A new recyclable Pd catalyst supported on vertically aligned carbon nanotubes for microwaves-assisted Heck reactions

Article abstract of DOI:10.1016/j.crci.2011.04.007

Palladium supported on vertically aligned multi-walled carbon nanotubes (Pd/VA-CNTs) is used as catalyst for the C-C coupling reactions of p-iodonitrobenzene with styrene and ethyl acrylate under microwaves irradiation. Pd/VA-CNTs catalyst exhibits higher

Full text of DOI:10.1016/j.crci.2011.04.007

C. R. Chimie 14 (2011) 663–670
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Full paper/M e´ moire
A new recyclable Pd catalyst supported on vertically aligned carbon
nanotubes for microwaves-assisted Heck reactions
a,
a
b
b
Izabela Janowska *, Kambiz Chizari , Jean-Hubert Olivier , Raymond Ziessel ,
a
a
Marc Jacques Ledoux , Cuong Pham-Huu
a
Laboratoire des mat e´ riaux, surfaces et proc e´ d e´ s pour la catalyse (LMSPC), UMR 7515 du CNRS, ECPM-universit e´ de Strasbourg, 25, rue Becquerel,
7087 Strasbourg cedex 02, France
6
b
Laboratoire de chimie organique et spectroscopies avanc e´ es (LCOSA), dans LMSPC UMR 7515 du CNRS, ECPM-universit e´ de Strasbourg, 25, rue Becquerel,
7087 Strasbourg cedex 02, France
6
A R T I C L E I N F O
A B S T R A C T
Article history:
Palladium supported on vertically aligned multi-walled carbon nanotubes (Pd/VA-CNTs) is
used as catalyst for the C-C coupling reactions of p-iodonitrobenzene with styrene and
ethyl acrylate under microwaves irradiation. Pd/VA-CNTs catalyst exhibits higher activity
compared to Pd supported on activated charcoal, under the same reaction conditions. Due
to the microwaves irradiation, the kinetics of the reaction is strongly accelerated
compared to that obtained with a traditional heating mode. The macroscopic form of
aligned CNTs support allows an easy recovery of the catalyst, avoiding a costly post-
reaction filtration. In addition, the interaction between the active phase and the support
leads to the negligible leaching of palladium during recycling tests. The observed results
indicate that Pd/CNTs is a recyclable and stable heterogeneous catalytic system.
ß 2011 Published by Elsevier Masson SAS on behalf of Acad e´ mie des sciences.
Received 8 June 2010
Accepted after revision 27 April 2011
Available online 24 June 2011
Keywords:
Heck coupling
Heterogeneous catalysis
Aligned carbon nanotubes
Microwaves
Palladium
1
. Introduction
The Heck reaction, based on the palladium catalysed
Recently, some reports have focused on the develop-
ment of new ‘‘ligand-free’’ catalysts using stabilized
palladium nanoparticles such as metallic palladium and
palladium nanoclusters immobilized on solid supports.
Few examples of stable palladium nanoparticles which are
active in Heck cross-coupling reactions have been reported
[4]. Ying and co-workers have shown that palladium-
grafted molecular sieves prepared by vapour grafting
method exhibit a high activity for the studied reaction [5].
Highly active palladium species were also generated in situ
olefination of aryl halides, is an important C-C coupling in
the synthesis of pharmaceuticals and conjugated molecu-
lar systems for materials science applications [1]. Accord-
ing to this and in view of industrial applications, many
efforts have been devoted to the development of new Pd-
based catalysts, either homogeneous or heterogeneous,
which could be operated under mild conditions and a
simple operating process. The C-C coupling reactions based
on the use of homogenous catalysts is well understood and
widely investigated [2]. However, phosphine and phos-
phane ligands are used to stabilize and activate in situ the
catalytically active Pd(0) and to prevent formation of
particles aggregates and catalytically inactive ‘‘palladium
black’’ [3].
by dissolution of the Pd active phase from the MO
x
support
(TiO , Al , NaY) during the course of the reaction [6].
2
2 3
O
These last leached palladium species, however, were
considered as homogeneous-like catalysts which re-
precipitates onto the surface of the support during the
reaction. According to the other studies in the area, the
leached palladium species are considered as the real active
phase in the Heck reaction as well [7]. In order to obtain a
stable heterogeneous catalyst the choice of a support
which interacts strongly with the active phase is necessary.
Due to their specific chemical and physical properties,
carbon nanotubes and nanofibers (CNTs, CNFs) are
*
Corresponding author.
E-mail address: janowskai@unistra.fr (I. Janowska).
1
631-0748/$ – see front matter ß 2011 Published by Elsevier Masson SAS on behalf of Acad e´ mie des sciences.
doi:10.1016/j.crci.2011.04.007

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I. Janowska et al. / C. R. Chimie 14 (2011) 663–670
amongst the most explored new generation nanomaterials
equipped with a quartz tube reactor (diameter  length,
2
À1
À1
[
8]. Their large surface areas (> 100 m .g ), well-defined
4 Â 80 cm) in flowing argon (1.5 L.min ) at 850 8C. After
structures and the absence of micropores which could
induce diffusion phenomena are interesting properties to
be studied in catalysis. CNTs and CNFs have been studied as
catalyst supports for noble metals in several and mostly in
the hydrogenation and oxidation reactions [9].
These graphitic carbon nanomaterials have shown
improved catalytic efficiency compared to activated
charcoal or alumina-supported catalytic systems [10].
Recently, Yuan and co-workers have investigated a new
stable carbon nanofiber-supported palladium and pointed
that the high stability of this catalyst was related to the
presence of the small pores in this mesoporous system
behaving as containers for the metal particles [11].
Previous works from us and Corma’s group have also
reported highly active and stable palladium supported-
CNTs catalysts for C-C coupling reactions [12].
the reaction the reactor with VA-CNTs product was cooled
down under an argon flow to room temperature. The
product was stripped from the inner walls of the quartz
2
tube in pieces of average size of 3 cm . The CNTs were
further treated with nitric acid at 120 8C for 2 h in order to
remove the remaining Fe catalyst, washed several times
with distilled water and dry for 48 h at room temperature
and for 2 h at 100 8C.
The palladium catalyst was prepared by an incipient
wetness impregnation method. Twenty-five milligrams of
2
aligned nanotubes (0.5 cm of VA-CNTs plate) were
2
impregnated with a EtOH:H O solution (1:1) containing
Pd(NO and dried for 20 h at RT and for 2 h at 100 8C. The
3 2
)
sample was further calcinated in air at 300 8C, in order to
transform the Pd salt into Pd oxide and next, reduced in
2
flowing H at 400 8C for 2 h to obtain Pd (0).
Microwave (mW) heating has received an ever-increas-
ing interest during the last decade as it allows a significant
improvement of reaction conversion and selectivity along
with a drastic reduction of its duration. Up to now, the field
in which microwaves were mostly employed deals with
organic synthesis where short reaction time and usually
high product yield are gained [13]. Microwave-assisted
organic synthesis (MAOS), applied to C-C coupling reac-
tions, allowed a significant reduction of the reaction time
from several hours to a few minutes [14]. If the reagents or
2.2. Catalyst characterization
The specific surface area of the aligned CNTs was
2
measured by the BET method using N as adsorbant at
liquid nitrogen temperature (TriStar sorptometer). Before
measurement the sample was outgassed at 300 8C
overnight in order to desorb some moisture and impurities
from its surface.
The XPS measurements of the CNTs support were
performed on a MULTILAB 2000 (THERMO) spectrometer
the solvent do not absorb the
mW, sensitizers such as
graphite [15] or SiC materials are used [16]. Carbon
materials, possessing high electrical and thermal conduc-
tivity, are rapidly heated under microwave irradiation. It is
expected that their combination with the metal active
equipped with an Al ka anode (hn = 1486,6 eV) with
10 min of acquisition. Peak decomposition was made with
the ‘‘Avantage’’ program from Thermoelectron Company.
The morphology of the solid was examined by scanning
electron microscopy (SEM) on a JEOL 6700-FEG micro-
scope. The solid was fixed on the sample holder by a
graphite paste for examination.
The microstructure of the support and the average
particle size of the active phase were examined by
transmission electron microscopy (TEM) on a Topcon
002B-UHR microscope working with an accelerated
voltage of 200 kV and a point-to-point resolution of
0.17 nm. The sample was sonically dispersed in an ethanol
solution during 5 min and a drop of the solution was
deposited onto a perforated carbon membrane on a copper
grid for observation.
phase in
mW conditions should give rise to highly efficient
catalytic processes [12b,17].
In the present contribution, we were interested in the
use of the CNTs macroscopic pattern, i.e. vertically-aligned
carbon nanotubes (VA-CNTs) to support palladium parti-
cles and apply them as catalyst in the Heck reactions using
the microwave (mW) heating mode. The macroscopic form
of the catalyst avoids a cost incentive post-reaction
filtration which is a step forward with respect to the
separation and recycling problems of the catalyst, espe-
cially when the active phase is constituted of toxic or
precious metal [18]. The catalytic activity of Pd/VA-CNTs
was compared with a commercial Pd supported on
activated charcoal (AC).
Zeta potential measurements and pH titration were
carried out on the Nano-ZS (multipurpose titrator, Malvern
Instruments). Samples were sonicated in aqua NaClO
4
2
. Experimental
electrolyte solution for 30 min before measurements.
Titrations of the CNTs were performed using 10 M NaOH
À1
2.1. Catalyst preparation
and HCl aqua solutions. The Zeta potential values were
determinate from the particle velocities according to the
Pd/AC with a Pd loading of 10 wt.% is commercially
Helmholtz-Smoluchowskiequation:
z = 4pmh/Dwheremis
available (Alfa Aesar GmbH & Co KG) and was used as
received.
the electrophoretic mobility, is the viscosity, and D the
dielectric constant of the liquid in the boundary layer.
h
The aligned carbon nanotubes were obtained by
catalytic chemical vapour deposition (CCVD), using ferro-
cene as an iron-catalyst precursor and toluene as a carbon
2.3. Microwaves irradiation experiment
source [8b-e,19]. Ferrocene concentration in toluene
Microwaves irradiation experiments were performed
using a multi-mode Mars System from CEM Corporation
using standard Pyrex vessel. The temperature profiles for
À1
solution (Fe(C
5
H
5
)
2
/C
7
H
8
) was fixed at 20 g.L . The liquid
mixture was injected into the hot zone of the furnace,

I. Janowska et al. / C. R. Chimie 14 (2011) 663–670
665
microwaves experiments were recorded using a fiber-optic
probe protected by a sapphire immersion well inserted
directly into the reaction mixture. Pressure profiles were
recorded with a pressure sensor directly connected to the
reaction vessel.
J = 16.2 Hz), 4.30 (q, 2H, J = 7.2 Hz), 1.35 (t, 3H, J = 7.2 Hz).
1
3
3
C NMR (75 MHz, CDCl ): d 165.9, 148.3, 141.5, 140.5,
128.5, 124.0, 122.5, 60.9, 14.1.
2.6. (E)-1-nitro-4-styrylbenzene
2
.4. General procedure for the coupling between aryl iodide
The general procedure was followed using 4-iodo-
nitrobenzene (0.803 mmol, 200 mg) and styrene
and terminal olefins
(
2.007 mmol, 230 mL). The reaction was heated at 160 8C
Terminal
olefin
(2.007 mmol),
triethylamine
during 10 min. Purification by flash chromatography on
silica gel (30:70 dichloromethane: petroleum ether)
afforded the (E)-1-nitro-4-styrylbenzene as a yellow solid
(108 mg, 60%), and (Z)-1-nitro-4-styrylbenzene as a yellow
(
0
2.409 mmol, 333
m
L) and Pd(10%)/VA-CNTs (25 mg,
2
.5 cm plate) or Pd/AC (25 mg) were added to a solution
of aryl iodide (0.803 mmol) dissolved in 10 mL of EtOH. The
mixture was subsequently heated at 160 8C with stirring
during the appropriate time (300 W, 12–13 bars) in a
solid (16 mg, 9%).
1
(E)-1-nitro-4-styrylbenzene:
300 MHz):
7.56-7.54 (m, 2H), 7.43-7.31 (m, 3H), 7.27 (d, J = 16.3 Hz,
H
NMR
3
(CDCl ,
5
0 mL reactor vessel. After this, the mixture was cooled
d = 8.23-8.20 (m, 2H), 7.65-7.61 (m, 2H),
down to 60 8C, Pd(10%)/VA-CNTs plate was extracted with
tweezers or separated by filtration in the case of Pd/AC and
the remaining solution was evaporated to obtain a dark red
oil. The residue was purified by flash chromatography on
silica gel eluting with a mixture of dichloromethane and
petroleum ether to provide the desired product.
1
3
1H), 7.14 (d, J = 16.3 Hz); C NMR (CDCl
3
, 75 MHz, ppm):
d
= 146.9, 144.0, 136.3, 133.4, 129.0, 127.1, 127.0, 126.4,
124.2.
1
(E,E’)-1-nitro-4-styrylbenzene:
H
NMR (CDCl
3
,
300 MHz):
d = 8.21 (d, 2H, J = 7.9 Hz), 7.50 (d, 2H,
J = 8.2 Hz), 7.37-7.33 (m, 3H), 7.32-7.26 (m, 2H), 5.62 (s,
1
3
2.5. Ethyl (E)-3-(4-nitrophenyl)-2-propenate
1H), 5.55 (s, 1H); C NMR (CDCl
3
, 75 MHz, ppm): d = 146.9,
144.2, 135.9, 133.2, 129.4, 129.5, 126.9, 127.0, 126.3, 122.1.
The general procedure was followed using 4-iodo-
nitrobenzene (0.803 mmol, 200 mg) and ethyl acrylate
2.007 mmol, 218 L). The reaction was heated at 160 8C
3. Results and discussion
(
m
during 5 min. Purification by flash chromatography on
silica gel (40:60 dichloromethane:petroleum ether)
3.1. VA-CNTs characteristics
afforded the compound as a white solid (141 mg, 80%):
The self-supported vertically aligned nanotubes were
obtained in high yield in the form of macroscopic plates
(c.a. 3 cm ) with high mechanical strength (Fig. 1A). SEM
1
3
H NMR (300 MHz, CDCl ): d 8.25 (d, 2H, J = 8.7 Hz), 7.71 (d,
2
1
H, J = 16.2 Hz), 7.69 (d, 2H, J = 8.7 Hz), 6.57 (d, 1H,
Fig. 1. The aligned carbon nanotubes obtained by CVD method using ferrocene/toluene mixture. A. Macroscopic plate of aligned carbon nanotubes with
high mechanical strength obtained by stripping them from the inner walls of the reactor. B. High-resolution SEM images showing carbon nanotubes with a
homogeneous diameter centered at around 80 nm and no carbon particles. C, D. The CNTs carpet with a height of 0,6 mm before (C) and after (D)
irradiation. No damage of the aligned structure was observed.
mW-

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I. Janowska et al. / C. R. Chimie 14 (2011) 663–670
micrographs demonstrated aligned carbon nanotubes with
an average diameter of 80 nm and no carbon nanoparticles,
indicating a high selectivity of the synthesis method
walls with a narrow channel in the middle of c.a. 10 nm in
diameter. High-resolution TEM analysis (Fig. 2B) indicated
a high graphitisation degree of tubes walls and the
presence of small amount of defects on the CNTs surface,
which was in agreement with the low specific surface area
(Fig. 1B). A few experiments showed that the thickness of
VA-CNTs carpet can be controlled by varying a duration of
the CCVD process without modification of neither the
diameter nor the alignment of the tubes. The SEM
micrograph on the Fig. 1C corresponds to the 0.6 mm
thick carpet of VA-CNTs, used subsequently in the Heck
coupling reaction. The as-synthesized VA-CNTs were
2
À1
c.a. 30 m .g . The high crystallinity of the material with
low topological defects along the tube axis was related to
the relatively high synthesis temperature. High-resolution
TEM micrograph (Fig. 2B) also highlighted the almost
absence of amorphous carbon on the tube surface.
The following acid treatment of the synthesized CNTs
introduced oxygenated groups on the nanotubes surface.
The presence of the oxygenated groups rendered the CNTs
surface more hydrophilic, and thus increased the surface
wetability towards the polar solvents - used during the
subsequent impregnation step. The oxygenated groups
also provided anchorage sites for the active phase
precursor which resulted in a better dispersion and
stability of the Pd particles in the catalytic system
[10a,20]. The percentage of oxygen atoms decorating the
CNTs surface, estimated by XPS spectroscopy, steadily
increased after acid treatment from 2 to 16 at. %. According
to the C1s and O1s XPS spectra, a different type of the
oxygen species on the nanotubes surface was observed
(Fig. 3A and B).
3
treated in an acid medium (HNO , 1 M, 120 8C) in order
to remove the residual iron-based catalyst. As one can see
in Fig. 1A, patterned CNTs were mechanically resistant
which allowed an easy manipulation with a pair of
tweezers. The macroscopic form of nanotubes avoided
as well the formation of nanotube dust in the atmosphere
during the manipulation which was good for health safe.
The pattern of VA-CNTs remained unaltered after the acid
treatment according to the optical photos (Fig. 1A). Similar
mechanical resistance of VA-CNTs have also been reported
by Ajayan and co-workers [8d] during their study of CNTs
based filters. This very important mechanical resistance
was attributed to high degree of tube entanglement and
the van der Waals force between the tubes.
The morphology of the nanotubes from VA-CNTs carpet
was investigated by TEM. The TEM micrograph (Fig. 2A)
confirmed the presence of nanotubes with an average
diameter of 80 nm. The tubes consisted of relatively thick
The presence of oxygen groups was also reflected by the
nanotubes surface charge modification observed by Zeta
potential measurements. The results for untreated and
Fig. 2. A. TEM micrographs of carbon nanotubes obtained by pyrolysis of a mixture of ferrocene and toluene at 850 8C. B. High graphitization degree and
small amount of defects of the nanotube wall were observed.
Fig. 3. C1s (A) and O1s (B) XPS spectrum of the carbon nanotubes after acid treatment at 120 8C for 2 h showing the presence of several oxygenated
functional groups on the solid surface.

I. Janowska et al. / C. R. Chimie 14 (2011) 663–670
667
The negatively charged acid treated CNTs surface at the pH
of subsequent infiltration process (pH of nitrate of
palladium solution), ca. pH = 1.0 favored the anchorage
of palladium cations, when one should expect repulsive
interactions if the positively charged (untreated by acid)
CNTs are used.
3.2. Pd/VA-CNTs catalyst characteristics
A deposition of Pd nanoparticles on the CNTs surface
was carried out by the incipient wetness impregnation
method with an aqueous/ethanol solution containing
palladium nitrate, followed by appropriate calcinations
and reduction processes (see Experimental section). The
TEM micrographs indicated the formation of relatively
small palladium particles on the surface of the carbon
nanotubes. The average palladium particle size deduced
from TEM analysis ranged between 5 and 10 nm and no
aggregates were observed on the sample (Figs. 5 and 6A).
The HR-TEM micrograph in the middle-top on Fig. 5
shows a palladium particle strongly attached to the surface
of the CNT. Previous results on palladium supported on
commercial carbon nanotubes reported by our group have
evidenced the strong interaction between the palladium
particles and the tube surface through oxygenated
functional groups, leading to relatively high palladium
dispersion [22]. The homogeneous graphitic morphology
Fig. 4. Zeta potentials for pristine CNTs (-ƒ-) and acid treated CNTs (-&-)
as a function of pH of nanotubes aqua suspension.
acid treated CNTs as a function of pH of the nanotube aqua
suspension are presented in Fig. 4. The Zeta potentials of
pristine CNTs were negative in the major range of pH and
decreased gradually with increasing pH, with an isoelectric
point (IEP) at around 3.7 pH unities. The acid treated CNTs
displayed negative Zeta potential values over the whole pH
range demonstrating the presence of a high density of
acidic sites [21]. A negligible change of Zeta potential
values along the pH axis means a better solvatation by
polar solvents and higher stability of such a suspension.
Fig. 5. TEM micrographs of the CNTs supported Pd after calcinations and reduction. (Middle-top) HR-TEM of palladium particle on the CNT surface. The
average size of palladium particles are 5-10 nm.
Fig. 6. A. COMPO – SEM micrograph of aligned CNTs supported palladium. The high dispersion of metal particles was attributed to the presence of
oxygenated functional groups on tubes surface and homogeneous morphology of the tubes. B. Low magnification TEM micrograph showing the weak
dispersion of Pd particles on activated charcoal (AC).

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I. Janowska et al. / C. R. Chimie 14 (2011) 663–670
Scheme 1.
of CNTs is also an important factor which influences the
dispersion of metal on the tube surface.
under microwave irradiation which provides a rapid heat
supply to the catalyst regardless of the surrounding liquid
medium. A preliminary test, carried out in water, resulted
in the decomposition of ethyl acrylate. Thus water was
On another hand, representative TEM micrograph
(Fig. 6B) of the commercial Pd/AC (Pd/activated charcoal)
catalyst reveals a heterogeneous dispersion of palladium
particles. Two different populations of Pd particles are
observed: the small ones and aggregates of large size,
despite much higher specific surface area of AC compared
to that of CNTs (Fig. 6A). Similar results were observed in
previous works [22] and the heterogeneous dispersion was
attributed to the heterogeneous surface character of the
support. One can easily find a portion of the support, free of
Pd (left part in Fig. 6B) and others with large Pd
agglomerates (right part in Fig. 6B).
2 3
replaced by ethanol and the mineral base -K CO by
triethylamine. The reaction pathways are sketched in
Schemes 1 and 2.
The complete conversion of p-iodonitrobenzene was
obtained after 10 min of reaction at 160 8C with styrene
and after 5 min at 160 8C with ethyl acrylate when the Pd/
VA-CNTs catalyst was used. The difference in reaction time
was likely due to a different reactivity of the olefins.
The yields of the Heck coupling products performed
with both catalysts are presented in Table 1. The yield of
product from reaction with ethyl acrylate approached 80%
on Pd/VA-CNTs whereas the same product has been
isolated with the yield of 70% for the Pd/AC catalyst. The
reaction ran stereoselectively, since only the most
thermodynamically stable ‘‘E’’ isomer was isolated.
In the case of the reaction between p-iodonitrobenzene
and styrene, two isomers were present in the reaction
mixture with a larger amount of the ‘‘E’’ isomer. The first
isomer, (E)-1-nitro-4-styrylbenzene and the second prod-
3.3. Catalytic evaluation
To test Pd/VA-CNTsas catalyst in the Heck reaction, p-
iodonitrobenzene as an aryl halide was chosen to react
with styrene (Scheme 1) and ethyl acrylate (Scheme 2) as
an olefin. For comparison, commercial Pd supported on an
activated charcoal (Pd/AC) catalyst was also tested in the
same reaction conditions. All experiments were performed
Scheme 2.

I. Janowska et al. / C. R. Chimie 14 (2011) 663–670
669
Table 1
The yields of the Heck reactions products obtained with Pd/VA-CNTs and Pd/AC catalysts.
Catalyst
Yield (%)
E)-3-(4-nitrophenyl)-2-propenate
Yield (%)
Yield (%)
(
(E)-1-nitro-4-styrylbenzene
(E,E’)-nitro-4-styrylbenzene
Pd-10%/VA-CNT
Pd-10%/AC
80
70
60
27
9
5
uct, (E,E’)-nitro-4-styrylbenzene were isolated with the
yield of: 60%-9% on Pd/VA-CNTs and 27%–5% on Pd/AC,
respectively. The nature of the E,E’ isomer versus the
potential Z isomer was ascertained by proton NMR. Two
singlets were characteristic of the gem proton whereas two
doublet of an AB system was expected for the Z isomer
mass spectroscopy. Different values of Pd particles
leaching from CNTs and AC resulted from different
metal-support interactions. CNT walls present a well-
defined graphene structure and the important presence of
oxygenated groups provides a higher anchorage site
density for the deposited metal active phase than on AC.
Activated charcoal is formed from different aromatic
hydrocarbon platelets, connected through different het-
eroatom (O, N, S) bridges, and then weak interactions with
Pd ions are partially responsible for the major Pd leaching
during the course of the reaction and less homogeneous
dispersion of Pd particles on the solid surface. At the same
time high leaching and subsequent re-deposition phe-
nomenon during the cooling of the system, called the
‘‘boomerang effect’’, is often observed for the metal/C
systems [26]. We inferred that superior stability of Pd/VA-
CNTs catalyst over Pd/AC resulted from the stronger
interaction between the active phase and the support, and
the mesoporous character of CNTs [11]. It should be taken
into account as well that a difference in the porosity of both
‘‘carbons’’ (CNTs versus AC) influences diffusion rate of
reactants towards an active phase, which consequently
modify the activity and selectivity of the catalyst.
Additionally, the good conductivity of the carbon nano-
tubes favour the transport and vectorization of micro-
waves leading to a rapid and homogeneous heat transfer
through the catalyst body.
[
23]. Furthermore the absence of any H,H coupling
constant confirmed the E,E’ nature of the isomer.
Although the Pd/VA-CNTs catalyst exhibited similar
selectivity compared to Pd/AC, its activity was much higher
under similar reaction conditions: more than twice, when
styrene was used as an olefin. It was worth noting that the
reaction was very sloppy in the case of Pd/AC and several
purification steps were required to isolate pure derivatives.
In all cases the thermodynamically less stable ‘Z’ isomer
was not formed. The palladium size, dispersion and
stabilization against agglomeration were important factors
for the catalytic efficiency, since it is known that the C-C
couplings are sensitive to the nature of the active phase
[
24].
The low activity, in the C-C coupling reactions, of Pd/
activated charcoal compared to Pd supported on SWNTs in
the traditional heating mode was already reported [12b].
The Sheldon tests [25], performed after the catalytic
reactions, indicated a small leaching of palladium from the
tested catalysts, i.e. 1.0% from VA-CNTs and 3.8% from
activated charcoal. After the catalytic experiment, the
residue, obtained after separation of the catalyst and
evaporation of the solvent was mineralised with concen-
trated acid. After evaporation of the acid, the resulting solid
was re-dissolved in aqueous solution and analysed by ICP-
After the catalytic tests, the platelet of Pd/VA-CNTs was
easily recovered with tweezers from the reactor and re-
tested four times without any additional loss of activity.
The SEM analysis of re-used Pd/VA-CNTs showed no
Fig. 7. XRD spectra of Pd/VA-CNTs catalyst before and after Heck coupling reaction.

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I. Janowska et al. / C. R. Chimie 14 (2011) 663–670
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and no mass change. The Pd nanoparticle crystallinity or
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analysis before and after catalytic test (Fig. 7).
TEM observation of the catalyst after Heck reaction (not
reported) shows no evidence of sintering of Pd particles;
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