Heterogeneous cobalt catalysts for the acceptorless dehydrogenation of alcohols

Article abstract of DOI:10.1039/c2gc36555c

A series of transition metal(M)-loaded TiO₂ catalysts (M/TiO₂) and Co-loaded catalysts on various support materials were prepared by an impregnation method, followed by in situ H₂-reduction at 400 °C, and tested for the acceptor-free oxidation of cyclododecanol in the liquid phase. Among the catalysts including noble metal catalysts, Co/TiO₂ showed the highest activity. In the presence of Co/TiO ₂ (0.1-3 mol%) the dehydrogenation of various aliphatic secondary alcohols proceeded to afford the corresponding ketones. The catalyst was recoverable and was reused after the H₂-reduction treatment. Based on the spectroscopic characterization of the catalyst combined with the studies on the effect of the Co oxidation states on the catalytic activity, it is clarified that the surface metallic Co sites with electron deficiency are the catalytically active species.

Full text of DOI:10.1039/c2gc36555c

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Heterogeneous cobalt catalysts for the acceptorless
dehydrogenation of alcohols†
Cite this: Green Chem., 2013, 15, 418
Ken-ichi Shimizu,*a,b Kenichi Kon,a Mayumi Seto,a Katsuya Shimura,a
Hiroshi Yamazakic and Junko N. Kondoc
A series of transition metal(M)-loaded TiO2 catalysts (M/TiO2) and Co-loaded catalysts on various support
materials were prepared by an impregnation method, followed by in situ H2-reduction at 400 °C, and
tested for the acceptor-free oxidation of cyclododecanol in the liquid phase. Among the catalysts includ-
ing noble metal catalysts, Co/TiO2 showed the highest activity. In the presence of Co/TiO2 (0.1–3 mol%)
the dehydrogenation of various aliphatic secondary alcohols proceeded to afford the corresponding
ketones. The catalyst was recoverable and was reused after the H2-reduction treatment. Based on the
spectroscopic characterization of the catalyst combined with the studies on the effect of the Co oxidation
states on the catalytic activity, it is clarified that the surface metallic Co sites with electron deficiency are
the catalytically active species.
Received 2nd October 2012,
Accepted 29th November 2012
DOI: 10.1039/c2gc36555c
www.rsc.org/greenchem
synthesis and manipulation, difficulties in catalyst reuse and
high price. To overcome these problems, heterogeneous
Introduction
During the last few decades, noble metal-based organometallic Ru,29–31 Ag,32,33 and Au34 catalysts have been recently deve-
and heterogeneous catalysts have been proven to be effective loped, but these systems require the use of noble metals. From
for a large number of organic reactions. One area of recent pro- environmental and economic viewpoints, use of first-row tran-
gress is the oxidation of alcohols to carbonyl compounds.1–4 sition-metal catalysts for this reaction is ideal. There are a few
To meet environmental and economical acceptability, much examples based on Ni,35–37 Co38 and Cu39 based hetero-
effort has been devoted to develop heterogeneous catalysts, geneous catalysts, but they suffer from drawbacks such as high
mainly based on noble metals1–4 (such as Pd,5 Ru,6,7 Pt,8 and catalyst loading, a limited scope35–37 or high reaction temp-
Au9,10) which are highly effective for the aerobic oxidation of erature (200 °C).37,38 Although a few cobalt (Co) catalysts are
alcohols. Non-noble metal based heterogeneous catalysts also known to be effective for the aerobic oxidation of alcohols,11–15
catalyze the aerobic oxidation of alcohols,1–3,11–16 but their to the best of our knowledge, Co catalyzed acceptor-free oxi-
efficiencies are generally lower than the noble metal catalysts dation of alcohols is not known. Our ongoing interest in the
in terms of activity and substrate scope. From the viewpoint of catalysis of supported metal nanoparticles (NPs) in organic
atom efficiency and safety of the reaction, an oxidant-free cata- synthesis40 motivated us to investigate possibilities of sup-
lytic dehydrogenation of alcohols to carbonyl compounds and ported Co metal NPs as cheaper alternatives to the well esta-
molecular hydrogen is ideal. This process is also of current blished noble-metal catalysts. We report herein our findings
interest as a potential method for hydrogen production from on the Co-based heterogeneous catalyst for the acceptor-free
biomass products.17,18 Several systems for the dehydrogenative alcohol dehydrogenation of secondary alcohols to ketones. The
oxidation of alcohols using homogeneous Rh,19 Ru,20–25 and effect of Co oxidation state on the activity is studied to clarify
Ir26–28 catalysts were reported, but most of them suffer from the structure of active Co species.
requirements of acid or base additives, difficult catalyst
Experimental
aCatalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021,
General
Japan. E-mail: kshimizu@cat.hokudai.ac.jp; Fax: +81-11-706-9163
bElements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura,
Commercially available organic and inorganic compounds
(from Tokyo Chemical Industry, Wako Pure Chemical Indus-
Kyoto 615-8520, Japan
cChemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,
tries, Kishida Chemical, or Mitsuwa Chemicals) were used
Midori-ku, Yokohama, 226-8503, Japan
without further purification. The GC (Shimadzu GC-14B) and
GCMS (Shimadzu GCMS-QP2010) analyses were carried out
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c2gc36555c
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with an Ultra ALLOY capillary column UA+-5 (Frontier Labo- monitored by GC analysis of aliquots (ca. 0.05 g). The initial
ratories Ltd) or an Rtx-65 capillary column (Shimadzu) using rate of the reaction was measured under the conditions where
nitrogen as the carrier gas.
the conversion was below 40%. The products were identified
by GC-MS equipped with the same column as GC and by com-
parison with commercially pure products. GC analysis of the
Catalyst preparation
TiO2 (JRC-TIO4, 50 m2 g−1), MgO (JRC-MGO-3, 19 m2 g−1), gas phase product (H2) was carried out by a Shimadzu GC-6A
CeO2 (JRC-CEO3, 81.4 m2 g−1), and SiO2–Al2O3 (JRC-SAL-2, instrument with a TCD detector, molecular sieve 5A column
Al2O3 = 13.75 wt%, 560 m2 g−1) were supplied from Catalysis and an Ar carrier.
Society of Japan. γ-Al2O3 (with a surface area of 124 m2 g−1
)
was prepared by calcination of γ-AlOOH (Catapal B Alumina
Characterization
purchased from Sasol) for 3 h at 900 °C. SiO2 (Q-10, 300 m2
g−1) was supplied from Fuji Silysia Chemical Ltd. Active Temperature programmed reduction under H2 (H2-TPR) was
carbon (296 m2 g−1) was purchased from Kishida Chemical. carried out with BELCAT (BELL Japan Inc.). The sample,
Hydroxides of Zr or Sn were prepared by hydrolysis of zirco- Co3O4/TiO2 (Co loading = 5 wt%, 20 mg), was mounted on a
nium oxynitrate 2-hydrate or SnCl4·6H2O in distilled water by quartz tube and was heated with a temperature ramp-rate of
gradually adding an aqueous NH4OH solution (1.0 mol dm−3), 5 °C min−1 in a flow of 5% H2/Ar (20 cm3 min−1). The effluent
followed by filtration of the precipitate, washing with distilled gas was passed through a trap containing MS4A to remove
water three times, drying at 100 °C for 12 h. Nb2O5·nH2O was water, then through the thermal conductivity detector. The
commercially supplied (CBMM). ZrO2, Nb2O5, and SnO2 were amount of H2 consumed during the experiment was detected
prepared by calcination of these hydroxides at 500 °C for 3 h.
Co3O4/TiO2 was prepared by an impregnation method; a
by a thermal conductivity detector.
X-ray diffraction (XRD) patterns of the powdered catalysts
mixture of TiO2 and an aqueous solution of Co(NO3)2·6H2O were recorded under ambient conditions with a Rigaku Mini-
was evaporated at 50 °C, followed by drying at 90 °C for 12 h Flex II/AP diffractometer with Cu Kα radiation.
and calcination in air at 300 °C for 1 h. Before each catalytic
Co K-edge X-ray absorption measurements were performed
experiment, a pre-reduced catalyst (Co/TiO2) was prepared by in transmission mode at the BL01B1 in the SPring-8 (Hyogo,
in situ pre-reduction of Co3O4/TiO2 in a glass (pyrex or quartz) Japan, proposal no. 2012A1734). The storage ring energy was
tube under a flow of H2 (20 cm3 min−1) at the reduction temp- operated at 8 GeV with a typical current of 100 mA. The catalyst
erature (TH ) of 400 °C for 0.5 h. Co (5 wt%)-loaded CeO2, ZrO2, pre-reduced in a flow of 100% H2 (20 cm3 min−1) for 0.5 h at
2
Nb2O5, SiO2–Al2O3, SnO2, SiO2, and C were prepared by the various temperatures was cooled to room temperature in the
impregnation method in the same manner as the preparation flow of H2 and was sealed in cells made of polyethylene under
of Co/TiO2. Al2O3- or MgO-supported Co (5 wt%) catalysts were N2, and then the XAFS spectrum was taken under ambient
prepared by the impregnation method, followed by drying at conditions. The analysis of X-ray absorption near-edge struc-
90 °C for 12 h, and by in situ pre-reduction of the precursor tures (XANES) and extended X-ray absorption fine structure
under H2 at 400 °C. TiO2- or Al2O3-supported metal (Ni, Cu, (EXAFS) was performed using the REX version 2.5 program
Ru, Pd, Rh, Ag) catalysts with a metal loading of 5 wt% were (RIGAKU). The Fourier transformation of the k3-weighted
prepared by the impregnation method in a similar manner to EXAFS oscillation from k space to r space was performed over
the preparation of Co/TiO2 using an aqueous solution of metal the range 24–157 nm−1 to obtain a radial distribution func-
nitrates (Ni, Cu), RuCl3, or an aqueous HNO3 solution of Rh tion. The inversely Fourier filtered data were analyzed with a
(NO3)3 and Pd(NO3)2. Au/TiO2 synthesized by a deposition–pre- usual curve fitting method in the k range of 24–157 nm−1. The
cipitation procedure (Au = 1.5 wt%, average Au particle size = parameters for the Co–O and Co–Co shells were provided by
3.6 0.28 nm) was purchased from the World Gold Council the FEFF6. The number of free parameters for curve fitting can
and used in the catalytic experiment without any pretreatment. be estimated as Pfree = 2Δk ΔR/π ≈ 13, indicating that we can
model the EXAFS data with three shells.
Catalytic reactions
IR spectra were obtained by use of a Jasco Fourier transform
The catalysts, e.g., Co/TiO2 (Co loading = 5 or 10%), were pre- infrared (FT-IR) 4100 spectrometer equipped with an MCT
reduced to Co3O4/TiO2 in the glass tube under a flow of H2 detector at a 4 cm−1 resolution. 64 scans were collected to
(20 cm3 min−1) at 400 °C for 0.5 h. After the pre-reduction, we obtain each spectrum. The real temperature of the sample was
carried out catalytic tests without exposing the catalyst to air estimated from a calibration curve of temperatures measured
as follows. The mixture of o-xylene (1 g), alcohol (1 mmol), and inside and outside the IR cell. The catalyst precursor (Co3O4/
n-dodecane (0.5 mmol) was injected onto the pre-reduced cata- TiO2) was pre-reduced under H2 (20 kPa) at 500 °C for 0.5 h,
lyst inside a reactor (cylindrical glass tube) through a septum followed by evacuation at the same temperature for 0.5 h. IR
inlet, followed by filling with N2. Then, the resulting mixture spectra of clean samples before CO adsorption experiments
was stirred under reflux; the reaction temperature was ca. were measured under evacuation at −180 °C and were used
144 °C and the heating temperature was 155 °C. Conversion as background spectra. Subtracted IR spectra of adsorbed
and yields of products were determined by GC using n- CO were measured after the sample was exposed to CO
dodecane as an internal standard. Progress of the reaction was (500 Pa) at −180 °C.
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Table 1 Dehydrogenation of cyclododecanola
Entry
Catalyst
Conv. (%)
Yield (%)
1b,c
2b,c
3b
4c
Co3O4
CoO
1
1
20
0
39
1
15
12
10
9
9
9
2
2
2
5
2
2
5
3
11
5
4
0
1
12
0
37
1
15
12
9
9
9
2
1
1
2
5
2
2
5
3
11
1
Co powder
Co3O4/TiO2
Co/TiO2
Co/TiO2–air
Co/Al2O3
Co/CeO2
Co/C
Co/ZrO2
Co/SiO2
Co/SiO2–Al2O3
Co/SnO2
Co/Nb2O5
Co/MgO
Ni/TiO2
Cu/TiO2
Ag/TiO2
Pd/TiO2
Ru/TiO2
Au/TiO2
Ag/Al2O3
Pd/Al2O3
Rh/Al2O3
Ru/Al2O3
5
6d
7
Fig. 1 Effect of Co loading of Co/TiO2 (TH = 400 °C) on TOF (based on the
8
9
2
total number of Co atoms) for the dehydrogenation of cyclododecanol at
144 °C.
10
11
12
13
14
15
16
17
18
19
20
21c
22
23
24
25
dehydrogenation of 1. It is found that the loading of 5–10 wt%
is the optimal value. A series of transition metal-loaded TiO2
and noble metal-loaded Al2O3 catalysts were also tested for the
dehydrogenation of 1. Among the first-row transition metals
(entries 5, 16, 17), Co/TiO2 showed the highest yield of 2.
Notably, Co/TiO2 showed higher yield than noble metal (Ag,
Pd, Ru, Au)-loaded TiO2 catalysts (entries 18–21) and noble
metal (Ag, Pd, Rh, Ru)-loaded Al2O3 catalysts (entries 22–25).
From the above results, TiO2-supported Co (5 or 10 wt%) cata-
lysts were found to be the optimal catalysts.
1
4
1
4
6
a Catalysts were pre-reduced in H2 at 400 °C. Conversion and yields
were determined by GC. b Catalyst = 10 mol%. c Catalysts were not pre-
reduced. d Pre-reduced Co/TiO2 was exposed to air at room temperature
for 0.5 h.
Structure of active Co species
Next, we studied the effect of oxidation states of Co on the
activity. The catalytic results in Table 1 showed that the catalyst
precursor (Co3O4/TiO2) and commercial cobalt oxides (CoO
and Co3O4) exhibited no activity when they were tested without
H2-reduction pretreatment (entries 1, 2, 4). The TiO2 support
showed no activity even after the H2-reduction pretreatment.
These results indicate that the support itself and cobalt oxides
are completely inert. By changing the H2-reduction temp-
Results and discussion
Catalyst screening
As summarized in Table 1, catalyst screening tests were first
carried out to investigate the effect of various catalyst para-
meters on the catalytic activity for the dehydrogenation of
cyclododecanol 1 to cyclododecanone 2 under the same reac-
tion conditions (under N2 in reflux conditions for 0.5 h). The
metal-loaded catalysts, prepared by H2-reduction (400 °C) of
metal oxide-loaded precursors, were used in the catalytic tests
without exposing the catalyst to air. First, commercial Co com-
pounds (10 mol% with respect to 1) and supported Co catalysts
(1 mol%) were tested (entries 1–15). Among various support
materials (MgO, Al2O3, CeO2, ZrO2, TiO2, Nb2O5, SiO2–Al2O3,
SiO2, SnO2, C), the TiO2-supported Co catalyst showed the
highest yield of 2. Use of a large amount (10 mol%) of com-
mercial Co compounds (entries 1–3) resulted in lower yield
than Co/TiO2. Co/TiO2 with different Co loadings (1–20 wt%)
were also prepared. Under the standard conditions shown in
Table 1, the initial formation rate of 2 was measured under the
conditions where the conversion of 1 was below 40%, and
the turnover frequency (TOF) of these catalysts (based on the
total number of Co atoms) was estimated. Fig. 1 shows
the effect of Co loading on the TOF of Co/TiO2 catalysts for the
erature (TH = 300–600 °C) of Co3O4/TiO2, we prepared a series
2
of Co/TiO2 catalysts with various Co oxidation states. Under
the conditions shown in Table 1, the initial formation rates of
2 by these Co/TiO2 catalysts were measured, and the TOF
based on the total number of Co atoms was estimated. Fig. 2
shows the effect of TH on the TOF. The TOF value steeply
2
increased with TH in the TH range from 300 °C to 400 °C and
2
2
then gradually decreased with further increase in TH
.
2
The structure of Co species in a series of Co/TiO2 catalysts
reduced at various temperatures was studied by various charac-
terization methods. The crystal phase was confirmed by XRD
(Fig. 3). In the XRD pattern of the as-calcined sample, lines
due to CoO and Co metal were absent, and a broad line due to
Co3O4 was observed. Thus the sample is referred to as Co3O4/
TiO2. The degree of reduction can be estimated from the TPR
profile of Co3O4/TiO2 under a flow of H2/Ar (Fig. 4). Two
different regions of reduction were observed: a lower temp-
erature region (200–340 °C) and a higher temperature region
located between 340 and 520 °C. XRD result shows that
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H2-reduction at 300 °C resulted in complete reduction of
Co3O4 to CoO. XRD patterns of the samples after the
H2-reduction at higher temperatures (400–600 °C) resulted in
complete reduction of CoO to metallic Co. These results
suggest that the reduction of Co3O4 species in the catalyst pre-
cursor proceeds in a two step process, Co3O4 to CoO (at
200–340 °C) and CoO to Co (at 340–520 °C).
Fig. 5A shows the XANES spectra of representative samples,
and Fig. 6 shows the XANES spectra of the Co/TiO2 catalysts
reduced at various temperatures. The XANES feature of Co3O4/
TiO2 is nearly identical to that of Co3O4, indicating that Co3O4
is the predominant Co species in the precursor. The spectral
features gradually change with increase in the reduction temp-
erature. The XANES spectrum of Co/TiO2 reduced at 600 °C
was nearly identical to that of Co foil. These results are quali-
tatively in agreement with the XRD and TRP results. Curve-
fitting analysis of XANES spectra is a well established method
to quantify the ratio of several metal species in different oxi-
dation states. As illustrated in Fig. S1 (in the ESI†), we obtained
the fractions of the Co metal, CoO, and Co3O4 by the least-
squares fitting of the XANES spectrum of the Co/TiO2 sample
with a linear combination of XANES spectra of Co foil, CoO,
and Co3O4/TiO2. The fraction of metallic Co, CoO and Co3O4
species in each catalyst is plotted in Fig. 2 as a function of the
reduction temperature. The result is qualitatively consistent
with the XRD result except for the result of the sample reduced
at 400 °C. The XRD result showed that the Co metal is the only
Co species in this sample, but the XANES result showed the
co-presence of CoO and the Co metal. This conclusion is sup-
ported by the following EXAFS result (Fig. 5B, Table 2). The
EXAFS of the sample reduced at 400 °C consists of a Co–O
shell (CN = 2.2 at RCo–O of 2.13 Å) and a Co–Co shell (CN = 6.3
at RCo–Co of 2.49 Å). The Co–O distance is consistent with that
of CoO, and the Co–Co distance is consistent with that of
Co foil, indicating the co-presence of CoO species and the
Co metal. The absence of CoO diffraction lines for this sample
may be due to the presence of X-ray amorphous CoO species
such as highly dispersed CoO species or ionic Co(II) species.
Finally, a comparison between structural and catalytic results
Fig. 2 Effect of H2-reduction temperature of Co/TiO2 (Co = 5 wt%) on TOF
(based on the total number of Co atoms) for the dehydrogenation of cyclodo-
decanol at 144 °C and fractions of Co species (XANES analysis).
in Fig. 2 shows that the steep increase in the activity in the TH
2
range from 300 °C to 400 °C corresponds to the formation of
metallic Co species. This suggests that the metallic Co species
are the active species.
Fig. 3 XRD patterns of Co/TiO2 (Co = 5 wt%) after H2-reduction at various
temperatures.
The catalyst named Co/TiO2–air (Table 1, entry 6) was pre-
pared by exposing the Co/TiO2 catalyst, pre-reduced at 400 °C,
to the ambient conditions for 0.5 h. This catalyst showed no
activity. This suggests that the metallic Co atoms on the
surface of Co particles are the active species and their re-oxi-
dation by O2 under ambient conditions results in the catalyst
deactivation. This hypothesis is confirmed by the following
results. Upon exposure to the ambient conditions, the percen-
tage of CoO species in the Co/TiO2 (TH = 400 °C) sample, esti-
2
mated by the H2-TPR experiment, increased from 37% to 54%.
EXAFS results (Table 2) showed that air-exposure of the
Co/TiO2 catalyst pre-reduced at 400 °C resulted in an increase
in the Co–O coordination number from 2.2 to 3.4. These
Fig. 4 H2-TPR profile of Co3O4/TiO2 (Co = 5 wt%).
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Fig. 5 (A) XANES and (B) EXAFS Fourier transforms at Co K-edge for Co/TiO2 (Co = 5 wt%) and reference compounds.
Fig. 6 Co K-edge XANES spectra of Co/TiO2 (Co = 5 wt%) after H2-reduction at
various temperatures.
Fig. 7 IR spectra of CO adsorbed on various catalysts at −180 °C: (a) TiO2, (b)
Table 2 Curve-fitting analysis of Co K-edge EXAFS
Co/TiO2 (Co loading = 10 wt%, TH = 500 °C), (c) Co/TiO2 after re-oxidation in
2
20 kPa O2 at room temperature.
Sample
Shell
Na
Rb/Å
σc/Å
Rf d/%
Co3O4/TiO2
O
6.0
1.92
2.85
3.34
2.13
2.49
2.10
2.52
3.09
2.49
2.14
3.00
1.92
2.85
3.35
0.064
0.045
0.064
0.050
0.069
0.094
0.094
0.105
0.075
0.083
0.069
0.054
0.049
0.057
2.5
H2-reduction at 500 °C are shown in Fig. 7. Adsorption of CO
on the reduced Co catalysts leads to the appearance of peaks
in the region of 1800–2200 cm−1 which can be attributed to
adsorbed CO species on different sites. The intense peak at
2179 cm−1 can be due to the CO adsorbed on TiO2 and oxi-
dized Co species. In the spectrum of Co/TiO2 bands in a range
of 2000–2100 cm−1 generally assigned to linearly adsorbed CO
on metallic Co are observed, while those in a range of
1800–2000 cm−1 due to bridge-type CO adsorption on large
Co0 crystallites are nearly absent. For metal oxide-supported
Co metal NPs, peaks centered around 2070 cm−1 have been
assigned to CO linearly adsorbed on metallic Co sites with
electron deficiency (Coδ+ sites)41–43 at the support interface or
near unreduced cobalt ions. CO adsorbed on the ideal metal
Co
Co
O
Co
O
Co
Co
Co
O
Co
O
3.6
5.9
2.2
6.3
3.4
5.2
1.6
11.7
4.9
8.8
4.5
3.6
5.1
Co/TiO2
0.6
1.8
Co/TiO2–air
Co foil
CoO
0.7
2.0
Co3O4
0.4
Co
Co
a Coordination numbers. b Bond distance. c Debye–Waller factor.
d Residual factor.
results indicate that metallic Co species on the surface of Co surface is expected to be observed at lower wavenumbers. The
metal NPs are the active species and these Co species are re- spectrum of Co/TiO2 below 2079 cm−1 shows an absorption
oxidized by O2 under ambient conditions to yield inactive CoO maximum at 2070 cm−1. Upon exposure to O2 (20 kPa) at room
species.
temperature for 0.5 h, the band due to CO adsorbed on the
The presence of surface Co0 species in the Co/TiO2 catalyst Coδ+ sites disappeared. Combined with the result of catalytic
(Co loading = 10 wt%) was confirmed by a CO adsorption IR tests, the result indicates that the surface metallic Co sites
study. IR spectra of adsorbed CO species on the catalyst after with electron deficiency are the catalytically active species.
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Table 3 Dehydrogenation of various alcoholsa
selectively converted to the corresponding ketones with good to
high yields (64–96%). For example, exo-norborneol was con-
verted to the corresponding ketone in 96% yield by 0.5 mol% of
the catalyst, corresponding to a TON of 192. For the reaction of
primary alcohols (benzylalcohol and 1-octanol), yields of the
corresponding aldehydes were low (10–14%). To the best of our
knowledge, this study represents the first example of acceptor-
free oxidation of various secondary alcohols by a Co catalyst. It
should be noted that the previous examples of the Co-based hete-
rogeneous catalysts for the aerobic alcohol oxidation were
effective for activated (benzyl) alcohols but not generally
effective for less activated aliphatic alcohols.11–15 Therefore,
this study represents the rare example of a Co-based catalyst
effective for the oxidation of aliphatic secondary alcohols to
ketones.
Entry Alcohol
Co (mol%) t/h Conv. (%) Yield (%)
1
2
Cyclooctanol
1st reuse
1
1
6
6
6
6
6
48
24
24
24
24
24
24
12
24
24
98
100
99
95
97
88
96
78
67
92
96
98
85
12
19
98
95
99
95
97
88
96
78
64
90
96
96
78
10
8
3
4
2nd reuse
3rd reuse
1
1
5
Cyclooctanol
Cyclooctanol
Cyclododecanol
Cycloheptanol
Cyclohexanol
Cyclopentadecanol
exo-Norborneol
4-Octanol
0.2
0.1
0.2
3
1
3
0.5
0.5
1
1
1
6b
7
8
9
10
11
12
13
14
15c
Conclusions
2-Octanol
1-Octanol
Benzylalcohol
In summary, we have shown that TiO2-supported Co metal NPs
heterogeneously catalyze the acceptor-free oxidation of second-
ary alcohols. The catalyst, readily prepared from commercial
compounds, is easily recoverable and reused, and shows higher
activity in the oxidation of less activated aliphatic alcohols than
the previous Co-based catalysts. Studies on the structure–activity
relationship showed that the surface metallic Co sites with elec-
tron deficiency are the catalytically active species.
a Co/TiO2 (Co = 10 wt%) pre-reduced at 400 °C was used. Conversion
and yields were determined by GC. b Solvent-free conditions:
cyclooctanol (10 mmol) was heated at 155 °C in a flow of N2 (10 mL
min−1) in the presence of Co/TiO2 (Co = 5 wt%) pre-reduced at 400 °C.
c T = 80 °C. Toluene (4% yield) was produced as a byproduct.
Catalytic properties
Next, we examined the catalytic properties of Co/TiO2 (10 wt%
Co) under the conditions shown in Table 3. The dehydrogena- Acknowledgements
tion of cyclooctanol by Co/TiO2 (1 mol%) for 6 h resulted in
This work was supported by a “Grant for Advanced Industrial
98% yield of cyclooctanone. After 1 h of the reaction the yields
of gas phase H2 (27%) and cyclooctanone (29%) were nearly
identical to the alcohol conversion (30%), indicating that H2
was generated quantitatively. The reaction was completely
terminated by a removal of the catalyst from the reaction
mixture after 1 h; further heating of the filtrate for 5 h under
the reflux conditions did not increase the yield. ICP analysis of
the filtrate confirmed that the content of Co in the solution
was below the detection limit. These results confirm that
the reaction is attributed to the heterogeneous catalysis of
Co/TiO2. After the reaction, the catalyst was easily separated
from the reaction mixture by centrifugation. The filtered cata-
lyst was washed with acetone, followed by drying in air at
90 °C for 12 h, and by reducing in H2 at 400 °C for 0.5 h. As
shown in Table 3 (entries 2–4), the recovered catalyst was
reused at least three times.44 In the low catalyst loading con-
dition (Co = 0.2 mol%), the catalyst showed 97% yield of the
product after 6 h corresponding to a TOF of 81 h−1. Even
under solventless conditions, the reaction with 0.1 mol% of
the catalyst for 48 h resulted in 88% yield of the product, corres-
ponding to the turnover number (TON) of 880. These values
are higher than those for previous Co-based catalysts11–15 for
the liquid phase aerobic oxidation of alcohols.
Technology Development” in 2011 from the New Energy and
Industrial Technology Development Organization (NEDO) and
a MEXT program “Elements Strategy Initiative to Form Core
Research Center” (since 2012), Japan.
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