Hydrogenation of phenol using silica-supported Pd and PdAu catalysts in the presence of H2 and O2

Article abstract of DOI:10.1246/bcsj.20120118

2-Cyclohexen-1-one can be synthesized from phenol using silica-supported Pd catalyst by hydrogenation under coexisting hydrogen and oxygen gases. The effects of hydrogen and oxygen ratio, solvent, and alloying Pd with Au were investigated. Similar transfo

Full text of DOI:10.1246/bcsj.20120118

Short Articles
Bull. Chem. Soc. Jpn. Vol. 85, No. 9, 1057-1059 (2012) 1057
(b)
(a)
Hydrogenation of Phenol Using
Silica-Supported Pd and PdAu
Catalysts in the Presence
of H2 and O2
Sample
Pd/SiO2
12.6
Dispersion/%
Metal surface
area/m2 g-1
Average particle
size/nm
56.3
8.9
20 nm
Shusuke Okada, Kensei Fujiwara,
Takashi Kamegawa, Kohsuke Mori,
and Hiromi Yamashita*
Figure 1. (a) TEM image of Pd/SiO2 and (b) calculated
dates from CO adsorption measurement.
Table 1. Influence of H2 and O2 Ratio for Synthesis 2-
Cyclohexen-1-one (2)a)
Division of Materials and Manufacturing Science,
Graduate School of Engineering, Osaka University,
2-1 Yamada-oka, Suita, Osaka 565-0871
OH
O
O
Pd/SiO2
+
Received April 21, 2012
E-mail: yamashita@mat.eng.osaka-u.ac.jp
H2 and O2
1
2
3
Selectivity/%
H2
/mL min
O2
/mL min
Conversion 1
/%
Entry
2-Cyclohexen-1-one can be synthesized from phenol
using silica-supported Pd catalyst by hydrogenation under
coexisting hydrogen and oxygen gases. The effects of hydro-
gen and oxygen ratio, solvent, and alloying Pd with Au were
investigated. Similar transformations toward corresponding
cyclic enones were observed in other phenolic compounds.
¹1
¹1
2
3
1
2
3
4
5
20
20
20
10
0
0
10
20
20
20
15.2
15.9
11.9
9.7
0.0
11.9
17.9 12.3
36.6 20.1
7.8
®
0.3
®
0.0
a) Reaction conditions: Pd/SiO2 100 mg, phenol 1.0 mmol,
acetonitrile 10 mL, 333 K, 6 h, under atmospheric pressure.
2-Cyclohexen-1-one and other cyclic enones are often em-
ployed for asymmetric synthesis and are very important reac-
tion intermediates for pharmaceutical products.1 Well-known
production methods for 2-cyclohexen-1-one are oxidation of
cyclohexene using molecular oxygen,2 hydrogen peroxide,3 or
organic peroxides.4 However, the selectivity toward 2-cyclo-
hexenone is not sufficient in this manner because of the forma-
tion of by-products, such as cyclohexene oxide and 2-cyclo-
hexen-1-ol. Therefore, 2-cyclohexen-1-one is hugely expensive,
and an inexpensive alternative process is explored extensively.5
Our group is studying one-pot oxidation reactions, involving
direct synthesis of hydrogen peroxide (H2O2) at Pd nano-
particles from H2 and O2 gases and the subsequent oxidation
reaction at a titanium oxide moiety.6 During the course of these
studies, we detected small amounts of 2-cyclohexen-1-one
together with quinones (oxidized product) and cyclohexanone
(hydrogenated product) using phenol as reactant. There are an
enormous number of reports concerning hydrogenation of phe-
nol to synthesize cyclohexanone, in which relatively high pres-
sure and additives are required.7 On the other hand, the present
reaction proceeds under atmospheric pressure without any
cocatalysts. We predict that the presence of oxygen is a key fac-
tor to synthesize 2-cyclohexen-1-one under mild reaction condi-
tions. In this study, we describe the investigation of the reaction
parameters as well as utilization of PdAu alloy nanoparticles.
The silica (fumed silica, SBET = 390 m2 g¹1, Sigma-Aldrich)-
supported Pd catalysts were prepared by impregnation using
aqueous PdCl2 solution (Pd content was adjusted to 1.0 wt %).
The product was calcined in air at 673 K for 4 h, and then treated
with H2 at 473 K for 1 h before testing its catalytic performance.
The TEM image of Pd/SiO2 catalyst and the data calculated
from CO adsorption measurement are shown in Figure 1. From
TEM images, Pd nanoparticles with a mean diameter of 10 nm
were observed, which corresponds well with CO adsorption
measurement.
First, hydrogenation of phenol with various H2 and O2 ratio
using Pd/SiO2 catalyst was investigated (Table 1). In the
case of only H2 bubbling (20 mL min¹1), only cyclohexanone
(3) and brownish polymerized compound was obtained with-
out 2-cyclohexen-1-one (2). By contrast, under both H2 and
O2, 2-cyclohexen-1-one was synthesized together with cyclo-
hexanone. Apparently, it was proven that the oxygen influ-
enced the activity and selectivity in the hydrogenation of
phenol, and the H2 (20 mL min¹1) and O2 (20 mL min¹1) gives
the highest activity and selectivity for synthesis of 2.
It is well known that catalytic reactions are often effected
by the solvent employed (Table 2). Among those investigated,
the reaction occurred only in acetonitrile. And the addition of
methanol accelerated the reaction rate. The enhancement was
also observed by the addition of ethanol. Alcohols are known
to work as a proton source in solvent and are often used in
hydrogenation reactions.8 The donated proton may accelerate
the reaction.
To achieve higher reaction rate, the alloying of Pd with Au
was attempted. There are many reports that bimetallic PdAu
catalysts show higher activity than that of pure Pd catalysts in
hydrogenation reactions9 and oxidation reactions.10
Published on the web September 8, 2012; doi:10.1246/bcsj.20120118

1058 Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012)
© 2012 The Chemical Society of Japan
Table 2. The Effect of Solvent for Synthesis 2-Cyclohexen-
1-one (2)a)
(B)
(A)
Selectivity/%
Conversion 1
/%
Entry Solvent Additive
(f)
2
3
(e)
1
2
3
4
5
6
7
CH3CN
®
11.9
27.3
13.9
24.3
0
36.6 20.1
42.8 11.0
35.6 trace
39.8 13.1
(d)
CH3CN CH3OH (2 mL)
CH3CN CH3OH (5 mL)
CH3CN C2H5OH (2 mL)
CH3OH
THF
(d)
(c)
(b)
(c)
(b)
®
®
®
0
0
0
0
0
0
0
0
H2O
(a)
(a)
a) Reaction conditions: Pd/SiO2 100 mg, phenol 1.0 mmol,
solvent 10 mL, H2 20 mL min¹1, O2 20 mL min¹1, 333 K, 6 h,
under atmospheric pressure.
0
2
4
6
0
2
4
6
Interatomic distance / Å
Figure 3. (A) Au LIII-edge FT-EXAFS and (B) Pd K-edge
FT-EXAFS spectra of PdAu/SiO2 catalysts. (a) PdAu3 foil,
(b) Pd25Au75/SiO2, (c) Pd50Au50/SiO2, (d) Pd75Au25/
SiO2, (e) Au foil, and (f) Pd foil.
(a)
(b)
(c)
as shown in Figure 3. In the Au LIII-edge Fourier transform
extended X-ray absorption fine structure (FT-EXAFS) spectra,
PdAu/SiO2 and the reference PdAu samples showed intense
doublet peaks at 2.1 and 2.85 ¡. This was completely differ-
ent from that of Au foil which exhibits a single peak at
approximately 2.5 ¡. Both peaks can be ascribed to the nearest
metal-metal bonding. The splitting of the first shell peak arises
from the interference between the EXAFS oscillations of the
Au-Au and Au-Pd bonds, whose phase shift and amplitude
differ from each other. In the Pd K-edge spectra, PdAu/SiO2
samples except for Pd25Au75/SiO2 exhibited a peak due to the
contagious Pd-Pd bond in the metallic form together with the
additional peak at a shorter atomic distance. This indicated the
presence of heteroatomic bonding. In Pd25Au75/SiO2, the Pd-
Au bond (at 2.85 ¡) was observed instead of Pd-Pd bond.
Since the reduction potentials of Au ions are more positive than
Pd ions (E0(Pd2+/Pd0) = +0.63 V, E0(Au3+/Au0) = +1.0 V
vs. NHE), following the Au ions are reduced first and form
the core, and the Pd ions are reduced on the surface. The
Pd75Au25/SiO2 and Pd50Au50/SiO2 were thought to be Au
rich cores covered with Pd rich shells, while PdAu alloy shells
were formed on Au rich cores in Pd25Au75/SiO2. The above
speculation is supported by the catalytic results.
The catalytic results are shown in Table 3. In the case of
Pd25Au75/SiO2, the activity and the selectivity was higher
than the pure Pd/SiO2 catalyst or the other PdAu/SiO2
catalysts. These results indicate that a synergy exists between
Au and Pd atoms for the SiO2-supported catalysts. The main
origin of the enhanced catalytic activity may be explained by
the electronic promotion of Pd by the addition of Au.
As shown in Figure 4, the time course of hydrogenation
of phenol reveals that 2-cyclohexen-1-one was produced at
an early stage, and the induction period was observed for
the formation of cyclohexanone. From the previous research
on hydrogenation of phenol, it is known that the phenol is
converted to enol initially and it is further hydrogenated to
cyclohexanone.7 The hydrogenation of phenol in the presence
of H2 and O2 should be first initiated by partial hydrogenation
20 nm
20 nm
20 nm
(d)
Sample
Pd75Au25/SiO2 Pd50Au50/SiO2 Pd25Au75/SiO2
Dispersion/%
28.5
17.4
57.2
18.4
53.9
Metal surface
107.4
area/m2 g-1
Average particle
size/nm
4.1
6.7
6.4
Figure 2. TEM images of (a) Pd75Au25/SiO2, (b) Pd50
Au50/SiO2, (c) Pd25Au75/SiO2, and (d) and the dates
obtained from CO adsorption measurement.
The PdAu catalysts with different Pd and Au ratio were
prepared from an aqueous solution of PdCl2 and HAuCl4. After
the impregnation, the sample was calcined in air at 673 K for
4 h, and then treated with H2 at 473 K for 1 h.
The CO adsorption results and TEM images of PdAu/SiO2
are summarized in Figure 2. In all samples, the average particle
size of PdAu nanoparticle were estimated to be around 4-6 nm
by CO adsorption. On the other hand, it was proven that PdAu
nanoparticles exhibited a bimodal size distribution: the smaller
nanoparticles exhibited an average diameter of 5 nm, while
some larger particles that ranged in size from 20 to 50 nm were
observed. The large particles become small with increasing Au
ratio. This bimodal distribution was consistent with previous
studies.11 They investigated the PdAu composition as a function
of particle size by scanning transmission electron microscopy
(STEM)-X-ray energy-dispersive spectrometry (XEDS)11a or
transmission electron microscopy (TEM)-energy-dispersive
X-ray spectrometry (EDX),11b where they reported small parti-
cles were Pd-rich or only Pd and large particles were Au-rich
PdAu particle.
X-ray absorption fine structure (XAFS) measurements were
carried out to elucidate the structure of the alloy nanoparticles,

S. Okada et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 9 (2012) 1059
Table 3. Synthesis of 2-Cyclohexen-1-one by PdAu/SiO2
Catalysts with Various Au and Pd Ratioa)
In summary, 2-cyclohexen-1-one and phenolic enones were
synthesized from phenol and phenol derivatives using hetero-
geneous Pd/SiO2 or PdAu/SiO2 catalysts under a mixture of H2
and O2. The existence of oxygen is key to form 2-cyclohexen-
1-one from phenol. It was also showed that the alloying of
Pd with Au was effective to enhance the catalytic activity and
Pd25Au75/SiO2 showed the best performance. The reaction
pathway is still unclear but it is now under investigation in our
laboratory. This report showed an inexpensive new route to
synthesize enones under mild condition.
Selectivity/%
Pd
/wt %
Au
/wt %
Conversion 1
/%
Entry
2
3
1
2
3
4
5
1
0
27.3
29.9
30.5
41.6
0
42.8
35.1
23.6
56.6
®
11.0
14.6
3.5
24.2
®
0.75
0.50
0.25
0
0.25
0.50
0.75
1
a) Reaction conditions: catalyst 100 mg, phenol 1.0 mmol,
acetonitrile 10 mL, methanol 2 mL, H2 20 mL min¹1, O2 20
mL min¹1, 333 K, 6 h, under atmospheric pressure.
Experimental
All reagents were commercial materials of reagent grade and
used without further purification.
O
Synthesis of Pd/SiO2 or PdAu/SiO2.
Fumed silica
30
purchased from Sigma-Aldrich (1.0 g) was dispersed in distill-
ed water (50 mL) and ethanol (50 mL) by ultrasound. Then, the
necessary quantity of PdCl2 aqueous solution (PdCl2 (1.0 g)
dissolved in 5 M aqueous HCl solution (5 mL) and distilled
water (495 mL)) and aqueous HAuCl4 solution (HAuCl4 (1.0 g)
was dissolved in distilled water (250 mL)) and added sequen-
tially. The suspension was stirred at 323 K for an hour and
evaporated under vacuum at 353 K. The obtained powder was
dried at 373 K in air, calcined at 823 K for 5 h in air, and
afterward, treated with H2 at 473 K for 1 h before testing its
catalytic performance.
20
O
10
0
0
2
4
6
Reaction Time/h
Figure 4. Time course of synthesis of 2-cyclohexen-1-one
and cyclohexanone from phenol by Pd25Au75/SiO2.
Reaction conditions: Pd25Au75/SiO2 100 mg, phenol 1.0
mmol, acetonitrile 10 mL, methanol 2 mL, H2 20 mL min
O2 20 mL min¹1, 333 K, under atmospheric pressure.
Characterization.
Pd K-edge EXAFS spectra were
¹1
,
recorded using fluorescence-yield collection at the beam line
01B1 station with an attached Si(311) monochromator at
SPring-8 of the Japan Synchrotron Radiation Research Institute
(JASRI). The EXAFS data were normalized by fitting the
background absorption coefficient, around the energy region
higher than the edge of approximately 35-50 eV, with the
smoothed absorption of an isolated atom. The EXAFS data
were examined using the Rigaku EXAFS analysis program.
OH
OH
O
O
H2
O2
H2
-1/2H2O
Scheme 1. Possible reaction root to synthesis 2-cyclohex-
ene-1 one from phenol.
References
a)
Table 4. Synthesis of Cyclic Enones by Pd25Au75/SiO2
1
2
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Reactant
Conversion 1¤
/%
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1¤
2¤
3¤
2
3
OH
O
O
O
3
C. M. Chanquía, A. L. Cánepa, J. Bazán-Aguirre, K. Sapag, E.
CH3
CH3
CH3
22.6
12.6
30.5
29.7
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4
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23.5
27.9
CH3
CH3
CH3
Chem.®Eur. J. 2011, 17, 9047.
7
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a) Reaction conditions: Pd25Au75/SiO2 100 mg, reactant
1.0 mmol, acetonitrile 10 mL, methanol 2 mL, H2 20 mL min
O2 20 mL min¹1, 333 K, 6 h, under atmospheric pressure.
¹1
326, 1250. b) J. Matos, A. Corma, Appl. Catal., A 2011, 404, 103.
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9
157.
of phenol to enol. Such extremely unstable reaction intermedi-
ates may undergoes oxidative dehydrogenation to produce 2-
cyclohexen-1-one, which is further hydrogenated to cyclo-
hexanone (Scheme 1).
Finally, other phenol derivatives, such as o-cresol and p-
cresol were used as reactants. Although the reaction rates were
very low compared to phenol, the partial hydrogenated prod-
ucts were obtained as in the phenol (Table 4).
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Brett, A. F. Carley, R. C. Tiruvalam, A. A. Herzing, C. J. Kiely, D. W.
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