Synthesis of Pd-CNT nanocomposites and investigation of their catalytic behavior in the hydrodehalogenation of aryl halides

Article abstract of DOI:10.1016/j.tetlet.2009.08.121

A catalytic system based on Pd-CNT nanocomposites for the hydrodehalogenation of aryl halides is described. Thiol groups were utilized as linkers to secure the Pd nanoparticles without agglomeration. The Pd-CNT nanocomposites effectively promoted the hydrodehalogenation of aryl halides at a low Pd content (～2.3%) and in the absence of any ligand. The results suggest that the CNTs could significantly influence the catalytic activities of CNT-supported metal catalysts for hydrodehalogenation.

Full text of DOI:10.1016/j.tetlet.2009.08.121

Tetrahedron Letters 50 (2009) 6290–6292
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of Pd–CNT nanocomposites and investigation of their catalytic
behavior in the hydrodehalogenation of aryl halides
*
*
Ja Young Kim, Youngshin Jo, Sunwoo Lee , Hyun Chul Choi
Department of Chemistry, Chonnam National University, Gwangju 500-757, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A catalytic system based on Pd–CNT nanocomposites for the hydrodehalogenation of aryl halides is
described. Thiol groups were utilized as linkers to secure the Pd nanoparticles without agglomeration.
The Pd–CNT nanocomposites effectively promoted the hydrodehalogenation of aryl halides at a low Pd
content (ꢀ2.3%) and in the absence of any ligand. The results suggest that the CNTs could significantly
influence the catalytic activities of CNT-supported metal catalysts for hydrodehalogenation.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 15 July 2009
Revised 27 August 2009
Accepted 31 August 2009
Available online 2 September 2009
Palladium is an attractive catalyst for the hydrodehalogenation
reaction because it promotes the cleavage of carbon–halogen
bonds and facilitates hydrogenation.1 In Pd-catalyzed hydrode-
halogenations, the reactivity is highly affected by both Pd content
and the ligand. Many researchers have reported that a number of
ligands such as N-heterocyclic carbene,2 triphenyl phosphine,3
and phosphite4 could be utilized to accomplish Pd-catalyzed hyd-
rodehalogenations. Although these catalysts demonstrated excel-
lent catalytic performance, the high Pd loading (5–10 wt %) and
use of ligands are detrimental due to high cost, ligand contamina-
tion, and difficulties with respect to reuse and recovery. Recently,
Sanjiki group reported Pd/C-catalyzed hydrodehalogenation, how-
ever, they employed hydrogen gas as a hydrogen source.5 There-
fore, the development of inexpensive, convenient, and ligand-free
heterogeneous catalysts for hydrodehalogenation reactions re-
quires further study.
Recently, carbon nanotubes (CNTs) have received considerable
attention as catalyst supports in both heterogeneous catalysis
and electrocatalysis due to their high mechanical strength, large
surface area, good electrical conductivity, and durability under
harsh conditions.6 Some researchers have reported CNT-supported
catalysts (e.g., Pt, Pd, Au, Ru, and RuO2) exhibiting good catalytic
behaviors under various chemical reaction conditions, involving
methanol electro-oxidation,7 selective hydrogenation,8 Suzuki
coupling,9 CO oxidation,10 and Fischer–Tropsch synthesis.11 How-
ever, few studies have centered upon CNTs as catalyst supports
for hydrodehalogenation reactions, a key approach in preventing
environmental pollution.12
In the present work, the high-yielding hydrodehalogenations
accomplished at low Pd contents (ꢀ2.3%) and in the absence of
any ligand are reported. Catalytic behaviors were also compared
with the reference systems. The Pd–CNT nanocomposites used in
this work were prepared by depositing Pd2(dba)3 on thiolated mul-
tiwall carbon nanotube (MWNT) surfaces.
MWNTs were obtained from Carbon Nano Tech. Co., Ltd. (South
Korea). Sodium hydrogensulfide (NaSH) and tris(dibenzylideneace-
tone)dipalladium(0) [Pd2(dba)3] were purchased from Aldrich. Ni-
tric acid (HNO3), sulfuric acid (H2SO4), and tetrahydrofuran (THF)
were of analytical grade and were used as received.
MWNTs were stirred in an acid solution of HNO3 and H2SO4
(1:3 by volume) at 90 °C for 3 h. The MWNTs were then filtered,
washed with distilled water, and dried in an oven at 110 °C. The
acid-treated MWNTs were dispersed in THF and then the NaSH
aqueous solution was added to produce the thiol groups on the
MWNT surfaces. The thiolation was confirmed using the XPS
spectrum in the sulfur 2p region. Finally, the thiolated MWNTs
were dispersed in THF and then the Pd2(dba)3/THF solution was
added. The mixture was stirred for 20 h until all Pd2(dba)3 precur-
sors were anchored onto the MWNTs. The Pd–CNT nanocompos-
ites were separated from the mixture by filtration, washed several
times with pure ethanol and DI water, and dried in a vacuum
oven at 50 °C for 4 h. To verify the support effect, the mixture
of Pd2(dba)3/CNT was also prepared by the following method. A
2.3 mg of Pd2(dba)3 and 100 mg of pristine CNT were added in
THF and the reaction mixture was stirred at room temperature
for 3 h. The solvent was evaporated and the residue was dried
in vacuum for 12 h.
The size and structure of Pd–CNT nanocomposite were exam-
ined by scanning electron microscopy (SEM, Jeol JSM-7500F) and
transmission electron microscopy (TEM, Jeol JEM-2010, 200 kV).
X-ray photoelectron spectroscopy (XPS) was measured with VG
* Corresponding authors. Tel.: +82 62 530 3385/3491; fax: +82 62 530 3389.
E-mail addresses: sunwoo@chonnam.ac.kr (S. Lee), chc12@chonnam.ac.kr (H.C.
Choi).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.08.121

J. Y. Kim et al. / Tetrahedron Letters 50 (2009) 6290–6292
6291
multilab 2000 equipment (ThermoVG scientific) using the Mg K
X-ray line of 1253.6 eV excitation energy.
The typical hydrodehalogenation reactions were preformed in a
round-bottomed flask fitted with water-cooled condenser.
4-Bromotoluene (3.0 mmol), Pd–CNT (0.069 mmol), and Cs2CO3
(3.6 mmol) were added in 20 mL cyclohexanol solvent. The reac-
tion mixture was stirred and heated at 100 °C for 10 h. The product
was separated by distillation method and identified by GC mea-
surements. The remained Pd–CNT was washed with Et2O and dried
in vacuum, and used again for the second run.
a
C 1s
a
Pd MVV
O 1s
O KLL
Pd 3d
Pd-CNT
Figure 1a shows a typical SEM image of pristine CNT with a
diameter in the range of 10–20 nm. All of the tubes had a clean sur-
face. For the Pd–CNT nanocomposite, the SEM image clearly shows
that nano-sized particles were highly dispersed on the CNTs
(Fig. 1b). Smaller and highly dispersed nanoparticles were much
more abundant than larger aggregated ones. The average particle
size was estimated to be ꢀ8.0 nm. The TEM images further reveal
the nanoparticles attached to the sidewall of the CNTs (Fig. 1c).
According to EDXS analysis (Fig. 1d), the species supported on
the CNT was Pd.
Pristine CNT
200 0
1000
800
600
400
Binding energy (eV)
Figure 2. XPS survey spectra of pristine CNT and Pd–CNT nanocomposite.
Figure 2 shows the XPS survey spectrum of the Pd–CNT nano-
composites as well as the spectrum of the reference material (pris-
tine CNT). The XPS results indicated that C, O, and Pd elements
exist on the Pd–CNT surface. The relative surface atomic ratio
was estimated from the corresponding peak areas, corrected with
tabulated sensitivity factors. The estimated value of the Pd content
was about 2.3 atomic %.
In order to investigate the activity of Pd–CNT nanocomposites in
the hydrodehalogenations, reactions of aryl halides with Pd–CNT
nanocomposites in the presence of base and a solvent bearing a
beta-hydrogen, isopropanol, and cyclohexanol, were initially carried
out. Based on our previous work,4 arylchloride was reacted with
NaOtBu in the isopropanol (entries 1–3) and aryl bromide was re-
acted with Cs2CO3 in the cyclohexanol (entries 4–8). As expected,
the reactions with both electron-withdrawing and electron-donat-
ing substituents gave the desired hydrodehalogenated products in
good to excellent yields. The results are summarized in Table 1.
When the catalyst was recovered and used again in the hydrode-
halogenation of aryl halides, the yields were acceptable.
To verify the support effect upon hydrodehalogenation, a com-
parative experimental using Pd–CNT nanocomposite, Pd2(dba)3,
and a Pd2(dba)3/CNT mixture under identical conditions was
conducted. Figure 3 shows the relationship between the reaction
time and product yield of the hydrodehalogenation of 4-bromo-
toluene. The curves clearly indicate that the activity of Pd–CNT
nanocomposites was much higher than that of the others. This
difference can be attributed to the metal–CNT interaction and
mass transfer induced by the CNTs. Some researchers suggested
that CNT-supported catalysts showed much better activity and/
or selectivity than other supports due to the metal–CNT interac-
tion.13 This interaction induces a peculiar microstructure or mod-
ification of the electron density in the metal cluster, and enhances
Figure 1. (a) SEM image of pristine CNT. (b) SEM image of the Pd–CNT nanocomposite. (c) TEM image of the Pd–CNT nanocomposite. (d) EDXS spectrum of the Pd–CNT
nanocomposite.

6292
J. Y. Kim et al. / Tetrahedron Letters 50 (2009) 6290–6292
Table 1
100
Hydrodehalogenation of aryl halides using Pd–CNT nanocomposites
90
80
70
60
50
40
30
20
10
0
cat. CNT-Pd
Ar
X
Ar H
Base, solvent
Temp, 10 h
Entry
1
Ar-X
Methoda
Product
Yieldb (%)
99(95)c
Pd-CNT nanocomposites
Pd2(dba)3
Pd2(dba)3/CNT mixture
Cl
H
A
Me
Me
Cl
H
2
3
A
A
98(94)c
96(92)c
Cl
H
MeO
Me
MeO
Me
0
2
4
6
8
10
12
Reaction time (h)
Br
H
4
5
B
B
94(92)c
95(89)c
Figure 3. Catalytic activities of Pd–CNT, Pd2(dba)3, and a Pd2(dba)3/CNT mixture in
the hydrodehalogenation of 4-bromotoluene.
Br
H
Acknowledgments
t-Bu
t-Bu
This work was supported by the Korea Research Foundation
Grant funded by the Korean Government (MOEHRD, Basic Research
Promotion Fund) (KRF-2008-331-C00192).
Me
Me
Br
Me
Br
H
6
B
98(95)c
Me
Me
Me
H
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Reaction method A: 2.3 mol % of Pd, Aryl chloride 3.0 mmol, and NaOtBu
a
3.6 mmol were reacted in the isopropanol at 80 °C for 10 h. Reaction method B:
2.3 mol % of Pd, aryl bromide 3.0 mmol, and Cs2CO3 3.6 mmol were reacted in the
cyclohexanol at 100 °C for 10 h.
b
All compounds are characterized by comparison of gas chromatography (GC)
analysis, 1H and 13C NMR spectra with authentic samples or literature data.
c
Yields were obtained from the second use of Pd–CNT. In this study, the product
yield is determined by GC, 1H and 13C NMR spectra.
the catalytic activity. Adsorption of aryl halides is also favored by
van der Waals interactions between the CNTs and aromatic rings,
which could show favorable reactant–product mass trans-
portation.14
In summary, we examined the hydrodehalogenation of aryl ha-
lides with Pd–CNT nanocomposite under ligandless conditions. The
Pd–CNT nanocomposites were prepared by depositing Pd2(dba)3
on thiolated nanotube surfaces. They effectively promoted the
hydrodehalogenation of aryl halides at low Pd content (ꢀ2.3%) in
the absence of any ligand, and exhibited higher activity than that
of the reference systems. The results suggest that the CNTs could
significantly influence the catalytic activities of the CNT-supported
metal catalysts for hydrodehalogenation.
14. (a) Vermisoglou, E. C.; Georgakilas, V.; Kouvelos, E.; Pilatos, G.; Viras, K.;
Romanos, G.; Kanellopoulos, N. K. Micropor. Mesopor. Mater. 2007, 99, 98–105;
(b) Vermisoglou, E. C.; Romanos, G. E.; Tzitzios, V.; Karanikolos, G. N.; Akylas,
V.; Delimitis, A.; Pilatos, G.; Kanellopoulos, N. K. Micropor. Mesopor. Mater.
2009, 120, 122–131.