Catalytic oxidation of formic acid by dioxygen with an organoiridium complex

Article abstract of DOI:10.1039/c4cy00957f

Catalytic oxidation of formic acid by dioxygen occurred efficiently using an organoiridium complex ([Ir^III(Cp^?)(4-(1H-pyrazol-1-yl-κN²)benzoic acid-κC³)(H₂O)]₂SO₄, 1) as a catalyst in a water-containing organic solvent as well as in water at ambient temperature. The catalytic cycle is composed of the reduction of 1 by formate to produce the hydride complex, which reduces dioxygen to water to regenerate 1. This journal is

Full text of DOI:10.1039/c4cy00957f

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Catalytic oxidation of formic acid by dioxygen
with an organoiridium complex†
Cite this: Catal. Sci. Technol., 2014,
, 3636
4
Tomoyoshi Suenobu, Satoshi Shibata and Shunichi Fukuzumi*
Received 24th July 2014,
Accepted 11th August 2014
DOI: 10.1039/c4cy00957f
www.rsc.org/catalysis
1
3,14
Catalytic oxidation of formic acid by dioxygen occurred efficiently
formic acid fuel cells.
The theoretical output potential is
(1.23 V) and methanol
Hence, the overall reaction for the
III
*
2
using an organoiridium complex ([Ir (Cp )(4-(1H-pyrazol-1-yl-κN )
1.45 V, which is higher than those of H
2
3
13,14
benzoic acid-κC )(H
2
O)]
2
SO
4
, 1) as a catalyst in a water-containing
(1.21 V) fuel cells.
organic solvent as well as in water at ambient temperature. The
catalytic cycle is composed of the reduction of 1 by formate to
produce the hydride complex, which reduces dioxygen to water to
regenerate 1.
cathodic oxidation of formic acid and the anodic reduction
of oxygen is expressed in eqn (2), which is largely exergonic
(Δ H = −255 kJ mol ).
c
0
−1
2
HCOOH + O
2
→ 2H
2
O + 2CO
2
(2)
1
Formic acid (HCOOH) is liquid at room temperature with a
−
3
relatively high volumetric density (d = 1.22 g cm ) and is
widely utilised as a preservative and an antibacterial additive
for livestock feed. HCOOH can be formed by reduction of CO
2
and the catalytic interconversion between HCOOH
and H (eqn (1)) has been reported to be ideal for carbon-
Formic acid is the most aggressive contributor of
atmospheric corrosion for indoor environments, being
also contained as a hazardous compound in wastewaters.
The best way of removing formic acid is through oxidation
into H O and CO (eqn (2)). Heterogeneous catalysts
2 2 2
have been reported to act as catalysts for oxidation of
HCOOH by O2. From an economical point of view, there
is still a need to improve the catalytic activity of oxidation
15
2
2
16,17
with H
2
by O
3
–6
neutral storage and transportation of H
2
.
1
7
H + CO ⇄ HCOOH
(1)
2
2
of HCOOH by O at temperatures and pressures as low as
possible. There has been no report so far on the use of
a homogeneous catalyst for efficient oxidation of HCOOH
2
7
In a natural enzymatic system, formate oxidase and
17
8
formate:oxygen oxidoreductase reduce dioxygen (O ) to reac-
tive oxygen species, e.g., superoxide and hydrogen peroxide
that would be further reduced to water. Formate is often used
as an electron donor for reductive activation of O to conduct
enzymatic oxygenation and for reduction of NAD and FAD
respectively. Subsequently,
NADH or FADH is supplied as an electron donor to either
2
by O at ambient pressure and temperature or its catalytic
2
mechanism.
2
We report herein the catalytic oxidation of HCOOH by O
2
9
+
in water and a water-containing protic solvent, ethylene glycol,
in the presence of a water-soluble iridium aqua complex
1
0
to regenerate NADH and FADH
2
,
III
*
2
3
2
(
(
[Ir (Cp )(4-(1H-pyrazol-1-yl-κN )benzoic acid-κC )
H O)] SO , [1] ·SO ; see the ESI†) acting as an efficient catalyst
reductase or oxidase, enabling regioselective oxidation such
2
2
4
2
4
1
1
as epoxidation.
for the removal of HCOOH by O . A mixture solvent of water
2
In addition to its importance as a renewable hydrogen
source for both enzymatic and non-enzymatic useful synthetic
reactions, formic acid is also utilised as a fuel for direct
and ethylene glycol was examined because ethylene glycol has
been used to improve the energy density of electric capacitors,
1
2
18
in which a trace of HCOOH has to be removed. 1 reacts
with HCOOH to produce the corresponding Ir–hydride
4
,19
Department of Material and Life Science, Graduate School of Engineering, Osaka
University and ALCA, Japan Science and Technology Agency (JST), 2-1 Yamada-oka,
Suita, Osaka 565-0871, Japan. E-mail: fukuzumi@chem.eng.osaka-u.ac.jp;
Fax: +81 6 6879 7370; Tel: +81 6 6879 7368
complex (3),
which can reduce O to H O.
2 2
Synthesis and characterization of 1 were carried out
according to the previous reports and are briefly described
5
,19
in the Experimental section in the ESI.†
The carboxylate
†
Electronic supplementary information (ESI) available: Experimental and
+
kinetic details. See DOI: 10.1039/c4cy00957f
form 1-H is protonated to give the carboxylic acid group
3636 | Catal. Sci. Technol., 2014, 4, 3636–3639
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in 1, as shown in eqn (3), at pH 2.8 since the pK of 1 was
a
5
,19
determined to be 4.0.
(
3)
Under an N atmosphere at pH 2.8 in the presence of 1,
2
formic acid decomposed efficiently to produce CO
2
(Fig. 1a)
Scheme 1 Catalytic cycle for decomposition of formic acid to form
H and CO by using 1 under an N atmosphere.
2 2 2
4
and H
2
(Fig. 1b) according to eqn (4). When the reaction
4
was conducted under an O atmosphere,
2
2 2
HCOOH → H + CO
(4)
the oxo[5,10,15,20-tetra(4-pyridyl)porphyrinato]titanium(IV)
2
0
the stoichiometric CO
2
was evolved with a TON of 170 at
complex in water, indicating that the four-electron reduc-
tion of O occurred to produce H O, as expressed by eqn (2).
9
h, exhibiting the same time course shown in Fig. 1a,
2
2
indicating that formic acid was decomposed according to
The conversion of formic acid was determined to be higher
than 99%.
−
eqn (4). Since the pK
dioxide (CO
the pH of the reaction solution (2.8), carbon dioxide may
evolve as a form of gas when dissolved CO gas is saturated
in the solution. However, the amount of H was largely
suppressed under an O2 atmosphere, as shown in Fig. 1b.
No H was detected by spectral titration with the use of
a
of bicarbonate (HCO
3
) to form carbon
2
) is 6.35, which is significantly higher than
In the catalytic reaction at pH 2.8 under an N atmosphere,
2
4
−
1 reacted with HCOO to afford the formate complex 2, which
2
is converted to the hydride complex 3 via β-hydrogen elimina-
+
2
tion from 2. Then, 3 reacts with H O to produce H , accom-
3
2
5
panied by the regeneration of 1, as shown in Scheme 1.
2
O
2
The formation of the hydride complex (3) was confirmed by
1
comparison with the H NMR spectrum of the isolated
hydride complex in DMSO-d
6
obtained by the reaction of
1
−
with H , which showed a typical hydride peak at δ =
14.74 ppm. Because the iridium hydride complex (3) is a
2
5
neutrally charged complex, the solubility of 3 in water is too
1
low to be detected by H NMR in D O.
2
On the other hand, the hydride complex (3) reacts with
O
2
to produce H
2
O and reproduce 1. The overall catalytic
−
cycle for the four-electron reduction of O by HCOO with
2
1
2
in competition with H evolution is shown in Scheme 2.
The rate-determining step of this catalytic oxidation cycle
was independently examined by the deuterium kinetic
isotope effect (KIE) on the catalytic oxidation of formic acid-d
(
DCOOH) vs. HCOOH. By comparing the time course of
oxidation of HCOOH by O2 with that of DCOOH in Fig. 2
also see Fig. S1 in the ESI†), the KIE was determined to be
.1 ± 0.2 at pH 2.8 at 298 K. This value is nearly equal to the
(
4
Fig. 1 (a) Time courses of CO
(
2
evolution from an aqueous formic acid
2.0 mM) solution (1.0 mL) in the presence of 1 (10 μM) under N and
atmosphere (green circle and red square, respectively) at pH 2.8 at
evolution from a formic acid (2.0 mM)
solution in the presence of 1 (10 μM) under N and O atmosphere
green circle and red square, respectively) at pH 2.8 at 298 K. The red
and green lines correspond to the reactions [(eqn (2) + eqn (4)) and
2
O
2
2
98 K. (b) Time courses of H
2
2
2
(
eqn (4), respectively]. The amounts of H
2
and CO
2
were analysed by
Scheme 2 Catalytic cycle for the formation of H
2 2 2
, CO and H O from
GC (see the ESI†).
formic acid in the presence of 1 under an O atmosphere.
2
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FMN in competition with the four-electron reduction of O
2
1
9c
and the reduced FMN reacts with O to produce H O .
2
2 2
In conclusion, a water-soluble iridium(III) complex (1) can
efficiently catalyse the oxidation of HCOOH by O to mainly
2
generate water with evolution of a little amount of H2
under acidic conditions at 298 K. This reaction occurred in
both water and water-containing ethylene glycol. The rate-
determining step of the catalytic cycle is the β-hydrogen
elimination of the formate complex (2) to form the hydride
complex (3) in the same manner as the hydrogen evolution
from HCOOH catalysed by 1. This study provides an efficient
way to remove undesired formic acid in water as well as in
water-containing ethylene glycol.
Fig. 2 Time courses of oxidation of HCOOH (2.0 mM; black circle)
and DCOOH (2.0 mM; blue square) by O
under an O atmosphere in water (1.0 mL) at pH 2.8 at 298 K. R
.99 and 0.98 for the linear correlations (black and blue, respectively).
2
in the presence of 1 (10 μM)
2
2
=
0
Acknowledgements
value (4.0) reported for the hydrogen evolution reaction
under an N atmosphere under otherwise the same experimental
2
1
This work was supported by the Advanced Low Carbon
Technology Research and Development (ALCA) program of
Japan Science Technology Agency (JST) (to S.F.) and Grants-in-
Aid (no. 24550077 to T.S.) from MEXT, Japan.
9
conditions. This indicates that the rate-determining step in
the overall catalytic cycle for oxidation of HCOOH by O is the
2
β-hydrogen elimination of the formate complex (2) to form the
hydride complex (3).
The catalytic oxidation of HCOOH by O2 also occurred
in a mixed solution (3.0 mL) of ethylene glycol and water
Notes and references
[
2
4 : 1 (v/v)] and the yield of H was decreased as compared
with that under an N2 atmosphere (Fig. 3). In the same
manner, various concentrations of HCOOH were oxidised by
O under an O atmosphere by using 1 in water-containing
2 2
1 (a) D. L. Royer, R. A. Berner and J. Park, Nature, 2007, 446,
530; (b) R. Masel, Nature, 2006, 442, 521; (c) S. Y. Reece,
J. A. Hamel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J. H. Pijpers
and D. G. Nocera, Science, 2011, 334, 645.
ethylene glycol at various pH values (Fig. S2 and S3 in the
ESI†). The amount of the remaining HCOOH was quantified
by H NMR, in which no oxidized product of ethylene glycol
2 W. Reutemann and H. Kieczka, Formic acid in Ullmann's
Encyclopedia of Industrial Chemistry, online edition, Wiley
VCH, Weinheim, 7th edn, 2011, DOI: 10.1002/14356007.
a12_013.pub2.
3 (a) S. Enthaler, J. von Langermann and T. Schmidt, Energy
Environ. Sci., 2010, 3, 1207; (b) M. Beller, A. Boddien,
F. Gartner, C. Federsel, P. Sponholz, D. Mellmann,
R. Jackstell and H. Junge, Angew. Chem., Int. Ed., 2011, 50,
6411; (c) Y. Himeda, S. Miyazawa and T. Hirose,
ChemSusChem, 2011, 4, 487; (d) G. Papp, J. Csorba,
G. Laurenczy and F. Joo, Angew. Chem., Int. Ed., 2011,
50, 10433.
1
was detected. The amount of H O produced was analysed
2
2
by spectral titration using the oxo[5,10,15,20-tetra(4-pyridyl)
2
0
porphyrinato]titanium(IV) complex in water. Neither H
2
O
2
nor H was formed in the absence of HCOOH. The amount
2
of H
3 μM at 5 h, which is also negligible as in the case of the
reaction in water. The TON reached 1300 at 22 h (Fig. 3). The
amount of H was significantly increased by adding flavin
2 2
O generated in the reaction of 2.0 mM HCOOH was
1
2
O
2
mononucleotide (FMN), as shown in Fig. S4 in the ESI.† In
the presence of FMN, the hydride complex (3) reacts with
4
(a) M. Grasemann and G. Laurenczy, Energy Environ. Sci.,
2
1
4
012, 5, 8171; (b) S. Fukuzumi, Eur. J. Inorg. Chem., 2008,
351; (c) S. Fukuzumi and T. Suenobu, Dalton Trans., 2013,
2, 18.
5
6
Y. Maenaka, T. Suenobu and S. Fukuzumi, Energy Environ.
Sci., 2012, 5, 7360.
(a) J. F. Hull, Y. Himeda, W.-H. Wang, B. Hashiguchi,
R. Periana, D. J. Szalda, J. T. Muckerman and E. Fujita,
Nat. Chem., 2012, 4, 383; (b) Y. Manaka, W.-H. Wang,
Y. Suna, H. Kambayashi, J. T. Muckerman, E. Fujita and
Y. Himeda, Catal. Sci. Technol., 2014, 4, 34.
7
(a) D. Doubayashi, T. Ootake, Y. Maeda, M. Oki,
Y. Tokunaga, A. Sakurai, Y. Nagaosa, B. Mikami and
H. Uchida, Biosci., Biotechnol., Biochem., 2011, 75, 1662; (b)
Y. Maeda, M. Oki, Y. Fujii, A. Hatanaka, M. Hojo, K. Hirano
and H. Uchida, Biosci., Biotechnol., Biochem., 2008, 72, 1999.
Fig. 3 Time courses of H
ethylene glycol and water [4 : 1 (v/v)] mixed solution (3.0 mL) in the
presence of 1 (18 μM) under N and O atmosphere (red circle and
green triangle, respectively) at pH 5.9 at 298 K.
2
evolution from formic acid (0.50 M) in an
2
2
3638 | Catal. Sci. Technol., 2014, 4, 3636–3639
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8
(a) P. M. F. Sousa, M. A. M. Videira and A. M. P. Melo, FEBS
Lett., 2013, 587, 2559; (b) P. M. F. Sousa, M. A. M. Videira,
A. Bohn, B. L. Hood, T. P. Conrads, L. F. Goulao and
A. M. P. Melo, Microbiology, 2012, 158, 2408.
V. Kohler, Y. M. Wilson, M. Durrenberger, D. Ghislieri,
E. Churakova, T. Quinto, L. Knorr, D. Haussinger, F. Hollmann,
N. J. Turner and T. R. Ward, Nat. Chem., 2013, 5, 93.
B. B. Blizanac, B. Piela, J. R. Davey, P. Zelenay and
P. Atanassov, Fuel Cells, 2009, 9, 547.
14 (a) X. Ji, K. T. Lee, R. Holden, L. Zhang, J. Zhang,
G. A. Botton, M. Couillard and L. F. Nazar, Nat. Chem., 2010,
2, 286; (b) L. Zeng, Z. K. Tang and T. S. Zhao, Appl. Energy,
2014, 115, 405.
9
15 M. Forslund, C. Leygraf, P. M. Claesson, C. Lin and J. Pana,
J. Electrochem. Soc., 2013, 160, C423.
1
1
0 (a) H. C. Lo, O. Buriez, J. B. Kerr and R. H. Fish,
Angew. Chem., Int. Ed., 1999, 38, 1429; (b) H. C. Lo, C. Leiva,
O. Buriez, J. B. Kerr, M. M. Olmstead and R. H. Fish,
Inorg. Chem., 2001, 40, 6705.
16 (a) Hazardous Substances Data Bank (HSDB) database, United
States National Library of Medicine (NLM), CASRN: 64-18-6,
http://toxnet.nlm.nih.gov/; (b) ACROS Organics Material
Safety Data Sheet (MSDS) for formic acid (#66394).
17 (a) A. Pintar, J. Batista and T. Tišler, Appl. Catal., B, 2008, 84,
30; (b) S. Yang, M. Besson and C. Descorme, Appl. Catal., B,
2010, 100, 282.
18 (a) C. Ramasany, J. P. del Val and M. Anderson, Electrochim.
Acta, 2014, 135, 181; (b) C. Ramasany, J. P. del Val and
M. Anderson, J. Power Sources, 2014, 248, 370.
19 (a) Y. Maenaka, T. Suenobu and S. Fukuzumi, J. Am. Chem.
Soc., 2012, 134, 367; (b) Y. Maenaka, T. Suenobu and
S. Fukuzumi, J. Am. Chem. Soc., 2012, 134, 9417; (c)
S. Shibata, T. Suenobu and S. Fukuzumi, Angew. Chem.,
Int. Ed., 2013, 52, 12327.
1 (a) C. E. Paul, S. Gargiulo, D. J. Opperman, I. Lavandera,
V. Gotor-Fernandez, V. Gotor, A. Taglieber, I. Arends and
F. Hollmann, Org. Lett., 2013, 15, 180; (b) M. Mifsud Grau,
J. C. van der Toorn, L. G. Otten, P. Macheroux, A. Taglieber,
F. E. Zilly, I. W. C. E. Arends and F. Hollmann, Adv. Synth.
Catal., 2009, 351, 3279; (c) A. Taglieber, F. Schulz, F. Hollmann,
M. Rusek and M. T. Reetz, ChemBioChem, 2008, 9, 565.
2 (a) K. Hofstetter, J. Lutz, I. Lang, B. Witholt and A. Schmid,
Angew. Chem., Int. Ed., 2004, 43, 2163; (b) F. Hollmann,
P. C. Lin, B. Witholt and A. Schmid, J. Am. Chem. Soc., 2003,
1
1
1
25, 8209; (c) S. Unversucht, F. Hollmann, A. Schmid and
K.-H. van Pée, Adv. Synth. Catal., 2005, 347, 1163.
3 (a) R. Wang, J. Liu, P. Liu, X. Bi, X. Yan, W. Wang, X. Ge,
M. Chen and Y. Ding, Chem. Sci., 2014, 5, 403; (b) T. S. Olson,
20 C. Matsubara, N. Kawamoto and K. Takamura, Analyst, 1992,
117, 1781.
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