High catalytic performance of palladium nanoparticles supported on multiwalled carbon nanotubes in alkene hydrogenation reactions

Article abstract of DOI:10.1039/c3nj00183k

The synthesis of Pd nanoparticles (Pd-NPs) supported on multi-walled carbon nanotubes (MWCNTs) and the cataytic performance of the resulting material (Pd-NPs/MWCNTs) in hydrogenation reactions are presented. Facile preparation approaches based on the decomposition of Pd precursors in the presence of MWCNTs lead to homogeneous dispersions of supported Pd-NPs with an average size of 4 nm and Pd loads of about 12%. The catalytic performance of this material was evaluated in hydrogenation reactions of α,β-unsaturated ketones, alkenes, cyclic di-, tri- and tetraenes, aromatic compounds, terpenes and terpenoids, resulting in very high activity offering short reaction times, high conversion rates, notable selectivity, and acceptable recyclability under mild conditions.

Full text of DOI:10.1039/c3nj00183k

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High catalytic performance of palladium nanoparticles
supported on multiwalled carbon nanotubes in alkene
hydrogenation reactions†
Cite this: DOI: 10.1039/c3nj00183k
Manuela Cano,a Ana M. Benito,*a Wolfgang K. Masera and
Esteban P. Urriolabeitia*b
The synthesis of Pd nanoparticles (Pd-NPs) supported on multi-walled carbon nanotubes (MWCNTs) and
the cataytic performance of the resulting material (Pd-NPs/MWCNTs) in hydrogenation reactions are
presented. Facile preparation approaches based on the decomposition of Pd precursors in the presence
of MWCNTs lead to homogeneous dispersions of supported Pd-NPs with an average size of 4 nm and
Pd loads of about 12%. The catalytic performance of this material was evaluated in hydrogenation
reactions of a,b-unsaturated ketones, alkenes, cyclic di-, tri- and tetraenes, aromatic compounds,
terpenes and terpenoids, resulting in very high activity offering short reaction times, high conversion
rates, notable selectivity, and acceptable recyclability under mild conditions.
Received (in Porto Alegre, Brazil)
15th February 2013,
Accepted 24th March 2013
DOI: 10.1039/c3nj00183k
www.rsc.org/njc
excellent catalytic behavior.16–26 Their large surface/volume ratio
makes them ideal supports for the incorporation of large amounts
Introduction
The hydrogenation of organic molecules is probably one of the
most important chemical reactions for the synthesis of new
compounds.1 This process can be catalyzed homogeneously or
heterogeneously, but it is well recognized that the heterogeneous
version is by far more interesting from an industrial point of view,2
offering well-known benefits in terms of waste reduction, separation
of the catalysts and recyclability.3 With the aim of improving
efficiencies, new catalysts and supports are being developed con-
tinuously. A metal displaying a high catalytic activity is palladium,4
especially when present in the form of nanoparticles (Pd-NPs).
Controlling the sizes and shapes of the nanoparticles is considered
key to improved catalytic reactions.5,6 This is closely linked to the
development of suitable stabilizing agents and supporting materials:
polymers,7–9 ionic liquids,10–12 dendrimers,13 metal oxides14,15 and
carbonaceous materials16,17 are widely used for this purpose. Among
the latter, carbon nanotubes (CNTs) have received increasing atten-
tion in the last few years due to their fascinating properties and
of metal nanoparticles. However, in order to achieve a robust
catalyst/support ensemble, stable under the catalytic conditions,
the use of functionalized CNTs is often required. Unfortunately,
the pre-functionalization of the CNTs involves numerous
synthetic steps and/or tedious manipulations, which make
them less attractive.
We have recently reported a simple and fast microwave-
assisted synthesis method to prepare Pd-NPs–MWCNTs hybrid
materials.27–30 Without applying any pre-functionalization step
the resulting Pd-NPs/MWCNTs displayed a high stability and an
impressive catalytic activity in C–C coupling reactions (Heck,
Suzuki), while achieving full recovery of the catalyst.30 Continuing
this work, in this contribution we present results on the catalytic
performance and recyclability behaviour of the hybrid Pd-NPs–
MWCNTs in hydrogenation reactions of a wide variety of
industrially relevant substrates. The results obtained show a very
high activity and notable selectivity under mild reaction conditions,
avoiding the use of high pressures and/or temperatures.
a
´
´
Instituto de Carboquımica, ICB-CSIC, Miguel Luesma Castan 4, E-50018
Zaragoza, Spain. E-mail: abenito@icb.csic.es; Fax: +34 976733318;
Tel: +34 976733977
Results and discussion
b
Synthesis and characterization of the Pd-NP–MWCNT hybrid
catalysts
In a previous communication,30 we reported that the reaction
of MWCNTs with the palladium complex [Pd2(dba)3ÁCHCl3]
(dba = di(benzylidene)acetone) under conventional or microwave
´
´
´
´
Instituto de Sıntesis Quımica y Catalisis Homogenea, CSIC-Universidad de
Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain.
E-mail: esteban@unizar.es; Fax: +34 976761187; Tel: +34 976762302
† Electronic supplementary information (ESI) available: Graphics and tables for
the measurement of conversion versus time by GC for the hydrogenation of
cyclohex-2-enone. See DOI: 10.1039/c3nj00183k
c
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carbon nanomaterial. This Pd hybrid material was subse-
quently used in catalytic hydrogenation reactions as will be
described in the following section.
Catalytic activity of the Pd-NP/MWCNT materials for the
hydrogenation of olefinic substrates
The behaviour of the Pd-NPs/MWCNTs materials as hydrogena-
tion catalysts has been studied in depth. Preliminary results
were promising, since a high activity was found for the hydro-
genation of styrene and methylacrylate.30 Aiming to expand the
scope of the reaction we have used these materials as catalysts
for the hydrogenation of substrates shown in Table 1.
Fig. 1 Two different TEM images of the Pd-NP/MWCNT material.
As a general trend, we have observed full conversions in
all cases with amounts of catalyst ranging 1–5%. The hydro-
genation of acetophenone using 5% Pd under mild conditions
heating afforded the hybrid material Pd-NPs–MWCNTs with
high loading of Pd (up to 40 wt%) at reaction times of a few
minutes (microwave). For this work here we reacted MWCNTs
and [Pd2(dba)3ÁCHCl3] in a 1 :1 ratio for 2 min microwave heating,
affording Pd-NP–MWCNT hybrid material with a Pd load of about
12.5 wt%. The efficiency of this reaction is about 70%.
TEM images of the material (Fig. 1) show clearly that a large
amount of Pd-NPs were deposited on MWCNTs. The nanoparticles
are homogeneously distributed, and no large nanoclusters were
observed. Moreover, the statistical analysis of the NPs size reveals
that most of the nanoparticles are comprised in a very narrow range
of diameters, whose average value is 4 nm. The high thermal
conductivity of the MWCNTs allows for efficient transmission of
the microwave radiation along the whole material, avoiding the
formation of hot spots or, in other words, of preferential nucleation
sites. Therefore, the nanoparticles are produced under the same
conditions throughout the material resulting in a high dispersity of
the particles and a homogeneous distribution of sizes.
The X-ray diffractogram (Fig. 2a) is dominated by the
presence of Pd (111), (200) and (220) peaks, evidencing that
the Pd(0) nanoparticles are crystallized in a face-centred cubic
structure. In addition the diffraction peak at 261, corresponding
to the graphite (002) interlayer distance, indicates the presence
of MWCNTs. The Raman spectrum (Fig. 2b) corroborates that
the incorporation of the Pd nanoparticles into the MWCNT
skeleton does not promote any important alteration of the
latter, as previously observed.30
We thus have achieved the synthesis of a hybrid Pd-NPs–
MWCNTs material through a straightforward, facile and effec-
tive preparation route. In this material Pd nanoparticles with
an average size of 4 nm were homogeneously supported on the
(T = 25 1C, pH = 1 atm) gives ethylbenzene (entry 1) without any
2
detectable traces of phenylethanol. It is worth noting that these
mild conditions clearly differed from the high pressures and
temperatures usually needed for the hydrogenation of this type
of substrates.31 When competing unsaturations are present in
the same substrate the hydrogenation occurs selectively. The
hydrogenation of methylvinyl ketone (entry 2) or cyclohex-2-
enone (entry 3) takes place with regioselective reduction of the
CQC double bond, while the CQO moiety remains unchanged,
this selectivity being always difficult to be achieved.26,32,33 In
addition to the selectivity found in these hydrogenations,
we have also noticed a high activity of the Pd-NP/MWCNT
catalysts, similar to that reported in the literature for conven-
tional Pd/C catalysts (same order of magnitude), and higher
than that described for other systems based on SWCNTs.26
Moreover, our hydrogenation processes take place under
milder reaction conditions than previously reported, which is
an additional advantage.
The hydrogenation of cyclohexene (entry 4) and 1,3-cyclo-
hexadiene (entry 5) always afforded cyclohexane as the final
product without detectable traces of intermediates. The hydro-
genation of 1,5-cyclooctadiene (entry 6) shows again excellent
conversions and selective formation of saturated cyclooctane.
It is noteworthy that the hydrogenation of 1,3,5,7-cyclooctate-
traene (COT) afforded almost selectively the partially hydro-
genated product, cyclooctene (COE), in good yield, when short
reaction times are used (entry 7). As expected, when longer
reaction times are allowed the fully hydrogenated derivative
cyclooctane is obtained (entry 8). Although several authors report
on the determination of the heat of hydrogenation of COT,34 the
selective metal-catalysed hydrogenation of COT to COE has never
been reported so far, to the best of our knowledge.
Even more interesting is the Pd-catalysed hydrogenation of
1,3,5-cycloheptatriene (CHT), which has been scarcely investi-
gated.35 As can be seen, by only changing the amount of catalyst
it is possible to control the target-hydrogenated product: while
the Pd-NP/MWCNT catalyst at a concentration of 5% (Pd) gives
the fully hydrogenated cycloheptane (entry 10), a concentration
of 2.5% of Pd selectively affords cycloheptene at the same
reaction time (entry 9). In both cases quantitative yields are
obtained. Due to the interest in cycloheptene (raw material in
Fig. 2 Characterization of the Pd-NP/MWCNT material: (a) XRD spectrum;
(b) Raman spectrum.
c
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Table 1 Hydrogenation of relevant substrates using Pd-NP–MWCNT hybrid catalysts
our catalysts are not able to hydrogenate the strongly aromatic
benzene-like structures. Probably the different resonance
energy of the azulene compared with that of benzene could
account for the distinct reactivity. Although the absolute values
of energy differ as a function of the bibliographic source,
it seems accepted that azulene is less aromatic than benzene
and that this lower aromaticity, expressed as the difference
Catalysta,
Yieldb
(%)
TOFc
(hÀ1
Entry Substrate % mmol (time, h) Product
)
1
2
3
5 (8 h)
5 (2 h)
1 (1 h)
100
100
100
2.5
10
in resonance energy, amounts to around 10 kcal molÀ1 36
.
Therefore, being less aromatic, azulene is hydrogenated easier
than benzene.
100
50
We have also carried out the hydrogenation of a naturally
occurring product, such as R-(+)-limonene, a cyclic terpene. The
hydrogenation of R-(+)-limonene (entry 12) occurs with total
conversion of the starting material although, as it has been
observed in preceding cases, the reaction affords mainly a
mixture of three different isomers. Interestingly, the reaction
occurred with some degree of selectivity, because usually up to
six different products can be observed upon the hydrogenation
of limonene.37 The main components of the mixture (71%) are
the two diastereoisomers of p-menthane (cis and trans), while
the interesting p-menthene amounts 27%, and a very small
amount of p-cymene (2%) is detected.
4
1 (2 h)
100
5
6
7
8
9
1 (2 h)
100
100
62d
100
100
50
50
50
40
20
1 (2 h)
1 (1.25 h)
1 (2.5 h)
2.5 (2 h)
Finally, the hydrogenation of the industrially relevant
R-carvone (entry 13) affords a mixture of cis- and trans-
carvomenthone and carvacrol (keto/alcohol molar ratio =
2.5/1) in 24 h under the usual mild conditions (T = 25 1C,
pH = 1 atm). Full conversion has been achieved, this fact being
2
10
11
5 (2 h)
1 (5 h)
100
100
10
20
an improvement with respect to previous contributions using
Pd7,38 or Pt38,39 as catalysts. Other systems have proved to be
slightly more efficient,40,41 but need somewhat more sophisti-
cated equipment due to the use of sCO2 as solvent and high
pressures. As expected, the hydrogenation of the CQC bonds
occurs preferentially, affording the cycloaliphatic ketone, while
the valuable carvacrol41 results from the isomerization of the
starting carvone.38 Therefore, the simple catalytic procedure
described here is quite active and selective, since only two of
the seven possible reaction products have been obtained, with
an observed molar ratio typical for this type of process.7,38–41
Another important aspect is the study of the recyclability of
the catalyst. In the following we report on one representative
case, namely the hydrogenation of cyclohex-2-enone (entry 3).
For this purpose we have subjected the same catalyst to several
consecutive batch additions of starting compound and tested
the conversion for each step. After each run is completed the
catalyst is isolated by filtration, washed and dried before a new
batch of reactants is added. The results are collected in Table 2.
As it can be seen, the catalyst remains totally active for 4 complete
runs. The activity drops during the fifth run to a still remarkable
value of 60%. These results are similar to those reported pre-
viously by our group for this type of Pd-NP/MWCNT catalysts in
Heck coupling reactions.30
12
1 (24 h)
1 (24 h)
71/2/27e
72/28e
13
a
Catalyst (%) = [(mmol Pd)/(mmol substrate)] Â 100. The amount of the
hybrid Pd-NPs–MWCNTs containing the required mmol of Pd will
depend on the amount of Pd in this particular sample (typically 12%).
b
d
c
Isolated yield. TOF
The product also contains small (o5%), but detectable amounts
=
mmol product/[mmol Pd
Â
time(h)].
e
of 1,5-cyclooctadiene and cyclooctane. Molar ratios determined by
integration of the corresponding peaks in the NMR spectra.
organic chemistry and a monomer in polymer synthesis), a
further optimization was attempted. However, all changes in
the reaction time and/or temperature afford higher amounts of
the fully hydrogenated cycloheptane.
The hydrogenation of azulene (entry 11) affords the aliphatic
decahydroazulene upon reduction of all the five CQC bonds
under mild conditions. This is noteworthy, since azulene is
aromatic and, in principle, it should be more reluctant than
non-aromatic substrates to be hydrogenated. Moreover, in the
case of acetophenone we have not observed hydrogenation of
the arylic ring, even at the level of traces. This means that,
although active enough to hydrogenate aromatic compounds,
A detailed inspection of the two first runs of this catalysis
has been carried out following the conversion of the reaction as
a function of time using gas chromatography. The results show
(see ESI†) that both runs 1 and 2 present a sigmoidal shape.
This fact, altogether with the TEM characterization, is in good
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Table 2 Recyclability of the Pd-NP/MWCNT in the hydrogenation of cyclohex-2-
190915-413 (HP-5) (30 m  320 mm  25 mm). The temperature of
the injector and the detector was 250 1C in both cases. The
temperature program used was: 5 min at 50 1C; from 50 1C to
250 1C heating at 25 1C minÀ1; 2 min at 250 1C. Under these
conditions, the retention times observed were: cyclohexanone
3.88 min, cyclohexenone 5.38 min and decane 6.67 min.
enone
Run
1
2
3
4
5
Conversion (%)
100
100
100
100
60
agreement with the heterogeneous nature of the catalysis.42 How-
ever, it is evident the presence of a slight induction period in the
first run, which is not observed in the second one. This induction
period could be due to the presence of a small amount of oxidized
Pd2+ in the supported NPs, which are reduced by H2 to elemental
Pd0 in the first steps of the reaction.43 Thus, although with some
slight differences, runs 1 and 2 show the same basic features in
terms of recyclability. Therefore, the use of these materials is not
limited to the first run and can be reused several times with similar
characteristics and efficiency. Further improvements in their
designs are necessary in order to make them still more attractive
from an application point of view.
Preparation of Pd-NPs–MWCNTs hybrid materials
Pd-NP/MWCNT catalysts were prepared using microwave assisted
synthesis in a CEMt S-class Microwave Synthesis System according
to a recently published procedure.30 In brief, MWCNTs and the
commercially available Pd complex [Pd2(dba)3ÁCHCl3] were reacted
in a 1 : 1 weight percent ratio as follows: 100 mg of MWCNTs and
100 mg of [Pd2(dba)3ÁCHCl3] in 20 mL of freshly distilled toluene
were reacted at 110 1C under an Ar atmosphere applying a
microwave power of 15–20 W. Reaction was completed after
2 min of microwave heating. The resulting yellow suspension
was centrifuged at 4000 rpm for 10 min and the precipitate was
re-suspended in 20 mL of toluene and filtered. The black solid
material was washed with 5 mL of CH2Cl2 and 15 mL of water, and
was dried at 110 1C for one day, affording powder-like Pd-NP/
MWCNT material containing about 12.5 wt% of Pd.
Conclusions
High performance catalysts based on palladium nanoparticles
deposited on MWCNTs have been prepared by decomposition of
[Pd2(dba)3ÁCHCl3] in the presence of MWCNTs under microwave
irradiation in a simple one-pot procedure. The resulting hybrid
material is characterised by Pd(0) nanoparticles with an average
size of 4 nm homogeneously distributed throughout the support.
Its catalytic activity has been tested in the hydrogenation of
different substrates. Full conversions are achieved for a broad
range of materials, such as a,b-unsaturated ketones (acetophenone,
MVK), olefins, poly-olefins (CHD, COD, CHT, COT), aromatic
substrates (azulene) and natural products (limonene, carvone).
Showing as well acceptable recyclability behaviour the developed
catalysts may offer promise for further industrial exploitation.
Characterization of the Pd-NPs–MWCNTs hybrid materials
The morphology of the prepared hybrid materials was studied by
transmission electron microscopy (TEM, JEOL-2000 FXII working at
200 kV). Samples were dispersed in ethanol in an ultrasound bath
for 10 min, and a drop of the suspension was placed on a copper
grid coated with carbon film. Powder X-ray diffraction (XRD)
measurements were carried out at room temperature on a Bruker
D8 Advance diffractometer using a Cu Ka X-ray radiation. The metal
loading in the carbonaceous samples was determined by Inductive
Coupled Plasma Spectroscopy (ICPS) using a Jobin-Ybon 2000
Ultrace Analyzer. Raman spectra were recorded using a Horiba Jobin
Yvon HR800 UV spectrometer at an excitation wavelength of 532 nm.
Catalytic hydrogenation experiments
Experimental details
General considerations
The Pd catalyst supported on MWCNTs, in the appropriate amount
(Table 1), was dispersed in CHCl3 (2 mL) under stirring in a 5 mL
Solvents were dried and distilled using standard procedures before round-bottomed flask at room temperature and atmospheric pres-
use. MWCNTs were purchased from Nanocyl Co, Belgium (Nanocylt sure. To this suspension 1 mmol of the substrate to be hydrogenated
NC7000, 90% carbon purity). Reagents were used as purchased from was added. The system was then evacuated and backfilled with H2 in
various commercial sources. The products of the catalytic reactions cycles at least five times. The pressure of H2 was kept constant (pH
=
2
were characterized by NMR spectroscopy and mass spectrometry. 1 atm) using a balloon system during the whole reaction time. Once
The 1H and 13C{1H} NMR spectra were recorded in CDCl3 solutions the hydrogenation has been performed, the resulting suspension
at 25 1C on a Bruker Avance-400 spectrometer (d, ppm; J, Hz), and was filtered through a 3 mm pore polycarbonate membrane filter,
were referenced using the solvent signal as internal standard. ESI/ and the solid catalyst material was washed with CH2Cl2 and water,
APCI mass spectra were recorded using an Esquire 3000 ion-trap and dried at 110 1C overnight. The solvent was removed from the
mass spectrometer (Bruker Daltonic GmbH, Bremen, Germany) organic solution by fractional distillation, affording the hydroge-
equipped with a standard ESI/APCI source. Samples were introduced nated products as oily residues, which were characterized by com-
by direct infusion with a syringe pump. Nitrogen served both as the parison of their 1H and 13C NMR spectral data with the
nebulizer gas and the dry gas. Helium served as a cooling gas for the corresponding reference samples found in the literature.
ion trap and collision gas for MSn experiments. The chromato-
Recyclability of the catalysts
graphic analysis of the products of the catalytic reaction was carried
out on a Gas Chromatograph Agilent 7890A equipped with a flame The tests of recyclability were performed following the general
detector and using He as a carrier. The column used was an Agilent experimental procedure for hydrogenation. Therefore, 1 mmol
c
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of cyclohex-2-enone was dissolved in 2 mL of CDCl3, and the
appropriate amount of the composite Pd-NPs–MWCNTs to get
0.01 mmol of Pd was added. The system was then evacuated and
backfilled with H2 keeping its pressure constant at 1 atm. The
reaction was followed by GC chromatography. The conversions
were determined every 15 min using decane as internal standard.
After 1 h of reaction time the conversion reached the 100% value.
At this moment the catalyst was isolated by filtration, it was
washed with CDCl3 and water, and dried at 110 1C overnight. The
catalyst thus isolated was evaluated in a second run using strictly
the same reaction conditions as those described for the first run
18 J. K. Lee and M. J. Kim, Tetrahedron Lett., 2011, 52, 499.
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´
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19 L. Rodrıguez-Perez, C. Pradel, P. Serp, M. Gomez and
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Acknowledgements
´
Financial support from the Spanish Ministerio de Economıa y
Competitividad (MINECO) and the European Regional Devel-
opment Fund (ERDF) under projects CTQ2011-22589 and
MAT2010-15026, CSIC under project 201080E124, and the
Regional Government of Aragon and the European Social Fund
(ESF) under projects E-97 and DGA-ESF T66 CNN is gratefully
acknowledged. M. C. thanks MICINN and ESF for her Grant No.
BES-2008-003503.
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