.
for the next reaction between iodobenzene and methylacrylate
under optimized condition. After similar purification methods, the
product obtained from Heck coupling reaction is negligible. It,
therefore, confirms that Pd does not leach out from the supported
matrix and thus exhibits a robust character of the catalyst.
The reduction in catalyst amount as well as reaction time is
possibly due to highly active Pd0-nanoparticles as catalyst. The
presence of robust and high surface area (422 m2/g) modified Mont-
morillonite as support for Pd0-nanoparticles also might have a
beneficial effect on the rate and selectivity of the reaction.
The recovered catalyst was further investigated through N2
adsorption–desorption and powder XRD studies. The identical
shape of the isotherms and the hysteresis loop reveal that the
characteristics of the catalysts are retained even after successive
reactions (Fig. 1). The specific surface areas of the recovered cata-
lysts decrease compared to 315 m2/g of freshly prepared catalyst
(Table 1). The BJH pore size distribution curves of the recovered
catalysts retain almost same distribution pattern compared to the
fresh catalyst also indicating the robustness of the catalyst (Fig. 2).
The decrease of surface area of the recovered catalysts after each
reaction may be due to the blockage of pores by the reactant
molecules. Likewise, the powder XRD analysis exhibited nearly
identical diffraction peaks for both the fresh and recovered Pd0-
nanoparticles (Fig. 4(B)). These experimental results clearly suggest
that the reactions undergo in a heterogeneous process.
4. Conclusion
The specific surface area, the pore diameter as well as the pore
size distribution of Montmorillonite were tuned by controlled acid
activation. The pores provided the space for nanoparticle forma-
tion and limited the growth of the particles up to desired nano size
range. Electron microscopic and spectroscopic data confirmed the
formation of Pd0-nanoparticles into the pores of micro- and meso-
porous support. The Pd0-nanoparticles were spherical in shape
and size below 10 nm. These nanoparticles demonstrated high
catalytic activities in Heck and Sonogashira reactions for synthe-
sizing some of the alkene and alkyne derivatives with high yield
under mild reaction conditions in a heterogeneous system. Further,
the nanocatalysts were reused for new batch of reactions with-
out significant loss of their activity. The operational simplicity and
To further confirm the robustness of the catalyst and also to
test the leaching of Pd from the support, we carried out the hot
filtration experiment. When the reaction between iodobenzene
and methylacrylate was complete and the hot filtration had been
accomplished, the obtained solution from the filtrate was applied

B.J. Borah, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 366 (2013) 202–209
209
robustness of the catalyst provides great promise toward further
useful applications in other palladium promoted transformations
in the future.
[25] L.O. Brown, J.E. Hutchison, J. Am. Ceram. Soc. 121 (1999) 882–883.
[26] M. Brust, M. Walker, D. Bethell, J.D. Schiffrin, R. Whyman, J. Chem. Soc. Chem.
Commun. (1994) 801–802.
[27] K.R. Gopidas, J.K. Whitesell, M.A. Fox, J. Am. Ceram. Soc. 125 (2003) 6491–6502.
[28] A. Fukuoka, T. Higuchi, T. Ohtake, T. Oshio, J.-I Kimura, Y. Sakamoto, N. Shimo-
mura, S. Inagaki, M. Ichikawa, Chem. Mater. 18 (2006) 337–343.
[29] D.K. Dutta, D. Dutta, P.P. Sarmah, S.K. Bhorodwaj, B.J. Borah, J. Biomed. Nan-
otechnol. 7 (2011) 76–77.
Acknowledgements
The authors are grateful to Dr. P.G. Rao, Director, CSIR-North
East Institute of Science and Technology, Jorhat, Assam, India, for
his kind permission to publish the work. The authors appreciate the
opportunity for a preliminary exploration under the proposed CSIR
XII Five Year Plan projects: Chemical Cluster (CSC and SPECS). The
authors are thankful to CSIR, New Delhi for financial support (In-
house Project: MLP-6000/01). Thanks are also due to Dr. H.C. Bajaj,
Scientist, CSMCRI, Bhavnagar for arranging the ICP-AES experiment.
[30] E. Negishi, Handbook of Organopalladium Chemistry for Organic Synthesis,
Wiley-Interscience, New York, 2002, p. 229.
[31] J. Tsuji, Palladium Reagents and Catalysts: New Perspectives for the 21st Cen-
tury, John Wiley & Sons, Weinheim, 2003.
[32] N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457–2483.
[33] N.T.S. Phan, M.V.D. Sluys, C.W. Jones, Adv. Synth. Catal. 348 (2006) 609–679.
[34] L. Xue, Z. Lin, Chem. Soc. Rev. 39 (2010) 1692–1705.
[35] A.M. Trzeciak, J.J. Ziolkowski, Coord. Chem. Rev. 249 (2005) 2308–2322.
[36] V.F. Slagt, A.H.M. de Vries, J.G. de Vries, R.M. Kellogg, Org. Process Res. Dev. 14
(2010) 30–47.
[37] B.J. Borah, K. Saikia, P.P. Saikia, N.C. Barua, D.K. Dutta, Catal. Today (2012),
http://dx.doi.org/10.1016/j.cattod.2012.03.083.
[38] P. Das, U. Bora, A. Tairai, C. Sharma, Tetrahedron Lett. 51 (2010) 1479–1482.
[39] M.N. Birkholz, Z. Freixa, P.W.N.M. van Leeuwen, Chem. Soc. Rev. 38 (2009)
1099–1118.
[40] M. Guo, F. Jian, R. He, Tetrahedron Lett. 46 (2005) 9017–9020.
[41] A. Molnar, Chem. Rev. 111 (2011) 2251–2320.
[42] B. Tamami, H. Allahyari, S. Ghasemi, F. Farjadian, J. Organomet. Chem. 696
(2011) 594–599.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2012.09.024.
[43] M.J. Climent, A. Corma, S. Iborra, M. Mifsud, Adv. Synth. Catal. 349 (2007)
1949–1954.
References
[44] Z. Yinghuai, S.C. Peng, A. Emi, S. Zhenshun, Monalisa, R.A. Kemp, Adv. Synth.
Catal. 349 (2007) 1917–1922.
[45] S. Sawoo, D. Srimani, P. Dutta, R. Lahiri, A. Sarkar, Tetrahedron 65 (2009)
4367–4374.
[46] A. Khalafi-Nezhad, F. Panahi, Green Chem. 13 (2011) 2408–2415.
[47] K. Sonogashira, in: F. Diedrich, A. de Meijere (Eds.), Metal Catalyzed Cross-
coupling Reactions, 1, Wiley-VCH, Weinheim, 2004, p. 319.
[48] D. Astruc, Inorg. Chem. 46 (2007) 1884–1894.
[49] A. Molnar, A. Papp, K. Miklos, P. Forgo, Chem. Commun. (2003) 2626–2627.
[50] K.R. Reddy, N.S. Kumar, P.S. Reddy, B. Sreedhar, M. Lakshmi Kantam, J. Mol.
Catal. A: Chem. 252 (2006) 12–16.
[51] N. Erathodiyil, S. Ooi, A.M. Seayad, Y. Han, S.S. Lee, J.Y. Ying, Chem. Eur. J. 14
(2008) 3118–3125.
[52] B.C. Ranu, K. Chattopadhyay, Org. Lett. 9 (2007) 2409–2412.
[53] V.L. Budarin, J.H. Clark, R. Luque, D.J. Macquarrie, R.J. White, Green Chem. 10
(2008) 382–387.
[54] L. Wang, A. Lu, C. Wang, X. Zheng, D. Zhao, R. Liu, J. Colloid Interface Sci. 295
(2006) 436–439.
[55] P.J. Wallis, W.P. Gates, A.F. Patti, J.L. Scott, E. Teoh, Green Chem. 9 (2007)
980–986.
[56] J.M. Huggett, Clays Clay Miner. 43 (1995) 682–692.
[57] C.M. Shen, Y.K. Su, H.T. Yang, T.Z. Yang, H.J. Gao, Chem. Phys. Lett. 373 (2003)
39–45.
[58] X. Yang, Q. Li, H. Wang, J. Huang, L. Lin, W. Wang, D. Sun, Y. Su, J.B. Opiyo, L.
Hong, Y. Wang, N. He, L. Jia, J. Nanopartic. Res. 12 (2010) 1589–1598.
[59] J. Demel, J. Cejka, S. Bakardjieva, P. Stepnicka, J. Mol. Catal. A: Chem. 263 (2007)
259–265.
[60] J. Demel, J. Cejka, P. Stepnicka, J. Mol. Catal. A: Chem. 329 (2010) 13–20.
[61] G. Budroni, A. Corma, H. Garcia, A. Primo, J. Catal. 251 (2007) 345–353.
[62] C. Baleizao, A. Corma, H. Garcy´a, A. Leyva, J. Org. Chem. 69 (2004) 439–446.
[63] R.F. Heck, J.P. Nolley, J. Org. Chem. 37 (1972) 2320–2322.
[64] T. Mizoroki, K. Mori, A. Ozaki, Bull. Chem. Soc. Jpn. 44 (1971) 581–582.
[65] A. Soheili, J. Albaneze-Walker, J.A. Murry, P.G. Dormer, D.L. Hughes, Org. Lett. 5
(2003) 4191–4194.
[1] H. Goesmann, C. Feldmann, Angew. Chem. Int. Ed. 49 (2010) 1362–1395.
[2] G. Schmid, B. Corain, Eur. J. Inorg. Chem. (2003) 3081–3098.
[3] C.N.R. Rao, G.U. Kulkarni, P.J. Thomas, P.P. Edwards, Chem. Soc. Rev. 29 (2000)
27–35.
[4] D.L. Feldheim, C.A. Foss Jr. (Eds.), Metal Nanoparticles: Synthesis, Characteri-
zation and Application, CRC Press, New York, 2002.
[5] J.M. Campelo, D. Luna, R. Luque, J.M. Marinas, A.A. Romero, ChemSusChem 2
(2009) 18–45.
[6] C. Templeton, W.P. Wuelfing, R.W. Murray, Acc. Chem. Res. 33 (2000) 27–36.
[7] O.S. Ahmed, D.K. Dutta, Langmuir 19 (2003) 5540–5541.
[8] B.J. Borah, D. Dutta, P.P. Saikia, N.C. Barua, D.K. Dutta, Green Chem. 13 (2011)
3453–3460.
[9] P.P. Sarmah, D.K. Dutta, Green Chem. 14 (2012) 1086–1093.
[10] B.J. Borah, D. Dutta, D.K. Dutta, Appl. Clay Sci. 49 (2010) 317–323.
[11] B.J. Borah, A. Yadav, D.K. Dutta, J. Biomed. Nanotechnol. 7 (2011) 152–153.
[12] D. Dutta, B.J. Borah, L. Saikia, M.G. Pathak, P. Sengupta, D.K. Dutta, Appl. Clay
Sci. 53 (2011) 650–656.
[13] L. Saikia, D. Dutta, D.K. Dutta, Catal. Commun. 19 (2012) 1–4.
[14] S. Wang, Z. Wang, Z. Zha, Dalton Trans. (2009) 9363–9373.
[15] K.K.R. Datta, B.V. Subha Reddy, K. Ariga, A. Viru, Angew. Chem. Int. Ed. 49 (2010)
5961–5965.
[16] S. Jammi, S. Sakthivel, L. Rout, T. Mukherjee, S. Mandal, R. Mitra, P. Saha, T.
Punniyamurthy, J. Org. Chem. 74 (2009) 1971–1976.
[17] L.D. Rogatis, M. Cargnello, V. Gombac, B. Lorenzut, T. Montini, P. Fornasiero,
ChemSusChem 3 (2010) 24–42.
[18] K. Kaneda, T. Mitsudome, T. Mizugaki, K. Jitsukawa, Molecules 15 (2010)
8988–9007.
[19] J. Virkutyte, R.S. Varma, Chem. Sci. 2 (2011) 837–846.
[20] L.D. Pachon, J.H. Van Maarseveen, G. Rothenberg, Adv. Synth. Catal. 347 (2005)
811–815.
[21] M. Kidwai, V. Bansal, A. Kumar, S. Mozumdar, Green Chem.
742–745.
9 (2007)
[22] C.-J. Jia, F. Schuth, Phys. Chem. Chem. Phys. 13 (2011) 2457–2487.
[23] W.W. Weare, S.M. Reed, M.G. Warner, J.E. Hutchison, J. Am. Ceram. Soc. 122
(2000) 12890–12891.
[66] A. Arques, D. Aunon, P. Molina, Tetrahedron Lett. 45 (2004) 4337–4340.
[67] M. Larhed, A. Hallberg, J. Org. Chem. 61 (1996) 9582–9584.
[24] R. Tatumi, T. Akita, H. Fujihara, Chem. Commun. (2006) 3349–3351.