Direct synthesis of phenol from benzene over platinum-loaded tungsten(VI) oxide photocatalysts with water and molecular oxygen

Article abstract of DOI:10.1246/cl.2011.1405

Tungsten(VI) oxide loaded with nanoparticulate platinum (Pt/WO₃) was demonstrated for the first time to exhibit photocatalytic activity for direct synthesis of phenol from benzene using water and molecular oxygen as reactants under ultraviolet or visible light irradiation; the selectivity for phenol (e.g., 74% at 69% of benzene conversion) on Pt/WO₃ photocatalysts was much higher than those on platinum-loaded titanium(IV) oxide (Pt/TiO₂) photocatalysts.

Full text of DOI:10.1246/cl.2011.1405

doi:10.1246/cl.2011.1405
Published on the web December 3, 2011
1405
Direct Synthesis of Phenol from Benzene over Platinum-loaded Tungsten(VI)
Oxide Photocatalysts with Water and Molecular Oxygen
Osamu Tomita,1 Ryu Abe,*1,2 and Bunsho Ohtani1,2
1Graduate School of Environmental Science, Hokkaido University, North 10, West 5, Kita-ku, Sapporo, Hokkaido 060-0810
2Catalysis Research Center, Hokkaido University, North 21, West 10, Kita-ku, Sapporo, Hokkaido 001-0021
(Received August 23, 2011; CL-110710; E-mail: ryu-abe@cat.hokudai.ac.jp)
Tungsten(VI) oxide loaded with nanoparticulate platinum
(Pt/WO3) was demonstrated for the first time to exhibit
photocatalytic activity for direct synthesis of phenol from
benzene using water and molecular oxygen as reactants under
ultraviolet or visible light irradiation; the selectivity for phenol
(e.g., 74% at 69% of benzene conversion) on Pt/WO3 photo-
catalysts was much higher than those on platinum-loaded
titanium(IV) oxide (Pt/TiO2) photocatalysts.
used. Since the WO3-K sample consists of mixture of fine and
large aggregated particles, fine particulate WO3 with a particle
size of 50-200 nm was separated from WO3-K by the method
reported previously,12 which will be denoted as WO3-K
(10 m2 g¹1) hereafter. TiO2 powders such as TiO2-P25 (Degussa
(Evonik) P 25, 59 m2 g¹1), TiO2-M (Merck, anatase, 11 m2 g¹1),
and TiO2-J (JRC-TIO-8 donated from the Catalysis Society of
Japan, anatase, 338 m2 g¹1) were also used for comparison. The
modification of photocatalysts with nanoparticulate Pt metal
cocatalysts (0.1 wt %) was accomplished by photodeposition
from hexachloroplatinic acid (H2[PtCl6]¢6H2O) according to the
method reported previously, which affords a highly uniform
dispersion of platinum particles (average size, 5 nm) on the
photocatalyst surface.12 Photocatalytic oxidation (hydroxylation)
of benzene was carried out in a Pyrex reaction cell with internal
volume of 15 mL containing a suspension of the photocatalyst
powder (50 mg) in an aerated aqueous benzene solution (2.5
mmol L¹1, 7.5 mL) with continuous stirring using a magnetic
stirrer. The reaction temperature was kept at 279 K by circulating
cooling water around the cell. Sample aliquots were withdrawn
from the reactor cell after each irradiation and filtered through
a PVDF filter to remove photocatalyst particles. Quantitative
analysis of solution was carried out using a high-performance
liquid chromatograph (Shimadzu, LC-10AT VP) equipped with
a C-18 column (Shodex, ODP2 HP4E) and a photodiode-array
detector. Generation of carbon dioxide (CO2) in the gas phase
was analyzed by gas chromatography.
Hydroxylated aromatic compounds are the most important
raw materials in chemical industry. For example, phenol is the
major source of phenol resins, which are utilized in many
commodities throughout the world. However, its industrial
production still requires multistep reaction processes, namely
cumene method, which consumes considerably large energy and
yields a by-product, acetone.1 Direct synthesis of phenol from
benzene in a one-step reaction, especially using environmentally
benign oxidants such as molecular oxygen (O2) or water, is
highly desirable, and thus have been extensively studied.2-4
However, there have been few catalytic process that enables high
benzene conversion with high phenol selectivity by using such
environmentally benign oxidants (O2 and/or water). Photo-
catalysis on semiconductor materials may be one of the
candidates for such clean and direct synthesis of phenol from
benzene. Although several research groups have reported direct
synthesis of phenol from benzene using titanium(IV) oxide
(TiO2) photocatalysts suspended in a water-benzene mixture in
the presence of molecular O2,5-10 the selectivity for phenol was
generally low mainly due to the occurrence of subsequent
peroxidation on photocatalyst. Yoshida et al. have demonstrated
that the selectivity for phenol can be significantly improved by
applying deaerated condition, i.e., the absence of O2, to Pt-
loaded TiO2 photocatalyst system, however, the efficiency was
not so high possibly due to the lower capability of water (or
proton) to capture the photoexcited electrons compared to O2.11
In the present study, we report highly selective phenol
production (ca. 74% at 69% of benzene conversion) on a Pt-
loaded tungsten(VI) oxide (Pt/WO3), which was recently
developed as a highly efficient visible-light-responsive photo-
catalyst,12 in an aqueous solution containing benzene and O2
under the ultraviolet or visible light irradiation. The Pt/WO3
photocatalysts showed much higher selectivity for phenol
production than platinized (or bare) TiO2 photocatalysts. The
different reactivity between WO3 and TiO2 photocatalysts
toward benzene oxidation is discussed.
Figure 1 shows the time-course curves of photocatalytic
reactions over Pt/WO3-K and Pt/TiO2-P25 in aqueous solutions
of benzene (2.5 mmol L¹1, 7.5 mL) in the presence of molecular
oxygen under the full arc irradiation with a 300-W xenon lamp
(- > 300 nm). As seen in Figure 1a, phenol was produced with
almost constant rate over Pt/WO3-K photocatalyst after 30 min
25
20
15
10
5
25
20
15
10
5
(a) Pt/WO3
phenol
(b) Pt/TiO2
phenol
in dark
in dark
benzene
CO2
dihydroxylated
benzene
CO2
dihydroxylated
0
0
0
60
120
180
240
300
0
60
120
180
240
300
Time / min
Time / min
Commercially available WO3 powders such as WO3-K
(Kojundo Chemical Laboratory, monoclinic, 2.2 m2 g¹1), WO3-
Y (Yamanaka Chemical Industries, monoclinic, 2.2 m2 g¹1), and
WO3-S (Soekawa Chemicals, monoclinic, 1.6 m2 g¹1) were
Figure 1. Time courses of photocatalytic oxidation of benzene over
(a) Pt/WO3-K and (b) Pt/TiO2-P25 in aerated aqueous solutions of
benzene (18.8 ¯mol) under ultraviolet and visible light (300 <
- < 500 nm).
Chem. Lett. 2011, 40, 1405-1407
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1406
25
20
15
10
5
25
20
15
10
5
Table 1. Photocatalytic oxidation of benzene by WO3 and TiO2
photocatalystsa
phenol
benzene
(a) Pt/WO3
(b) Pt/TiO2
phenol
benzene
CO2
in dark
in dark
Selectivity/%c
Conversion/%b
(Time/min)
Amount of
CO2/¯mol
OH
OH
OH
OH
O
Entry Sample
Conditions
OH
OH
OH
O
- > 300 nm O2
- > 300 nm O2
- > 400 nm O2
- > 400 nm O2
- > 300 nm O2
22.2 (60)
68.9 (240)
26.6 (60)
52.5 (240)
16.4 (240)
40.6 (60)
32.4 (60)
38.0 (60)
59.1 (240)
13.3 (60)
33.8 (240)
82.5 (240)
43.0 (60)
48.9 (60)
11.5 (15)
38.6 (60)
79.3
73.7
83.8
75.1
84.6
58.8
48.7
25.9
21.8
60.8
34.0
20.6
31.0
26.5
63.0
35.4
1.9
0
1.2 4.0
0 8.8
<0.1
1.4
1
2
Pt/WO3-K
2.3
1.4
1.4
1.6
1.2
1.2
0
0.7
0
CO2
dihydroxylated
1.2 2.8
2.9 1.2
3.3 1.1
1.3 1.1
2.0 1.2
0.6 1.1
0.8 0.1
<0.1
0.6
Pt/WO3-K
0
dihydroxylated
180 240
3
4
5
WO3-K
0
<0.1
<0.1
0.6
0
0
Pt/WO3-Y - > 300 nm O2
Pt/WO3-S - > 300 nm O2
0
0
60
120
180
240
0
60
120
0
Time / min
Time / min
- > 300 nm O2
Pt/TiO2-P25
0
4.1
6
7
- > 300 nm O2
0
<0.1
0
13.6
0.4
- > 300 nm Ar
Pt/TiO2-P25
0
0.9
0.7
0
0
Figure 2. Time courses of photocatalytic reaction over (a) Pt/WO3-
K and (b) Pt/TiO2-P25 in aerated aqueous solutions containing both
benzene (18.8 ¯mol) and phenol (18.8 ¯mol) under ultraviolet and
visible light (300 < - < 500 nm).
- > 300 nm Ar
0
0.1
0
0.3
8
9
TiO2-P25
- > 300 nm O2
<0.1
2.8
0
0.8 0.1
13.4
0.8
Pt/TiO2-M - > 300 nm O2
0
1.9
0.1
0.7
0.7
0
0
0
0
10 Pt/TiO2-J
- > 300 nm O2
- > 300 nm Ar
- > 300 nm Ar
1.3
0
1.2
0
<0.1
0.1
11 Pt/TiO2-J
0
0.2
aInitial concentration of benzene: 2.5 mmol L¹1, Solvent: H2O
7.5 mL, Light source: 300 W Xe lamp. bConversion:
areas (ca. 10 m2 g¹1), showed apparently different reactivity.
Thus, the factor dominating the reactivity undoubtedly rests on
the difference in the composition (WO3 or TiO2) but not on the
surface areas. As expected by the photoabsorption property of
TiO2, visible light irradiation did not yield any appreciable
products over Pt/TiO2 photocatalysts.
(Cbenzene,0 ¹ Cbenzene,t)/Cbenzene,0 © 102. cSelectivity: Cproducts,t
(Cbenzene,0 ¹ Cbenzene,t) © 102.
/
of irradiation along with the consumption of benzene. Small
amounts of dihydroxylated benzenes such as catechol or
hydroquinone were also produced (Entry 1 in Table 1). Al-
though appreciable generation of CO2 (ca. 1.4 ¯mol) was
observed in gas phase after the prolonged irradiation (240
min), the amount of CO2 produce was much smaller than that of
phenol (ca. 9.5 ¯mol). The production of phenol was also
observed over the Pt/TiO2-P25 as seen in Figure 1b, while the
phenol production was saturated within 60 min of irradiation
accompanied by significant increase of CO2 generation, sug-
gesting that the once-produced phenol was subsequently
oxidized over the photocatalyst.13-17
As summarized in Table 1, the photocatalytic reaction over
Pt/WO3-K resulted in highly selective phenol production with
79% of selectivity at 22% of benzene conversion in initial period
(60 min). Even after the long time irradiation of 240 min, a high
selectivity of 74% was still obtained at 69% of benzene
conversion (Entry 1) along with the production of catechol
(2.3%), resorcinol (0.7%), and p-benzoquinone (8.8%), resulting
in 85.5% of selectivity for hydroxylated benzene or quinone
products from benzene. It should be noted here that the products
without aromatic ring could not be quantified in the present
study. Therefore, other unidentified parts (ca. 14%) are certainly
corresponding to the various intermediate compounds produced
through the cleavage of aromatic ring, as well as to the final
oxidative product CO2. Visible light irradiation (- > 400 nm)
afforded better phenol selectivity (Entry 2) on Pt/WO3-K
photocatalyst compared to the case of full arc irradiation
(Entry 1). Other Pt/WO3 samples also showed relatively high
selectivity for phenol production above 48% (Entries 4 and 5).
On the other hand, all the Pt/TiO2 photocatalysts showed much
lower selectivity for phenol production below 31% in the
presence of O2 even at low conversions of benzene (Entries 6, 9,
and 10). In all cases H2 production was negligible, indicating
that almost all of the photoexcited electrons were consumed to
reduce molecular O2, not water (or H+). It should be noted that
the Pt/WO3-K and the Pt/TiO2-M, which have similar surface
These results suggested that the rates of subsequent
oxidations of phenol on Pt/WO3 photocatalysts are much lower
than those on Pt/TiO2 photocatalysts, enabling the high
selectivity of phenol to be obtained in Pt/WO3 photocatalyst
systems. To examine this, the photocatalytic reactions were
carried out in the coexistence of benzene and phenol. As shown
in Figure 2a, the amount of phenol was first decreased and then
increased with steady rate over Pt/WO3-K photocatalyst along
with decrease of benzene. This indicates that the rate of phenol
production on Pt/WO3-K is higher than that of subsequent
oxidation of phenol, possibly due to the preferential reaction of
benzene over phenol on Pt/WO3-K photocatalyst. On the other
hand, both benzene and phenol decreased on the Pt/TiO2-P25
photocatalyst, while benzene decreased much faster than phenol
(Figure 2b). The CO2 generation on Pt/TiO2-P25 was observed
from the beginning of the photoirradiation with much higher
rate compared to that of dihydroxylated products, indicating
that both the phenol and dihydroxylated products are readily
oxidized with cleavage of the aromatic ring producing various
oxidized intermediates and CO2. The faster decrease of benzene
suggests another possible reaction pathway on Pt/TiO2-P25;
benzene molecules are directly oxidized into CO2 without
forming any hydroxylated benzenes. When the photocatalytic
oxidation of benzene was carried out in the absence of water,
i.e., in dehydrated acetonitrile, no hydroxylated product was
obtained on Pt/WO3-K nor Pt/TiO2-P25 photocatalysts, indi-
cating that the hydroxyl groups were originated from water
molecules. It has been reported that hydroxyl radicals (•OH) are
produced through the reaction of photogenerated holes with
water molecules adsorbed on the photocatalyst surface.18,19
photocatalyst þ h¯ ! eꢀ þ hþ
hþ þ H2O ! Hþ þ •OH
ð1Þ
ð2Þ
The hydroxyl radical certainly reacts with benzene (or phenol) to
generate hydroxylated benzene radical, which are then oxidized
by a hole on a photocatalyst surface and deprotonated producing
phenol (or dihydroxylated benzenes), as shown below.20
Chem. Lett. 2011, 40, 1405-1407
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1407
which an appreciable amount of H2 was produced during the
photoirradiation, showed much higher selectivity for phenol
(60.7% at 13.3% of conversion) than that in the presence of O2,
while the reaction rate was lowered considerably due to the
lower capability of H2O (or H+) to capture the photoexcited
electrons compared to that of O2. These results strongly suggest
that the different behavior of O2 in the reactions is one of the
reasons for the different reactivity in benzene oxidation between
WO3 and TiO2 photocatalysts. However, the selectivity for
phenol drastically decreased in prolonged reactions on the Pt/
TiO2 photocatalyst even in the absence of O2 (Entries 7 and 11),
indicating that the different behavior of O2 is not the sole reason
for the different reactivity between WO3 and TiO2 photo-
catalysts. As suggested above, the different reactivity of holes
toward benzene and water certainly may contribute to the
different reactivity. Further investigation is now under way to
clarify the reaction mechanism for hydroxylation of benzene on
Pt/WO3 photocatalysts.
In conclusion, it was first demonstrated that Pt/WO3
photocatalysts showed activity for direct production of phenol
from benzene using water and O2 as reactants under UV or
visible light. The selectivity for phenol on Pt/WO3 photo-
catalysts was much higher than those on Pt/TiO2 (or TiO2)
photocatalysts. These results demonstrate the potential of Pt/
WO3 photocatalysts for highly selective organic synthesis with
environmentally benign oxidants such as water and/or O2 using
abundant visible light included in solar radiation.
H
OH
+
•OH
ð3Þ
ð4Þ
OH
H
OH
+
h+
Interestingly, stoichiometric amount of CO2 to the decreased
benzene was found to generate from benzene-acetonitrile
solution with a relatively high rate when Pt/TiO2-P25 photo-
catalyst was irradiated in the presence of O2, while no CO2 gas
was generated on Pt/WO3-K in a same time scale of irradiation
(µ120 min). This result indicates the presence of reaction
pathway of direct oxidation of benzene on Pt/TiO2-P25 surface
by photogenerated holes into CO2 without forming any
hydroxylated benzenes. It appears that the direct oxidation of
benzene occurs preferentially on Pt/TiO2-P25 photocatalyst
even in aqueous solution, resulting in the significant generation
of CO2 from the initial period (Figure 1b). On the other hand,
the holes generated on Pt/WO3 photocatalysts possibly react
with water preferentially over benzene, resulting in the prefer-
ential generation of •OH that can produce phenol from benzene
without cleavage of aromatic ring. The different participation of
molecular oxygen (O2) to the reactions seems to be another
reason for the different reactivity between WO3 and TiO2
photocatalysts. It is well known that bare WO3 is not an
efficient photocatalyst for oxidation of organic compounds in the
presence of O2 because the conduction band (CB) bottom (ca.
+0.5 VNHE) potential is insufficient for the reduction of O2
This study was supported by the 2007 Industrial Technology
Research Grant Program from the New Energy and Industrial
Technology Development Organization (NEDO) of Japan.
•¹
[E0(O2/O2 ) = ¹0.33 VNHE and E0(O2/HO2•) = ¹0.05 VNHE
]
in a one-electron process. The incapability of O2 to scavenge the
electrons in CB of WO3 results in the fast recombination and the
lower photocatalytic activity. Indeed, bare WO3 photocatalyst
showed much lower conversion for the photocatalytic reaction
with benzene (Entry 3) compared to that on Pt/WO3, while the
selectivity for phenol production was high. We have recently
demonstrated that Pt-loaded WO3 exhibits high photocatalytic
activity for decomposition of various aliphatic compounds due
to the promotion of multielectron reduction of O2 on the Pt
cocatalysts.12 It was also confirmed that the reduction product of
O2 was mainly hydrogen peroxide (H2O2) generated via two-
electron process [E0(O2/H2O2) = +0.55 VNHE]. Negligible re-
action occurred by the simple mixture of benzene and H2O2 in
water, indicating that the H2O2 itself cannot participate to the
oxidation (hydroxylation) of benzene. On the other hand, TiO2
photocatalysts possess sufficiently negative CB bottom for one-
electron reduction of O2. Therefore TiO2 can reduce O2 even
without Pt cocatalyst resulting in efficient oxidation of organic
compounds by holes. Interestingly, bare TiO2-P25 showed
higher rate for benzene oxidation than Pt/TiO2-P25 (Entries 6
and 8), implying that most of the photoexcited electrons were
consumed on the TiO2 surface, not on Pt, via single electron
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© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/