B.J. Borah et al. / Applied Catalysis A: General 469 (2014) 350–356
351
Table 1
Surface properties of different Montmorillonite clay based support/catalysts.
Samples
Surface properties of support/catalysts
Specific surface
area (m /g)
Average pore
diameter (nm)
BJH pore size
distribution (nm)
Specific pore
volume (cm /g)
2
3
AT-Mont.
422
4.28
3.77
0.58
Catalyst
o
After run
Fresh
Pd -Mont.
315
251
202
188
6.47
7.12
8.15
3.81
3.90
3.97
4.09
0.43
0.40
0.38
0.32
1
2
3
12.53
of success for the betterment of the Suzuki–Miyaura cross cou-
pling reaction conditions has been achieved by using different types
of organic ligands such as phosphines, amines, N-heterocyclic car-
benes, dibenzylidineacetone (dba) [24–38]. In most of the ligands
systems, excellent conversions and selectivities were observed.
However, the homogeneous catalytic systems often encountered
several problems such as separation and reusability of catalyst
after reaction and more importantly, the high cost of the ligands
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 colour changed from yellow to black, due to
−
o
the conversion of Pd(II) into Pd -nanoparticles. The black solid mass
[
24–38]. In addition to these, most of the phosphine based lig-
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 Pd -Mont. and stored in airtight bottle for further
ands are sensitive to air and therefore, the reactions require inert
atmosphere and majority of the reactions are performed in organic
solvents [18,36]. Moreover, due to stringent and growing environ-
mental regulations, chemical industries need to introduce greener
solvents instead of organic solvents as well as catalysts which are
efficient in those solvent systems. Among the different greener
solvents, water has gained tremendous importance due to its avail-
ability, cost and environmental acceptability. The Suzuki–Miyaura
coupling reaction proceeds efficiently in aqueous medium due to
excellent stability of boronic acids in aqueous medium and also the
ability of water to dissolve most of the bases favours the trans-
metalation step of the catalytic cycle. As a consequence, the overall
rate of the Suzuki–Miyaura coupling reaction increases in aqueous
medium [31–38]. Recently, considerable efforts have been made to
utilize water as the solvent in the Suzuki–Miyaura cross-coupling
reaction and excellent yields were reported [39–43]. However, in
most of the cases, either a harsh reaction condition or phase transfer
agent or promoter/additive are required to perform the reactions
o
use.
2.3. General procedure for the Suzuki–Miyaura coupling reactions
Aryl halide (1 mmol), arylboronic acid (1.2 mmol), K CO
3 mmol) and 25 mg (0.07 mol%) Pd -catalyst (Pd -Mont.) were
2
3
o
o
(
added to 10 ml of water and the reaction mixture was stirred at
◦
6
0 C for a stipulated time period. The progress of the reactions
was monitored by TLC. After completion of the reaction, the mix-
◦
ture was cooled to room temperature (∼25 C), and ethyl acetate
(10 ml) was added to the mixture followed by separation of the
solid catalyst by filtration through sintered funnel (G-3). The recov-
ered catalyst was washed with acetone, dried in a desiccator and
stored for another run. The organic extract was washed with water,
dried over Na SO4 and concentrated to give the crude products,
2
which was finally purified by silica gel column chromatography
using ethyl acetate and hexane as eluents. The products were
[
44–53]. Therefore, the development of new greener catalytic sys-
tem for the Suzuki–Miyaura reaction, which is efficient in water, is
a challenge for researchers. Herein, we report a well-defined het-
erogeneous catalytic system i.e. the modified Montmorillonite clay
1
identified by H NMR, mass spectrometry and melting point deter-
mination followed by their comparison with the standard literature
data [54,55].
o
supported Pd -nanoparticles catalyst for the Suzuki–Miyaura cou-
pling reactions under a mild reaction condition with water as the
solvent. These nanoparticles, retaining their catalytic efficiency for
several cycles, may serve as potential reusable catalysts.
3
. Results and discussion
3.1. Characterization of support
2
. Experimental
The naturally occurring Montmorillonite clay is a cheap, robust,
easily available and environmentally benign catalyst and cata-
lyst support [11–16,56,57]. The purified Montmorillonite clay was
modified by mineral acids treatment under controlled conditions
in order to generate high surface area and porous matrix. The
characterization of modified Montmorillonite clay was thoroughly
carried out with the help of different analytical instruments like
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 sulphuric acid
was added to it. The resulting dispersion was refluxed for 1 h. After
cooling, the supernatant liquid was discarded and the activated
Montmorillonite clay was repeatedly washed with deionised water
Powder-XRD, FTIR, N adsorption–desorption, SEM, TEM, 29Si and
2
2
7
◦
Al MAS-NMR and were well documented in our earlier reports
11–16]. The parent Montmorillonite clay (Parent Mont.) is 2:1
and finally dried in a hot air oven at 50 ± 5 C over night to obtain the
[
solid product. The activated Montmorillonite clay was designated
as AT-Mont.
layered dioctahedral aluminosilicate [58,59] and consists of two
tetrahedral silicate sheets which are bonded to either side of an
◦
o
octahedral aluminate sheet having basal reflection at 7.06 2Â cor-
2.2. Preparation of the supported Pd -nanoparticles
responding to a basal spacing of 12.5 A˚ . The powder XRD data reveal
0
.5 g of AT-Mont. was taken into a 100 ml beaker and 16 ml
that after 1 h acid activation, the layered structure is disrupted and
turned into an amorphous matrix containing high silica. During acid
(
0.5 mmol) aqueous solution of K PdCl4 was added slowly under
2