Rhodium nanoparticles supported on carbon nanofibers as an arene hydrogenation catalyst highly tolerant to a coexisting epoxido group

Article abstract of DOI:10.1021/ol902018g

Rhodium nanoparticles supported on a carbon nanofiber (Rh/CNF-T) show high catalytic activity toward arene hydrogenation under mild conditions in high turnover numbers without leaching the Rh species; the reaction is highly tolerant to epoxido groups, which often undergo ring-opening hydrogenation with conventional catalysts.

Full text of DOI:10.1021/ol902018g

ORGANIC
LETTERS
2009
Vol. 11, No. 21
5042-5045
Rhodium Nanoparticles Supported on
Carbon Nanofibers as an Arene
Hydrogenation Catalyst Highly Tolerant
to a Coexisting Epoxido Group
Yukihiro Motoyama,* Mikihiro Takasaki, Seong-Ho Yoon, Isao Mochida, and
Hideo Nagashima
Institute for Materials Chemistry and Engineering, Graduate School of Engineering
Sciences, Kyushu UniVersity, Kasuga, Fukuoka 816-8580, Japan
motoyama@cm.kyushu-u.ac.jp
Received August 31, 2009
ABSTRACT
Rhodium nanoparticles supported on a carbon nanofiber (Rh/CNF-T) show high catalytic activity toward arene hydrogenation under mild
conditions in high turnover numbers without leaching the Rh species; the reaction is highly tolerant to epoxido groups, which often undergo
ring-opening hydrogenation with conventional catalysts.
Arene hydrogenation over heterogeneous catalysts has taken
part in the facile production of substituted cyclohexane
derivatives on both laboratory and industrial scales.1 Among
various transition metal catalysts, rhodium supported on
activated carbons (Rh/C) generally shows higher catalytic
activity for arene hydrogenation than heterogeneous catalysts
containing other transition metals.1c However, one drawback
to the use of expensive rhodium is that the catalyst is often
quickly deactivated due to facile sintering of the metal
particles and leaching of metallic species from the support,
making the use of recycled Rh/C difficult.2 Since catalytic
properties of heterogeneous catalysts are highly dependent
on the surface design of the support,3 a search for carbon
materials that promote arene hydrogenation under mild
conditions with high turnover frequency and are robust
enough to be reusable may provide a more suitable support
for the rhodium catalysts. In this context, carbon nanofibers
(CNFs) having controlled nanostructures consisting of stacked
graphene sheets are attractive supports for metal particles.
CNFs are classified into three types, where graphite layers
are either perpendicular (platelet, CNF-P), parallel (tubular,
CNF-T), or stacked obliquely (herringbone, CNF-H) to the
fiber axis.4 We have recently established selective synthetic
methods for these CNFs4b,c and explored methods to im-
mobilize ruthenium, palladium, and platinum nanoparticles
on their surface by decomposition of organometallic precur-
(1) Reviews: (a) Weissernicl, K.; Arpe, H. J. Industrial Organic
Chemistry, 2nd ed.; VCH: New York, 1993; p 343. (b) Stanislaus, A.;
Cooper, B. H. Catal. ReV. Sci. Eng. 1994, 36, 75. (c) Augustine, R. L.
Heterogeneous Catalysis for the Synthetic Chemist; Marcel Dekker: New
York, 1996; Chapter 17. (d) Harman, W. D. Chem. ReV. 1997, 97, 1953.
(e) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 191, 187.
(f) Dyson, P. J. Dalton Trans 2003, 2964.
(3) (a) Feldheim, D. L.; Foss, C. A., Jr. Metal Nanoparticles: Synthesis,
Characterization, and Application; Marcel Dekker: New York, 2002. (b)
Catalyst Supports and Supported Catalysts; Stiles, A. B., Ed.; Butterworths:
Boston, 1987. (c) Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨z-
inger, H., Weitkamp, J., Eds.; VCH: Weinheim, 1997.
(4) (a) Rodriguez, N. M. J. Mater. Res. 1993, 8, 3233. These three CNFs
can be synthesized selectively in large scales; see: (b) Tanaka, A.; Yoon,
S.-H.; Mochida, I. Carbon 2004, 42, 591. (c) Tanaka, A.; Yoon, S.-H.;
Mochida, I. Carbon 2004, 42, 1291.
(2) Halttunen, M. E.; Niemela¨., M. K.; Krause, A. O. I.; Vuori, A. I. J.
Mol. Catal. A: Chem. 1996, 109, 209.
10.1021/ol902018g CCC: $40.75
Published on Web 09/29/2009
 2009 American Chemical Society

sors.5 The Ru, Pd, and Pt/CNFs thus produced act as efficient
catalysts for arene hydrogenation and the reduction of nitro
compounds in high turnover numbers. It is important that
neither sintering nor leaching of metallic species is observed;
this results in possible reuse of the catalyst without loss of
the activity. In this paper, we wish to report that rhodium
nanoparticles can be immobilized on the surface of the three
types of CNFs by a procedure similar to that used for Ru/
CNF and that one of the resulting Rh/CNFs, Rh/CNF-T,
behaves as a highly efficient reusable catalyst for arene
hydrogenation under mild conditions.6 Of particular interest
is that the use of Rh/CNF-T allows the hydrogenation of
aromatic compounds having glycidyl moiety without pro-
moting the ring-opening hydrogenation of the epoxido group.
Scheme 1 shows a synthetic procedure of the CNF-
supported rhodium nanoparticles (Rh/CNFs). Thermal de-
composition of Rh4(CO)12 (3.0 mg; [Rh] ) 1.65 wt % for
the support) in the presence of three types of carbon
nanofibers (100 mg) in refluxing toluene under an argon
atmosphere followed by filtration and washing with toluene
and ether afforded the corresponding Rh/CNF-P, Rh/CNF-
H, and Rh/CNF-T. The formation of rhodium nanoparticles
on the CNFs was confirmed by transmission electron
microscopy (TEM), and these nanoparticles were dispersed
on the surface in all CNFs with an average particle size of
2.4 nm for Rh/CNF-P and 3.8 nm for both Rh/CNF-T and
Rh/CNF-H, respectively (see Supporting Information). The
rhodium content of the Rh/CNFs was determined by
inductively coupled plasma-mass (ICP-MS) analysis; 0.8 wt
% for Rh/CNF-P, 0.2 wt % for Rh/CNF-T, and 0.4 wt %
for Rh/CNF-H, respectively.
Table 1. Hydrogenation of Benzene with Rh Catalystsa
entry
catalyst
S/Cb
PH (atm) temp (°C) TOFc
2
1
2
Rh/CNF-P
Rh/CNF-H
Rh/CNF-T
Rh/CNF-T
2,500
5,100
10,300
116,000
1
1
1
4
1
4
rt
rt
rt
75
22
75
120
d
1,060
7,750
200
3
4e
5
Rh/AlO(OH)f 100
6e
Rh/AlO(OH)f 10,000
1,250
a All reactions were carried out with benzene (1 mmol) and Rh/CNF
catalyst (5 mg) in hexane (1 mL) at room temperature. b S/C ) mol
(benzene)/mol (Rh). c TOF ) mol (product)/mol (Rh)·h. The product yield
was determined by GLC analysis. d The product was not detected. e 1 mL
of benzene and no solvent was used. f Reference 7.
(TOF) of the benzene hydrogenation over Rh/AlO(OH)
catalyst was 200 [mol (benzene)/mol (Rh)·h] at 22 °C under
1 atm of H2.9 Hydrogenation of benzene (1 mmol) in the
presence of the Rh/CNFs (5 mg) was carried out in hexane
(1 mL) at room temperature under a hydrogen atmosphere.
After 3 h, the conversion was determined by GC, from which
the TOF was calculated. The results are summarized in Table
1. The TOF was dependent on the CNF; the reaction
smoothly proceeded with both Rh/CNF-P and Rh/CNF-T,
whereas no reaction took place using Rh/CNF-H as the
catalyst. In particular, Rh/CNF-T showed high catalytic
efficiency, and the TOF of the reaction reached over 1,000
[mol (benzene)/mol (Rh)·h] (entries 1-3). The TOF of the
solventless hydrogenation at 75 °C under H2 pressure (initial
pressure: PH ) 4 atm) reached 7,750 (entry 4). These TOFs
2
have catalytic efficiencies 5 times higher than those of Park’s
Rh/AlO(OH) catalyst at ambient temperature under 1 atm
of H2 (entry 5) and at 75 °C under 4 atm of H2 (entry 6). As
previously stated, the hydrogenation of benzene is efficiently
catalyzed by Ru/CNFs; however, the reactions required
application of slightly higher temperature (>40 °C) and
hydrogen pressure (>10 atm). The activity of Ru/CNFs was
dependent on the CNF used; the order of activity for the
hydrogenation was Ru/CNF-P > Ru/CNF-H . Ru/CNF-T,5b
different from Rh/CNF-T > Rh/CNF-P . Rh/CNF-H. In the
experiments shown in entries 1 and 3, the catalyst was
recovered and subjected to repeated experiments for hydro-
genation of benzene (5 times). Neither loss of catalytic
activity nor leaching of the metallic species was observed.
Phenol is a good substrate for investigating the perfor-
mance of the catalyst in the selectivity. It is known that
cyclohexanol (1) is a final product in the hydrogenation;
however, incomplete hydrogenation results in the formation
of intermediary cyclohexen-1-ol (3), which is isolated as
cyclohexanone (2). Three commercially available Rh/C
catalysts were subjected to hydrogenation of phenol at room
temperature under 3 atm of H2. In all cases, the catalyst was
recycled and used for a second run under the same condi-
tions. One of them showed no activity (entry 5) in both the
first and second run, whereas one of the others exhibited
low activity (TOF ) ca. 11) to give a 3:7-2:8 mixture of 1
and 2 in the first and second run (entry 4). Moderate activity
(TOF ) 35) and exclusive formation of 1 were seen in the
Scheme 1. Preparation of Rh/CNFs
The catalytic activity of Rh/CNFs is compared with those
reported in the literature using benzene as a simple and
standard substrate.7,8 To our knowledge, the most active
arene hydrogenation catalyst in the literature was reported
by Park and co-workers: rhodium nanoparticles entrapped
in boehmite matrix, Rh/AlO(OH).7 The turnover frequency
(5) (a) Motoyama, Y.; Takasaki, M.; Higashi, K.; Yoon, S.-H.; Mochida,
I.; Nagashima, H. Chem. Lett. 2006, 35, 876. (b) Takasaki, M.; Motoyama,
Y.; Higashi, K.; Yoon, S.-H.; Mochida, I.; Nagashima, H. Chem. Asian J.
2007, 2, 1524. (c) Takasaki, M.; Motoyama, Y.; Yoon, S.-H.; Mochida, I.;
Nagashima, H. J. Org. Chem. 2007, 72, 10291. (d) Takasaki, M.; Motoyama,
Y.; Higashi, K.; Yoon, S.-H.; Mochida, I.; Nagashima, H. Org. Lett. 2008,
10, 1601.
(6) Representative papers for arene hydrogenation under mild conditions:
(a) Yang, H.; Gao, H.; Angelici, R. J. Organometallics 2000, 19, 622. (b)
Maegawa, T.; Akashi, A.; Sajiki, H. Synlett 2006, 1440. (c) Maegawa, T.;
Akashi, A.; Yaguchi, K.; Iwasaki, Y.; Shigetsura, M.; Monguchi, Y.; Sajiki,
H. Chem. Eur. J. 2009, 15, 6953.
(7) Park, I. S.; Kwon, M. S.; Kim, N.; Lee, J. S.; Kang, K. Y.; Park, J.
Chem. Commun 2005, 5667
.
Org. Lett., Vol. 11, No. 21, 2009
5043

Table 2. Recycle Experiments in Hydrogenation of Phenol with
Table 3. Hydrogenation of Various Arenes with Rh/CNF-Ta
Rh Catalystsa
convn (%); [TOFb]; {1/2}
entry catalyst
1st run
2nd run
3rd run
1
2
Rh/CNF-P 78 [170] {45:55} 70 [150] {31:69}
>99 [>860]
{>99:<1}
>99 [>35]
{>99:<1}
>99 [>860]
{>99:<1}
>99 [>860]
{>99:<1}
Rh/CNF-T
3
4
5
Rh/Cc
Rh/Cc
Rh/Cc
17 [5] {12:88}
31 [11] {22:78}
<1 [-] {-}
31 [11] {32:68}
<1 [-] {-}
a All reactions were carried out with phenol (1 mmol) and Rh/CNF
catalyst (5 mg) in hexane (1 mL) at room temperature for 12 h under H2
(3 atm). The yield was determined by GLC analysis. b TOF ) mol (product)/
mol (Rh)·h. c Three commercially available Rh/C catalysts purchased from
three different sources (5 wt %) were used.
a All reactions were carried out with aromatic compound (1 mmol) and
Rh/CNF-T (5 mg; 0.2 wt % Rh; S/C ) 10,300) in hexane (1 mL) at rt for
12 h under H2 (3 atm). b Determined by GLC analysis with n-decane or
n-dodecane as an internal standard because these products are volatile. c The
reaction was accompanied by hydrogenation to a ketone to alcohol to give
1-phenylethanol (13%). d At 70 °C under 10 atm of H2. e Determined by
1H NMR analysis.
Rh/C shown in entry 3; however, significant retardation of
the catalytic activity and a decrease in selectivity of 1 were
apparently problematic in the second run. Two Rh/CNFs
were subjected to the hydrogenation under the same condi-
tions. As shown in entry 2, Rh/CNF-T afforded the best
results among those we examined in the catalyst efficiency
and selectivity of 1, and of particular importance is that the
good catalyst performance was reproduced in the second and
third runs.10 Furthermore, ICP-MS analysis of the product
obtained in each cycle has shown that the amount of Rh-
leaching from CNF-T was below the detection limit (<0.3
ppm).
of the starting material. Starting materials were completely
recovered in the reaction of benzonitrile and nitrobenzene
(entries 4-7). Nitrogen-containing substituent seemed to
poison the catalyst;11 the reaction of pyridine required both
higher temperature (70 °C) and hydrogen pressure (10 atm).
No leaching of Rh was observed in the product, whereas
TEM analysis indicates no sintering of the nanoparticles on
the support after the reaction (entry 8). Reaction of a CdC
bond was preceded by the arene hydrogenation; trans-stilbene
was selectively converted to 1,2-diphenylethane (entry 9).
Disubstituted o-xylene and o-cresol were also hydrogenated
in good to high yields under these conditions. (entries 10
and 11).
A highlight of the catalyst performance of Rh/CNF-T is
successful hydrogenation of arenes containing a glycidyl
ether moiety, in which aromatic hydrogenation proceeds with
the epoxido group remaining intact. Epoxides are important
synthetic intermediates in organic synthesis.12 From an
idustrial point of view, epoxides exemplified by easily
accessible glycidyl ethers are important components of
epoxido resins, and various epoxides are subjected to
practical usage as monomers, comonomers, and bridging
reagents.13 Selective hydrogenation of an aromatic ring in
The utility of Rh/CNF-T is demonstrated by the hydro-
genation of substituted aromatic compounds as shown in
Table 3. The reaction was carried out in the presence of Rh/
CNF-T as a catalyst (S/C ) 10,300) at ambient temperature
under H2 (initial pressure: PH ) 3 atm). It should be noted
2
that the substrates underwent hydrogenation over Ru/CNF-P
(S/C e 14,000, 100 °C, PH ) 30 atm) as previously
2
reported;5b however, the activity of Ru/CNFs is in most cases
lower than that of Rh/CNF-T under the reaction conditions
shown in Table 3. The substituents on an aromatic ring
affected the reaction rate. The hydrogenation of arenes with
electron-donating groups such as toluene, phenol, and anisole
smoothly proceeded under the conditions to afford the
corresponding cyclohexane derivatives in quantitative yields
(entries 1-3). In contrast, the reaction of ethyl benzoate was
relatively slower, whereas that of acetophenone was ac-
companied by formation of 1-phenylethanol and recovery
(8) (a) Pellegatta, J.-L.; Blandy, C.; Collie`re, V.; Choukroum, R.;
Chaudret, B.; Cheng, P.; Philippot, K. J. Mol. Catal. A: Chem. 2002, 178,
55. (b) Roucoux, A.; Schulz, J.; Patin, H. AdV. Synth. Catal. 2003, 345,
222.
(11) Reviews: (a) Weissernicl, K.; Arpe, H. J. Industrial Organic
Chemistry, 2nd ed.; VCH: New York, 1993; p 343. (b) Stanislaus, A.;
Cooper, B. H. Catal. ReV. Sci. Eng. 1994, 36, 75. (c) Augustine, R. L.
Heterogeneous Catalysis for the Synthetic Chemist; Marcel Dekker: New
York, 1996; Chapter 17. (d) Harman, W. D. Chem. ReV. 1997, 97, 1953.
(e) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 191, 187.
(f) Dyson, P. J. Dalton Trans. 2003, 2964.
(9) Similar TOF (250 [mol (benzene)/mol (Rh)·h]) was reported in
experiments using polymer-stabilized rhodium nanoparticles (Rh/PVP) at
30 °C under 7 atm of H2; see ref 8a.
(10) Other recyclable rhodium catalysts: (a) Mu, X.-D.; Meng, J.-Q.;
Li, Z.-C.; Kou, Y. J. Am. Chem. Soc. 2005, 127, 9694. (b) Falini, G.;
Gualandi, A.; Savoia, D. Synthesis 2009, 2440.
(12) Aziridines and Epoxide in Organic Synthesiss; Yudin, A. K., Ed.;
Wiley-VCH: Weinheim, 2006.
5044
Org. Lett., Vol. 11, No. 21, 2009

the vicinity of an epoxido group is a challenge to be
developed for preparation of the corresponding cyclohexane
derivatives containng the epoxido group. However, the
reaction has a problem of epoxide opening, which is easily
promoted by acid, base, and H2 activated by transition metal
catalysts.14 Accordingly, there have been only a few ex-
amples of catalytic arene hydrogenation with good to high
selectivity. Lemaire and co-workers reported that the hy-
drogenation of phenyl glycidyl ether by a RuCl3/trioctylamine
system (S/C ) 40) proceeded at room temperature to give
cyclohexyl glycidyl ether selectively in 70% yield.15a Hara
and Inagaki reported that Rh/graphite catalyzed the hydro-
genation of phenyl glycidyl ether (S/C ) 1,370) and
bisphenol A bisglycidyl ether (S/C ) 600) at 85 °C to afford
the corresponding alicyclic compounds with 91% and 97%
selectivity.15b In both cases, however, application of high
hydrogen pressure (50-70 atm) was necessary. Hydrogena-
tion of benzyl glysidyl ether over Ru/CNF-P, which as we
reported shows high activity toward arene hydrogenation,5a-c
could not solve the problems of either activity or selectivity.
Catalytic hydrogenation of various arenes containing an
epoxido group were carried out with Rh/CNF-T (S/C )
2,620) under H2 (10 atm), and the results are summarized in
Table 4. Although the reaction of styrene oxide (4a) afforded
a mixture of products containing ethylcyclohexane and
phenylethanols due to concomitantly occurring hydrogenoly-
sis of C-O bonds (entry 1), that of allylbenzene oxide (4b)
gave the corresponding allylcyclohexane oxide (5b). The
reaction was not fast, and the yield reached 30% after 24 h
as shown in entry 2. In sharp contrast, glycidyl ethers 4c-g,
smoothly underwent the hydrogenation under these condi-
tions to afford the corresponding alicyclic epoxides (5c-g)
in almost quantitative yields. Besides Rh/CNF-T, the Rh/
CNF-P was also useful for hydrogenation of 4d (entry 5). It
is noteworthy that reductive cleavage of a benzylic C-O
bond, which potentially occurs in the hydrogenation of 4d
and 4g, was not observed at all (entries 4 and 9). Further-
more, the reaction of optically active benzyl glycidyl ether
4d (>99% ee, R) involved no racemization, affording (R)-
cyclohexylmethyl glycidyl ether 5d (entry 6).
Table 4. Hydrogenation of Various Arenes 4a-g Containing an
Epoxido Groupa
a All reactions were carried out with 4 (0.5 mmol) and Rh/CNF-T (10
mg; [Rh] ) 0.2 wt %) in hexane at room temperature under H2 (10 atm).
b Isolated yield. c Complicated mixtures were obtained. d For 24 h.
e Determined by 1H NMR analysis. f Rh/CNF-P was used as a catalyst.
g Determined by chiral HPLC analysis using Daicel CHIRALPAK AS-H.
h The product is tetrahydrofurfuryl glycidyl ether (ca. 1:1 diastereo mixtures).
prepared from Rh4(CO)12 and CNFs. Among them, Rh/
CNF-T is a highly active and reusable catalyst for arene
hydrogenation, which proceeds under mild conditions in high
turnover numbers. The catalytic activity is not decreased in
repeated experiments, and there is no leaching of the rhodium
species to the product. Utility of the Rh/CNF-T is demon-
strated in hydrogenation of arenes containing epoxido group,
which proceeds without promoting the epoxido opening. We
are now investigating the further use of CNFs as effective
supports for various transition-metal nanoparticles.
Acknowledgment. This work was partially supported by
a Grant-in Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
In summary, we have found that Rh/CNFs are easily
(13) (a) Potter, W. G. Epoxide Resins; Butterworth: London, 1970. (b)
Liska, V. Prog. Org. Coatings 1983, 11, 109.
Supporting Information Available: Experimental pro-
cedures and copies of NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
(14) Sajiki, H.; Hattori, K.; Hirota, K. Chem.sEur. J. 2000, 6, 2200,
and references therein.
(15) (a) Fache, F.; Lehuede, S.; Lemaire, M. Tetrahedron Lett. 1995,
36, 885. (b) Hara, Y.; Inagaki, H. Chem. Lett. 2002, 1116.
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5045