Rhodium and iridium nanoparticles entrapped in aluminum oxyhydroxide nanofibers: Catalysts for hydrogenations of arenes and ketones at room temperature with hydrogen balloon

Article abstract of DOI:10.1002/adsc.200600651

The recyclable metal nanoparticle catalysts, rhodium in aluminum oxyhydroxide [Rh/ AlO(OH)] and iridium in aluminum oxyhydroxide [Ir/A1O(OH)], were simply prepared from readily available reagents. The catalysts showed high activities in the hydrogenation of various arenes and ketones under mild conditions. Selective hydrogenation was possible for bicyclic and tricyclic arenes in high yields. The catalysts were active at room temperature even with a hydrogen balloon. Also, the catalysts showed high turnover frequency (TOF) values under solventless conditions at 75 °C under 4 atm hydrogen pressure: ca. 1700h ¹ in the hydrogenation of benzene. Furthermore, Rh/A1O(OH) can be reused forat least 10 times without activity loss. The catalysts were characterized by the transmission electron microscopy (TEM), powder X-ray diffraction (XRD), inductively coupled plasma (ICP), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption and hydrogen chemisorption experiments. The sizes of rhodium and iridium particles were estimated to be 3-4 nm and 2-3 nm, respectively. Aluminum oxyhydroxide nanofibers of these catalysts have surface areas of 500-600 m² g ^-1.

Full text of DOI:10.1002/adsc.200600651

FULL PAPERS
DOI: 10.1002/adsc.200600651
Rhodium and Iridium Nanoparticles Entrapped in Aluminum
Oxyhydroxide Nanofibers: Catalysts for Hydrogenations of
Arenes and Ketones at Room Temperature with Hydrogen
Balloon
In Soo Park,a Min Serk Kwon,a Kyung Yeon Kang,b Jae Sung Lee,b
and Jaiwook Parka,*
a
Center for Integrated Molecular System, Department of Chemistry, Pohang University of Science and Technology
(POSTECH), San 31, Hyojadong, Pohang, Kyeongbuk 790-784, Republic of Korea
Fax : (+82)-54-279-3399; e-mail: pjw@postech.ac.kr
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyojadong,
b
Pohang, Kyeongbuk 790-784, Republic of Korea
Fax : (+82)-54-279-5528; e-mail: jlee@postech.ac.kr
Received: December 29, 2006; Revised: May 8, 2007
Abstract: The recyclable metal nanoparticle cata- at least 10 times without activity loss. The catalysts
lysts, rhodium in aluminum oxyhydroxide [Rh/ were characterized by the transmission electron mi-
AlO(OH)] and iridium in aluminum oxyhydroxide croscopy (TEM), powder X-ray diffraction (XRD),
[Ir/AlO(OH)], were simply prepared from readily inductively coupled plasma (ICP), energy dispersive
available reagents. The catalysts showed high activi- X-ray analysis (EDX), X-ray photoelectron spectros-
ties in the hydrogenation of various arenes and ke- copy (XPS), nitrogen adsorption and hydrogen
tones under mild conditions. Selective hydrogenation chemisorption experiments. The sizes of rhodium
was possible for bicyclic and tricyclic arenes in high and iridium particles were estimated to be 3–4 nm
yields. The catalysts were active at room temperature and 2–3 nm, respectively. Aluminum oxyhydroxide
even with a hydrogen balloon. Also, the catalysts nanofibers of these catalysts have surface areas of
showed high turnover frequency (TOF) values under 500–600 m2 gÀ1.
solventless conditions at 758C under 4 atm hydrogen
pressure: ca. 1700 hÀ1 in the hydrogenation of ben- Keywords: arenes; heterogeneous catalysis; hydroge-
zene. Furthermore, Rh/AlO(OH) can be reused for nation; iridium; ketones; rhodium
Introduction
materials including SBA-15, MCM-41 and activated
carbon have been used as supports for metal nanopar-
Developing efficient catalytic systems has been inten- ticles, and extensive research efforts have been fo-
sively pursued, not only for high catalytic activity and cused on their applications to catalytic reactions.[2]
selectivity, but also for catalyst recycling. Indeed, vari-
The hydrogenation of arenes to non-aromatic com-
ous homogeneous catalysts which are immobilized on pounds and that of ketones to alcohols are important
heterogeneous supports such as polymers or inorganic organic transformations. Although a large number of
materials, suffer from the low activity, difficult synthe- stoichiometric metal hydride reagents have been de-
sis and limited stability. Recently, nanometer sized veloped, these reagents produce a large amount of
metal particles with the strictly controlled geometry undesirable wastes and also cause problems in large
and uniform size show unexpected catalytic activities scale processes. Traditionally, arene hydrogenation
as well as physical and chemical properties. Thus, the has been performed with heterogeneous catalysts
design of solid-phase metal nanoparticle catalysts is such as Rh/Al2O3, Raney nickel, and metal sulfide
an interesting research topic for organic synthesis and under drastic reaction conditions.[3] Rothwell et al.
for chemical and pharmaceutical processes.[1] Due to have used homogenous niobium and tantalum cata-
the high tendency toward agglomeration and recycling lysts for the hydrogenation of arenes and arylphos-
problems, the metal nanoparticle itself is not suitable phines.[4] Finke et al. reported that metal nanoparti-
for catalytic reactions. Therefore, several mesoporous cles act as catalysts for the hydrogenation of arenes
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and ketones.[5] As recyclable catalysts, molecular rho- From the high-resolution TEM images, the sizes of
dium catalysts tethered to a palladium-silica support[6] the rhodium nanoparticles are 3–4 nm. The powder
or ionic copolymer[7] have been reported. Arene hy- XRD pattern for 1 shows the existence of metallic
drogenation has been performed under various condi- rhodium. The particle size is calculated to be 3.3 nm
tions such as liquid-liquid biphasic conditions,[8] ionic from the half-width of the diffraction line (111)
liquid conditions,[9] and supercritical CO2 condi- (Figure 3). In the case of the iridium catalyst 2, the
tions.[10]
particle size is 2–3 nm from the high-resolution TEM
Recently, we have reported a simple synthetic image, but the XRD pattern shows the presence of
method for recyclable metal nanoparticle catalysts.[11] not only metallic iridium but also iridium dioxide.[13]
Among the catalysts, rhodium nanoparticles entrap- The XPS analysis for 2 reveals that iridium particles
ped in aluminum oxyhydroxide [Rh/AlO(OH) (1)] are covered with iridium dioxide (Figure 4). The bind-
showed excellent catalytic activity for the hydrogena- ing energy from the XPS spectrum shows two peaks
tion of various arenes even under 1 atm H2 at room at 61.1 eV and 64.4 eV that were assigned to the 4f7/2
temperature. Herein we detail the preparation of Rh/ and 4f5/2 peaks of metallic iridium, respectively. Also,
AlO(OH) (1) and Ir/AlO(OH) (2) and their catalytic two peaks at 62.3 eV and 65.7 eV were assigned to the
activities in arene hydrogenation and in ketone hy- 4f7/2 and 4f5/2 peaks of iridium dioxide, respectively.
drogenation. Also, the catalysts were characterized by Compared to previously reported iridium XPS
TEM, powder XRD, ICP analysis, EDX, XPS, N2 ad- data,[13,14] our data certainly confirm the formation of
sorption and H2 chemisorption experiments. Notably, iridium dioxide on the surface of iridium nanoparti-
the catalysts are recyclable and stable in water as well cles in aluminum oxyhydroxide.
as in the air. Both catalysts showed high activities in
The nitrogen adsorption experiments conducted for
arene hydrogenation and in ketone hydrogenation 1 and 2 have shown their specific surface areas, pore
under mild conditions. The turnover number (TON) volumes and the pore sizes (Table 1). Also, the metal-
was over 20,000 in the hydrogenation of anisole and lic surface areas, active particle diameters and metal
benzene under solventless conditions at 758C under 4 dispersions were estimated by the H2 chemisorption
atm H2 pressure. This activity is the highest one experiments. Zeng et al. had already reported high
among those observed previously under similar condi- surface-area alumina using the sol-gel method with
tions.
aluminum tri-sec-butoxide.[12b] When compared to the
results obtained by the Zeng group, the surface areas
of 1 and 2 possess similar values. Recently, the fibrous
structure of aluminum oxyhydroxide was fully charac-
terized by the Seal group.[12a] We confirmed the exis-
Results and Discussion
Rh/AlO(OH) (1) and Ir/AlO(OH) (2) were prepared tence of Rh or Ir by EDX, and the metal contents
by a one-pot procedure from RhCl3 hydrates and were controlled at between 0.1 and 8 wt%, which
IrCl3 hydrates similar to that reported for the corre- could be analyzed by ICP. It is notable that the metal
sponding palladium catalyst.[11] The metal nanoparti- entrapment in our synthetic method was almost quan-
cles were generated by heating a mixture of the metal titative on the basis of the ICP data.
chloride, 2-butanol and Al
A
We tested the catalytic activity of 1 in the hydroge-
(Scheme 1). When the reaction mixture turned to nation of benzene and anisole and compared the ac-
black suspension after 3 h, excess water was added to tivity with those reported previously (Table 2). Ani-
make gel. Heating the reaction mixture above 1208C sole was selected as the standard substrate for arene
or prolonging the reaction time caused agglomeration hydrogenation.[5] Anisole hydrogenation using 1 (1.0
of the metal nanoparticles.
mol% Rh) gave methoxycyclohexane in 100% con-
A bluish-gray powder was obtained after aging the version within 1 h at room temperature under 1 atm
gel for 1 day, washing it with acetone, and drying at H2. Even with 0.010 mol% of Rh, 20 mmol of neat
1208C. The TEM images of 1 and 2 show nanofiber anisole were hydrogenated completely (TON: 10,000)
morphologies which are characteristic for the alumi- in 26 h at 758C under 4 atm H2. Importantly, this ac-
num oxyhydroxide matrix (Figure 1 and Figure 2).[12] tivity is the highest one among those observed previ-
Scheme 1. Preparation of Rh/AlO(OH) and Ir/AlO(OH).
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Figure 1. TEM images of Rh/AlO(OH) (1) and size distribution of particles.
Figure 2. TEM images of Ir/AlO(OH) (2).
ously under similar conditions. A commercial rhodi- The catalyst was active at 408C under 1 atm H2, but
um catalyst (Rh/Al2O3) showed only about 4% of the ten times more palladium than rhodium were re-
activity of 1 under similar conditions. Schulz et al. quired. Recently, rhodium nanoparticles immobilized
have reported that rhodium nanoparticles stabilized on carbon nanotubes[15] and entrapped in ionic liquid
by surfactants act as hydrogenation catalysts in aque- copolymers[10] have been employed as arene hydroge-
ous medium.[6e] Although a high activity in arene hy- nation catalysts. The former was tested in anthracene
drogenation under biphasic conditions was observed, hydrogenation under 10 atm H2, and the latter in ben-
it was very difficult to recover the catalyst from the zene hydrogenation under 40 atm H2.
reaction mixture and the recovered catalyst showed
The scope of arene hydrogenation with 1 was inves-
lowered activity. Angelici et al. have used a hybrid tigated at room temperature with a hydrogen balloon
catalyst consisting of a homogeneous rhodium catalyst (Table 3). Mono- and disubstituted benzene deriva-
tethered to a silica-supported palladium catalyst and tives were hydrogenated to their corresponding cyclo-
observed a synergy effect on arene hydrogenation.[8] hexane derivatives in 1–5 h with 1.0 mol% of Rh. An
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tively. However, in a separate experiment, cyclohexyl
methyl ketone was not hydrogenated to 1-cyclohexyl-
ethanol under the same conditions. Generally, the hy-
drogenation of electron-rich arenes is faster than that
of electron-poor ones. Particularly, the hydrogenation
of dimethyl terephthalate took 46 h. Selective hydro-
genation was possible for bicyclic arenes. Tetrahydro-
naphtalene was obtained in 97% yield after 3 h, while
decalin was produced in 99% yield after 10 h. From
2-methoxynaphthalene, 1,2,3,4-tetrahydro-6-methoxy-
naphthalene was obtained in 96% yield after 5 h,
while 2-methoxydecalin was produced in 99% yield
after 30 h. Quinoline was hydrogenated selectively to
1,2,3,4-tetrahydroquinoline in 94% after 6 h. These
results from the hydrogenation of bicyclic arenes are
worthy of note, compared to those obtained by using
a polymer-incarcerated palladium (PI-Pd) catalyst.[16]
For example, only a 7:3 mixture of 5,6,7,8-tetrahydro-
2-methoxynaphthalene and 1,2,3,4-tetrahydro-2-me-
thoxynaphthalene was obtained in the hydrogenation
of 2-methoxynaphthalene with PI-Pd (5.0 mol% Pd)
under 70 atm H2 at 508C. In the hydrogenation of dis-
ubstituted benzenes and bicyclic arenes, about 9:1 cis/
trans selectivities were observed except in the case of
o-cresol, where a 1:1 cis/trans mixture was obtained.
Selective hydrogenation is also possible for anthra-
cene: 9,10-dihydroanthracene was obtained in 97%
yield. Dehalogenation accompanied the hydrogena-
tion of 4-chloroanisole to give methoxycyclohexane.
The use of triethylamine facilitated the dehalogention
by quenching the hydrochloride formed during the re-
action. The activity that 1 shows in the dehalogena-
tion is much higher than the recent one that requires
high hydrogen pressure (40 atm) and high reaction
temperature (758C).[17]
Figure 3. XRD spectra of 1 and 2.
To show the effectiveness of 1 in large-scale reac-
tions, we performed the hydrogenation of liquid
arenes without solvent under 4 atm H2 at 758C
(Table 4). Benzene was hydrogenated to cyclohexane
completely after 1.5 h by using 0.10 mol% of Rh. The
hydrogenation proceeded also smoothly with 0.010
mol% of Rh to result in a TOF of 1700 hÀ1. Even
with 0.0050 mol% of Rh, the hydrogenation was com-
pleted in 15 h (TOF=1300 hÀ1). Anisole was convert-
ed completely to methoxycyclohexane with 0.10
mol% of Rh, although the reaction was about four
times slower than that of benzene. In the reaction
with 0.0050 mol% of Rh, a mixture of anisole and hy-
drogenation products (57% of methoxycyclohexane
and 11% of 1-methoxycyclohexene) was found after
24 h, but the mixture was converted to methoxycyclo-
hexane completely after 42 h (TOF=480 hÀ1). The
hydrogenation of toluene was slightly slower than
that of benzene, while that of o-xylene was about two
Figure 4. XPS spectra of 2.
Table 1. Properties calculated from N2 adsorption and H2
chemisorption of 1, 2.
Property
Rh/AlO(OH)
(1)
Ir/AlO(OH)
(2)
BET surface area (m2 gÀ1) 616
516
1.1
3.7
36.2
124
Pore volume (cm3 gÀ1)
Pore size (nm)
Dispersion (%)
Metallic surface area
(m2 gÀ1)
0.85
2.9
28.7
126
Active particle diameter
(nm)
3.8
2.1
interesting observation was that the hydrogenation of times slower than that of benzene. Heteroatom-con-
acetophenone resulted in a 65:35 mixture of 1-cyclo- taining substrates such as ethyl benzoate, o-cresol,
hexylethanol and cyclohexyl methyl ketone, respec- and pyridine were hydrogenated in comparable rates
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Table 2. Arene hydrogenation for various catalysts.
Catalyst
Substrate
S/C[a]
Conditions
Product (Yield [%][b])
TOF[c] [hÀ1]
Rh/AlO(OH) (1)
Rh/AlO(OH) (1)
Rh/AlO(OH) (1)
Rh/AlO(OH) (1)
Ir/AlO(OH) (2)
anisole
benzene
anisole
benzene
anisole
100
100
r.t., 1 atm H2, n-hexane
r.t., 1 atm H2, n-hexane
758C, 4 atm H2, solventless
758C, 4 atm H2, solventless
758C, 4 atm H2, solventless
methoxycyclohexane (100) 100 (340)
cyclohexane (100)
200 (690)
10000
10000
1000
4000
10000
methoxycyclohexane (100) 380 (1320)
cyclohexane (100)
methoxycyclohexane (39)
cyclohexane (100)
cyclohexane (100)
methyl cyclohexanoate
(100)
1700 (5920)
16
250
4000
10
Rh/PVP [BMI]
Ru/MMT[e]
Rh/CNT[f]
A
758C, 40 atm H2, [BMI]
A
benzene
methyl ben- 38
zoate
benzene
anisole
1108C, 80 atm H2, solventless
558C, 10 atm H2, MeOH
Rh/PVP[g]
Rh-CNC/
2000
2810
308C, 7 atm H2, biphasic
408C, 1 atm H2, n-heptane
cyclohexane (100)
methoxycyclohexane (42)
250
130
[h]
Pd-SiO2
Rh-HEA16X[i]
anisole
anisole
100
2600
208C, 1 atm H2, biphasic
228C, 3 atm H2, propylene
carbonate
methoxycyclohexane (100) 28
methoxycyclohexane (91)[l] 18
Rh-polyoxoanion[j]
Ir [BMI]
A
anisole
250
758C, 4 atm H2, solventless
methoxycyclohexane (62); 10 (18)
cyclohexane (12)
Ir [BMI]
Rh/Al2O3 Aldrich
G
benzene
anisole
250
100
758C, 4 atm H2, solventless
r.t., 1 atm H2, n-hexane
cyclohexane (100)
methoxycyclohexane (4.1) 4.1
125 (219)
[a]
Substrate/catalyst (moles/moles).
Determined by GC.
TOF is defined as moles of product by moles of total metal per hour, and the value in parenthesis is corrected for the ex-
[b]
[c]
posed metal which is estimated by the H2 chemisorption.
[d]
[e]
[f]
[g]
[h]
[i]
Ref.[7]
Ref.[9i]
Ref.[15]
Ref.[8c]
Ref.[6a]
Ref.[8e]
[j]
Ref.[5b]
[k]
[l]
Ref.[9d]
1-Methoxycyclohexene was also formed in 8%.
(TOF ca. 100 hÀ1). However, the TOF of phenol hy- drogenation, but gave only a 7:93 mixture of 9,10-di-
drogenation was only 38. The hydrogenation of quino- hydroanthracene and 1,2,3,4,5,6,7,8-octahydroanthra-
line was much slower (TOF=16 hÀ1), but gave cene.
1,2,3,4-tetrahydroquinoline selectively in 94% yield.
We tested the activities of 1 and 2 for the hydroge-
Anthracene was hydrogenated after dissolving in cy- nation of cyclohexanone under conditions
A
clohexane. In contrast to the production of 9,10-dihy- (0.50 mmol of a substrate dissolved in 1.0 mL of n-
droanthracene in the hydrogenation at room tempera- hexane at room temperature with a hydrogen bal-
ture under 1 atm H2, 1,2,3,4,5,6,7,8-octahydroanthra- loon) or under conditions B (20 mmol of a neat sub-
cene was the major product in the reaction with 5 strate at 758C under 4 atm H2). The iridium catalyst 2
mol% of Rh at 758C under 4 atm H2. Furthermore, showed a lower activity than the rhodium catalyst 1
perhydroanthracenes (cis/trans mixture) were pro- under conditions A. However, under conditions B,
duced quantitatively after 5 h when the amount of 1 the activity of 1 decreased considerably, while that of
was increased to 10 mol% of Rh. These results are 2 rather increased. Thus, the activity of 2 was about
remarkable, compared with previous results reported ten times higher than that of 1 under condition B
by the Wai group[15] and by the Kobayashi group.[16] (Table 5).
Wai et al. have used carbon nanotube-supported
Then we carried out the hydrogenation of various
metal nanoparticles (M/CNT) as the catalyst for the ketones under conditions A with 1 (1.0 mol% of Rh)
hydrogenation of anthracene. When they employed a and under conditions B with 2 (0.10 mol% of Ir)
rhodium catalyst (Rh/CNT) at room temperature (Table 6). As expected, the reactions under conditions
under 10 atm H2, 1,2,3,4-tetrahydroanthracene was B with 2 showed much higher TOF than those under
the major product in 62% yield with a low selectivity. conditions A with 1. Particularly, the TOF of 2-hepta-
A bimetallic catalyst (Pd-Rh/CNT) improved the hy- none hydrogenation increased about 70 times. Cyclo-
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Table 3. Hydrogenation of various arenes at room temperature with a hydrogen balloon.[a]
Entry
Substrate
Time [h]
Product (Yield [%][b])
1
2
3
toluene
anisole
phenol
1
1
1
methylcyclohexane (100)
methoxycyclohexane (100)
cyclohexanol (100)
4
5
6
7
8
9
ethyl benzoate
acetophenone
1-phenylethanol
o-xylene
4.5
4.5
4
2.5
3
3
10
5
ethyl cyclohexanoate (100)
1-cyclohexylethanol (65), cyclohexyl methyl ketone (35)
1-cyclohexylethanol (100)
1,2-dimethylcyclohexane (100) cis/trans=94/6
2-methylcyclohexanol (100) cis/trans=52/48
tetralin (97), decalin (3)
decaline (100) cis/trans=88/12
1,2,3,4-tetrahydro-6-methoxynaphthalene (96), 2-methoxy-1,2,3,4-
tetrahydronaphthalene (4)
2-methoxydecalin (99) cis/trans=85/15
1,2,3,4-tetrahydroquinoline (94)
9,10-dihydroanthracene (97), 1,2,3,4,5,6,7,8-octahydroanthracene (3)
dimethyl cyclohexanedicarboxylate (100) cis/trans=85/15
methoxycyclohexane (99)
o-cresol
naphthalene
naphthalene
2-methoxynaphthalene
10
11[c]
12[c]
13[d]
14[e]
15[f]
16 g]
2-methoxynaphthalene
quinoline
anthracene
dimethyl terephthalate
4-chloroanisole
30
6
3
46
3
[a]
At room temperature, 0.50 mmol substrate in n-hexane (1.0 mL) was reacted with 1 (1.0 mol% Rh).
Determined by GC.
3.0 mol% of Rh was used.
[b]
[c]
[d]
[e]
[f]
5.0 mol% of Rh was used.
0.10 mmol anthracene, 5.0 mol% of Rh, and 5.0 mL cyclohexane were used at 708C.
5.0 mol% of Rh and 3.0 mL ethyl acetate were used.
3.0 mol% of Rh and 1.0 equiv triethylamine were used
[g]
Table 4. Hydrogenation of various arenes at 758C under 4 atm H2 pressure without solvent.[a]
Entry
Substrate
Mol% Rh
TON
Time [h]
Product (Yield [%][b])
TOF[c]
1
2
3
4
5
6
benzene
benzene
benzene
anisole
anisole
anisole
0.10
1000
1.5
6.0
15
6.0
26
cyclohexane (100)
cyclohexane (100)
cyclohexane (100)
670
0.010
0.0050
0.10
0.010
0.0050
10000
20000
1000
10000
20000
1700
1300
170
390
475
methoxycyclohexane (100)
methoxycyclohexane (100)
anisole (32), 1-methoxycyclohexene (11),
methoxycyclohexane (57)
methoxycyclohexane (100)
methylcyclohexane (100)
1,2-dimethylcyclohexane (100) cis/trans=92/8
ethyl cyclohexanoate (100)
2-methylcyclohexanol (100) cis/trans=54/48
piperidine (100)
24
42
2.0
2.5
12
12
10
26
30
3
480
500
330
83
83
100
38
16
6
2
7
8
9
10
11
12
13
14[d]
15[d]
toluene
o-xylene
ethyl benzoate
o-cresol
pyridine
0.10
0.10
0.10
0.10
0.10
0.10
0.20
5.0
1000
1000
1000
1000
1000
1000
500
phenol
cyclohexanol (100)
quinoline
anthracene
anthracene
1,2,3,4-tetrahydroquinoline (94)
1,2,3,4,5,6,7,8-octahydroanthracene(86)
perhydroanthracenes (100)
20
10
10
5
[a]
Neat substrate (20 mmol) was reacted with 1 at 758C under 4 atm H2.
Determined by GC.
TOF is defined as the moles of hydrogenated products per the moles of total Rh per hour.
Anthracene (0.10 mmol) was dissolved in cyclohexane (5.0 mL).
[b]
[c]
[d]
pentanone was hydrogenated selectively to cyclopen- drogenated quantitatively to ethyl 2-hydroxypropa-
tanol in quantitative yield under conditions A with 1, nate under both conditions. The hydrogenation of
but was transformed into a mixture of cyclopentanol acetone was more than 20 times faster than that of
and dicyclopentyl ether in a 92:8 ratio under condi- 2,6-dimethyl-4-heptanone under conditions B, indicat-
tions B with 2. A keto ester, ethyl pyruvate, was hy- ing the steric effect on the metal nanoparticle-cata-
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Table 5. Hydrogenation of cyclohexanone to cyclohexanol
with 1 or 2.
as well as after 1 h under conditions A to show no ac-
tivity loss; 50–60% conversions and complete conver-
sions were observed after 30 min and after 1 h, re-
spectively. We analyzed the metal contents in the re-
covered catalysts also by ICP to confirm no metal
leaching in the recycling tests. The observed high ac-
tivity and recyclability of 1 would result from the fi-
brous aluminum oxyhydroxide matrix that provides a
highly porous structure and stabilizes metal nanopar-
ticles. There is a previous report that also emphasizes
the fibrous morphology and hydroxy groups of alumi-
na for high catalytic activities.[18]
Entry Catalyst
Conditions[a] Time Yield
TOF[c]
[hÀ1]
[h]
[%][b]
1
2
3
4
5
6
1 (0.10
mol% Rh)
1 (1.0
mol% Rh)
1 (0.10
mol% Rh)
2 (0.10
mol% Ir)
2 (1.0
mol% Ir)
A
A
B
10
15
87
99
79
99
5.2
9.8
63
99
75
99
59
99
87
66
240
200
17
16
63
50
150
100
200
170
0.33
0.50
3.0
6.0
10
A
A
B
20
0.50
1.0
3.0
6.0
2 (0.10
mol% Ir)
Conclusions
In summary, we have developed a simple synthetic
method for recyclable Rh and Ir catalysts and demon-
strated their high activities in arene hydrogenation
and ketone hydrogenation. The catalysts are active
for a wide range of substrates at room temperature
even with a hydrogen balloon. They are also active
under solventless conditions, and show high TOF
values (upto 1700 h À1). Highly selective hydrogena-
tions are possible for bicyclic and tricyclic aromatic
[a]
Conditions A: cyclohexanone (0.50 mmol) dissolved in
1.0 mL of n-hexane was reacted at room temperature
with hydrogen balloon. Conditions B: neat cyclohexa-
none (20 mmol) was reacted at 758C under 4 atm H2.
The yield of cyclohexanol was determined by GC.
TOF is defined as the moles of cyclohexanol produced
per the moles of total metal per hour
[b]
[c]
lyzed hydrogenation. It is noticeable that the activity compounds in high yields by controlling the reaction
of 2 shown in the hydrogenation of acetone is five conditions. Easy separation and robustness in recy-
times higher than that of the previous iridium nano- cling are practical advantages in addition to the easy
particle catalyst prepared in ionic liquid.[7i]
catalyst synthesis.
Recyclability is an essential advantage of heteroge-
neous catalysts over homogeneous ones. Hence, we
tested the recyclability of 1 in the hydrogenation of
anisole under conditions A and under conditions B
(Table 7). The catalyst 1 was recovered simply by fil-
tration or by careful decantation after sedimentation.
The recovered catalyst retained its original activity
even in the 10th cycle under conditions A and at least
in the 5th cycle under conditions B. The reaction mix-
tures in the recycling tests were analyzed after 30 min
Experimental Section
General
Commercial chemicals except anthracene were used without
further purification. Anthracene was purified by recrystalli-
zation from cyclohexane at room temperature. The wide-
angle XRD spectrums were obtained on a diffractometer
(M18XHF, MAC Science Co.) with Cu Ka radiation. The
Table 6. Hydrogenation of various ketones with 1 and 2.
Entry
1
Substrate
Catalyst[a]
Conditions
Time [h]
Product (Yield [%][b])
TOF[c] [hÀ1]
cyclopentanone
1
2
1
2
1
2
2
2
A
B
A
B
A
B
B
B
6.0
7.0
20
3.0
1.0
4.0
1.5
36
cyclopentanol (99)
17
130
5
330
99
250
660
28
cyclopentanol (92)[d]
2
3
2-heptanone
2-heptanol (97)
2-heptanol (99)
ethyl 2-hydroxypropanoate (99)
ethyl 2-hydroxypropanoate (99)
isopropyl alcohol (99)
2,6-dimethyl-4-heptanol (99)
ethyl pyruvate
4
5
acetone
2,6-dimethyl-4-heptanone
[a]
Conditions A: substrate (0.50 mmol) dissolved in 1.0 mL of n-hexane was reacted at room temperature with hydrogen
balloon. Conditions B: neat substrate (20 mmol) was reacted at 758C under 4 atm H2. 1.0 mol% of Rh or 0.10 mol% of
Ir was used respectively.
[b]
[c]
[d]
Determined by GC.
TOF is defined as moles of hydrogenated products by moles of total metal per hour.
Dicyclopentyl ether was formed in 8%.
Adv. Synth. Catal. 2007, 349, 2039 – 2047
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2045

In Soo Park et al.
FULL PAPERS
Table 7. Recycling experiment of anisole hydrogenation with 1[a]
Conditions A
Cycle
1
51.5
100
2
56.8
100
3
57.7
100
4
50.6
100
5
65.1
100
6
58.7
100
7
51.3
100
8
51.3
100
9
52.6
100
10
50.5
100
% yield[b]
% yield[c]
Conditions B
Cycle
1
99.9
2
99.9
3
99.9
4
99.9
5
99.9
% yield[d]
[a]
Conditions A: anisole (0.50 mmol) dissolved in 1.0 mL of n-hexane was reacted at room temperature with hydrogen bal-
loon. Conditions B: neat anisole (20 mmol) was reacted at 758C under 4 atm H2.
After 30 min, determined by GC (1.0 mol% of Rh).
After 60 min, determined by GC (1.0 mol% of Rh).
After 6 h, determined by GC (0.10 mol% of Rh).
[b]
[c]
[d]
TEM images were taken on a field emission electron micro-
in the air for reuse. The filtrate was analyzed by gas chroma-
tography.
scope (JEM 2010F, JEOL) operated at 200 kV. X-ray photo-
electron spectra (XPS) were acquired with a VG-Scientific
ESCALAB 220iXL spectrometer equipped with a hemi-
spherical electron analyzer and an Mg Ka (1253.6 eV) X-
ray source. The binding energy calibration was performed
using the C1s peak at 284.6 eV as the reference energy. H2
chemisorption experiments were performed using an Au-
tochem II 2920 V 3.02 instrument. After a pre-treatment
procedure under argon flowing at 1008C for 2 h, H2 chemi-
sorption was conducted with 5% H2/Ar at 708C. Dispersion
and exposed metal were estimated by the well accepted 1:1
surface hydrogen metal atom stoichiometry.
Hydrogenation of Anisole at 758C under 4 atm H2
Anisole (2.2 mL, 20 mmol) and 1 (4.1 mg, 0.010 mol% Rh)
were added in a 15-mL pressure vessel, and the reaction
mixture was stirred at 758C for 26 h under 4 atm H2. The
stirring speed was 800–900 minÀ1. The catalyst was filtered,
then dried in the air for reuse. The filtrate was analyzed by
gas chromatography.
Hydrogenation of Cyclohexanone at Room
Temperature under 1 atm H2
Synthesis of 1
To a solution of cyclohexanone (52 mL, 0.50 mmol) in
hexane (1.0 mL) in a 10-mL round flask 2 (24 mg, 1.0 mol%
Ir) was added, and the reaction mixture was stirred at room
temperature for 1 h under an H2 balloon. The speed of stir-
ring was 800–900 minÀ1. The reaction mixture was analyzed
by gas chromatography.
RhCl3·xH2O (Rh content: 38 wt.%) (200 mg, 0.73 mmol of
Rh), 2-butanol (1.6 g, 22 mmol), Al(O-sec-Bu)3 (4.3 g,
A
18 mmol) were added into 25-mL round flask equipped with
a condenser. After being stirred at 1008C for 3 h to give a
black suspension, water (1.5 mL) was added. The reaction
mixture was further stirred for 30 min at 1008C, then kept at
room temperature for 24 h. The black solid was filtered,
washed with acetone and dried at 1208C for 1 h to give 1 as
a bluish-gray powder; yield: 1.5 g (5.0 wt.% of Rh). The
rhodium content was estimated by ICP analysis.
Acknowledgements
We are grateful for the financial supports from Korea Re-
search Foundation (KRF-2006–005 J01201).
Synthesis of 2
IrCl3·xH2O (Ir content: 55 wt.%) (140 mg, 0.39 mmol of Ir),
2-butanol (1.6 g, 22 mmol), Al(O-sec-Bu)3 (5.6 g, 23 mmol)
G
were added into 25-mL round flask equipped with a con-
denser. After being stirred at 1008C for 3 h to give a black
suspension, water (1.5 mL) was added. The reaction mixture
was further stirred for 30 min at 1008C, then kept at room
temperature for 24 h. The black solid was filtered, washed
with acetone and dried at 1208C for 1 h to give 2 as a
bluish-gray powder; yield: 1.9 g (4.1 wt.% of Ir). The iridi-
um content was estimated by ICP analysis.
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2047