Hydrogenation of aryl ketones using palladium nanoparticles on single-walled carbon nanotubes in an ionic liquid

Article abstract of DOI:10.1016/j.tetlet.2010.11.081

Single-walled carbon nanotubes (SWNTs) are used as supporting materials for palladium (Pd) nanoparticles generated in situ in ionic liquid (IL); Pd nanocatalysts on SWNTs exhibit superior reactivity for hydrogenation of aryl ketones in IL under mild conditions (1 atm of H₂ (g) and room temperature) and can be reused above 10 times without any loss of catalytic activity.

Full text of DOI:10.1016/j.tetlet.2010.11.081

Tetrahedron Letters 52 (2011) 499–501
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Hydrogenation of aryl ketones using palladium nanoparticles
on single-walled carbon nanotubes in an ionic liquid
Jae Kwan Lee a, , Mahn-Joo Kim b,
⇑
⇑
a Department of Green Energy & Research Center for Convergence Technology, Hoseo University, Chungnam 336-795, Republic of Korea
b Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Single-walled carbon nanotubes (SWNTs) are used as supporting materials for palladium (Pd) nanopar-
ticles generated in situ in ionic liquid (IL); Pd nanocatalysts on SWNTs exhibit superior reactivity for
hydrogenation of aryl ketones in IL under mild conditions (1 atm of H2 (g) and room temperature) and
can be reused above 10 times without any loss of catalytic activity.
Received 27 October 2010
Accepted 15 November 2010
Available online 18 November 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Hydrogenation
Palladium nanoparticle
SWNT
Ionic liquid
1. Introduction
cules because of their large surface area to volume ratio and unique
properties.9 However, their applications in chemical reactions have
The room temperature ionic liquids (RTILs) have been attracting
growing interest as environmentally benign reaction solvents in
the chemical synthesis and biotransformations.1,2 Recently, the io-
nic liquids (ILs) have offered special opportunities due to their un-
ique characteristics in new applications, such as electrochemical
device3 and dye-sensitized solar cell.4 Nonetheless, a great effort
has been devoted for developing new green solvents for chemical
reactions up to now. The alternative use of ionic liquids as green
solvent enhanced the activity, selectivity, and stability of catalysts.
Moreover, the impressive progress for generation of transition-
metal-nanoparticles in ionic liquid and their efficient catalysis
have been reported in recent years.5 Among them, palladium nano-
particles/ionic liquid (IL) system is very useful for various organic
synthesis.6 However, the palladium nanoparticles in IL were read-
ily aggregated in hydrogenation using H2, resulting in the decrease
of their catalytic activity. To overcome this problem, Huang et al.
used the phenanthroline ligand-stabilized Pd nanoparticles in
hydrogenation of olefins.7 Recently, Chun et al. reported the Pd
nanoparticles deposited on imidazolium bromide-functionalized
ionic multi-walled carbon nanotube (MWCNT) for hydrogenation
of olefin.8
been a little explored, because they are essentially insoluble in any
solvent and are present in bundles or ropes. Interestingly, we have
found that SWNTs are well dispersed in ionic liquids. And Fukushi-
ma et al. reported that SWNTs formed gels when mixed with ionic
liquid, and entangled nanotube bundles were found to untangle
within the gel to form much finer bundles.10 These observations
led us to envisage that the SWNTs would be suitable as the sup-
porting materials for immobilization of metal-nanoparticles in
ionic liquid. We herein wish to report that Pd nanoparticles on
SWNTs are useful for hydrogenation of aryl ketones in IL, 1-
butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]).
2. Results and discussion
Figure 1a shows the scanning electron microscopy (SEM) image
of SWNTs used in this study which were produced by a chemical
vapor deposition (CVD) method.11 And Pd nanoparticles were gen-
erated by hydrogen reduction of a palladium(II) acetate, followed
by direct adsorption on SWNTs in [BMIM][PF6] in situ.12 Figure 1b
shows the SEM image and the energy dispersive X-ray spectros-
copy (EDS) spectrum of these palladium nanoparticles attached
on SWNTs.13
In order to evaluate the catalytic reactivity of Pd nanoparticles
on SWNTs synthesized in IL, the hydrogenation of various aryl ke-
tones was investigated. In typical experiments, the hydrogenation
of aryl ketone was performed with a solution containing substrate
(0.3 mmol), palladium nanoparticles (6 mol %), H2 (1 atm), and
SWNTs (5 mg) in IL (1 ml) at room temperature. After the >99%
The carbon nanotubes (CNTs) are very attractive as supporting
materials for immobilization of metal-nanoparticles and biomole-
⇑
Corresponding authors. Tel./fax: +82 41 540 9639 (J.K.L.); tel.: +82 54 279 2766;
fax: +82 54 279 0654 (M.-J.K.).
E-mail addresses: jklee@hoseo.edu, leejk@sogang.ac.kr (J.K. Lee), mjkim@poste-
ch.ac.kr (M.-J. Kim).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.11.081

500
J. K. Lee, M.-J. Kim / Tetrahedron Letters 52 (2011) 499–501
Table 1
Hydrogenation of aryl ketones catalyzed by [Pd nanoparticles on SWNT] in
[BMIM][PF6]a
Entry
1
Substrate
Product
OH
Rxn. time (h)
24
Yieldb (%)
99
O
O
OH
2
3
4
72
30
30
94c
99
O
O
OH
OH
99
O
OH
5
6
7
30
72
72
99
99
99
MeO
F
MeO
F
O
OH
OH
O
F
F
O
OH
8
24
99
F3C
N
F3C
N
Figure 1. (a) SEM image of single-walled carbon nanotubes (SWNTs) and (b) SEM
and EDAX image of palladium (Pd) nanoparticles on SWNTs.
O
OH
9
10
11
24
24
80
99
O
OH
completion of the reaction was checked by TLC, the solution mix-
ture was extracted with ethyl ether and the ethereal phase was
concentrated. The yield was determined by 1H NMR. The results
were given at Table 1.
99
N
O
N
O
OH
OH
OH
85d
The hydrogenation of the nonsubstituted acetophenone was
completed in 24 h at room temperature with a quantitative yield
(entry 1). The acetophenones substituted by methyl group, which
is electron-donating motif, on meta, para positions needed a little
bit longer reaction time for the reaction completion but gave the
quantitative yields, (entries 2 and 3) while the reaction of aceto-
phenone ortho-substituted by a methyl group exhibited 94% yield
in 72 h and was not completed with much longer reaction time
without any by-product (entry 3). These may be caused by the ste-
ric effect of ortho-substituted methyl group near the carbonyl reac-
tion center. The 40-methoxy acetophenone containing stronger
electron-donating strength gave similar reactivity to 40-methyl
acetophenone (entry 5). The substrates having electron withdraw-
ing fluorine exhibited around 2.4 times lower reactivity than those
having electron-donating substituents (entries 7 and 8). Interest-
ingly, the substrate with a trifluoromethyl substituent showed bet-
ter reactivity, resulting in faster reaction (entry 8). In case of
heteroaromatic substrates, 2-acetyl- and 3-acetylpyrdine, similarly
good reactivity was observed (entries 9 and 10). Also, the diketone
substrate such as acetyl acetophenone can be efficiently hydroge-
nated by Pd nanoparticles on SWNTs in IL, but partially reduced
products were detected (entries 11 and 12). The fused aryl ketone
substrates exhibited similar reactivities to 20-methly acetophenone
(entries 13 and 14).
O
12
80
86d
O
OH
O
OH
OH
13
14
72
72
97c
94c
O
a
The reaction conditions: 0.3 mmol substrate, 1 atm H2, 5 mg SWNT, 1 ml
[BMIM][PF6], rt.
The yields were determined by 1H NMR.
b
c
The reactions were not completed and no by-products were detected.
The reactions were completed, but some partially reduced products were
d
detected.
time, 24 h. These results suggest that the palladium nanoparticles
were not aggregated under H2, but remained stable and reactive
on the large surface of SWNTs.
The stability of palladium nanoparticles on SWNTs was further
examined by repeating its catalytic reaction, which is the hydroge-
nation of acetophenone (Table 2). Interestingly, the catalytic activ-
ity of the palladium nanoparticles on SWNTs was not reduced even
after the tenth run. This clearly shows that the SWNTs in IL can sta-
bilize the palladium nanoparticles by preventing their aggregation.
Moreover, these reactions reached completion without produc-
ing by-products which result from the C–O bond cleavage and
over-reduction of aromatic ring. Also, the [Pd on SWNT]/IL system
(99% yield) is more reactive than Pd/IL (39% yield) and [Pd on acti-
vated carbon]/IL (92% yield) systems14 within the same reaction

J. K. Lee, M.-J. Kim / Tetrahedron Letters 52 (2011) 499–501
501
Table 2
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Reuse of palladium nanoparticles on SWNTs in IL
Pd nanoparticles (6 mol%)
H2 (1 atm)
O
OH
SWNT (5mg)
[BMIM][PF 6] (1 ml)
r.t. 24h
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Runa
1
99
2
99
3
99
4
99
5
99
6
99
7
99
8
99
9
99
10
99
Yieldb (%)
a
The first run was carried out with fresh Pd nanoparticles on SWNTs and the
second to tenth runs were done with recovered Pd nanoparticles on SWNTs.
The yields were determined by 1H NMR.
b
3. Conclusion
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In conclusion, this work has demonstrated that SWNTs could
serve as supporting materials for immobilization of palladium
nanoparticles in IL. The palladium nanoparticles on SWNT in IL
were very reactive and stable for hydrogenation of aryl ketones.
We believe that SWNTs are particularly promising as the support-
ers for immobilization of other metal nanoparticles or of biomole-
cules. Further studies on the immobilization on SWNTs in IL will
expand the scope of the utilization of various catalysts for organic
synthesis.
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Acknowledgment
This work was supported by the National Research Foundation
of Korea (grant numbers: KRF-2007-313-C00413 and KRF-2008-
314-C00203). We thank Professor Cheol Jin Lee in Korea University
for kindly providing the SWNTs.
12. The palladium nanoparticles on SWNTs were prepared in [BMIM][PF6]. The
SWNTs (5 mg) was grounded in IL (1 ml) for 30 min, and then Pd(II) acetate
(0.018 mmol) was dissolved in the solution. The Pd(II) acetate was in situ
reduced in IL with 1 atm of hydrogen for 5 min at room temperature. The aryl
ketone (0.3 mmol) was added to this solution under 1 atm of hydrogen at room
temperature. After the >99% completion of the reaction was checked by TLC,
the products were extracted with ethyl ether. The ethereal phase was
concentrated and analyzed by 1H NMR.
References and notes
13. Scanning Electron Microscopy and EDAX were recorded in a Hitachi S-4300
FEG. The palladium nanoparticles on SWNT samples were prepared by
removing IL after hydrogenation.
14. [Pd on activated carbon]/IL system was prepared by above method with using
activated carbon instead of SWNTs.
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