Photo-Electrochemical C?H Bond Activation of Cyclohexane Using a WO3 Photoanode and Visible Light

Article abstract of DOI:10.1002/anie.201805079

The photo-electrochemical C?H bond activation of cyclohexane to produce cyclohexanol and cyclohexanone (KA oil) with high partial oxidation selectivity (99 %) and high current utilization ratio (76 %) was achieved in air at room temperature at atmospheric pressure. The production rate of KA oil was accelerated by applying a bias. The incident photon to current efficiencies at 365 and 420 nm were 57 % and 24 %, respectively.

Full text of DOI:10.1002/anie.201805079

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201805079
German Edition: DOI: 10.1002/ange.201805079
Oxidation
ꢀ
Photo-Electrochemical C H Bond Activation of Cyclohexane Using
a WO3 Photoanode and Visible Light
Hiroyuki Tateno, Shoji Iguchi, Yugo Miseki, and Kazuhiro Sayama*
ꢀ
Abstract: The photo-electrochemical C H bond activation of
cyclohexane to produce cyclohexanol and cyclohexanone (KA
oil) with high partial oxidation selectivity (99%) and high
current utilization ratio (76%) was achieved in air at room
temperature at atmospheric pressure. The production rate of
KA oil was accelerated by applying a bias. The incident photon
to current efficiencies at 365 and 420 nm were 57% and 24%,
respectively.
the WO3 conduction band (ca. + 0.5 V vs. RHE) has the
potential to reduce O2. Shiraishi et al. reported that the
partial oxidation selectivity of KA oil among KA oil and CO2
reached about 93% using a Pt-WO3 powder photocatalyst
under ambient conditions.[5] The photocatalytic reaction
system is simple, however, the reaction activity was generally
limited by the poor charge separation ability of the particles.
In contrast, with the photoelectrode reaction system, the
quantum efficiency was higher than that for the powder
photocatalyst because the charge separation could be signifi-
cantly accelerated by applying an external bias.[6] Therefore, it
is expected that photoelectrode systems would also be useful
for the oxidative transformation of organic compounds into
valuable chemicals. However, only a few reactions have been
reported. Choi et al. reported the photo-electrochemical
oxidation of 5-hydroxymethylfurfural to 2,5-furandicarbox-
ylic acid using a BiVO4 photoanode.[7] We have successfully
achieved dimethoxylation of furan and oxidative acetophe-
none production with excellent faradaic efficiencies using
a BiVO4/WO3 photoanode.[8]The next step towards further
developing an organic synthesis method using an oxide
semiconductor photoelectrode under visible light and its
ꢀ
D
irect activation of the C H bond has attracted widespread
attention with respect to improving the atom- and step-
economies of organic synthesis when considering green
chemistry. In particular, C H bond activation of simple
alkanes to produce basic materials remains an important
ꢀ
ꢀ
subject. It is well known that an inert C H bond has a high
dissociation energy (439 kJmolꢀ1) compared with functional-
ized regions such as a C X bond (X = I, 238 kJmolꢀ1; Br,
ꢀ
294 kJmolꢀ1; Cl, 350 kJmolꢀ1).[1] For instance, the oxidative
activation of cyclohexane to cyclohexanol (C-ol) and cyclo-
hexanone (C-one), the mixture of which is often called KA
oil, and they are important intermediates for manufacturing
textiles such as nylon-6 and nylon-6,6 polymers, generally
requires high temperature and pressure.[2] Therefore, much
effort has been devoted to achieving KA oil production with
high selectivity under ambient conditions.
ꢀ
application for barely oxidizable materials, such as the C H
activation of cyclohexane, is urgently needed.
Herein we investigated the production of KA oil by
ꢀ
Photo-electrochemical reactions such as powder photo-
catalyst and photoelectrode systems have been intensively
investigated under ambient conditions not only for solar
energy conversion such as water splitting, but also for organic
synthesis of various valuable chemicals. There are many
oxidative C H bond activation of cyclohexane using a porous
WO3 photoanode in air at room temperature and atmospheric
pressure (Figure 1). The production rate was easily acceler-
reports[3] on the C H bond activation of cyclohexane to
ꢀ
produce KA oil with oxide photocatalysts, mainly using TiO2-
based photocatalysts under UV light. Modification of TiO2 by
doping and loading with transition metals was investigated to
utilize visible light,[4] but the photocatalytic activities in the
visible-light region were insufficient. WO3 is a visible-light-
active semiconductor which absorbs light up to 480 nm, and
its absorption coefficient is high because of the direct photon
transition. The oxidation ability of h+ at the valence band
potential of WO3 is almost identical to that of TiO2. The eꢀ in
[*] H. Tateno, S. Iguchi, Y. Miseki, K. Sayama
Advanced Functional Material Team, Research center for photo-
voltaics (RCPV), National Institute of Advanced Industrial Science
and Technology (AIST)
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan)
E-mail: k.sayama@aist.go.jp
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201805079.
Figure 1. Schematic illustration of a photo-electrochemical system for
cyclohexane oxidation to produce KA oil. CB=Conduction band,
RHE=reversible hydrogen electrode, VB=valence band.
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ated, even at low external bias, with high partial oxidation
selectivity (99%) and a high current utilization ratio (appar-
ent faradaic efficiency, 76%) of KA oil. The incident photon
to current efficiencies (IPCE) at 365 and 420 nm were 57%
and 24%, respectively.
The oxidation current and accumulation of products in the
dark and under simulated solar-light irradiation in the
constant potential method (cell voltage of 0.5 V vs. Pt) were
determined as shown in Figure 3. The oxidation current of
A porous WO3 photoanode was prepared on a conducting
glass support according to the literature,[9] and characterized
as fine particles of a monoclinic crystal using X-ray diffraction
spectroscopy and scanning electron microscopy (see Fig-
ures S1 and S2 in the Supporting Information). Detailed
experimental conditions are given in the Supporting Infor-
mation.
Linear sweep voltammograms in a cyclohexane-contain-
ing electrolyte were measured using a WO3 photoelectrode in
the dark and under simulated solar-light irradiation as shown
in Figure 2. The oxidation current was above + 1.2 V vs. the
Figure 3. The effect of light irradiation on photo-electrochemical oxida-
tion of cyclohexane to KA oil (total amount of C-ol + C-one) using
a WO3 electrode under air. Composition of the electrolyte: 18 mL
cyclohexane, 12 mL tBuOH, and 2 mL HNO3; working electrode: WO3
electrode (2ꢀ2 cm2); counter electrode: Pt mesh; light source: simu-
lated solar light (100 mWcmꢀ2). Electrolysis was conducted in con-
stant potential mode at a cell voltage of 0.5 V vs. counter electrode.
cyclohexane was negligible in the dark. In contrast, KA oil
was clearly observed upon photoirradiation, and no by-
products, such as bicyclohexyls, were detected during this
period. Figure 4 shows the applied bias dependence of the
generation rate of KA oil under ambient conditions and
simulated solar-light irradiation. KA oil was produced even at
0 V vs. CE, but the production rate was relatively low
Figure 2. Linear sweep voltammograms of a cyclohexane-based elec-
trolyte solution using a WO3 photoelectrode in the dark and under
simulated solar-light irradiation (100 mWcmꢀ2) under air. Composition
of the electrolyte: 18 mL cyclohexane, 12 mL tBuOH, and 2 mL HNO3;
working electrode: WO3 disk electrode (F=5 mm); counter electrode:
Pt mesh; scan rate: 10 mVsꢀ1
.
counter electrode (CE) in the dark, and it clearly shifted to
ꢀ0.7 V vs. CE. under photoirradiation. The applied bias of
above 2.0 V could be reduced by light energy at a current
density of 0.5 mAcmꢀ2. The oxidation current reached around
2.0 mAcmꢀ2 at a cell voltage above 0.5 V vs. CE. According to
the negative potential of the WO3 conduction band compared
with that of O2 reduction, it is reasonable that a small
photocurrent was observed even at 0 V vs. the Pt counter
electrode (no applied bias), as shown in Figure 2. However,
the photocurrent gradually decreased with time with the
constant potential method (see Figure S3). It was found that
the potential of the WO3 photoanode relative to the reference
electrode under light irradiation was unchanged, and that the
potential of the Pt cathode to the reference electrode was
negatively shifted (see Figure S4), suggesting that the initial
decrease in the photocurrent-potential property in the two-
electrode system was caused by the limitation of O2 reduction
on the Pt cathode, rather than by the photoanodic reaction.
Figure 4. Applied bias dependence of the electrochemical oxidation of
cyclohexane to KA oil (total amount of C-ol + C-one) using a WO3
electrode with simulated solar-light irradiation (100 mWcmꢀ2) under
air. Composition of electrolyte: 18 mL cyclohexane, 12 mL t-BuOH, and
2 mL HNO3; working electrode: WO3 electrode (2ꢀ2 cm2); counter
electrode: Pt mesh. Electrolysis was conducted in constant potential
mode (0 to 2.0 V vs. counter electrode).
2
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(< 8 mmolhꢀ1). The production rate of KA oil increased with
the applied bias and became saturated at cell voltages above
1.5 V. The generation rate of KA oil then reached
41.2 mmolhꢀ1 at a cell voltage of 2.0 V, although KA oil was
scarcely observed below 2.0 V in the dark. It should be noted
that no oxidation products derived from cyclohexane were
obtained by the conventional electrochemical oxidation
system in the dark using either the Pt, glassy carbon, or
graphite electrodes, even when the anodic current was
observed at high anodic potential. These results indicate
that the selective oxidation of cyclohexane to KA oil was
performed on a WO3 photoanode at a low cell voltage using
simulated solar-light assistance.
The presence of O2 is important for KA oil production on
the photoanode as well as for O2 reduction on the Pt CE. The
oxidative transformation of cyclohexane to KA oil generally
required O2. It was reported that the selectivity for producing
KA oil decreased with the O2 concentration, and bicyclohexyl
was mainly formed at low oxygen concentrations by thermal
catalytic reactions over solid supported CuII, FeIII, CrIII, and
CoIII salts.[2] The reaction mechanism shown in Equation (1)
Figure 5. Reaction time dependence of the accumulation of cyclohex-
anol (diamonds), cyclohexanone (circles), and the mole ratio of
cyclohexanone/cyclohexanol (open squares) using a WO3 electrode
under an O2 (a) and N2 (b) atmosphere with simulated solar-light
irradiation (100 mWcmꢀ2). Composition of the electrolyte: 18 mL
cyclohexane, 12 mL tBuOH, and 2 mL HNO3; working electrode: WO3
electrode (2ꢀ2 cm2); counter electrode: Pt mesh. Electrolysis was
conducted in constant potential mode at 0.5 V vs. counter electrode.
Partial oxidation selectivity ¼
½ðC-ol þ C-oneÞ=ðC-ol þ C-one þ ð1=6Þ CO2Þꢁ 100
ð2Þ
Evaluation of the faradaic efficiency of this reaction is
difficult when the reaction mechanism is not clear, because
there is a possibility that reaction of a radical intermediate
with O2 will take place through chain reactions. The apparent
faradaic efficiency was defined as the current utilization ratio
of KA oil to the charge passed [Eq. (3); see the Supporting
was proposed.[10] The oxidation of cyclohexane proceeds via
a cyclohexyl radical by a single-electron transfer process, and
it reacts with O2 to form the cyclohexylperoxyl radical.
Bicyclohexyl was often detected by coupling of a cyclohexyl
radical when O2 was absent. Subsequently, the generated pair
of cyclohexyperoxyl radicals are disproportionated to form
the same amount of C-ol and C-one. C-ol possibly underwent
an additional two-electron oxidation to C-one.
Figure 5 shows the time dependence of C-ol and C-one
production by oxidation of cyclohexane on a WO3 photo-
anode under O2 and N2 atmospheres with simulated solar-
light irradiation. Under an N2 atmosphere, a negligible
amount of KA oil was detected and a small amount of
bicyclohexyl (10.6 mmol) was formed, although the photo-
current was almost identical to that under the O2 atmosphere.
In the presence of O2, both C-ol and C-one were produced
effectively. The ratio of produced C-one to C-ol increased
with the reaction time. We confirmed that C-ol was oxidized
to C-one in the tBuOH/HNO3 electrolyte (without cyclohex-
ane) on the WO3 photoanode, suggesting that excess C-one is
obtained by the oxidation of produced C-ol [Eq. (1)].
Current utilization ratio of KA oil ðapparent faradaic efficiencyÞ ¼
½ð3C-oneꢀC-olÞ=ðCharge passed=96485Þꢁ 100
ð3Þ
Information], and this was calculated as appproximately
76%. All these results indicate that the hole in the valence
band of WO3 selectively oxidized the cyclohexane to KA oil,
while the solvent might be partially oxidized. The prepared
WO3 photoanode was suitable and was reused at least five
times without necessitating a regeneration process. A turn-
over number (moles of cylohexanol and cyclohexanone per
moles of WO3) of 57 was obtained.
Finally, the incident photon to current efficiency (IPCE,
the apparent quantum efficiency to photocurrent) of this
photoanode reaction method was measured under mono-
chromatic light using a band pass filter (Figure 6, diamonds).
The IPCE spectrum was similar to the absorption spectrum of
the WO3 photoelectrode in the visible-light region (see
Figure S5). The IPCE at 400 and 420 nm were 40% and
24%, respectively. The maximum IPCE of 57% was obtained
at a wavelength of 365 nm. These high values indicate the
advantages of the efficient photo-electrochemical reaction for
KA oil production. The IPCE in the shorter wavelength
region at less than 365 nm was low mainly because of the
absorption of the electrolyte solution (see Figure S6). For
comparison, the IPCE spectrum in 1.0m aqueous HClO4
without any organic substrate for O2 evolution from H2O is
The production of CO2 gas was reported by the complete
oxidation of cyclohexane with O2.[3,4,11] Under our experi-
mental conditions (charge passed: 21 C, cell voltage: 0.5 V,
reaction time: 5 h), the amount of CO2 produced was small
(7 mmol) compared with that of C-ol (34 mmol) and C-one
(67 mmol). The partial oxidation selectivity, which is defined
by Equation (2),[3e,5,12] was 99%. This value is high compared
with that for the photocatalytic reaction on Pt-WO3 (93%).[5]
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Figure 6. IPCE spectra of cyclohexane-based electrolyte solution and
1m HClO4(aq) under monochromatic light irradiation from the front
side of the WO3 electrode at 2.0 V vs. Ag/AgNO3. Composition of the
electrolyte: 18 mL cyclohexane, 12 mL t-BuOH, and 2 mL HNO3;
working electrode: WO3 electrode (2ꢀ2 cm2); counter electrode: Pt
mesh, reference electrode: Ag/AgNO3.
also shown in Figure 6 (circles). The spectral shape for water
oxidation was similar to that for cyclohexane oxidation, and
the IPCE at 400 and 420 nm was 38% and 26%, respectively.
These results indicate that the oxidation reaction of cyclo-
hexane was as fast as water oxidation on the WO3 photoanode
under the reaction conditions.
In conclusion, efficient photo-electrochemical production
of KA oil from cyclohexane was demonstrated for the first
time. Excellent partial oxidation selectivity (99%) and
apparent faradaic efficiency (76%) were achieved using
a porous WO3 photoanode in air at room temperature and
atmospheric pressure. The IPCE at 365 and 420 nm were 57%
and 24%, respectively. All results indicated that photo-
electrochemical processes had significant potential for vari-
ꢀ
ous organic syntheses including C H activation.
Acknowledgements
The present work was partially supported by the International
Joint Research Programs for Innovative Energy Technology.
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Conflict of interest
The authors declare no conflict of interest.
ꢀ
Keywords: alkanes · C H activation · electrochemistry ·
oxidation · photocatalysis
Manuscript received: May 1, 2018
Version of record online: && &&, &&&&
4
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These are not the final page numbers!

Angewandte
Communications
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Communications
Oxidation
Light and electricity: The photo-electro-
ꢀ
chemical C H bond activation of cyclo-
H. Tateno, S. Iguchi, Y. Miseki,
hexane to produce cyclohexanol and
cyclohexanone (KA oil) with high partial
oxidation selectivity (99%) was achieved
in air at room temperature and at
atmospheric pressure. The production
rate of KA oil was accelerated by applying
a bias. The incident photon to current
efficiencies at 365 and 420 nm were 57%
and 24%, respectively.
K. Sayama*
&&&& — &&&&
ꢀ
Photo-Electrochemical C H Bond
Activation of Cyclohexane Using a WO3
Photoanode and Visible Light
Angew. Chem. Int. Ed. 2018, 57, 1 – 5
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
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