Unexpected Pd-catalyzed hydrogenation of phenol to 2-cyclohexene-1-one: Enhanced activity and selectivity assisted by molecular oxygen

Article abstract of DOI:10.1039/c2cc31995k

Addition of O₂ enables the liquid-phase hydrogenation of phenol using H₂ at low temperatures under atmospheric pressure with an unexpected selectivity towards 2-cyclohexene-1-one rather than cyclohexanone. As a catalyst, as-synthesized Pd/MCM-41 including the residual templates within the mesoporous channels exhibits a prominent positive effect on the catalytic performance.

Full text of DOI:10.1039/c2cc31995k

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Showcasing research from the Laboratory of
Profs. Hiromi Yamashita and Kohsuke Mori
at the Division of Materials and Manufacturing
Science, Graduate School of Engineering,
Osaka University
As featured in:
Unexpected Pd-catalyzed hydrogenation of phenol to
2-cyclohexene-1-one: enhanced activity and selectivity assisted
by molecular oxygen
Addition of O enables the liquid-phase hydrogenation of phenol
2
using H at low temperatures under atmospheric pressure with an
2
unexpected selectivity towards 2-cyclohexene-1-one rather than
cyclohexanone. As a catalyst, as-synthesized Pd/MCM-41 including
the residual templates within the mesoporous channels exhibits a
prominent positive effect on the catalytic performance.
See Hiromi Yamashita et al.,
Chem. Commun., 2012, 48, 8886.
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COMMUNICATION
Unexpected Pd-catalyzed hydrogenation of phenol to 2-cyclohexene-1-one:
enhanced activity and selectivity assisted by molecular oxygenw
Kohsuke Mori, Ken Furubayashi, Shusuke Okada and Hiromi Yamashita*
Received 19th March 2012, Accepted 17th April 2012
DOI: 10.1039/c2cc31995k
Addition of O enables the liquid-phase hydrogenation of phenol
2
into the reaction mixture, together with molecular hydrogen (H2)
as a reducing reagent, substantially enhances the catalytic
conversion of phenol as well as selectivity towards 2-cyclohexene-
1-one. To the best of our knowledge, this is the first synthesis
of 2-cyclohexene-1-one from the hydrogenation of phenol. It
should also be noted that this paradoxical new reaction
proceeds at low temperatures under atmospheric pressure
without the need for co-catalysts.
using H at low temperatures under atmospheric pressure with
2
an unexpected selectivity towards 2-cyclohexene-1-one rather
than cyclohexanone. As a catalyst, as-synthesized Pd/MCM-41
including the residual templates within the mesoporous channels
exhibits a prominent positive effect on the catalytic performance.
Catalytic hydrogenation processes are employed in the
chemical industry as a promising way to obtain several
1
essential products and intermediates. Among these processes,
The mesoporous silica material (MCM-41) was prepared via
the sol–gel method according to a literature procedure, using
cetyltrimethylammonium bromide (CTABr) as a structure-
directing agent (SDA) and tetramethylorthosilicate (TMOS)
the hydrogenation of phenol to cyclohexanone is one of the
most crucial reaction pathways for the manufacture of nylon-6
2
and nylon-66. The process is performed in either gas or liquid
14
2+
as a Si source. The incorporation of the Pd precursor was
done using the template ion exchange (TIE) method by the
treatment of as-synthesized MCM-41 with an aqueous
phase with the use of supported Pd catalysts. The gas phase
route can be carried out easily in a continuous reactor for
higher throughput, but generally requires elevated temperatures
1
5
solution of (NH
4 2 4
) PdCl . Finally, reduction with an aqueous
3
–6
(
423–573 K). The liquid phase route allows cost and energy
NaBH solution in an Ar atmosphere afforded Pd/MCM-41
4
savings because the reaction proceeds at relatively low temp-
eratures. However, high hydrogen pressures (0.5–4 MPa) and
(Pd 0.5 wt%) as a pale gray powder. The residual SDA
templates inside the mesoporous channel are generally removed
by calcination or extraction prior to the catalytic reaction;
however, this process was intentionally skipped. As discussed
later, the residual SDA plays a significant role in enhancement
of the catalytic activity during phenol hydrogenation. This new
phenomenon is the other important issue we wish to report here.
The low-angle X-ray diffraction (XRD) pattern of Pd/MCM-41
reveals a diffraction peak at around 2y = 2–31 associated with
the d100 spacing, which indicates a hexagonally-packed meso-
porous structure. The amount of residual SDA template in the
as-synthesized MCM-41 was determined to be 40 wt% by TG
analysis. The Brunauer–Emmett–Teller (BET) surface area
Lewis acid additives (AlCl
3 2
and SnCl ) are often required, thus
the process sometimes suffers from subsequent hydrogenation
–11
to cyclohexanol.
7
a,b-Unsaturated ketones are commonly known to be pharmaco-
phores and versatile building blocks in organic synthesis,
and numerous strategies have been documented for their
1
2
production. Unfortunately, most methods either involve
several reaction steps or are often complicated by low yield
and undesired by-products. For example, the production of
2
-cyclohexene-1-one on a commercial scale is performed by
selective hydrogenation of benzene to cyclohexene followed
by allylic oxidation, which simultaneously produces the corre-
(SBET), pore volume (V
adsorption–desorption were 734 m g , 0.63 cm g , and
2.6 nm, respectively. The SBET and V of the as-synthesized
p
), and pore diameter calculated from
1
3
2
À1
3
À1
sponding allylic alcohol, epoxide, and diol.
N
2
In this communication, we present a new protocol for the
one-step synthesis of 2-cyclohexene-1-one by the liquid-phase
selective hydrogenation of phenol using a Pd-supported meso-
porous silica (Pd/MCM-41) catalyst. The most important
discovery here is that the addition of molecular oxygen (O2)
p
2
À1
3
À1
3
Pd/MCM-41 are lower than 974 m g and 0.87 cm g of
2
the calcined Pd/MCM-41 as well as 886 m g and 0.81 cm g
À1
À1
as synthesized Pd/MCM-41 before reduction. These results
suggest that the SDA template and Pd nanoparticles are
present within the mesoporous channel. The Pd K-edge Fourier
transform-extended X-ray absorption fine structure (FT-EXAFS)
Division of Materials and Manufacturing Science, Graduate School of
Engineering, Osaka University, 2-1 Yamada-oka, Suita,
˚
exhibited a single peak at approximately 2.5 A due to the
Osaka 565-0871, Japan. E-mail: yamashita@mat.eng.osaka-u.ac.jp;
Fax: +81-6-6879-7457; Tel: +81-6-6879-7457
w Electronic supplementary information (ESI) available: Experimental
procedures, Pd K-edge XAFS spectra, and TEM analysis. See DOI:
contiguous Pd–Pd bond in the metallic form with an inter-
˚
atomic distance and a coordination number of 2.76 A and 9,
respectively. These results are in good agreement with those of
the transmission electron microscopy (TEM) analysis, where a
10.1039/c2cc31995k
8
886 Chem. Commun., 2012, 48, 8886–8888
This journal is c The Royal Society of Chemistry 2012

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Table 1 Results of phenol hydrogenation using Pd/MCM-41 under
different conditions
a
Yield (%)
Entry
H
2
/mL
O
2
/mL
(2)
(3)
1
2
3
4
5
6
7
20
20
20
20
10
5
0
0
47
63
72
42
28
0
0
42
36
21
3
5
10
20
20
20
20
1
0
0
a
Catalytic tests were performed using catalyst (0.05 g, Pd: 2.3 Â
À3
1
0
mmol), phenol (1.0 mmol), and acetonitrile (20 mL) at 333 K
for 6 h with magnetic stirring. Yields were determined using a gas
chromatograph.
Fig. 2 Kinetic profile in the hydrogenation of 2-cyclohexene-1-one
2 2 2
under a H –O atmosphere (K) and H atmosphere (&).
narrow size distribution of spherical Pd nanoparticles with a
mean diameter (d) of 2.4 nm was observed, which is almost
consistent with the pore diameter of MCM-41.
atmosphere during the course of the reaction. Fig. 1 shows
that the initial stage of reaction under H and O flow
proceeded smoothly with high selectivity towards 2. After 2 h,
conversion of phenol completely ceased by the removal of O
2
2
The hydrogenation of phenol (1) using Pd/MCM-41 was
conducted by flowing several sets of gaseous H and O for 6 h
2
2
2
from the reaction mixture accompanied with a significant
at 333 K in the presence of CH CN solvent under atmospheric
3
decrease in selectivity toward 2, due to the formation of 3
pressure (Table 1). No reaction occurred in the presence
of either H2 or O ; however, both H and O2 flows were
2
through subsequent hydrogenation. Upon addition of O , the
2
2
catalytic activity and selectivity returned to the same levels as
those at the initial stage. When 2 was used as the substrate,
hydrogenation using only H2 proceeded rapidly to give 3,
whereas significant retardation was observed under an atmo-
indispensable to achieve hydrogenation of phenol. 2-Cyclo-
hexene-1-one (2) and cyclohexanone (3) were the only
products detected over the entire range of conditions studied,
and the highest yield (72%) with a high selectivity of 2 (77%)
sphere of the H
Treatment of as-synthesized Pd/MCM-41 with H
mixture of H and O had no significant effect on the mean
2 2
–O mixture (Fig. 2).
was achieved with H
2 2 2
/O = 20 (mL)/20 (mL). Excess O flow
2
or a
relative to H increased the selectivity toward 2 up to 94%, but
2
2
2
the reaction rate was substantially decreased. Among the
solvents examined, acetonitrile yielded favorable results,
diameter of the Pd nanoparticles, which indicates that the
reaction did not involve structural change of the Pd nano-
whereas ethanol, toluene, THF, and H O were completely
2
particles. The addition of H
the hydrogenation rate of phenol and selectivity toward 2,
which suggests that the H produced in situ from H and O
2 2
O had no significant influence on
ineffective.
The prominent effect of O2 on the catalytic activity and
selectivity can also be well demonstrated by switching the
2
O
2
2
2
on Pd sites does not participate in the reaction mechanism.
Although the detailed reaction mechanism is now under
investigation in our laboratory, a possible reaction pathway
is illustrated in Scheme 1. In the presence of H and O , the
2
2
Fig. 1 Kinetic profile in the hydrogenation of phenol under a H
atmosphere (white zone) and H atmosphere (gray zone).
2
–O
2
Scheme 1 A plausible reaction pathway for the Pd-catalyzed hydro-
genation of phenol under a H –O atmosphere.
2
2
2
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Table 2 Results of phenol hydrogenation using various Pd catalysts
under the H –O mixture
2 2
Pd/MCM-41 and CTABr was employed (entry 5), which
suggests that the close proximity of the periphery of the active
Pd nanoparticles to SDA within the mesoporous channels is
an essential condition. The use of Al O , fumed SiO , TiO ,
a
2
3
2
2
CeO , carbon, and Na-Y–zeolite (SiO /Al O = 5) gave
2
2
2
3
poor results with respect to both conversion and selectivity
entries 6–11). This unique phenomenon can be explained by
(
the weak basicity dispersed inside the mesoporous channel,
19
which is in favor of interaction with the weak acidic phenol.
Yield (%)
Entry
Sample
Template
(2)
(3)
Preliminary investigation provided evidence that the as-
1
2
3
4
5
Pd/MCM-41
CTABr
—
CTACl
CTABr
—
72
18
54
71
0
21
8
30
16
0
synthesized Pd/MCM-41 exhibited high adsorption capacity
À1
Calcined Pd/MCM-41
Pd/MCM-41
Pd/MCM-48
Physical mixture of calcined
Pd/MCM-41 and CTABr
for phenol (0.4 mmol g ) in CH
CN solution over 6 h, while
À1
3
À3
negligible adsorption (o4 Â 10 mmol g ) was observed for
the calcined Pd/MCM-41.
In conclusion, the combination of H and O allows selec-
2
2
6
7
8
9
Pd/Al
Pd/SiO
Pd/TiO
Pd/CeO
Pd/Carbon
Pd/Na-Y zeolite
2
O
3
—
—
—
—
—
—
33
9
0
0
0
46
1
0
0
0
tive hydrogenation of phenol to 2-cyclohexene-1-one under
mild reaction conditions. The use of as-synthesized uncalcined
Pd/MCM-41 including the SDA template also plays a key role
in achieving efficient hydrogenation that could potentially con-
tribute to saving energy and time. The unprecedented catalytic
performance demonstrated in this study holds a significant
promise for the achievement of novel catalyst systems.
2
2
2
1
1
0
1
0
0
a
Catalytic tests were performed using catalyst (0.05 g, Pd: 2.3 Â
À3
1
6
0
mmol), phenol (1.0 mmol), and acetonitrile (20 mL) at 333 K for
2 2
h with H /O = 20 (mL)/20 (mL). Yields were determined using a
gas chromatograph.
Notes and references
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888 Chem. Commun., 2012, 48, 8886–8888
This journal is c The Royal Society of Chemistry 2012