Photocatalytic hydroxylation of aromatic ring by using water as an oxidant

Article abstract of DOI:10.1039/b811555a

Electrophilic oxygen species photocatalytically derived from water molecules can selectively react with the aromatic ring of both benzene and its derivatives to produce the corresponding phenols and hydrogen over platinum-loaded titanium oxide when illuminated with light of appropriate wavelength in the absence of oxygen. The Royal Society of Chemistry.

Full text of DOI:10.1039/b811555a

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Photocatalytic hydroxylation of aromatic ring by using water
as an oxidant
a
a
a
a
Hisao Yoshida,* Hayato Yuzawa, Masanori Aoki, Kazuko Otake,
b
a
Hideaki Itoh and Tadashi Hattori
Received (in Cambridge, UK) 7th July 2008, Accepted 7th August 2008
First published as an Advance Article on the web 26th August 2008
DOI: 10.1039/b811555a
Electrophilic oxygen species photocatalytically derived from water
molecules can selectively react with the aromatic ring of both
benzene and its derivatives to produce the corresponding phenols
and hydrogen over platinum-loaded titanium oxide when illumi-
nated with light of appropriate wavelength in the absence of oxygen.
Pt precursor in a Pyrex glass vessel with vigorous stirring,
followed by photoirradiation from a 300 W Xe lamp, which
emitted both UV and visible light, for 3 h with continuous
stirring. The suspension was then filtered off with suction,
washed with distilled water, and dried at 333 K. The samples
are referred to as Pt(x)/TiO
based on the weight of TiO
2
, where x is the Pt content (wt%)
2
. The Pt content was confirmed by
As we know, selective aromatic-ring oxidation of benzene
derivatives such as alkylbenzenes is very difficult since the
usual oxidants tend to react with the substituents first. In
industrial processes for the production of phenols such as the
ICP (inductively coupled plasma emission spectroscopy, Perkin-
Elmer Optima 3300 DV). The diffuse reflectance UV–visible
spectrum of the catalyst was recorded at room temperature on
a JASCO V-570 equipped with an integrating sphere covered
1
cumene process, benzene and alkylbenzenes are indirectly
oxidized by using molecular oxygen at the expense of propene
with acetone formation. Even in these cases, undesirable
oxidation of side chains reduces the selectivity when we
employ molecular oxygen as an oxidant. Thus, the develop-
ment of novel selective oxidation systems has been desired
from the viewpoint of green and sustainable chemistry.
Heterogeneous photocatalyses for the organic syntheses have
with BaSO . Before the photocatalytic reaction, the photocata-
4
3
lyst (0.2 g) in a quartz photoreactor (99 cm ) was photoirradiated
2
from beneath (18 cm ) by the Xe lamp in air for 1 h to clean up
the catalyst surface. After introducing substrate (1 ml) and
distilled water (1 ml) in this order, the gas phase was purged
with flowing Ar. Then, the mixture of substrate, water and the
photocatalyst was magnetically stirred and photoirradiated from
the Xe lamp under Ar atmosphere in the closed system at around
room temperature for 3 h. In many cases in the present study, a
band-pass filter was used between the reactor and the lamp to
limit the irradiation light wavelength. After the photoreaction,
first, the gas phase was collected with a syringe and analysed by
GC–TCD. Then, methanol was added to the liquid phase and it
was filtered. The filtrate was analysed by GC–MS and HPLC. In
the present study, no reaction proceeded in the dark. In the
absence of photocatalyst, no reaction occurred except for some
photochemical reactions such as coupling. These results indicate
that all the hydroxylation reactions observed in the present study
proceed photocatalytically. In the presence of gaseous oxygen
and photocatalyst, the degradation of substrates mainly occurred
with low selectivity to phenols.
2
received much attention because of their unique potential. In
recent decades, many studies on photocatalytic oxidation of both
benzene and its derivatives have been carried out to yield corre-
sponding phenols by using metal-loaded and unloaded TiO
5
2
3
,4
6
2 2
photocatalyst, mesoporous TiO , TiO -pillared clay, ion-ex-
7
changed zeolites and so on. However, the selective aromatic-ring
hydroxylation of benzene derivatives has not been reported. It is
believed that some kinds of active oxygen species would be
generated in these photocatalytic oxidation systems, such as
8
superoxide, hydrogen peroxide and hydroxy radical, which
would not provide high selectivity. For the direct and selective
oxidation of the aromatic ring, selective generation of electrophilic
active oxygen species in the system would be important. In the
present study, we thus examined photocatalytic oxidation of
benzene and its derivatives by using water as an oxidant in the
absence of molecular oxygen. We discovered a very interesting
fact, namely that irradiation with light of selected wavelength
provided highly selective hydroxylation of the aromatic ring.
Platinum-loaded titanium dioxide was prepared by a photo-
Table 1 shows the results for the photocatalytic reactions of
benzene and alkylbenzenes with water in the absence of
molecular oxygen. The TiO
inactive for selective photooxidation of benzene though only
small amounts of H and CO were observed (Entry 1). When
Pt(0.1)/TiO was employed without wavelength limitation of
without Pt loading was almost
2
2
2
deposition method from TiO (donated by the Catalysis Society
2
2
of Japan as JRC-TIO-8, anatase, BET specific surface area:
3
the irradiation light (Entry 2), phenol was obtained as
the major organic product while other minor organic
products such as biphenyl via coupling, cyclohexanol and
cyclohexanone via consecutive hydrogenation of phenol, and
2
38 m g , crystallites size estimated from X-ray diffraction:
ꢀ1
7
nm) and H PtCl ꢁ6H O (Kishida). The TiO powder was
2
6
2
2
dispersed into MeOH (50%) aqueous solution containing the
2 2
large amounts of H and CO wwere also obtained. In the case
of toluene, obtained products were cresol, bibenzyl and
a
Department of Applied Chemistry, Graduate School of Engineering,
Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan.
E-mail: yoshidah@apchem.nagoya-u.ac.jp; Fax: +81-52-789-3178
Division of Environmental Research, EcoTopia Science Institute,
methylcyclohexanol with a larger amount of H ,z and the
2
b
cresol selectivity was quite low, such as 38% (Entry 4). When
we selected the wavelength of the irradiation light by using a
Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan
4
634 | Chem. Commun., 2008, 4634–4636
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a
Table 1 Photocatalytic oxidation of benzene and alkylbenzenes by TiO
Organic products/mmol
2
and Pt/TiO
2
photocatalysts
Gas
products/
mmol
Coupling Carbonyl Hydrogenated
Irradiation Phenols products products compounds
Total
yield /
CO2 mmol Y (10 %)
Phenols
h
Conversion selectivity
c
d
e
f
g
CO
Y
2
b
2
Entry Substrate
Catalyst
light/nm
Y
OH
Y
C
Y
CHO
Y
HCP
H
2
(%) SOH
i
1
2
3
4
5
6
7
8
9
1
Benzene
TiO
2
Alli
All
n.d.
8.9
3.7
11
1.5
3.5
5.7
6.1
3.4
2.0
n.d.
0.4
0.4
7.8
0.1
1.7
1.4
0.1
0.3
n.d.
—
—
—
0.3
n.d.
0.1
0.1
n.d.
n.d.
n.d.
n.d.
0.4
0.5
0.5
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
2.4 3.1
62 11
9.4 n.d. 4.7
240 11 29
3.0 n.d. 1.7
31 n.d. 7.0
31 n.d. 6.6
7.4 n.d. 6.3
6.0 n.d. 3.7
6.3 n.d. 1.2
0.5
12
0.5
11
4.3
32
1.9
7.8
7.3
7.0
4.1
1.3
0
Pt(0.1)/TiO
Pt(0.1)/TiO
Pt(0.1)/TiO
Pt(0.1)/TiO
Pt(0.1)/TiO
Pt(0.1)/TiO
2
2
2
2
2
2
75
76
38
88
50
86
97
85
499
j
405 ꢃ 20
i
Toluene
All
j
j
j
j
j
j
254 ꢃ 20
365 ꢃ 20
405 ꢃ 20
405 ꢃ 20
405 ꢃ 20
405 ꢃ 20
Pt(1)/TiO
Ethylbenzene Pt(1)/TiO
tert-
Butylbenzene
2
2
2
0
Pt(1)/TiO
a
Catalyst: 0.2 g, reaction time: 3 h, H O: 1 ml, benzene: 1 ml (11 mmol) or alkylbenzene: 1 ml (9 mmol). The detection limits for organic products
2
b
c
contains x wt% of Pt. Biphenyl for benzene, bibenzyl etc. for toluene,
and CO
2
were 0.1 mmol and 1 mmol, respectively. Pt(x)/TiO
2
d
e
diphenylbutanes etc. for ethylbenzene. Benzaldehyde for toluene. Cyclohexanol and cyclohexanone for benzene, methylcyclohexanol for
f
g
toluene. Y = YOH + Y
h
C
ꢄ 2 + YCHO + YHCP + YCO2/(number of carbons in a substrate molecule). Calculated from the total yield.
i
ꢀ2
ꢀ2
ꢀ2
S
OH (%) = YOH/Y ꢄ 100. No filter was used. Light intensity: 10 mW cm (220–300 nm), 50 mW cm (310–400 nm), 176 mW cm
j ꢀ2
2
(360–470 nm). Band-pass filter was used. Intensity of the irradiation light, which could be absorbed by TiO , was 3 mW cm . n.d. = not detected.
band-pass filter, it was found that the product selectivity
varied with the wavelength. Fig. 1 depicts the effect of the
irradiation light wavelength on the yield of cresol (the hydro-
xylation product) and the coupling products (the represen-
tative byproducts such as bibenzyl) in the reaction of toluene
photoexcitation mechanism should be carried out. The plati-
num content also affected the selectivity. For the hydroxyla-
tion of toluene, the sample loaded with 1.0 wt% of platinum
showed higher selectivity (97%) than the sample loaded with
0.1 wt% of platinum did (86%) (Entries 7 and 8), while for
benzene hydroxylation the suitable platinum loading amount
was lower than 1 wt%, typically around 0.1 wt%. In the
optimum conditions with the moderate photocatalyst upon
photoirradiation of selected wavelength around the absorp-
tion edge (405 ꢃ 20 nm), the selectivity for the aromatic-ring
hydroxylation of toluene was as high as 97%, where the
production ratio of hydrogen to phenols was rather close to
unity (Entry 8).y In addition, aromatic-ring hydroxylation was
also confirmed for ethylbenzene and tert-butylbenzene (En-
tries 9 and 10). These results suggest that the direct aromatic-
ring hydroxylation with water occurs photocatalytically in
these conditions, generally as shown in eqn (1).z Here, the
water seems to act as an oxidant.
(
i.e., Table 1, Entries 5–7), where the absorption spectrum of
the Pt(0.1)/TiO photocatalyst is also shown. Since the inten-
2
sity of the light that can be absorbed by TiO (less than 400 nm)
2
ꢀ
2
was adjusted to 3 mW cm for each experiment, we can
compare the product selectivity. It was found that the highest
yield and selectivity of cresol were obtained when light around
3
85–425 nm was used. Since the main bandgap absorption of
TiO is less than around 380 nm and very small and broad
2
bands are also found even around 400 nm, it is suggested that
some kinds of photoexcitation other than the usual bandgap
excitation are efficient for the selective hydroxylation. It
is speculated that some surface defects such as hydroxyl
groups or coordinatively unsaturated sites might function
as photocatalytic active sites. Further investigation on the
ð1Þ
Considering the product selectivity, the active oxygen
species for the hydroxylation are suggested to be electrophilic.
To confirm this and to establish the generality of this photo-
catalytic hydroxylation of the aromatic ring, various substi-
tuted benzenes were examined for this photocatalytic reaction.
The yield and selectivity of the hydroxylated products are
shown in Table 2 with the distribution of isomers, i.e., ortho-,
meta-, and para-directing substituted phenols. Corresponding
phenols were obtained as the main products from every
substrate listed here. The yield of hydroxylation products
increased with increasing electron donating ability of the
substituent from nitrobenzene to toluene (Table 2, Entries
1–6), except for anisole (Entry 7). The hydroxylation occurred
Fig. 1 Product yield of (a) cresol and (b) coupling products when
illuminated with light of selected wavelength by using band-pass
filters, and (c) a diffuse reflectance UV–visible absorption spectrum
of Pt(0.1)/TiO
2
. The permitted wavelength of light was 254 ꢃ 20, 365 ꢃ
20 and 405 ꢃ 20 nm as indicated by horizontal bars. The intensity of
ꢀ2
the irradiation light, which can be absorbed by TiO , was 3 mW cm .
2
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a
Table 2 Photocatalytic hydroxylation of various substituted benzenes
water molecule acts as the main oxidant in the hydroxylation.
2
At the same time, the participation of the oxygen from TiO ,
Hydroxylation products
probably the surface hydroxy groups, is also suggested.
Further investigation would be required to clarify the detailed
reaction mechanism.
Distribution of isomers
%)
(
Yield YOH Selectivity
2
Entry Substrate (10 %)
b
b
S
OH (%) ortho-
meta- para-
In conclusion, it was clarified that the direct aromatic-ring
hydroxylation of benzene, alkylbenzenes and substituted ben-
zenes could be promoted by platinum-loaded titanium oxide
photocatalyst by using water as an oxidant in the absence of
molecular oxygen. Photoexcitation with light of the appro-
priate wavelength slightly longer than the absorption edge, i.e.,
above 380 nm, and the optimisation of the platinum loading
amount were effective for improvement of the selectivity to
produce phenols. It is suggested that the active oxygen species
in this system are electrophilic ones derived from water and a
photoformed hole, like hydroxy radicals, on the surface. The
electrophilic active oxygen species would interact not with the
substituents but with the electron-rich aromatic nucleus.
1
2
3
4
5
6
7
a
Ph–NO
Ph–CN
Ph–Cl
Ph–F c
Ph–H
2
1.6
3.0
7.3
8.4
8.8
70
499
86
77
77
n.d.
n.d.
37
42
—
499 n.d.
499 n.d.
22
19
—
23
11
41
39
—
39
14
Ph–CH
Ph–OCH
3
11
7.1
80
53
38
75
3
Pt(0.1)/TiO : 0.2 g, H
Irradiation light wavelength: 405
2
2
O: 1 ml, substrate: 1 ml (9.2–11 mmol),
ꢃ
20 nm, Light intensity:
ꢀ2
b
mW cm (measured at 310–400 nm), Reaction time: 3 h. See
5
the footnote of Table 1. Benzene. n.d. = not detected.
c
only at the meta-position for the substrates having an electron-
withdrawing group such as nitrobenzene and benzonitrile
(
Entries 1 and 2). On the other hand, the hydroxylation
preferably occurred at the ortho- and para-position to some
extent in the cases of the substrates having an electron-donat-
ing group such as chlorobenzene, fluorobenzene, toluene, and
anisole (Entries 3–7). These results confirm that the reaction
proceeds by a mechanism similar to that of electrophilic
aromatic substitution and the active oxygen species in this
photocatalytic reaction have an electrophilic character. For
the substrates having a strongly electron-donating group such
as phenol and aniline, the hydroxylation did not occur in
the present conditions, while the side reactions preferably
occurred through the activation of the electron-rich substitu-
ent. Anisole would give a lower yield of hydroxylation pro-
ducts for the same reason. These results were quite a contrast
to the reported facts that, in an oxygen atmosphere on TiO2
photocatalyst, nitorobenzene and benzonitrile did not show
such a clear orientation effect, and phenol and aniline were
hydroxylated clearly at ortho- and para-orientation before
Notes and references
w Platinum is considered to reduce the recombination of photoexcited
electrons and holes on TiO
by the excited electron to produce hydrogen. H2 and CO2 would be
formed as follows: C + 12H O - 6CO + 15H
was much larger than the
2
, and to accelerate the reduction of protons
6
H
6
2
2
2
.
z In many cases, the amount of detected H
2
amount expected from the detected products, where other compounds
that were undetectable in the present method might be formed to some
extent.
y The apparent quantum yield, roughly estimated from the number of
the incident photons and the produced hydrogen, was ca. 0.8%.
z The reaction was confirmed to continue at least over 12 h and the
photocatalyst was reusable: Even when the color of the photocatalyst
became pale yellow due to the formation of byproducts from side
reactions such as oligomerization, the photocatalyst could be refreshed
by washing with benzene or toluene.
1
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3
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1
1
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present conditions would be different from the major one in
the presence of oxygen. It is also confirmed that the substi-
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the reaction would be much influenced by the interaction
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´
´
2
´
´
between the substrate and the catalyst surface. From these
ꢀ
facts, it is proposed that H O, OH or the surface hydroxy
2
4 I. Izumi, W. W. Dunn, K. O. Wilbourn, F. F. Fan and A. J. Bard, J.
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group of TiO is oxidized by the photoformed hole on the
2
surface to form an electrophilic oxygen species such as surface
hydroxyl radical suitable for the photocatalytic hydroxylation,
while molecular oxygen would be reduced by the photoexcited
electron to form a nucleophilic oxygen species such as super-
oxide. In the absence of molecular oxygen, the active oxygen
species would be limited to the electrophilic oxygen species
derived from water and the aromatic-ring hydroxylation
would proceed selectively.
5
Y. Shiraishi, N. Saito and T. Hirai, J. Am. Chem. Soc., 2005, 127,
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1
6
To confirm the origin of the active oxygen species, labeled
1
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8
water containing 95.4% of H2 O was employed in the reac-
tion of toluene under light of the appropriate wavelength
8
M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann,
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(
405 ꢃ 20 nm). As a result, 89% of the produced ortho-cresol
18
contained the labeled oxygen atom O. It is suggested that the
4
636 | Chem. Commun., 2008, 4634–4636
This journal is ꢂc The Royal Society of Chemistry 2008