Efficient three-component coupling reactions catalyzed by Cu 0-nanoparticles stabilized on modified montmorillonite

Article abstract of DOI:10.1039/c3cy00639e

The in situ generation of Cu⁰-nanoparticles into the nanopores of modified montmorillonite and their catalytic performance in the three component (A³) coupling reactions of aldehyde, amine and alkyne to synthesize propargylamines have been described here. The modification of montmorillonite was carried out with HCl under controlled conditions for generating nanopores into the matrix and these pores act as a host for the in situ generation of Cu⁰-nanoparticles. The Cu ⁰-nanoparticles were generated in the nanopores of the acid activated clay mineral matrix by the successful loading of a Cu(CH₃COO) ₂ metal precursor using an incipient wetness impregnation technique followed by reduction with NaBH₄. The TEM study reveals that Cu ⁰-nanoparticles of below 10 nm in size are evenly distributed on the support. The synthesized Cu⁰-nanoparticles exhibit a face centered cubic (fcc) lattice. The Cu⁰-nanoparticles serve as an efficient green and heterogeneous catalyst for three-component coupling via C-H alkyne-activation to synthesize propargylamines with excellent yields (82-94%) and 100% selectivity under mild reaction conditions without requiring any additives or an inert atmosphere. The nanocatalysts can be recycled and reused several times without significant loss of their catalytic activity. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy00639e

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Efficient three-component coupling reactions
catalyzed by Cu0-nanoparticles stabilized on
modified montmorillonite†
Cite this: Catal. Sci. Technol., 2014,
4, 1047
*
Bibek Jyoti Borah, Subrat Jyoti Borah, Lakshi Saikia and Dipak Kumar Dutta
The in situ generation of Cu0-nanoparticles into the nanopores of modified montmorillonite and their
catalytic performance in the three component (A3) coupling reactions of aldehyde, amine and alkyne to
synthesize propargylamines have been described here. The modification of montmorillonite was carried
out with HCl under controlled conditions for generating nanopores into the matrix and these pores act
as a “host” for the in situ generation of Cu0-nanoparticles. The Cu0-nanoparticles were generated in the
nanopores of the acid activated clay mineral matrix by the successful loading of a Cu(CH3COO)2 metal
precursor using an incipient wetness impregnation technique followed by reduction with NaBH4. The
TEM study reveals that Cu0-nanoparticles of below 10 nm in size are evenly distributed on the support.
The synthesized Cu0-nanoparticles exhibit a face centered cubic (fcc) lattice. The Cu0-nanoparticles
serve as an efficient green and heterogeneous catalyst for three-component coupling via C–H alkyne-
activation to synthesize propargylamines with excellent yields (82–94%) and 100% selectivity under mild
reaction conditions without requiring any additives or an inert atmosphere. The nanocatalysts can be
recycled and reused several times without significant loss of their catalytic activity.
Received 27th August 2013,
Accepted 10th December 2013
DOI: 10.1039/c3cy00639e
www.rsc.org/catalysis
classical methods are available for the synthesis of
propargylamines through the addition of metallated alkynes
1. Introduction
Recently, in the route of the development of green processes
for the synthesis of fine chemicals and bio-active molecules,
the concept of atom economy/efficiency finds considerable
attention.1 In this regard, an increasing number of multi-
component reactions have been designed for the synthesis of
diverse complex molecules through a combination of three or
more starting materials in one-pot, which reduces waste and
increases safety as well as reaction yield.2 Among the different
multicomponent reactions, the catalytic coupling reaction of
aldehyde, amine and alkyne (so-called A3 coupling) is one of
the best examples, where propargylamines are produced as
the main product. Propargylamines are versatile intermediates
for the preparation of various nitrogen-containing compounds
and are the key components of biologically active pharmaceu-
ticals and natural products.3 Propargylamines derivatives have
gained much importance after the pivotal discovery of
rasagiline (Azilect™), which is molecularly a propargylamine
derivative used as a monotherapy in early Parkinson's disease,
and some propargylamine derivatives are also used as potent
anti-apoptotic agents and neuroprotective agents.4 Several
to an imine. Metallated alkynes are generated by using differ-
ent strong bases such as alkylmetals, metallated amides,
alkoxides and hydroxides.5 However, these metal alkynylides
are relatively difficult to handle, because they are moisture
sensitive and require strictly controlled reaction conditions
otherwise they form large amounts of waste materials.
In recent years, various mid- and late transition metal cata-
lyzed couplings of aldehydes, amines and alkynes have
become a powerful tool that facilitates the coupling of alkynes
to imines (generated in situ) via terminal alkyne C–H activa-
tion in a more efficient manner.3e–h,6 Among the different
metals, Cu(I) and Cu(II) complexes, and also various simple
and inexpensive copper halide salts have been extensively uti-
lized as catalysts for the A3-coupling reaction with secondary
amines.7 However, most of these catalytic systems are homo-
geneous in nature, and only a few reports on heterogeneous
catalysts are available in the literature. Li and Wang reported
an efficient and reusable heterogeneous Cu(I) silica based
organic–inorganic composite material for the A3-coupling
reaction under solvent free conditions.8 Li, Wang and co-
workers prepared a silica supported complex of Cu(I) and an
N-heterocyclic carbene and used it to achieve a high yield in
the A3-coupling reaction.9 Besides silica based materials, other
supports like molecular sieves, zeolites, etc., have also been
used for immobilizing Cu metal.6d,10 But these catalytic systems
Materials Science Division, CSIR-North East Institute of Science and Technology,
Jorhat 785006, Assam, India. E-mail: dipakkrdutta@yahoo.com;
Fax: +91 376 2370011
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cy00639e
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often show several disadvantages. Most of the catalyst supports
are synthetic and require laborious and time-consuming efforts
in their synthesis as well as surface functionalization, which
may poison the catalytic active sites of the supports and the
particles, moreover, molecular sieves or zeolites, like meso-
porous matrixes in general, exhibit low or restricted pore sizes.
In the last decade, supported metal nanocatalytic systems
have been considered as a sustainable and competitive alter-
native to conventional catalysis, due to their high surface
reactivity and high surface-to-volume ratio of metal nano-
particles, which enhances their activity and selectivity and
more importantly they satisfied the stringent criteria of a
heterogeneous catalyst.11 Nowadays, nanoparticles of metals
such as Cu, Au, Fe, In, etc., have been used as substitutes for
bulk metal in the various C–H activation reactions in order to
reduce catalyst loading, reaction time and also to minimize
the undesired products.2a,3g,6a,11,12 Particularly, Cu0-nano-
particles supported on different supports have been exten-
sively utilized in the 1,3-dipolar cycloaddition of azides and
terminal alkynes to synthesize 1,2,3-triazoles.13 But, limited
numbers of reports are available in the literature which
describe the use of supported Cu0-nanoparticles in the A3
coupling reaction.12a,14 Albaladejo et al. reported an efficient
catalytic system based on Cu0-nanoparticles supported on
TiO2 for the A3 coupling reaction and widely illustrated the
substrate tolerance as well as recyclability of the catalyst.12a
Although, homogeneous and heterogeneous catalysts studied
in A3 coupling reactions show good activity, there are still
some drawbacks which restrict their acceptance, such as high
prices of the used precious metals and tedious catalyst prepa-
ration methods. In addition, some catalytic systems used
environmentally unacceptable substances, harsh reaction
conditions, long reaction times, and required additives or
inert atmospheres. Therefore, the development of a novel,
efficient and recoverable catalytic system for A3 coupling
reactions is a great challenge for researchers. With these pre-
cedents in mind and encouraged by our recent works,15
herein, we have reported a well-defined heterogeneous cata-
lytic system i.e. Cu0-nanoparticles supported on modified
montmorillonite for A3 coupling reaction under mild reaction
conditions. These nanoparticles maintain their catalytic effi-
ciency for several cycles and serve as a potential reusable cat-
alyst. The modified montmorillonite is considered to be
environmentally benign, cheap, easily available and a robust
support/stabilizer material for the synthesis of different metal
nanoparticles.15 Laszlo et al. reported interesting montmoril-
lonite based catalytic systems for organic synthesis.16 This
montmorillonite was collected from an indigenous source,
i.e. natural deposits from the western part of India, and was
purified and activated with mineral acid under controlled
conditions to generate a matrix with a high surface area and
which contained micro- and mesopores with diameters in the
range 0–10 nm on the surface. These nano range pores act as
a “host” for the in situ generation of Cu0-nanoparticles and
also limit the growth of the particles up to a desired range.
The other added advantages of this catalyst support is that it
requires no surface functionalization and the pore size can
be customized by applying controlled acid activation.15a–d
2. Experimental section
2.1. Preparation of support
Purified Na+-montmorillonite (10 g) was added into a 250 mL
three necked round bottom flask and 200 mL of 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 was repeatedly
washed with deionised 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 are
presented in Table 1.
2.2. Preparation of Cu0-nanoparticles
0.5 g of AT-Mont. was added into a 100 mL beaker and
15 mL of the (0.254 mmol) aqueous solution of Cu(CH3COO)2
was added slowly under vigorous stirring. The stirring
was continued for another 6 h followed by evaporation to
the dried solid in a rotary evaporator. 0.3 g of the dry
clay–Cu(CH3COO)2 composite was dispersed in 50 mL of water
in a 100 mL two necked round bottom flask and 10 mL of an
aqueous solution of NaBH4 (134 mg, 3.54 mmol) was then
added slowly over 15 min in a nitrogen environment under
constant stirring. The reaction started immediately and the
colour changed from blue to black, due to conversion of
Cu(II) into Cu0-nanoparticles. The black solid mass was recov-
ered and washed with distilled water several times and then
Table 1 Surface properties of different montmorillonite based support/catalysts
Surface properties of support/catalysts
Specific surface area Average pore diameter
BJH pore size distribution Specific pore volume
Samples
(m2 g−1
422
)
(nm)
(nm)
(cm3 g−1
0.58
)
AT-Mont.
4.28
3.77
Cu0-Mont.
299
255
210
192
5.18
6.43
7.25
8.53
3.81
3.90
4.27
3.89
0.48
0.38
0.32
0.30
1
2
3
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dried in a desiccator for 12 h. The sample thus prepared was
designated as Cu0-Mont.
2.3. General procedure for the one-pot synthesis of
propargylamines
The aldehyde (1 mmol), amine (1 mmol), alkyne (1.2 mmol),
20 mg of catalyst and 5 mL of toluene were put in a 25 mL
round bottom flask and the reaction mixture was refluxed at
110 °C for a stipulated time period. The progress of the reac-
tions was monitored by TLC. After completion of the reaction,
the solid catalyst was separated from the mixture by filtration
through a sintered funnel (G-3) and the solvent was removed
under low pressure in a rotavapor. The recovered catalyst was
washed with acetone (30 mL), dried in a desiccator and stored
for another consecutive reaction run. The crude product
obtained was then purified by silica gel column chromatogra-
phy using ethyl acetate and hexane as eluents. The products
were characterized by (1H, 13C) NMR and mass spectrometry
and all gave satisfactory results [ESI†].
Fig. 1 N2 adsorption–desorption isotherm and BJH pore size distribution
curve (inset) of AT-Mont.
3. Results and discussion
3.1. Characterization of support
Naturally occurring montmorillonite is considered to be a
cheap, robust, easily available and environmentally benign
catalyst and catalyst support.15a–f Purified montmorillonite
was modified with a mineral acid treatment under con-
trolled conditions in order to generate a high surface area
and porous matrix. The characterization of the modified
montmorillonite (AT-Mont.) was thoroughly carried out with
the help of different sophisticated analytical instruments
like Powder-XRD, FTIR, N2 adsorption–desorption, SEM, 29Si
and 27Al MAS-NMR studies and is well documented in our
earlier reports.15a–f The parent montmorillonite (Parent
Mont.) is 2 : 1 layered dioctahedral aluminosilicate and con-
sists of two tetrahedral silicate sheets which are bonded to
either side of an octahedral aluminate sheet having a basal
reflection at 7.06° 2θ corresponding to a basal spacing of
12.5 Å.17 The powder XRD reveals that after 1 h of acid acti-
vation, the layered structure is disrupted and becomes an
amorphous high silica containing matrix [ESI†]. During acid
activation, the surface area (up to 422 m2 g−1) (Table 1) as
well as pore volume (~0.6 cm3 g−1) increased due to the
leaching of Al from the octahedral sites of the montmoril-
lonite matrix, which also introduced permanent porosity on
the clay surface. The clay mineral matrix contained micro-
(<2 nm) and mesopores (2–50 nm) with average pore diame-
ters of ~4.28 nm and also the differential volume versus pore
diameter plot, i.e. BJH plot, indicated a relatively narrow
pore size distribution and these nano range pores were
advantageously utilized for the in situ synthesis of various
metal nanoparticles as reported by us recently.15a–f The shape
of the N2 adsorption–desorption isotherm (Fig. 1) was
of type-IV with a H3 hysteresis loop at P/Po ~ 0.4–0.9, indi-
cating a mesoporous solid. The FTIR, 29Si and 27Al MAS-NMR
measurements and SEM-EDX studies also confirmed
that during acid activation, nano range pores are formed on
the surface of the clay mineral matrix due to Al leaching
and predominant amounts of Si compared to Al were
present on the surface [ESI†].15a–e
3.2. Characterization of Cu0-nanoparticles
The evidence of formation of the Cu0-nanoparticles (Cu0-Mont.)
was obtained from powder XRD analysis. Fig. 2 shows three
broad peaks of 2θ values 43.3, 50.3 and 74.1° which are
assigned to the (111), (200) and (220) indices of the fcc lattice
of metallic Cu. The XRD peaks for any other oxidation states
of Cu were not observed, which could be attributed to the
presence of Cu only in its zero oxidation state.
The TEM image (Fig. 3) of the nanoparticle composite
shows that dispersed Cu0-nanoparticles were formed in the
Fig. 2 Powder XRD pattern of Cu0-Mont.
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Fig.
4
N2 adsorption–desorption isotherm and BJH pore size
distribution curve (inset) of Cu0-Mont.
Fig. 3 TEM and HRTEM (inset) images of Cu0-nanoparticles.
smaller pores to generate bigger ones during the formation
of the Cu0-nanoparticles in the pores.
The Cu contents in Cu0-Mont., as analyzed by ICP-AES,
reveals the presence of 0.05 mol% of Cu in the catalyst.
micro- and mesopores of AT-Mont. The supported Cu0-nano-
particles are spherical in shape, well separated from each
other and with sizes of below 10 nm. From the TEM study, it
was also observed that some of the Cu0-nanoparticles were
found to be larger than the pore size of the support which
may be due to the presence of Cu0-nanoparticles on the outer
surface of the support rather than inside the pores. The
HRTEM image (Fig. 3 (inset)) of a single Cu0-nanoparticle
showed regular lattice planes inside the nanoparticles and
these planes are continuously extended to the whole particle
without any stacking faults or twins, indicating its single
crystalline nature.
3.3. Catalytic activity
In order to determine the best reaction conditions (i.e. sol-
vent, reaction time, etc.) that are required to afford excel-
lent yields of propargylamines, a series of three-component
reactions were carried out using benzaldehyde, piperidine
and phenylacetylene as the model substrates. After screen-
ing a wide range of reactions, we found that our catalytic
system (i.e. Cu0-Mont.) is most efficient for the title reac-
tion in toluene as a solvent at refluxing temperature. The
reactions required 3 h time for completion. A control experi-
ment under the above conditions but in the presence of
only the acid activated clay (AT-Mont.) as catalyst was done
and gave no trace of product after 24 h. Using the opti-
mized reaction conditions, we explored the versatility and
limitations of some commercially available aldehydes,
secondary amines and alkynes as substrates as well as the
efficiency of our catalyst for the said three-component
coupling to synthesise propargylamine derivatives and the
results are summarized in Table 2. Firstly, phenylacetylene
and piperidine were allowed to couple with different aro-
matic aldehydes (e.g. benzaldehyde, 4-methoxybenzaldehyde,
4-chlorobenzaldehyde, 4-methylbenzaldehyde, 3-bromobenzaldehyde,
2-hydroxybenzaldehyde) and gave the corresponding propar-
gylamines in very good to excellent yields. The substituent of
the aldehyde seemed to have pronounced effects both on the
reaction time and on the isolated yield. The model reaction,
i.e. the coupling between benzaldehyde, phenylacetylene
and piperidine gave a 94% isolated yield (entry 1). In the
case of aldehydes containing an electron donating group
e.g. 4-methoxybenzaldehyde, 4-methylbenzaldehyde and
SEM-EDX analysis also substantiated the formation of
Cu0-nanoparticles in the well-tuned pores of AT-Mont. Fur-
thermore, EDX analysis indicates that Cu is present on the
surface of the modified montmorillonite along with the other
elements of clay [ESI†].
Although the encapsulation of the Cu0-nanoparticles into
the nanopores of the modified clay mineral changed some of
the textural parameters of the materials, from Fig. 4, it is
observed that Cu0-Mont. exhibits a similar type of isotherm
and hysteresis loop to that of AT-Mont. This reveals that the
highly ordered porous structure of clay is still maintained
even after the encapsulation of the Cu0-nanoparticles. An
appreciable decrease in specific surface area and specific
pore volume after supporting Cu0-nanoparticles (Table 1)
might be due to clogging of some of the pores by the Cu0-
nanoparticles. In addition, the presence of Cu0-nanoparticles
may cause complexities in the porosity measurements with
nitrogen sorption, because the electrostatic forces between
an adsorbate (i.e. nitrogen) and a metallic surface may affect
the measured values to some extent. However, the increase in
pore diameter may be due to the rupture of some of the
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Table 2 Cu0-nanoparticle catalyst A3 coupling reaction of aldehyde, amine and alkyne
Entry
1
Alkyne
Aldehyde
Amine
Product
Time (h)
3
Yielda (%)
94b
90c
88d
2
3
5
5
89
87
4
5
3
3
92
90b
88c
85d
6
7
8
5
3
3
87
82
87
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Table 2 (continued)
Entry
Alkyne
Aldehyde
Amine
Product
Time (h)
3
Yielda (%)
89
9
90b
87c
84d
10
11
3
5
85
Reaction conditions: aldehyde (1 mmol), amine (1 mmol), alkyne (1.2 mmol), 20 mg Cu0-Mont. (0.05 mol %), solvent: toluene, temperature:
110 °C. Yields are given for isolated products.a The reaction products were free from copper as checked by the analytical method followed from
Vogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis; revised by G. Svehla, Fifth Edition, 1979, Longman Group Limited,
London, page no.: 215–217. Methodology: 100 mg of each of the reaction products (from entries 1 or 2) were dissolved in 10 mL of medium
concentrated HNO3 (8 M). The solution was diluted by adding 10 mL of distilled water followed by passing H2S gas through the solution for a
period of 5 minutes. No black precipitate was observed. Hence, reaction products were free from Cu. The detection concentration limit is: 1 in
5 × 106. Selectivity: 100%.b 1st run. c 2nd run. d 3rd run.
2-hydroxybenzaldehyde, it was observed that they required
relatively longer times and gave slightly lower yields com-
pared to aldehydes containing electron withdrawing groups,
e.g., 4-chlorobenzaldehyde and 3-bromobenzaldehyde. We
also used an aliphatic alkyne, i.e., 3-butyne-1-ol and allowed
it to react with two different iminium ions which were gener-
ated in situ from benzaldehyde and piperidine (entry 8) and
benzaldehyde and morpholine (entry 9), it was found that they
formed the desired propargylamines in a high yield. In another
strategy, we compared the synthesis of propargylamines by
keeping the benzaldehyde and phenylacetylene constant
and using different secondary amines such as piperidine,
morpholine, and diethyl amine. The results obtained from
these studies revealed that an aliphatic amine, i.e. diethylamine
(entry 7), shows a lower conversion than the cyclic amines,
i.e. piperidine and morpholine (entries 1 and 10). Next, we
explored our methodology for the synthesis of symmetrical
bis-propargylamine derivatives by coupling dialkyne, viz.
1,6-heptadiyne, with benzaldehyde and piperidine (entry 11)
and as expected, the desired product formed with a good
isolated yield (85%) but the reactions needed a comparatively
longer time, i.e. 5 h, for completion.
3.4. Recyclability of the catalyst
For practical applications of heterogeneous catalysts, the level
of reusability is a very important factor. The recyclability of
our catalyst for the three component coupling reactions was
investigated (Table 2) in the coupling of phenylacetylene with
benzaldehyde and piperidine (entry 1), 4-chlorobenzaldehyde
and piperidine (entry 5) and benzaldehyde and morpholine
(entry 10). The catalyst was recovered by a simple filtration
technique after each experiment. The recovered catalyst was
washed with acetone, dried in a desiccator and reused
directly with fresh reaction mixture and without further puri-
fication for the desired coupling products synthesis up to a
3rd run. The results (Table 2, entries 1, 5 and 10) show that
in each case the catalyst remains active for several runs with-
out significant loss in efficiency.
The recovered catalyst was further investigated through N2
adsorption–desorption, powder XRD studies and HRTEM
analysis. The identical shape of the isotherms and the hyster-
esis loop reveal that the characteristics of the catalysts are
retained even after successive reactions (ESI†). The specific
surface areas of the recovered catalysts decrease compared to
299 m2 g−1 of the freshly prepared catalyst (Table 1). The
decrease in surface area of the recovered catalysts after each
reaction may be due to blockage of the pores by the reactant
molecules. In the HRTEM analysis of the recovered Cu0-Mont.
(after 3rd run), it was found that a few particles aggregate,
but the sizes are still below 10 nm (Fig. 5). In the powder XRD
In order to show the efficiency as well as the practical
applicability of the catalyst (Cu0-Mont.), we scaled up the
reaction of benzaldehyde to 10 mmol with 10 mmol of piperi-
dine and 13 mmol of phenylacetylene under similar
optimised reaction conditions. The reaction proceeded well
with a 90% isolated yield and 100% selectivity at 5 h.
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Scheme 1 Tentative reaction mechanism of A3 coupling reaction.
Fig. 5 HRTEM image of recovered Cu0-Mont. after 3rd run.
4. Conclusion
Nanoporous modified montmorillonite was found to be an
excellent eco-friendly support material for the in situ syn-
thesis of Cu0-nanoparticles. The Cu0-nanoparticles were
spherical in shape and had sizes in the range of 0–10 nm.
These nanoparticles demonstrated high catalytic activity in
promoting the three-component coupling of aldehyde,
amine and alkyne via C–H alkyne-activation to synthesize
propargylamines with excellent yields and selectivity under
mild reaction conditions without requiring any additives or
an inert atmosphere. Further, the nanocatalysts were reused
for new batch of reactions without a significant loss in
their activity under similar conditions. The operational sim-
plicity, robustness of the catalyst as well as utilization of
environmentally benign indigenous materials as the catalyst
support and use of mild reaction conditions, make it
attractive for the large scale synthesis of biologically active
propargylamine molecules.
of the recovered catalyst, the appearance of low intense broad
peaks at around 2θ values 36 and 54° along with other char-
acteristic peaks of metallic Cu indicates the formation of a
small amount of CuO after the 1st run of the reaction
(ESI†.18 However, the efficiency of the recovered catalyst was
not affected much though a fraction of Cu(0) is converted to
CuO. This may be because Cu(II) is also known for exhibiting
high alkynophilicity. These experimental results are adequate
enough to establish the robust character of the catalyst and
also prove that the reactions occur in a heterogeneous process.
A tentative reaction mechanism for the three component
(A3 coupling) synthesis of propargylamines catalyzed by Cu0-
nanoparticles under heterogeneous conditions is proposed.
The tentative mechanistic scheme is pictorially illustrated in
Scheme 1. It is proposed that, in the first step, a terminal
alkyne is coordinated to Cu0-nanoparticles supported on
AT-Mont., which activates the C–H bond. As a result, the
corresponding copper–alkylidine complex is formed on the
surface of the AT-Mont. This is a very favourable step because
Cu metal is well known to exhibit high alkynophilicity for
terminal alkynes.12a,15a In the second step, an addition of the
copper–alkylidine complex to the iminium ion generated
in situ from the reaction between the aldehyde and the amine
occurs to give the desired product, and the catalyst is regene-
rated. The only theoretical by-product formed from the reac-
tion is water. The regenerated catalyst continues the catalytic
cycle until the completion of the reaction. The drastic reduc-
tion in catalyst amount requirement as well as reaction time
in these reactions is possibly due to the presence of highly
active Cu0-nanoparticles as the catalyst. The presence of a
robust and high surface area (422 m2 g−1) modified montmo-
rillonite as the support for the Cu0-nanoparticles also might
have a beneficial effect on the rate and selectivity of the reaction.
Acknowledgements
The authors are grateful to Dr. R. C. Boruah, Director, CSIR-
North East Institute of Science and Technology, Jorhat,
Assam, India, for his kind permission to publish the work.
The authors also thank Council of Scientific and Industrial
Research (CSIR), New Delhi, India, for financial support
(XIIth FYP Network projects: BSC 0112, CSC 0125, CSC 0135
and In-house project: MLP-6000/1). SJB thanks UGC, New
Delhi for JRF.
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1054 | Catal. Sci. Technol., 2014, 4, 1047–1054
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