Direct synthesis of hydrogen peroxide from hydrogen and oxygen by using a water-soluble iridium complex and flavin mononucleotide

Article abstract of DOI:10.1002/anie.201307273

H₂, O₂ to H₂O₂: The direct synthesis of hydrogen peroxide from hydrogen and oxygen in water has been made possible by using an iridium(III) complex, [Ir^III(Cp)(4-(1H-pyrazol- 1-yl-κN²)benzoic acid-κC³)(H ₂O)]₂SO₄, and flavin mononucleotide. This method gives hydrogen peroxide with a high turnover number (847) and yield (19.2 %) under normal pressure and at room temperature. Copyright

Full text of DOI:10.1002/anie.201307273

Angewandte
Chemie
DOI: 10.1002/anie.201307273
Homogeneous Catalysis
Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen by
Using a Water-Soluble Iridium Complex and Flavin Mononucleotide**
Satoshi Shibata, Tomoyoshi Suenobu, and Shunichi Fukuzumi*
+
Hydrogen peroxide (H O ) is known to be a highly selective
complex 1-H reacts with H in an aqueous phosphate buffer
2
2
2
and environmentally friendly oxidant and is used in the
chemical industry for the manufacture of numerous organic
solution (pH 6.0) to produce the iridium(III) hydride complex
[
14]
2 (lmax = 336 nm; Equation (2) and Figure 1). The reaction
[
1,2]
+
and inorganic compounds.
process for H O production by sequential hydrogenation and
However, the current industrial
of 1-H with H proceeds rapidly to completion to form 2
2
within 3 s (Figure S1 in the Supporting Information). For the
2
2
oxidation of an alkyl anthraquinone is not environmentally
benign as a result of a number of disadvantages, such as the
requirement for toxic solvents, high energy consumption, and
[3,4]
multiple steps.
To provide an alternative, extensive efforts
have been devoted to achieving the direct synthesis of H O2
2
from hydrogen (H ) and oxygen (O ) by using heterogeneous
2
2
[
3–12]
precious-metal catalysts (mainly Pd, Au, or Au–Pd).
This
direct process suffers from serious problems such as unfav-
orable high-pressure conditions and relatively low yields
owing to undesired side reactions such as formation of H O
2
1
(
=
H + = O !H O), decomposition of H O (H O !H O +
2
2
2
2
2
2
2
2
2
1
[3–12]
O ), and hydrogenation of H O (H O + H !2H O).
2
2
2
2
2
2
2
2
For this reason, selectivity in the direct synthesis of H O from
2
2
H and O has been limited. In addition, it has been quite
2
2
Figure 1. Changes in the UV/Vis absorption spectrum during the
difficult to elucidate the heterogeneous catalytic mechanism
as compared with the homogeneous catalytic mechanism, in
which intermediates can be detected. However, there has to
date been no report of a homogeneous catalytic system for the
reaction of 1 (25 mm) with H (0.82 mm) in an aqueous phosphate
2
buffer (pH 6.0) at 298 K. An argon-saturated buffer solution of 1 (black
ꢀ1
line) was bubbled with H (1.0ꢀ10 MPa) for 5 min thus resulting in
2
the formation of 2 (red line).
[13]
direct synthesis of H O from H and O .
2
2
2
2
We report herein the direct synthesis of H O from H and
2
2
2
+
O in water by using a water-soluble iridium aqua complex
reaction of 1-H with H , the formation of 2 in the presence of
2
2
III
2
3
[
(
Ir (Cp*)(4-(1H-pyrazol-1-yl-kN )benzoic
acid-kC )-
O2 was confirmed by changes in the UV/Vis absorption
spectrum, which indicates the slower reaction of 2 with O2
(Figure S2 in the Supporting Information).
H O)] SO ([1] SO ), which can react with H to produce
2
2
4
2
4
2
[
14,15]
an iridium hydride complex (2),
and flavin mononucle-
otide (FMN) under normal pressure and at room temper-
ature. The synthesis and characterization of 1 were carried out
as reported and are briefly described in the Experimental
[
14]
Section.
At pH 6.0, the carboxylic acid group in 1 is deprotonated
+
[14]
III
to give the carboxylate form 1-H [Eq. (1)]. The Ir -OH2
[*] S. Shibata, Dr. T. Suenobu, Prof. Dr. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering, Osaka University
2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan)
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Homepage: http://www-etchem.mls.eng.osaka-u.ac.jp/
Prof. Dr. S. Fukuzumi
Department of Bioinspired Science, Ewha Womans University
Seoul 120-750 (Korea)
[
**] This work was supported by Grants-in-Aid (Nos. 20108010 and
4550077) from the Ministry of Education, Culture, Sports, Science
and Technology (Japan) and NRF/MEST of Korea through the WCU
R31-2008-000-10010-0) and GRL (2010-00353) Programs.
Complex 2 can efficiently reduce FMN (lmax = 373 and
2
4
45 nm) to the 1,5-dihydroflavin (FMNH : l = 291 nm and
2 max
(
390 nm) in aqueous phosphate buffer solution (pH 6.0) under
[
16]
N [Equation (3) and Figure 2]. FMNH was also generated
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307273.
2
2
by catalytic reduction of FMN with H in the presence of
2
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Angewandte
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(Figure 3). The rise of the absorption band of FMN at
lmax = 445 nm obeyed first-order kinetics (Figure S5 in the
Supporting Information). From the slope of the first-order
plot (inset of Figure S5 in the Supporting Information),
Figure 3. Changes in the UV/Vis absorption spectrum during the
oxidation of FMNH (50 mm) by O (0.68 mm) under an O
2
2
2
ꢀ2
ꢀ2
(
5.0ꢀ10 MPa)/N (5.0ꢀ10 MPa) gas mixture in an aqueous phos-
2
phate buffer (pH 6.0) after mixing an O -saturated buffer solution with
2
a buffer solution of FMNH under N . FMNH was formed in the
2
2
2
reduction of FMN (100 mm) by H (bubbling for 10 min) with
2
Figure 2. Changes in the UV/Vis absorption spectrum during the
1
(5.0 mm) under normal pressure at 298 K.
reduction of FMN (25 mm) by 2 (0.55 mm) under an H
2
ꢀ2
ꢀ2
(
5.0ꢀ10 MPa)/N (5.0ꢀ10 MPa) gas mixture after mixing a buffer
2
solution of FMN (50 mm) under N with that of 2. Complex 2 was
2
a pseudo-first-order rate constant (kobs) was obtained, and the
second-order rate constant (k ) for the reaction of FMNH
2
formed by the reduction of 1 (1.1 mm) by H (bubbling for 10 min)
2
under normal pressure in an aqueous phosphate buffer (pH 6.0) at
C
4
ꢀ1 ꢀ1
2
98 K. The black and blue solid lines correspond to FMN and FMNH2,
with O was determined to be 2.0 ꢀ 10 m s .
2
respectively.
FMNH2 can also be independently generated by the
[16a]
reduction of FMN with sodium hydrosulfite (Na S O ).
As
2
2
4
the reaction of FMNH with O proceeds, the formation of
2
2
1
(Figure S3 in the Supporting Information). A similar
FMN leads to an increase in the absorbance at lmax = 445 nm,
and this increase obeyed first-order kinetics (Figure S6a in
the Supporting Information). The pseudo-first-order rate
constant (kobs) proportionally increased with concentration of
spectral change has been reported in the reaction of FMN
with sodium dithionite in water. The wavelengths (lmax) and
extinction coefficients at lmax for the absorption spectrum of
[
15]
3
ꢀ1
ꢀ1
FMNH obtained in this work (e = 3.6 ꢀ 10 m cm at l
=
O (Figure S6b in the Supporting Information). From the
2
max
2
[
14]
3
90 nm) are consistent with those in the literature, thus
slope of the linear plot, the second-order rate constant for the
indicating the stoichiometric formation of FMNH . The decay
reaction of FMNH with O2 was determined to be 2.8 ꢀ
2
2
4
ꢀ1 ꢀ1
of the absorption band of FMN at lmax = 445 nm obeyed
second-order kinetics (Figure S4a in the Supporting Infor-
mation). Irrespective of the ratio of the initial amounts of 2 to
10 m s . This value agrees well with k determined for the
C
reaction in the presence of 1 and H . These values are more or
2
4
ꢀ1 ꢀ1
less consistent with the value (5.8 ꢀ 10 m s ) reported for
FMN, the observed second-order rate constants (k ) agree
the reaction of a reduced form of a flavoprotein oxidase with
O at pH 7.0.
2
B
5
5
ꢀ1 ꢀ1
ꢀ1 ꢀ1
[18]
well with each other (1.5 ꢀ 10 m
s
and 1.2 ꢀ 10 m s ,
Figure S4a and b in the Supporting Information, respec-
tively).
The amount of H O2 produced was determined by
2
titration with the oxo[5,10,15,20-tetra(4-pyridyl)porphyrin-
[
19]
FMNH can readily be oxidized by O to regenerate FMN,
ato]titanium(IV) complex. In the reaction consisting of the
stepwise reduction of FMN by H to form FMNH when using
2
2
[17]
accompanied by the formation of H O [Eq. (4)]. Upon
2
2
2
2
mixing an O -saturated phosphate buffer solution with
1 followed by the oxidation of FMNH by O to generate
2
2 2
H O , a stoichiometric amount of H O was produced when
2
2
2
2
the concentration of FMN was varied, thus demonstrating
a linear relationship between the concentrations of H O and
2
2
FMN with a slope of 1.0 (Figure 4).
Thus, the overall catalytic cycle for the selective direct
synthesis of H O from H and O by using 1 and FMN is
2
2
2
2
expected to proceed according to Scheme 1: the iridium(III)
+
III
a buffer solution of FMNH that was formed by the reduction
complex 1-H reacts with H to produce the Ir -H complex 2,
2
2
of FMN by H2 in the presence of 1, the characteristic
which reduces FMN to FMNH , followed by oxidation by O
2
2
absorption bands of FMN were completely regenerated
to produce H O , accompanied by the regeneration of FMN.
2 2
2
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Chemie
When Sc(NO ) (100 mm) was added to this system, the
3
3
amount of H O dramatically increased (Figure 5, red
&
). The
TON with respect to 1 and FMN reached 201 at 4 h. When
2
2
1
1
(1.0 mm) and FMN (50 mm) were used, the TON based on
reached 847 at 4 h. The product yield of H O based on the
2
2
total amount of H and O supplied in the catalytic reaction
2
2
system was 19.2% at 10 min (Figure S7 in the Supporting
Information). This value is more than three times larger than
that obtained in the nanocolloidal Pd–Au system under
[
11d]
normal pressure of a H /O gas mixture (6.1%).
The rate
2
2
3
+
of catalytic formation of H O2 is accelerated with [Sc ]
Figure 5) and reached a turnover frequency (TOF) of 50 h ,
2
ꢀ1
(
Figure 4. Plot of the concentration of H O produced by the catalytic
however, the rate remained unchanged on increasing the
concentration of Sc over 50 mm. The limited TOF might be
2
2
ꢀ1
3+
reduction of O (1.0ꢀ10 MPa) by FMNH (generated through the
2
2
reduction of FMN by 2 under H ) versus the concentration of FMN
2
caused by the loss of H O through the catalytic reduction of
2
2
(
25 mm–1.0 mm) loaded at the beginning of the reaction at 298 K. 2
H O by H with 1 to produce H O (Scheme 2). This was
2
2
2
2
ꢀ1
was produced by the reduction of 1 (25 mm) by H (1.0ꢀ10 MPa) in
2
independently confirmed by the reduction of H O by H
2
2
2
an aqueous phosphate buffer solution (pH 6.0).
when using 1 in water in the absence and presence of
Sc(NO ) (Figure S8 in the Supporting Information).
3
3
Scheme 1.
Scheme 2.
When the catalytic reduction of FMN (5.0 mm) followed
The reduction of H O to H O by H was catalyzed by
2 2 2 2
by the oxidation of FMNH by O was made possible by the
1 (Figure S8 in the Supporting Information, black *). How-
2
2
presence of 1 (5.0 mm) in an aqueous phosphate buffer
ever, this reaction was effectively retarded by the presence of
3
+
solution (pH 6.0), H O was catalytically produced from H
Sc (Figure S8 in the Supporting Information, blue ^ and red
~), thus indicating that the further hydrogenation of H O as
2
2
2
and O . The formation of H O stopped within a few minutes
2
2
2
2
2
at pH 6.0 and 100 min at pH 2.8, when the turnover number
shown in Scheme 2 could be inhibited by the presence of
strong acid; a result consistent with the fact that H O is
(
TON) of H O2 production with respect to 1 and FMN
2
2
2
[20]
reached 28 at pH 6.0 and 41 at pH 2.8 (Figure 5, black *). This
limited TON stands in sharp contrast to the stepwise catalytic
reduction of FMN and the oxidation of FMNH by O (see
known to be stabilized under acidic conditions. In the same
manner as in heterogeneous catalytic systems, the decom-
position of H O directly synthesized from H and O2 in
2
2
2
2
2
above).
reactions catalyzed by carbon- or TiO - supported Au, Pd, or
2
an Au–Pd alloy was strongly retarded by the pretreatment of
[
5a,c]
the catalyst with acid.
The rate-determining step of the catalytic scheme shown
in Scheme 1 was investigated by examining the dependence of
the overall rate of catalytic H O production on the concen-
2
2
tration of 1. The rate of formation of H O from H and O
2
2
2
2
when using 1 in the presence of Sc(NO ) increased with
3
3
increasing concentrations of 1 (Figure S9 in the Supporting
Information), thus indicating that the rate-determining step is
[
21]
the reduction of FMN to FMNH by 2 to regenerate 1. The
2
characteristic UV/Vis absorption bands of FMN in the
presence of 1 under N2 (Figure S10a in the Supporting
Information) remained unchanged under both H and O
throughout the catalytic reaction (Figure S10b), thus indicat-
ing that FMN reduction by 2 is the rate-determining step of
the overall catalytic reaction at pH 6.0. Under these con-
Figure 5. Time course of H O production by the reaction of H
2
2
2
2
2
ꢀ2
ꢀ2
(
5.0ꢀ10 MPa) with O (5.0ꢀ10 MPa) catalyzed by 1 (5.0 mm) and
2
FMN (5.0 mm) in water (pH 2.8) at 298 K in the absence (black *) or
presence of Sc(NO ) (25, 50, and 100 mm; green ~, blue ^, and red
3
3
&
, respectively). Data shown by * were obtained in the absence of
FMN but with 1 (5.0 mm) and Sc(NO ) (100 mm) under otherwise the
ditions, selective two-electron reduction of O to H O occurs
3
3
2
2
2
same experimental conditions.
without the further reduction of H O to H O.
2
2
2
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.
Angewandte
Communications
In conclusion, the water-soluble iridium(III) complex
can efficiently catalyze the direct synthesis of H O from H
2
and O2 when using a water-soluble flavin (FMN) under
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1
2
2
normal pressure in aqueous solution at 298 K. The catalytic
III
cycle consists of the reduction of 1 by H to form the Ir -H
2
complex 2, followed by the reduction by 2 of FMN to FMNH2,
which then reacts with O to produce H O , accompanied by
2
2
2
the regeneration of FMN and 1. The addition of Sc(NO ) led
3
3
to a high TON (847) and a reasonably high yield of H O2
2
(
19.2%). This is because Sc(NO ) can inhibit the catalytic
3 3
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2
2
2
Experimental Section
[
All experiments were performed under an Ar or N atmosphere by
2
using standard Schlenk techniques unless otherwise noted. The
limiting concentration of O2 in aqueous solution was prepared by
using a mixed gas flow of O and N under normal pressure (1.0 ꢀ
6221 – 6224.
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2
ꢀ1
1
0
MPa). The mixed gas was controlled by using a gas mixer (Kofloc
GB-3C, KOJIMA Instrument Inc.), which can mix two or more gases
at a defined partial pressure and mass flow rate. A defined
concentration of H2 in an aqueous solution was prepared by
a mixed gas flow of H2 and N (or O ) controlled by using a gas
[
2
2
1048.
flow meter (KOJIMA Instrument Inc.) appropriate for each gas
ꢀ1
[10] T. Garcꢁa, R. Murillo, S. Agouram, A. Dejoz, M. Lꢂzaro, L.
under normal pressure (1.0 ꢀ 10 MPa). UV/Vis absorption spectra
were recorded on a Hewlett–Packard 8453 diode array spectropho-
tometer with a quartz cuvette (light-path length = 1 cm) at 298 K.
Kinetic measurements for reactions with short half-lives (under 10 s)
were performed on a UNISOKU RSP-601 stopped-flow spectrometer
equipped with a MOS-type highly sensitive photodiode array at
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2
98 K. Because the O /H2 ratio is within the explosive range, dry
2
catalyst should not be added to the O /H gas mixture. Moreover,
2
2
precautions should be taken to contain the system in the event of an
explosion.
Details concerning chemicals, determination of the reaction yield,
detection of H O , and pH adjustment are described in the Support-
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Received: August 19, 2013
[13] In homogeneous catalytic systems for the reaction of H
with O ,
2 2
Revised: September 9, 2013
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O
2 2
Published online: && &&, &&&&
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Keywords: homogeneous catalysis · hydrogen ·
hydrogen peroxide · iridium · oxygen
2 2
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Angewandte
Chemie
[
20] Hydrogenation was reported to be a major pathway for H O
value is significantly smaller than that for the next catalytic
step (rC,max), the reduction of O2 by FMNH , which was
2
2
decomposition on Pd or Pd – Au alloy supported on SBA-15, see:
N. Gemo, P. Biasi, P. Canu, F. Menegazzo, F. Pinna, A.
Samikannu, K. Kordꢂs, T. O. Salmi, J.-P. Mikkola, Top. Catal.
2
determined
to
be
rC,max = k [FMNH ] [O ] = 2.8 ꢀ
C
2
max
2
4
ꢀ1 ꢀ1
ꢀ6
ꢀ3
ꢀ4
ꢀ1
10 (m
s
) ꢀ 5.0 ꢀ 10 (m) ꢀ 1.4 ꢀ 10 (m) = 2.0 ꢀ 10 m s
.
The
2013, 56, 540 – 549.
reaction rate of the first catalytic step (r ), the reaction of
A
[
21] The maximum rate of this reaction step (rB,max) in Scheme 1
under catalytic reaction conditions in the absence of Sc(NO3)3
was determined to be rB,max = k [FMN] [2]max = 1.5 ꢀ
1 with H , can be evaluated based on the data in Figure S1 in the
2
Supporting Information, and was determined to be r > 6.0 ꢀ
A
ꢀ
6
ꢀ1
10 m s > rB,max
.
B
max
ꢀ6
5
ꢀ1 ꢀ1
ꢀ6
ꢀ6
ꢀ1
10 (m s ) ꢀ 5.0 ꢀ 10 (m) ꢀ 5.0 ꢀ 10 (m) = 3.8 ꢀ 10 m s . This
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Homogeneous Catalysis
S. Shibata, T. Suenobu,
S. Fukuzumi*
&&&& — &&&&
Direct Synthesis of Hydrogen Peroxide
from Hydrogen and Oxygen by Using
a Water-Soluble Iridium Complex and
Flavin Mononucleotide
3
H , O to H O : The direct synthesis of
acid-kC )(H O)] SO , and flavin mono-
2 2 4
2
2
2
2
hydrogen peroxide from hydrogen and
oxygen in water has been made possible
nucleotide. This method gives hydrogen
peroxide with a high turnover number
(847) and yield (19.2%) under normal
pressure and at room temperature.
III
by using an iridium(III) complex, [Ir -
2
(
Cp*)(4-(1H-pyrazol-1-yl-kN )benzoic
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!