Photocatalytic hydroxylation of benzene by dioxygen to phenol with a cyano-bridged complex containing FeII and RuII incorporated in mesoporous silica-alumina

Article abstract of DOI:10.1021/acs.inorgchem.5b02909

Photocatalytic hydroxylation of benzene to phenol was achieved by using O₂ as an oxidant as well as an oxygen source with a cyano-bridged polynuclear metal complex containing Fe^II and Ru^II incorporated in mesoporous silica-alumina ([Fe(H₂O)₃]₂[Ru(CN)₆]@sAl-MCM-41). An apparent turnover number (TON) of phenol production per the monomer unit of [Fe(H₂O)₃]₂[Ru(CN)₆] was 41 for 59 h. The cyano-bridged polynuclear metal complex, [Fe(H₂O)₃]₂[Ru(CN)₆], exhibited catalytic activity for thermal hydroxylation of benzene by H₂O₂ in acetonitrile (MeCN), where the apparent TON of phenol production reached 393 for 60 h. The apparent TON increased to 2500 for 114 h by incorporating [Fe(H₂O)₃]₂[Ru(CN)₆] in sAl-MCM-41. Additionally, [Fe(H₂O)₃]₂[Ru(CN)₆] acts as a water oxidation catalyst by using [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) and Na₂S₂O₈ as a photosensitizer and a sacrificial electron acceptor as evidenced by ¹⁸O-isotope labeling experiments. Photoirradiation of an O₂-saturated MeCN solution containing [Fe(H₂O)₃]₂[Ru(CN)₆]@sAl-MCM-41 and scandium ion provided H₂O₂ formation, where photoexcited [Ru(CN)₆]^4- moiety reduces O₂ as indicated by laser flash photolysis measurements. Thus, hydroxylation of benzene to phenol using molecular oxygen photocatalyzed by [Fe(H₂O)₃]₂[Ru(CN)₆] occurred via a two-step route; (1) molecular oxygen was photocatalytically reduced to peroxide by using water as an electron donor, and then (2) peroxide thus formed is used as an oxidant for hydroxylation of benzene.

Full text of DOI:10.1021/acs.inorgchem.5b02909

Article
pubs.acs.org/IC
Photocatalytic Hydroxylation of Benzene by Dioxygen to Phenol
with a Cyano-Bridged Complex Containing FeII and RuII Incorporated
in Mesoporous Silica−Alumina
Yusuke Aratani,† Kohei Oyama,† Tomoyoshi Suenobu,† Yusuke Yamada,*,‡ and Shunichi Fukuzumi*,†,§,∥
†Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA and SENTAN, Japan Science
and Technology Agency, Suita, Osaka 565-0871, Japan
‡Department of Applied Chemistry and Bioengineering, Graduate School of Engineering, Osaka City University, Osaka 558-8585, Japan
§Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea
∥Faculty of Science and Engineering, Meijo University, ALCA and SENTAN, Japan Science and Technology Agency, Nagoya,
Aichi 468-0073, Japan
S
* Supporting Information
ABSTRACT: Photocatalytic hydroxylation of benzene to
phenol was achieved by using O2 as an oxidant as well as an
oxygen source with a cyano-bridged polynuclear metal complex
containing FeII and RuII incorporated in mesoporous silica−
alumina ([Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41). An apparent
turnover number (TON) of phenol production per the mono-
mer unit of [Fe(H2O)3]2[Ru(CN)6] was 41 for 59 h. The
cyano-bridged polynuclear metal complex, [Fe(H2O)3]2[Ru-
(CN)6], exhibited catalytic activity for thermal hydroxylation of
benzene by H2O2 in acetonitrile (MeCN), where the apparent
TON of phenol production reached 393 for 60 h. The apparent TON increased to 2500 for 114 h by incorporating
[Fe(H2O)3]2[Ru(CN)6] in sAl-MCM-41. Additionally, [Fe(H2O)3]2[Ru(CN)6] acts as a water oxidation catalyst by using
[Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) and Na2S2O8 as a photosensitizer and a sacrificial electron acceptor as evidenced by
18O-isotope labeling experiments. Photoirradiation of an O2-saturated MeCN solution containing [Fe(H2O)3]2[Ru(CN)6]@sAl-
MCM-41 and scandium ion provided H2O2 formation, where photoexcited [Ru(CN)6]4− moiety reduces O2 as indicated by
laser flash photolysis measurements. Thus, hydroxylation of benzene to phenol using molecular oxygen photocatalyzed by
[Fe(H2O)3]2[Ru(CN)6] occurred via a two-step route; (1) molecular oxygen was photocatalytically reduced to peroxide by using
water as an electron donor, and then (2) peroxide thus formed is used as an oxidant for hydroxylation of benzene.
Recently, photocatalytic systems producing H2O2 by reduction
of O2 with water as an electron donor have been constructed by
utilizing the water oxidation catalysis of cyano-bridged complexes
and O2 reduction catalysis of a Ru complex.24−26 Besides, some
metal complexes containing Fe ions have been reported to act
as a catalyst for hydroxylation of benzene by H2O2.27−31 Com-
bination of catalysis for photocatalytic H2O2 production and for
hydroxylation of benzene by H2O2 allows to construct a com-
posite catalyst for benzene hydroxylation by O2. A cyano-bridged
complex incorporating Ru and Fe ions seems to be a good
candidate for such a reaction.
We report herein photocatalytic hydroxylation of benzene
to phenol using O2 as an oxygen source as well as an oxidant
with a cyano-bridged polynuclear metal complex containing
FeII and RuII incorporated in mesoporous silica−alumina
([Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41), in MeCN under
photoirradiation at 298 K. As indicated in Figure 1, Fe ions
INTRODUCTION
■
Production of phenol from benzene in a one-step reaction has
merited significant interest from the viewpoints of an environ-
mentally benign green process and economical efficiency as
compared with the current three-step cumene process.1−3 Thus,
extensive efforts have so far been devoted to develop the one-step
direct hydroxylation of benzene using various oxidants such as
H2O2,4−7 N2O,8,9 O2 with reductants.10−15 Catalytic oxidation of
benzene using O2 as the oxidant is particularly of high potential
interest, because O2 is regarded as an ideal oxidant due to its
natural, inexpensive, and environmentally friendly characteristics.
However, catalytic aerobic oxidation of benzene via O2 activation
without reductants required harsh reaction conditions such as high
temperature and high pressures.16−19 Photocatalytic oxidation of
benzene with O2 without a reductant has also been reported using
organic photocatalysts.20−23 In such a case, water was used as an
oxygen source, whereas O2 acts only as an oxidant.20−23 Thus,
there has been no report on catalytic hydroxylation of benzene
with O2 used as an oxygen source as well as an oxidant under mild
conditions at ambient pressure and temperature.
Received: December 16, 2015
© XXXX American Chemical Society
A
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(1.33 g, 21.7 mmol) and stirred for 22 h at room temperature. The
obtained solid was collected by filtration and dried at 200 °C for 1 h.
The dried sample was calcined at 550 °C for 6 h under atmospheric
conditions to obtain sAl-MCM-41. The obtained powder was char-
acterized by N2 adsorption−desorption at 77 K and powder X-ray
diffraction.
Preparation of [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 and [Cu-
(H2O)n]2[Ru(CN)6]@sAl-MCM-41. [Fe(H2O)3]2[Ru(CN)6]@sAl-
MCM-41 and [Cu(H2O)n]2[Ru(CN)6]@sAl-MCM-41 were prepared
by a similar procedure. sAl-MCM-41 (0.20 g) was added to an aque-
ous solution (10 mL) of iron(II) perchlorate hydrate (0.20 g, 0.56 mmol)
or copper(II) sulfate pentahydrate (0.14 g, 0.56 mmol) and magnetically
stirred for 2 h at room temperature. The obtained solid collected by
centrifugation was washed with water two times and dried in vacuo.
The obtained Fe2+@sAl-MCM-41 or Cu2+@sAl-MCM-41 (50 mg) was
added to an aqueous solution of potassium hexacyanoruthenate(II)
hydrate (57 mg, 0.14 mmol) for 2 h on a magnetic stirrer. The obtained
solid was collected by filtration and washed with water and dried in vacuo
to obtain [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 or [Cu(H2O)n]2[Ru-
(CN)6]@sAl-MCM-41. The loading amounts of Fe and Ru on the
sAl-MCM-41 were determined by X-ray fluorescence (XRF) measure-
ments to be 8.7 × 10−4 and 5.7 × 10−5 mol g−1, respectively.34
N2 Adsorption−Desorption Isotherms. Nitrogen adsorption−
desorption isotherms at 77 K were recorded by an instrument for
volumetric adsorption measurements, Belsorp-mini II (BEL Japan, Inc.),
within a relative pressure range from 0.01 to 101.3 kPa. The mass of a
sample was ca. 100 mg for an adsorption analysis after pretreatment at
393 K for ca. 1 h under vacuum conditions and kept in N2 atmosphere
until N2-adsorption measurements. The sample was exposed to a mixed
gas of He and N2 with a programmed ratio, and adsorbed amount of N2
was calculated from the change of pressure in a cell after reaching the
equilibrium.
Figure 1. Partial structure of a cyano-bridged complex containing FeII
and RuII ([Fe(H2O)3]2[Ru(CN)6]). Three water molecules (in average)
bind to FeII ions to fulfill the octahedral coordination structure.
in [Fe(H2O)3]2[Ru(CN)6] have extra ligands such as water
molecules to fulfill octahedral coordination; however, the coor-
dinating water molecules are easily liberated during the catalytic
reaction. The FeII ions may react with H2O2 to produce hydro-
xyl radicals or iron oxo species, which oxidize benzene to pro-
duce phenol. The incorporated metal complex, [Fe(H2O)3]2-
[Ru(CN)6], acts as both an effective photocatalyst for oxida-
tion of water with O2 to produce H2O2 and also an excellent
catalyst for hydroxylation of benzene with H2O2 to produce
phenol.
EXPERIMENTAL SECTION
■
Materials. All chemicals used for synthesis were obtained from
chemical companies and used without further purification. Purified
water was provided by a water purification system, Millipore Direct-Q3
UV, where the electronic conductance was 18.2 MΩ cm. Tetraethyl
orthosilicate, copper(II) sulfate pentahydrate, cobalt(II) nitrate hexa-
hydrate, manganese(II) perchlorate hexahydrate, cetyltrimethylammo-
nium bromide, sodium aluminate, aqueous ammonia, benzene, sodium
iodide, and an aqueous solution of hydrogen peroxide (30 wt %) were
purchased from Wako Pure Chemical Industries. Iron(II) perchlorate
hydrate and potassium hexacyanoruthenate(II) hydrate were delivered
by Sigma-Aldrich. Acetonitrile was obtained from Nacalai tesque. Potas-
sium hexacyanorhodate(III) and scandium(III) nitrate was supplied by
Mitsuwa Chemicals Co, Ltd.
Powder X-ray Diffraction. Powder X-ray diffraction patterns
were recorded on a Rigaku MiniFlex 600. Incident X-ray radiation
was produced by a Cu X-ray tube operating at 40 kV and 15 mA
with Cu Kα radiation (λ = 1.54 Å). The scan rate was 1° min−1 from
2θ = 2−40°.
Physical Measurements. UV−vis absorption spectra of solutions
were recorded on a Hewlett-Packard 8453 diode array spectrometer.
UV−vis diffused reflectance spectra were measured by a Jasco V-670
spectrometer equipped with an SIN-768 attachment. IR spectra were
recorded on a Jasco FT/IR-6200 spectrometer for the samples pellet-
ized with KBr. The electron paramagnetic resonance (EPR) spectra
were taken on a JEOL X-band spectrometer (JES-REIXE) with a
quartz EPR tube (4.5 mm) at 150 K. The g values were calibrated
using a Mn2+ marker. Sub-nanosecond laser-induced transient absorp-
tion spectra were collected by a customized measuring system equip-
ped with a picosecond-pulse laser for the photoexcitation and a
photomultiplier detector developed by UNISOKU Co., Ltd.35 Nano-
second laser flash photolysis experiments were performed using an
Nd:YAG laser (Continuum, SLI-20, 4−6 ns fwhm) at λ = 266 nm
with the power of 0.5 mJ per pulse.36 The transient absorption
measurements were performed using a continuous wave xenon lamp
(150 W) and a photomultiplier (Hamamatsu 2949) as a probe light
and a detector, respectively. The output from a photomultiplier was re-
corded on a digitizing oscilloscope (Tektronix, TDS3032, 300 MHz).
XRF measurements were performed to determine the loading amount
of [Fe(H2O)3]2[Ru(CN)6] on sAl-MCM-41 by a Rigaku ZSX-100e,
which is a wavelength-dispersive spectrometer equipped with a 4 kW
Rh X-ray tube.
Syntheses of [Mn(H2O)3]2[Ru(CN)6], [Fe(H2O)3]2[Ru(CN)6],
[Co(H2O)3]2[Ru(CN)6], [Cu(H2O)n]2[Ru(CN)6].32 An aqueous solution
(10 mL) of manganese(II) perchlorate hexahydrate, iron(II) perchlo-
rate hydrate, cobalt(II) nitrate hexahydrate, or copper(II) sulfate penta-
hydrate (0.14 mmol) was slowly added to an aqueous solution (10 mL)
of potassium hexacyanoruthenate(II) hydrate (0.073 mmol) for 2 h with
stirring magnetically. The formed precipitate was collected by filtration,
washed with water, and dried in vacuo.
Synthesis of [Fe(H2O)2]1.5[Rh(CN)6].32 An aqueous solution
(10 mL) of iron(II) perchlorate hydrate (0.20 mmol) was slowly
added to an aqueous solution (10 mL) of potassium hexacyan-
orhodate(III) (0.13 mmol) for 2 h with stirring magnetically. The
obtained precipitate was collected by filtration, washed with water,
and dried in vacuo.
Synthesis of Spherical Mesoporous Silica−Alumina (sAl-
MCM-41).33 sAl-MCM-41 was synthesized by a reported method with
modifications.33 Cetyltrimethylammonium bromide (14.6 g) was dis-
solved to a mixed solution (814 mL) composed of water, aqueous
NH3, and ethanol [270:444:100 (v/v/v)]. Tetraethylorthosilicate
(27.4 g, 131 mmol) was added dropwise to the aqueous solution for
2 h and magnetically stirred for further 19 h at room temperature, and
the formed precipitate was collected by filtration. The obtained pre-
cipitate was washed with an ethanol solution (100 g) containing
ammonium nitrate (500 mg, 6.25 mmol) at 60 °C and then washed
with water and ethanol, successively. The precipitate was collected by
filtration and dried at 200 °C. The obtained spherical MCM-41 was
suspended to an aqueous solution (400 mL) of sodium aluminate
Hydroxylation of Benzene by H2O2. Typical reaction conditions
are as follows: [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 (1.0 mg, 17 μM
of [Fe(H2O)3]2[Ru(CN)6]) was suspended in MeCN (2.5 mL).
Benzene (0.40 mL, 1.4 M) and aqueous hydrogen peroxide (30 wt %;
0.40 mL, 1.2 M) was added to the suspension successively under
vigorous stirring at 298 K. A small portion of the reaction solution was
taken out and injected to a Shimadzu QP-5000 GC/MS to analyze
products.
Photocatalytic Production of H2O2. Typical reaction conditions
are as follows: [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 (1.0 mg, 17 μM
B
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water. When sAl-MCM-41 without loading Fe2+ was used
instead of the Fe2+ adsorbed sAlMCM-41, no incorporation of
[RuII(CN)6]4− was observed (Figure S3 in Supporting Informa-
tion). The formation and incorporation of [Fe(H2O)3]2[Ru(CN)6]
was confirmed by a diffuse reflectance UV−vis absorption spec-
trum, powder X-ray diffraction, and transmission electron micro-
scopy images with elemental mappings of Ru and Fe as shown
in Figures S4 and S5 in Supporting Information, where the
absorption band and diffraction patterns due to [Fe(H2O)3]2-
[Ru(CN)6] were observed. The amount of incorporated
[Fe(H2O)3]2[Ru(CN)6] was determined to be 5.7 × 10−5 mol
g−1, which corresponds to 2.1% of the cation exchange sites of
sAl-MCM-41, by XRF measurements. The BET surface area and
pore volume of [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 were
determined to be 526 m2 g−1 and 0.439 cm3 g−1, respectively,
which are lower than those of bare sAl-MCM-41 (758 m2 g−1 and
0.491 cm3 g−1; Figure S6 in Supporting Information). The lower
BET surface area mainly resulted from the decrease of the inner
surface area to 478 from 708 m2 g−1. No significant change in the
BJH pore size distribution for both samples suggested that
[Fe(H2O)3]2[Ru(CN)6] mainly located inside mesopores and
partly filled them.
of [Fe(H2O)3]2[Ru(CN)6]) was suspended to an O2-saturated mixed
solution (3.3 mL) composed of MeCN and water [2.9:0.40 (v/v)]
containing Sc(NO3)3 (0.10 M). The suspension was gently stirred by a
magnetic stirrer under photoirradiation with a xenon lamp (Ushio
Optical, Model X SX-UID 500X AMQ) at room temperature. The
amount of produced H2O2 at each reaction time was determined by the
titration of iodide ion. The photocatalytic production of H2O2 was also
performed in 16O2-saturated H218O containing [Fe(H2O)3]2[Ru(CN)6].
After removal of 16O2 and 18O2 in the gas phase by passing He gas,
the reaction solution was moved to a sealed vial containing MnO2,
which catalyzes H2O2 decomposition, via cannula under He atmosphere.
The evolved oxygen gas in the headspace was analyzed by a gas chro-
matograph [He carrier, TC-FFAP column (GL Science, 1010−15242)
at 313 K] equipped with a mass spectrometer (Shimadzu, QP-5000
GC/MS).
Photocatalytic Water Oxidation. Typical reaction conditions are
as follows: a hetero-polynuclear cyanide complex (0.54 mM) was
suspended to an Ar-saturated phosphate buffer (50 mM, pH 8.0,
2.0 mL) containing Na2S2O8 (10 mM) and [Ru(bpy)3]Cl2 (1.0 mM).
The solution was then irradiated with the xenon lamp through a color
filter glass (Asahi Techno Glass) transmitting λ > 390 nm at room
temperature. The concentration of oxygen gas in a headspace was
quantified by a Shimadzu GC-17A gas chromatograph [Ar carrier, a
capillary column with molecular sieves (Agilent Technologies,
19095PMS0, 30 m × 0.53 mm) at 313 K] equipped with a thermal
conductivity detector (TCD) at a certain reaction time.
First, catalytic hydroxylation of benzene (0.40 mL, 1.4 M) by
30 wt % aqueous H2O2 (0.40 mL, 1.2 M) to produce phenol
was examined by using [M(H2O)3]2[Ru(CN)6] without sAl-
MCM-41 (0.10 mg, 82 μM; M = Mn, Fe, Co, or Cu) as a
heterogeneous catalyst in MeCN (2.5 mL; Figure 2). Among
Thermal Water Oxidation. [RuIII(bpy)3](PF6)3 (0.50 mM) and
[Fe(H2O)3]2[Ru(CN)6] (1.0 mg) were dissolved to a phosphate
buffer (2.0 mL, 50 mM, pH 8.0) in a sealed vial under Ar atmosphere.
After a certain reaction time, the concentration of oxygen gas in the
headspace of the reaction vial was quantified by a gas chromatograph
with a TCD.
18O-Isotope Labeling Experiments. [Fe(H2O)3]2[Ru(CN)6]
(5.0 mg, 6.8 mM) was suspended to an He-saturated H218O (2.0 mL)
containing Na2S2O8 (10 mM) and [Ru(bpy)3]Cl2 (1.0 mM). The sus-
pension was then irradiated with a xenon lamp (λ > 390 nm) at
room temperature. The concentration of oxygen gas in a headspace was
quantified by a gas chromatograph equipped with a mass spectrometer
(Shimadzu, QP-5000 GC/MS).
Hydroxylation of Benzene with O2. Typical reaction conditions
are as follows: [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 (1.0 mg, 17 μM
of [Fe(H2O)3]2[Ru(CN)6]) was suspended to an O2-saturated mixed
solution (3.3 mL) composed of MeCN and benzene [2.9:0.40 (v/v)].
The suspension was magnetically stirred under photoirradiation at room
temperature. Product analyses were performed by a Shimadzu QP-5000
GC/MS.
Figure 2. Time profiles of production of phenol in catalytic
hydroxylation of benzene (0.40 mL, 1.4 M) with 30 wt % aqueous
H2O2 (0.40 mL, 1.2 M) in MeCN (2.5 mL) containing [M-
(H2O)3]2[Ru(CN)6] (M = Mn, Fe, Co, or Cu; 0.10 mg, 82 μM) at
323 K.
RESULTS AND DISCUSSION
■
Catalytic Hydroxylation of Benzene by H2O2 Using
[Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41. Nanosized spherical
mesoporous silica−alumina (sAl-MCM-41), which is highly
dispersible to a solvent and can efficiently prevent the growth of
[Fe(H2O)3]2[Ru(CN)6] particles in mesopores, was synthesized
by using a mixture of tetraethyl orthosilicate and sodium alumi-
nate as precursors under basic (aqueous NH3) conditions.33 The
addition of Al3+ to silica results in formation of cation-exchange
sites, which are important to enhance interaction between metal
ions and the surfaces. The Brunauer−Emmett−Teller (BET)
surface area of sAl-MCM-41 was determined to be 758 m2 g−1
based on the N2 adsorption−desorption isotherms [Figure S1a in
Supporting Information]. The powder X-ray diffraction pattern
of sAl-MCM-41 indicates that the pore diameter is 3.7 nm, which
is the same value determined by the Barrett−Joyner−Halenda
(BJH) pore size distribution (Figures S1b and S2 in Supporting
Information). [Fe(H2O)3]2[Ru(CN)6] was incorporated in sAl-
MCM-41 by the reaction of Fe2+ adsorbed on sAl-MCM-41 and
K4[RuII(CN)6] ([Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41) in
the catalysts, [Fe(H2O)3]2[Ru(CN)6] showed the highest
reactivity in terms of a yield and a rate of phenol production.
The apparent turnover number (TON) of phenol production
per the monomer unit of [Fe(H2O)3]2[Ru(CN)6] was 393 at
60 h. The overoxidation products such as p-benzoquinone were
not produced, and the product selectivity to phenol was >99%
(Figure 3). When the concentration of benzene was decreased
to 4.3 mM, the yield of phenol increased to 15% with >99%
selectivity. No further oxidation of phenol to p-benzoquinone
occurred at 323 K when the conversion of benzene was lower
than 15%. The high selectivity to phenol may result from the
interaction of formed phenol with Lewis acid sites, which
suppresses further oxidation.6a,37−39 The initial rate of for-
mation of phenol (<2 h) was proportional to concentrations
of benzene and [Fe(H2O)3]2[Ru(CN)6], whereas the rate
remains constant irrespective of different concentrations of
H2O2 (see Figure S7 in Supporting Information). The first-order
C
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or Ru2+@sAl-MCM-41) as a catalyst for benzene hydroxylation
under the same reaction conditions for the reaction system
employing [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41, suggesting
that formation of [Fe(H2O)3]2[Ru(CN)6] is important to exhibit
high catalytic activity.
Photocatalytic H2O2 Production from H2O and O2
Using [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41. It was also
found that [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 acted as an
effective photocatalyst for production of H2O2 from H2O and O2
by photoirradiation of an O2-satuated aqueous dispersion con-
taining [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 and Sc(NO3)3
as shown in Figure 5. The presence of Sc3+ has been reported to
●
Figure 3. Time profiles of production of phenol (red ) and
■
p-benzoquinone (black ) in catalytic hydroxylation of benzene
(0.40 mL, 1.5 M) with aqueous H2O2 (0.40 mL, 1.3 M) in MeCN
(2.5 mL) containing [Fe(H2O)3]2[Ru(CN)6] (0.10 mg, 82 μM) at 323 K.
dependence of the rate on the catalyst concentration suggests
that the monomer unit of [Fe(H2O)3]2[Ru(CN)6] acts as the
catalyst when no dissociation or association of [Fe(H2O)3]2-
[Ru(CN)6] may be involved in the catalytic hydroxylation of
benzene with H2O2. No dependence of the rate on concentration of
H2O2 suggests that the catalyst forms a complex with H2O2 such as
metal-peroxo or metal-oxo species during the catalytic reaction.
When [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 was employed
as a catalyst, the apparent TON of phenol production per the
monomer unit of incorporated [Fe(H2O)3]2[Ru(CN)6] reached
2500 at 114 h (Figure 4). The incorporation of [Fe(H2O)3]2[Ru-
Figure 5. Time course of production of H2O2 from H2O and O2 in an
O2-saturated MeCN (2.9 mL) containing [Fe(H2O)3]2[Ru(CN)6]@
sAl-MCM-41 (1.0 mg, 17 μM of [Fe(H2O)3]2[Ru(CN)6]), water
(0.40 mL), and Sc(NO3)3 (0.10 M) under photoirradiation with a
xenon lamp.
stabilize O2•−, which is converted to H2O2 and O2 by dis-
proportionation.25,26 The photocatalytic H2O2 formation was
also performed by using 16O2-saturated mixed solvent of MeCN
and H218O [2.9:0.4 (v/v)] under the same reaction conditions
other than the solvent. The formed H2O2, which was catalytically
decomposed to O2 by MnO2 for analyses, contained only 16O,
suggesting that 16O2 is an electron acceptor in this reaction
system (Figure S8 in Supporting Information). No H2O2
formation was observed when [Cu(H2O)n]2[Ru(CN)6]@sAl-
MCM-41 or K4[RuII(CN)6] was used instead of [Fe(H2O)3]2-
[Ru(CN)6]@sAl-MCM-41 in the reaction dispersion (Figure S9
in Supporting Information), indicating that the FeII moiety is
necessary to exhibit the catalysis for H2O2 production.
The role of FeII moiety seemed to be an active site for water
oxidation, because some cyano-bridged polynuclear metal com-
plexes have been reported to act as water oxidation catalysts,
in which N-bound metal ions are active sites.40 Catalysis of
[Fe(H2O)3]2[Ru(CN)6] was examined for the photocatalytic
water oxidation by S2O82− in the presence of [RuII(bpy)3]2+
(bpy = 2, 2′-bipyridine) as a photosensitizer. Steady O2 evolution
was observed for 30 min under visible light irradiation (λ >
390 nm) as shown in Figure 6. In a previous report, the inter-
mediate SO4•− formed by one-electron oxidation of S2O82− is a
strong oxidant, so it may oxidize water oxidation catalysts
(WOCs) directly.41 To exclude this possibility, [RuIII(bpy)3]3+
thermally produced by oxidation of [RuII(bpy)3]2+ with PbO2
was employed as an oxidant for water oxidation.42 O2 evolved
from a phosphate buffer (pH 8.0) contained both [RuIII(bpy)3]3+
and [Fe(H2O)3]2[Ru(CN)6], whereas no O2 evolved without
[Fe(H2O)3]2[Ru(CN)6] (Figure S10 in Supporting Information).
Figure 4. Time profiles of production of phenol in catalytic
hydroxylation of benzene (0.40 mL, 1.5 M) with 30 wt % aqueous
H2O2 (0.40 mL, 1.3 M) in MeCN (2.5 mL) containing [Fe(H2O)3]2-
[Ru(CN)6]@sAl-MCM-41 (1.0 mg, 17 μM of [Fe(H2O)3]2[Ru-
2+
2+
●
▲
(CN)6], red ), Fe @sAl-MCM-41 (1.0 mg, 34 μM of Fe , ), and
2+
2+
■
Ru @sAl-MCM-41 (2.3 mg, 17 μM of Ru , ) at 323 K.
(CN)6] in sAl-MCM-41 resulted in significant improvement of
the catalytic durability by increasing the number of true active
sites.
During the reaction, 100 mM of H2O2 was consumed for
producing 37 mM phenol, suggesting that the oxidation reac-
tion accompanied by the decomposition of H2O2 by dispro-
portionation as evidenced by tiny bubbles formation in the
reaction vial. Lower apparent TONs were achieved by employing
Fe2+ or Ru2+ incorporated in sAl-MCM-41 (Fe2+@sAl-MCM-41
D
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Article
water oxidation. The evolved oxygen in the headspace of a vial
after photoirradiation of a reaction suspension for a certain time
was analyzed by a gas chromatograph equipped with a molec-
ular sieve column and a mass spectrometer (Figure 7). The
evolved gas after photoirradiation for 30 min contained 18O18O
and 18O16O with the ratio of 60:40, while the amount of formed
16O2 was under the detection limit of the mass spectrometer.
The 16O atom of 18O16O originates from water molecules of
[Fe(H216O)3]2[Ru(CN)6] contained as the extra ligands.
Nanosecond laser-induced transient absorption spectra of
K4[RuII(CN)6] in water were measured to investigate a role of
[RuII(CN)6]4− moiety in the photocatalytic H2O2 formation
(Figure S11 in Supporting Information), where the absorption
band at 750 nm is assigned to the excited state of [RuII(CN)6]4−
moiety. Transient absorption spectra of [Fe(H2O)3]2[Ru(CN)6]
also exhibited the absorption band at 750 nm (Figure S12 in
Supporting Information). The lifetime of the excited state of
[RuII(CN)6]4− was determined to be 1.6 μs in deaerated MeCN.
The lifetime of the excited state of [RuII(CN)6]4− was shortened
by increasing concentration of O2 (Figure 8a). This suggests that
electron transfer from the excited state of [RuII(CN)6]4− to O2
occurs to produce [RuIII(CN)6]3− and O2•−. In the presence of
Sc3+ ions, O2•− is bound to a Sc3+ ion to afford the O2•−−Sc3+
complex, which was detected by EPR after photoirradiation
of an O2-saturated MeCN solution of [Fe(H2O)3]2[Ru(CN)6]
at 150 K as shown in Figure 9, where the signal due to
[RuIII(CN)6]3− was also observed (see reference spectra in
Figure S13 in Supporting Information).43 The rate constant of
electron transfer from the excited state of [RuII(CN)6]4− to O2
was determined from the linear plot of the decay rate constant
Figure 6. Time profiles of O2 evolution under photoirradiation
(λ > 390 nm) of an Ar-saturated phosphate buffer (2.0 mL, 50 mM,
pH 8.0) containing Na2S2O8 (10 mM), [RuII(bpy)3]2+ (1.0 mM) and
●
[Fe(H2O)3]2[Ru(CN)6] (0.54 mM, ) or [Fe(H2O)2]1.5[Rh(CN)6]
○
(0.54 mM, ).
18 16
Figure 7. Time profiles of 16O2 ( ), O O ( ), or 18O2
⧫
●
▲
( )
evolution under photoirradiation (λ > 390 nm) of a He-saturated
H218O (2.0 mL) containing Na2S2O8 (10 mM), [RuII(bpy)3]2+ (1.0 mM)
and [Fe(H2O)3]2[Ru(CN)6] (6.8 mM).
Thus, the [RuIII(bpy)3]3+ formed during the photocatalytic water
oxidation can oxidize the WOC to evolve O2.
Similarly, [Fe(H2O)2]1.5[Rh(CN)6], which possesses the FeII
moiety without the [Ru(CN)6] moiety, exhibits catalytic activity
for water oxidation when the reaction system employed
[Fe(H2O)2]1.5[Rh(CN)6], instead of [Fe(H2O)3]2[Ru(CN)6]
as the water oxidation catalyst. These results manifested that FeII
moiety acted as a catalytic active site for water oxidation in the
cyano-bridged complexes.
Figure 9. EPR spectra observed under dark (black broken line) and
photoirradiation (red solid line) with an Hg lamp of an O2-saturated
MeCN solution (2.0 mL) containing [Fe(H2O)3]2[Ru(CN)6] (0.20 mg,
0.27 mM) and Sc(NO3)3 (0.10 M) at 150 K.
18O-isotope labeling experiments using H218O instead of
H216O were conducted to obtain the direct evidence for the
Figure 8. (a) Decay time profiles of absorbance at 750 nm observed in N2-saturated MeCN containing K4[RuII(CN)6] (1.8 mM) and various
concentrations of O2 (0−0.24 mM). (b) Plot of the decay rate constant vs concentration of O2.
E
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Article
versus concentration of O2 (Figure 8b) to be 1.8 × 1010 M−1 s−1,
which is diffusion-limited.
The mechanism of the photocatalytic production of H2O2
from H2O and O2 with [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41
in MeCN is shown in Scheme 1. Photoexcitation of
Scheme 1. Overall Photocatalytic Cycle for H2O2 Production
Figure 10. Time course of formation of phenol under photoirradiation
of an O2-saturated MeCN solution (2.9 mL) containing [Fe(H2O)3]2-
[Ru(CN)6]@sAl-MCM-41 (1.0 mg, 17 μM of [Fe(H2O)3]2[Ru(CN)6])
and benzene (1.5 M, 0.40 mL) with a xenon lamp.
[Fe(H2O)3]2[Ru(CN)6] resulted in electron transfer from the
excited state of [Fe(H2O)3]2[Ru(CN)6] to O2 in the presence
of Sc3+ to produce [Fe(H2O)3]2[RuIII(CN)6]+ and O2•−−Sc3+.
The reduction potential of [RuIII(CN)6]3− moiety in the
presence of iron ions has been reported as E1/2 = 1.24 V versus
standard hydrogen electrode (SHE),44 which can oxidize the
FeII moiety to high-valent iron species suitable for water oxi-
dation, because the standard electrode potential for water oxi-
dation to evolve oxygen at the pH of the reaction solution (2.8)
is 1.06 V versus SHE. The O2•−−Sc3+ complex disproportion-
ates in the presence of H+ to produce H2O2.25,26
Photocatalytic Benzene Hydroxylation by O2 Using
[Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41. The catalytic hydroxyla-
tion of benzene with H2O2 to phenol (eq 1) was combined with
the photocatalysis of [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 for
production of H2O2 from H2O and O2 (eq 2). The overall
catalytic reaction is given by eq 3, which shows the photocatalytic
phenol with H2O2 and the photocatalytic production of H2O2
from H2O and O2 in this study provides a new strategy to
achieve one-pot hydroxylation of benzene with O2 as an ideal
oxygen source as well as an oxidant.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.5b02909.
■
S
N2-adsorption and desorption isotherms and BJH pore
size distribution of sAl-MCM-41 and [Fe(H2O)3]2[Ru-
(CN)6]@sAl-MCM-41, powder X-ray diffraction patterns,
UV−vis spectra, TEM images with elemental mappings,
time profiles of phenol production, distribution of O2 in
18O-labeling experiment, time profiles of H2O2 production,
time profiles of O2 evolution, transient absorption spectra,
and EPR spectra (PDF)
C6H6 + H2O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ C6H5OH + H2O
[Fe(H2O)3]2[Ru(CN)6]@sAl‐MCM‐41
(1)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: ymd@a-chem.eng.osaka-cu.ac.jp. (Y.Y.)
*E-mail: fukuzumi@chem.eng.osaka-u.ac.jp. (S.F.)
■
hν
H2O + 1/2O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H2O2
[Fe(H2O)3]2[Ru(CN)6]@sAl‐MCM‐41
(2)
hν
C6H6 + 1/2O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ C6H5OH
[Fe(H2O)3]2[Ru(CN)6]@sAl‐MCM‐41
(3)
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the manu-
script.
hydroxylation of benzene to phenol using O2 as an oxygen source
as well as an oxidant. The time profile of production of phenol by
photoirradiation of an O2-saturated MeCN solution of benzene
(0.40 mL, 1.5 M) containing [Fe(H2O)3]2[Ru(CN)6]@sAl-
MCM-41 (1.0 mg, 17 μM of [Fe(H2O)3]2[Ru(CN)6]) at 298 K
is shown in Figure 10. The TON reached 41 after photo-
irradiation for 59 h. Benzene hydroxylation to phenol was
catalyzed by [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 with H2O2
even at a low concentration (3.0 mM) (Figure S14 in Supporting
Information).
Funding
This work was supported by ALCA and SENTAN projects
from JST (to S.F.) and JSPS KAKENHI (No. 16H02268 to
S.F., Nos. 24350069 and 15K14223 to Y.Y.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
CONCLUSIONS
We sincerely acknowledge Dr. T. Nakagawa and Mr. K.
Okamoto, UNISOKU Co. Ltd, for the measurements of the
laser-induced transient absorption spectra. We are thankful to
Mr. Masatsugu Ishimoto, Osaka City University, for EDS-
STEM measurements.
■
Hydroxylation of benzene to phenol has been achieved by using
O2 as an oxygen source as well as an oxidant by combination of
the catalytic hydroxylation of benzene to phenol with H2O2 and
the photocatalytic production of H2O2 from H2O and O2
with use of [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41, which acts
as a catalyst for both benzene hydroxylation and water oxi-
dation as well as a photocatalyst for O2 reduction to H2O2.
The combination of the catalytic hydroxylation of benzene to
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