Efficient and selective oxidation of benzylic alcohol by O2 into corresponding aldehydes on a TiO2 photocatalyst under visible light irradiation: Effect of phenyl-ring substitution on the photocatalytic activity

Article abstract of DOI:10.1016/j.jcat.2010.06.006

The highly efficient and selective photocatalytic oxidation of benzyl alcohol and its derivatives substituted with-OCH₃,-CH ₃,-C(CH₃)₃,-Cl,-CF₃ and-NO ₂ into corresponding aldehydes has been successfully carried out on TiO₂ in the presence of O₂ under visible light irradiation. The photocatalytic activity for the formation of the aldehyde was evaluated by a pseudo-first-order reaction, and it was found that the activity is enhanced by phenyl-ring substitution with the electron-releasing groups (-OCH₃,-CH₃,-C(CH₃)₃) and the electron-withdrawing groups (-Cl,-CF₃ and-NO₂). The effects of the substituents and their orientation on the photocatalytic performance of selective oxidation reaction are discussed here. It was shown that the photocatalytic activities are influenced not only by the oxidative potentials of the reactants but also by the stability of the resonant structures of the benzylic alcohol radicals formed by oxidation with a hole, leading to further reactions to form corresponding aldehydes.

Full text of DOI:10.1016/j.jcat.2010.06.006

Journal of Catalysis 274 (2010) 76–83
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Efficient and selective oxidation of benzylic alcohol by O2 into corresponding
aldehydes on a TiO2 photocatalyst under visible light irradiation: Effect of
phenyl-ring substitution on the photocatalytic activity
a
a
a
b
Shinya Higashimoto a, , Nobuaki Suetsugu , Masashi Azuma , Hiroyoshi Ohue , Yoshihisa Sakata
*
a College of Engineering, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan
b Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The highly efficient and selective photocatalytic oxidation of benzyl alcohol and its derivatives substi-
tuted with –OCH3, –CH3, –C(CH3)3, –Cl, –CF3 and –NO2 into corresponding aldehydes has been success-
fully carried out on TiO2 in the presence of O2 under visible light irradiation. The photocatalytic
activity for the formation of the aldehyde was evaluated by a pseudo-first-order reaction, and it was
found that the activity is enhanced by phenyl-ring substitution with the electron-releasing groups
(–OCH3, –CH3, –C(CH3)3) and the electron-withdrawing groups (–Cl, –CF3 and –NO2). The effects of the
substituents and their orientation on the photocatalytic performance of selective oxidation reaction
are discussed here. It was shown that the photocatalytic activities are influenced not only by the oxida-
tive potentials of the reactants but also by the stability of the resonant structures of the benzylic alcohol
radicals formed by oxidation with a hole, leading to further reactions to form corresponding aldehydes.
Ó 2010 Elsevier Inc. All rights reserved.
Received 30 April 2010
Revised 5 June 2010
Accepted 7 June 2010
Keywords:
Selective photocatalytic oxidation
Titanium dioxide
Alcohol oxidation
Benzyl alcohol
Benzaldehyde
Oxidative potential
Visible light
Electron-releasing group
Electron-withdrawing group
Resonant structure
1. Introduction
der visible light irradiation [14]. In another application, the selec-
tive photocatalytic oxidation of benzylic alcohol by O2 into
The development of organic synthesis for the partial oxidation
of alcohols into aldehydes or ketones has been the focus of much
attention since such chemical reactions are a fundamental but sig-
nificant procedure with great potential in commercial applications
such as in the fragrance or pharmaceutical industries [1–3]. A
number of studies have been reported on the selective catalytic
oxidation of alcohol into aldehyde or ketone by O2 using various
kinds of catalysts: hydroxyapatite-supported Pd nanoclusters [4],
Pd clusters embedded on mesoporous carbon [5], polymer-stabi-
lized Au nanoclusters [6], monometallic Au [7], Au supported on
CeO2 [8] or bimetallic Au–Pd supported on TiO2 [9], Ru supported
on Al2O3, TiO2 or hydroxyapatite [10–12] and carbon-supported Pt
catalysts [13].
corresponding aldehydes was successfully carried out under visible
light irradiation on 2,4,6-triarylpyrylium salts [15] and 9-phenyl-
10-methylacridinium photocatalysts [16]. Shishido et al. have also
demonstrated the photocatalytic oxidation of alcohols into corre-
sponding aldehydes on a semiconductive Nb2O5 photocatalyst un-
der solvent free conditions [17].
Meanwhile, TiO2 photocatalysts have also been extensively uti-
lized for such photochemical applications as H2 production by
water splitting, the degradation of volatile organic compounds
(VOC), dye-sensitized solar cells, super-hydrophilic materials and
organic synthesis [18–21]. Among them, the photo-oxidation of
alcohol into aldehyde or ketone has been widely studied on TiO2
[21–34]. In particular, the selective photocatalytic oxidation of
benzylic alcohol by O2 under UV-light irradiation on TiO2 was pre-
viously reported in the gas phase [26], in acetonitrile [27,28] or in
aqueous solution [29–34]. However, it is known that anatase-type
TiO2 photocatalysts have a large band-gap transition of ca. 3.2 eV,
which only allows the utilization of a small percentage of the solar
radiation that reaches the earth’s surface. Therefore, the develop-
ment of effective and highly sensitive visible light responsive
TiO2 photocatalytic systems is strongly desired. Ohno et al. have
On the other hand, ‘‘green” photo-oxidation reactions have be-
come recognized for their fundamental and applied chemical utili-
zation of solar energy. Funyu et al. have reported the highly
efficient and selective epoxidation of alkenes by photochemical
oxygenation sensitized by ruthenium(II) porphyrin with water un-
* Corresponding author. Fax: +81 6 6957 2135.
E-mail address: higashimoto@chem.oit.ac.jp (S. Higashimoto).
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.06.006

S. Higashimoto et al. / Journal of Catalysis 274 (2010) 76–83
77
reported that the photo-induced epoxidation of alkene on TiO2 by
H2O2 proceeds under visible light irradiation [35].
ton energy was adjusted to a photon flow of 0.5–0.8 mW cmꢁ2, as
measured by a power meter (ORION/PD, Ophir).
Recently, we have shown that TiO2 exhibits efficient and selec-
tive photocatalytic oxidation of benzyl alcohol and its derivatives
into corresponding aldehydes by O2 under visible light irradiation
[36,37]. These findings lead to the new development of visible light
responsive photocatalystic reactions whose systems are able to
utilize clean and safe solar energy. In this study, we have investi-
gated the kinetics for the photocatalytic oxidation of benzylic alco-
hols substituted with –OCH3, –CH3, –C(CH3)3, –Cl, –CF3 and –NO2
to corresponding aldehydes by O2 on TiO2 under visible light irra-
diation. In particular, we have focused on providing an understand-
ing of the effects of the kinds of substituents and their orientation
on the efficiency of the photocatalytic oxidation reaction.
After the reaction, the catalysts were immediately separated
from the solution by filtration through a 0.20-lm membrane filter
(Dismic-25, Advantec). The solution was then analyzed by HPLC
(Shimadzu LC10ATVP, UV–Vis detector, column: Chemcopak, mo-
bile phase: a mixture of acetonitrile and 1.0% aqueous formic acid),
and the gas phase was analyzed by GC (Model-802, Ohkura; col-
umn: porapak Q).
2.3. Characterization
Diffuse reflectance UV–Vis spectroscopic measurements were
carried out using a UV–Vis spectrophotometer (UV-2200A, Shima-
dzu). The samples, TiO2-adsorbed benzyl alcohol or its derivatives,
were prepared in CH3CN/0.1 M benzylic alcohol, followed by wash-
ing with acetonitrile and drying in air at room temperature.
2. Experimental
2.1. Materials
2.4. Electrochemical measurements
All chemicals and solvents used in this study were of analytical
grade and were utilized as received without further purification.
Benzyl alcohol and its derivatives (para-benzylic alcohols substi-
tuted with –Cl and –C(CH3)3 groups and ortho-, meta- or para-
substituted benzylic alcohols with –OCH3, –CH3, –NO2 and –CF3
groups) were obtained from Wako Pure Chemical Industries, Ltd.
The photocatalyst used in this work was anatase TiO2 (BET surface
area: 320 m2/g) obtained from Ishihara Co., Ltd. (ST-01).
Electrochemical measurements were performed with a Poten-
tiostat/Galvanostat (HABF5001, HOKUTO DENKO). Platinum wire,
saturated calomel electrodes (SCE) and a 1-mm2 diameter glass
carbon disc were used as auxiliary, reference and working elec-
trodes, respectively. The electrolyte was bubbled vigorously by
N2 for 30 min prior to measurements. Cyclic voltammetry (CV)
was measured by sweeping at 50 mV sꢁ1 in CH3CN/0.1 M LiClO4/
5 mM benzylic alcohol. The oxidative potentials (Ep) of the benzylic
alcohols were estimated from the results of voltammogram
analysis.
2.2. Photocatalytic reactions
Photocatalytic reactions were carried out in a photoreaction cell
(20 mL in volume) made of pyrex glass. The TiO2 photocatalyst
(50 mg) was suspended in acetonitrile solution (10 mL) involving
3. Results
the reactant (50 lmol) in the cell and the gas phase was purged
3.1. Electronic properties of TiO2-adsorbed benzylic alcohols
by pure O2 of 1 atmospheric pressure. The photocatalytic reaction
was performed by irradiation with a blue LED lamp (OptiLED,
2.5 W, IMPACT EYE Co., Ltd.). The photo-intensity of the LED lamp
was measured by an illumination meter (TMS 870, TASCO Co., Ltd.)
adjusted to 1.8 ꢀ 104 lux. The energy distribution emitted from the
LED lamp is shown in Fig. 1.
The quantum yield of the photocatalytic reaction was estimated
under irradiation with monochromatic light of full width at half
maximum (FWHM) of ca. 6.7 nm through a monochrometer (SG-
100, Koken) from a 500 W Xe lamp (USHIO Co., Ltd.). The photocat-
alytic reaction was carried out in a photoreaction cell made of
quartz with the TiO2 photocatalyst (10 mg) suspended in acetoni-
The UV–Vis absorption spectra were examined, and the results
are shown in Fig. 2. As shown in Fig. 2a, TiO2 by itself exhibits
absorption only in the UV region, and the band-gap is estimated
to be ca. 3.2 eV (ca. 380 nm) from the absorption edge of the spec-
trum. On the other hand, absorption in the visible region can be ob-
served in the spectrum for the TiO2-adsorbed benzyl alcohol, as
shown in Fig. 2b, while TiO2-adsorbed with methanol or benzene
2
trile solution (2 mL) involving benzylic alcohol (10
lmol). The pho-
0.2
0.15
1.5
0.1
0.05
1
0
400
500
600
700
800
(d)
(c)
(b)
(a)
Wavelength / nm
0.5
0
300
400
500
600
700
800
400
450
500
550
600
Wavelength / nm
Wavelength / nm
Fig. 2. UV–Vis spectra of: (a) TiO2 by itself; and TiO2-adsorbed (b) benzyl alcohol,
(c) p-methoxybenzyl alcohol and (d) p-nitrobenzyl alcohol. Inset shows the
expansion of (a)–(d).
Fig. 1. Energy distribution emitted from the LED lamp (visible light) used in this
experiment.

78
S. Higashimoto et al. / Journal of Catalysis 274 (2010) 76–83
does not exhibit absorption in the visible light region. It could,
therefore, be assumed that this absorption in the visible light re-
gion was unique and was not caused by the band-gap transition
from the valence band to conduction band of TiO2. This absorption
was assigned to the ligand-to-metal charge transfer (LMCT) of the
surface complex formed by the interaction of benzyl alcohol with
the Ti sites [38,39]. The UV–Vis spectra of the TiO2-adsorbed ben-
zylic alcohols were also examined. Fig. 2c and d shows the UV–Vis
spectra of TiO2-adsorbed p-methoxybenzyl alcohol and p-nitro-
benzyl alcohol, respectively. Both of the spectra also show absorp-
tion in the visible light region originating with the LMCT of the
surface complex. The UV–Vis spectra of the other TiO2-adsorbed
substituted benzylic alcohols used in this study were examined,
and similar spectra, as shown in Fig. 2c and d, were observed.
These results show that the surface complex forms by the interac-
tion of the TiO2 surface with the adsorbed benzylic alcohols, gener-
ating photo-absorption in the visible light region. It was observed
that absorption in the visible region of the surface complex could,
in fact, lead to photocatalytic reactions under visible light
irradiation.
quently, it was confirmed that the total mass balance after the
reactions could be explained by the equilibrium adsorption of
the aldehydes on the TiO2 surface (Table S1, Supplementary
material).
3.3. Quantum yields of the photocatalytic selective oxidation of benzyl
alcohol and its derivatives on TiO2
Fig. 4 shows the quantum yields (U) for the photocatalytic oxi-
dation of (a) benzyl alcohol, (b) p-methoxybenzyl alcohol and (c) p-
nitrobenzyl alcohol into corresponding aldehydes on TiO2 as a
function of the photo-irradiation wavelength. The quantum yield
(U
) is defined in the following Eq. (1):
amount of product formed
amount of photons absorbed
U
¼
ꢀ 100
ð1Þ
The photo-response for the oxidation of benzyl alcohol into
benzaldehyde was confirmed up to ca. 700 nm, and the quantum
yields were estimated to be ca. 4.3% under photo-irradiation at
500 nm, ca. 5.5% at 460 nm and ca. 13.8% at 440 nm, as shown in
Fig. 4a. These results clearly indicate that visible light absorption
plays a significant role in the photocatalytic selective oxidation
of benzyl alcohol into benzaldehyde. The quantum yields for the
oxidation of p-methoxybenzyl alcohol into aldehyde are estimated
to be ca. 6.8% under photo-irradiation at 500 nm, ca. 17% at 460 nm
and ca. 29.1% at 440 nm, as shown in Fig. 4b, while those of p-nitro-
benzyl alcohol into aldehyde are estimated to be ca. 13.4% under
photo-irradiation at 500 nm, ca. 39.2% at 460 nm and ca. 67.4% at
440 nm, as shown in Fig. 4c. Thus, it was noted that the quantum
yields for the photocatalytic oxidation of benzylic alcohols substi-
tuted with –OCH3 and –NO2 are significantly higher than those of
benzyl alcohol.
3.2. Photocatalytic oxidation of benzyl alcohol and its derivatives on
TiO2 under visible light irradiation
The oxidation of benzyl alcohol and its derivatives on a TiO2
photocatalyst was carried out under visible light irradiation in
the presence of O2, and the results are shown in Fig. 3. It was con-
firmed that the oxidation reaction does not take place under visible
light without a TiO2 photocatalyst nor with a TiO2 photocatalyst
without irradiation, i.e., both TiO2 and irradiation are required in
combination for the oxidation reaction to occur. Before irradiation,
a decrease in the amount of benzylic alcohol originating from its
adsorption on TiO2 was observed. After confirming the equilibrium
adsorption of benzylic alcohol on TiO2, the light was turned on. As
shown in Fig. 3a, the amount of benzyl alcohol decreased with an
increase in irradiation time while that of benzaldehyde increased
with the formation of only negligible amounts of benzoic acid or
CO2. It should be noted that these reactions immediately ceased
when the light was turned off. When the light was then turned
back on, the reactions immediately proceeded again. These results
indicate that they are not an auto-oxidative chain reaction, but a
photo-responsive reaction system. After irradiation for 240 min,
the benzyl alcohol was completely converted into benzaldehyde
with high selectivity (>99%) without further oxidation into benzoic
acid or CO2.
The photocatalytic oxidation of various kinds of benzylic alco-
hol substituted with –OCH3, –CH3, –C(CH3)3, –Cl, –CF3 and –NO2
was also investigated, and the results are shown in Fig. 3b–o. Here,
the reactants, as shown above, were classified into two categories:
category A includes benzylic alcohols substituted with such elec-
tron-releasing groups as –OCH3, –CH3, –C(CH3)3, as shown in
Fig. 3b–h; while category B includes benzylic alcohols substituted
with such electron-withdrawing groups as –Cl, –CF3 and –NO2, as
shown in Fig. 3i–o. For all of the photocatalytic reaction systems,
as shown in Fig. 3, it was found that the selective photocatalytic
oxidation of benzylic alcohols by O2 into corresponding aldehydes
was successfully carried out on TiO2 under visible light irradiation,
while an induction period was appeared with its time dependence
on the kind of substituent. It was, thus, confirmed that benzylic
alcohols in both categories A and B are converted into correspond-
ing aldehydes with high conversion (>99%) and with high selectiv-
ity (>99%). However, the total mass balance was less than 100% in
the photocatalytic oxidation of benzylic alcohols belonging to cat-
egories A and B after the reactions were completed, as shown in
Fig. 3. Here, the uptakes of various kinds of aldehydes on TiO2 were
characterized after equilibrium adsorption in the dark. Conse-
3.4. Kinetics of the photocatalytic oxidation of benzyl alcohol and its
derivatives
Characterization studies of the photocatalytic activity for the
oxidation of benzylic alcohol into corresponding aldehydes were
carried out by examining the reaction rate constants. If the
photo-formed aldehyde increases without the induction period,
the photocatalytic activity can be simply evaluated by the first-
order kinetics. Here, it is considered that the induction period
(0–2000 s) as shown in Fig. 3 may be attributed to a surface re-
construction between benzylic alcohol and TiO2 during photo-
irradiation, leading to the formation of the more active surface
structure than the initial state. Therefore, the apparent reaction
rate constant (k) was confirmed to simulate the part of an incre-
ment except for the induction period by the first-order kinetic
analysis.
Under conditions of sufficient O2 concentration during the
photocatalytic reactions, the reaction kinetics were ruled by the
following pseudo-first-order Eq. (2) or (3).
kðt0 ꢁ tÞ ¼ lnfa0=ða0 ꢁ xÞg
x ¼ a0f1 ꢁ exp kðt0 ꢁ tÞg
ð2Þ
ð3Þ
Here, a0, x, k, t and t0 represent the amounts of added reactant
molecules (50 mol), photo-formed corresponding aldehyde, the
l
apparent reaction rate constant, reaction time, and induction per-
iod, respectively. For example, except for the induction period,
ln{a0/(a0 ꢁ x)} against t for the oxidation of benzyl alcohol exhibits
a linear relationship, as shown in Fig. 5I. The k and t0 values are ob-
tained from the slope and interpolated value at the x-axis of the
linear line. Using set k and t0 values, the amount of photo-formed

S. Higashimoto et al. / Journal of Catalysis 274 (2010) 76–83
79
50
40
30
20
10
0
50
50
40
30
20
10
0
(b)
(c)
(a)
40
30
20
10
0
0
60
120
Time / min
180
240
0
20
40
60
80
0
10
20
30
40
50
Time / min
Time / min
50
40
30
20
10
0
50
40
30
20
10
0
50
40
30
20
10
0
(f)
(e)
(d)
0
20
40
60
80
0
30
60
90
120
0
30
60
90
120
Time / min
Time / min
Time / min
50
40
30
20
10
0
50
40
30
20
10
0
50
40
30
20
10
0
(g)
(h)
(i)
0
30
60
90
120
0
30
60
90 120 150
0
30
60
90
120
Time / min
Time / min
Time / min
50
50
40
30
20
10
0
50
40
30
20
10
0
(k)
(j)
(l)
40
30
20
10
0
0
10
20
30
40
0
20
40
60
0
10
20
30
40
Time / min
Time / min
Time / min
50
40
30
20
10
0
50
40
30
20
10
0
50
40
30
20
10
0
(n)
(o)
(m)
0
10
20
30
40
0
20
40
60
80
0
20
40
60
80
Time / min
Time / min
Time / min
Fig. 3. Photocatalytic oxidation of various kinds of benzylic alcohols on TiO2 (50 mg) under visible light irradiation. The initial amount of alcohol added to the reaction cell
was 50 mol. Amounts of (s) alcohol; (d) aldehyde; and (h) total alcohol and aldehyde. Reactants: benzylic alcohols substituted with the following groups: (a) -H, (b) o-
OCH3, (c) m-OCH3, (d) p-OCH3, (e) o-CH3, (f) m-CH3, (g) p-CH3, (h) p-(CH3)3, (i) p-Cl, (j) o-CF3, (k) m-CF3, (l) p-CF3, (m) o-NO2, (n) m-NO2 and (o) p-NO2.
l
benzaldehyde (x) was simulated, as shown in Fig. 5II. The induction
period (t0) and the apparent reaction rate constants (k) for the oxi-
dation of different kinds of benzylic alcohols were thus estimated,
as shown in Fig. S1. The relationship between the photocatalytic
activities and the oxidative potentials of benzylic alcohols will be
discussed in the following section.

80
S. Higashimoto et al. / Journal of Catalysis 274 (2010) 76–83
80
60
40
20
0
Ep
(c)
(b)
(a)
50 µA mm-2
0
1.0
2.0
E / vs. SCE
3.0
Fig. 6. Cyclic voltammogram of benzyl alcohol. The first oxidative potential (Ep) of
the benzyl alcohol is shown by an arrow.
400
450
500
550
600
Wavelength / nm
3.5. Relationship between the photocatalytic activities and the
oxidative potentials of benzylic alcohols on TiO2 under visible light
irradiation
Fig. 4. Quantum yields for the photocatalytic oxidation of (a) benzyl alcohol, (b) p-
methoxybenzyl alcohol and (c) p-nitrobenzyl alcohol into corresponding aldehydes
on TiO2 under light irradiation of various wavelengths.
The cyclic voltammogram of benzyl alcohol was recorded in the
range of 0–3.0 V vs. SCE in the dark, and the results are shown in
Fig. 6. This voltammogram exhibits an irreversible curve and first
oxidative potential (Ep) of the benzyl alcohol at ca. +2.16 V vs.
SCE, as shown in Fig. 6. It is known that the Ep of benzyl alcohol
is attributable to the one-electron oxidation from the phenyl-ring
[40]. Similarly, the cyclic voltammogram of various kinds of ben-
zylic alcohols have been recorded (Fig. S2, Supplementary mate-
rial) and the Ep for these are listed in Table 1. It was observed
that the Ep is strongly influenced by phenyl-ring substitution: the
reactant molecules in category A exhibit lower Ep in the range of
1.66–2.08 V, while those in category B have higher Ep in the range
of 2.17–2.80 V than benzyl alcohol (+2.16 V), as shown in Table 1.
Moreover, it can be seen that the ortho-, meta- and para-oriented
methoxybenzyl alcohol and nitrobenzyl alcohol showed Ep at
1.76, 1.80 and 1.66 V for methoxybenzyl alcohol, and 2.80, 2.80
and 2.80 V vs. SCE for nitrobenzyl alcohol, respectively. Thus, it
was confirmed that the Ep of benzylic alcohols are significantly
influenced by different kinds of substituents, not so by their orien-
tation. Fig. 7 shows the apparent reaction rate constant (k) as a
function of the Ep. It was observed that the photocatalytic activity
was not governed by the Hammett rule, i.e., substitution with the
electron-releasing groups –OCH3, –CH3, –C(CH3)3 in category A as
well as with the electron-withdrawing groups –Cl, –CF3 and –
NO2 in category B increased the photocatalytic activity of benzyl
alcohol, as shown in Fig. 7.
3
[I]
2.5
2
1.5
1
t0 = 0
k = 0.000199
0.5
0
0
5000
10000
Time / sec
15000
50
40
30
20
10
0
[II]
Table 1
Oxidative potentials (Ep) of benzyl alcohol and its derivatives.
X
CH2OH
V (vs. SCE)
X
H
2.16
(ortho-
meta-
para-)
Category A
OCH3
CH3
1.76
2.08
–
1.80
2.08
–
1.66
1.95
2.06
0
5000
10000
15000
C(CH3)3
Time / sec
Category B
Cl
CF3
NO2
–
2.70
2.80
–
2.65
2.80
2.17
2.75
2.80
Fig. 5. Plots [I] for ln{a0/(a0 ꢁ x)} against the time for the formation of benzalde-
hyde. Plots [II] for the amount of photo-formed benzaldehyde with experiment (d)
and its simulation (s).

S. Higashimoto et al. / Journal of Catalysis 274 (2010) 76–83
81
with a ratio of kC–H/kC–D at ꢂ3.9 [36]. This result suggests that
0.0025
0.002
0.0015
0.001
0.0005
0
the photo-excitation of the surface complex plays an important
o-CF3
role in the abstraction of the
a-hydrogen atom from the –CH2OH
group as a rate-determining step. It can be, therefore, assumed that
the abstraction of H atom, as shown in Schemes 1I and II, is one of
the important steps, while the presence of O2 as an electron accep-
tor seems to assist de-protonation and elimination of the electron
from benzyl alcoholic radical cation as shown in Scheme 1II and III.
As shown in Fig. 7, the effect of the substituent on the photocat-
alytic activity as a function of the Ep of benzylic alcohols shows the
U-shaped curve, which is not governed by the Hammett rule. It is
known that the Hammett rule is often applied to the ionic reaction,
not to the photo-induced radical reaction. How can the relation-
ship between the photocatalytic activity and the Ep in Fig. 7 be ex-
plained? It was first confirmed that the photocatalytic activity does
not directly depend on the amount of adsorbed benzylic alcohol or
visible light absorbance of the TiO2-adsorbed benzylic alcohol
(Figs. S3 and S4, Supplementary material).
m-OCH3
p-NO2
p-CF3
p-OCH3
m-CF3
p-C(CH3)3
o-NO2
m-NO2
m-CH3
p-Cl
o-OCH3
p-CH3
o-CH3
H
1.5
2
2.5
Ep / V (vs. SCE)
3
Fig. 7. Relationship between the apparent reaction rate constants (k) for the
photocatalytic oxidation of benzylic alcohols and their oxidative potentials (Ep). The
substituents of the benzylic alcohols are shown in this figure.
In order to understand the contribution of the photo-generated
hole, Ag+ was applied as an electron scavenger. Fig. 8 shows the
UV–Vis spectra of TiO2 after visible light irradiation for 60 s in
the presence of the reactants: (a) p-nitrobenzyl alcohol, (b) p-(tri-
fluoromethyl)benzyl alcohol, (c) benzyl alcohol, (d) p-methylben-
4. Discussion
The photocatalytic oxidation of benzylic alcohols into corre-
sponding aldehydes was observed to proceed on TiO2 with high
efficiency and selectivity without further oxidation into benzoic
acid or CO2 under visible light irradiation. Scheme 1 shows the
reaction mechanism for the photocatalytic oxidation of benzylic
alcohol by O2 on TiO2 under visible light irradiation. This scheme
is derived from a well-known mechanism for the electrochemical
oxidation of benzyl alcohols into aldehydes [41].
In this photocatalytic system, the surface complexes, which are
formed by the interaction of the –CH2OH group of molecular ben-
zyl alcohol with the TiO2 surface, induce absorption in the visible
light region [36,37]. Therefore, upon visible light irradiation, the
surface complex is photo-excited to form electrons (eꢁ) and holes
(h+). It should be noted that they were not photo-induced by direct
excitation of the TiO2 band-gap.
[I]
(e)
0.1
(d)
(c)
(b)
On the other hand, the photocatalytic oxidation of benzyl alco-
hol under pure O2 (1 atm), air and N2 were examined. It was found
that the efficiency for the photocatalytic oxidation of benzyl alco-
hol depends on the concentration of O2: the photocatalytic activity
was higher in the presence of pure O2 than in air, and almost no
activity was observed under N2. Furthermore, the first-order iso-
(a)
400
500
600
700
800
Wavelength / nm
tope effect shows that the C–D bond of
a, a-d2 benzyl alcohol is
0.25
cleaved more slowly than the C–H bond of normal benzyl alcohol
(e)
[II]
0.2
0.15
0.1
CH2OH
CH2OH
CH2OH
- e-
+•
(I)
X
X
X
(d)
•
CHOH
+•
(b)
- H+
0.05
(c)
(II)
X
(a)
0
1.5
•
2
2.5
3
CHO
CHOH
(III)
Ep / V vs. SCE
-e-, - H+
X
X
Fig. 8. UV–Vis spectra [I] of TiO2 after 60 s visible light irradiation in the presence of
Ag+ ions (50
mol) with the benzylic alcohols (50 mol), and the absorbance, A [II]
as a function of Ep of benzylic alcohol. The benzylic alcohols used in this experiment
l
l
D
X: -H, -OCH3, -C(CH3)3, -Cl, -CF3 and -NO2
are (a) p-nitrobenzyl alcohol, (b) p-(trifluoromethyl)benzyl alcohol, (c) benzyl
Scheme 1. Reaction mechanism for the photocatalytic oxidation of benzylic alcohol
alcohol, (d) p-methylbenzyl alcohol and (e) p-methoxybenzyl alcohol. The
DA was
by O2 on TiO2 under visible light irradiation.
estimated by subtracting the un-irradiated spectrum from the photo-irradiated one.

82
S. Higashimoto et al. / Journal of Catalysis 274 (2010) 76–83
zyl alcohol and (e) p-methoxybenzyl alcohol, with Ag+ ions instead
of O2. As shown in Fig. 8I-c, when visible light irradiation of TiO2
was carried out in the presence of benzyl alcohol and Ag+ ions un-
der N2 atmosphere, a broad absorption at ca. 400–800 nm could be
observed. This absorption at ca. 400–800 nm is attributed to the Ag
particles [42]. It should be noted that the Ag+ ions by itself could
not be directly reduced by the reactant molecules but through
the TiO2 surface, suggesting that the electron transfer takes place
by way of the TiO2 surface to the electron scavenger such as O2.
It was also shown that the band intensity attributed to Ag depends
on the kinds of adsorbed benzylic alcohols, and their band
intensities were observed in the following order: p-methoxybenzyl
alcohol ꢃ p-methylbenzyl alcohol > benzyl alcohol > p-(trifluoro-
methyl)benzyl alcohol > p-nitrobenzyl alcohol after irradiation.
Fig. 8II shows the relationship between the absorbance (
500 nm, as shown in Fig. 8I, and the Ep of the reactants. From
Fig. 8II, it was observed that the A decreases with an increase
DA) at
0.0025
D
in the value of Ep of the reactant molecules. This indicates that
the lower the Ep value of the reactants interacting with TiO2, the
higher the reducibility of the Ag+ ions. In other words, one-electron
oxidation from benzylic alcohol with low Ep takes place more effi-
ciently by a photo-induced hole. However, the relationship be-
tween the photocatalytic activity and EP for the benzylic alcohols
belonging to both categories A and B, as shown in Fig. 7, has yet
to be fully understood by the one-electron oxidizability of benzylic
alcohols.
0.002
(d)
(c)
0.0015
0.001
(a)
(b)
According to a previous report [43], it is known that the rate
constant for a-C–H de-protonation from benzyl alcohol (and more
0.0005
0
generally alkylaromatic) radical cations decrease by increasing the
stability of the radical cation. As shown in Scheme 1II, de-proton-
ation from the radical cations is influenced by the substituents. In
particular, the radical cations de-stabilized by such electron-with-
drawing groups as –CF3 and –NO2 may favorably promote de-pro-
tonation, followed by de-protonation and elimination of an
electron from the benzylic alcohol radical, as shown in Scheme 1III.
ortho-
meta-
para-
Orientation
Fig. 9. Apparent reaction rate constants (k) for the photocatalytic oxidation of
benzylic alcohols substituted with ortho-, meta- and para-oriented (a) –OCH3, (b) –
CH3, (c) –CF3 and (d) –NO2 into corresponding aldehydes.
(I)
•
CHOH
CHOH
OMe
•
•
CHOH
OMe
•
OMe
(stable)
•
•
•
CHOH
CHOH
CHOH
MeO
MeO
MeO
•
CHOH
CHOH
MeO
MeO
CHOH
MeO
•
(stable)
(II)
•
CHOH
CHOH
CHOH
•
•
•
(unstable)
NO2
NO2
NO2
•
CHOH
CHOH
CHOH
•
O2N
O2N
O2N
•
•
CHOH
CHOH
CHOH
O2N
O2N
O2N
•
(unstable)
Scheme 2. Resonant structures of benzylic alcohol radicals belonging to I and II.

S. Higashimoto et al. / Journal of Catalysis 274 (2010) 76–83
83
Fig. 9 shows the effect of the electronic properties by the ortho-,
Appendix A. Supplementary data
meta- and para-orientation on the photocatalytic activity for the oxi-
dation of benzylic alcohols substituted with (a) –OCH3, (b) –CH3, (c)
–CF3 and (d) –NO2 into corresponding aldehyde. It was observed that
methoxybenzaldehyde and methylbenzaldehyde in meta-orienta-
tion are photo-formed more efficiently than in ortho- and para-ori-
entation, while nitrobenzaldehyde and (trifluoromethyl)-
benzaldehyde in ortho- and para-orientation are formed more effi-
ciently than in meta-orientation.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2010.06.006.
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