Efficient H2 generation from formic acid using azole complexes in water

Article abstract of DOI:10.1039/c3cy00830d

Iridium azole-containing complexes are demonstrated to catalyze the dehydrogenation of formic acid into H₂-CO₂ (1/1) mixtures in aqueous solution in the absence of organic additives, and with a maximum turnover frequency (TOF) of 34 000 h^-1 at 80 °C.

Full text of DOI:10.1039/c3cy00830d

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Efficient H2 generation from formic acid using azole
complexes in water†
Cite this: Catal. Sci. Technol., 2014,
4, 34
a
Yuichi Manaka, Wan-Hui Wang, ab Yuki Suna,a Hide Kambayashi,a
*
c
c
ab
*
James T. Muckerman, Etsuko Fujita and Yuichiro Himeda
Received 22nd October 2013,
Accepted 22nd October 2013
DOI: 10.1039/c3cy00830d
www.rsc.org/catalysis
Iridium azole-containing complexes are demonstrated to catalyze
the dehydrogenation of formic acid into H2–CO2 (1/1) mixtures in
aqueous solution in the absence of organic additives, and with a
maximum turnover frequency (TOF) of 34 000 h−1 at 80 °C.
organic solvents.4,5 Formic acid was selectively decomposed
into H2 and CO2 without CO generation. CO causes a major
problem for hydrogen fuel cells because it acts as a Pt anode
catalyst poison at concentrations on the order of ppm.
The strategy of increasing the catalytic activity of the iridium
catalyst with bpy derivative ligands is based on controlling the
electron donating ability of the ligand through introduction
of electron donating substituents. For example, complex 2
[Cp*Ir(4DHBP)(OH2)]2+ (4DHBP: 4,4′-dihydroxy-2,2′-bipyridine),
Fig. 1, bearing two electron donating hydroxyl groups, pro-
duced a remarkable initial TOF of 14 000 h−1 at 90 °C,4 and
we have achieved the best TOF value (TOF: 228 000 h−1, 90 °C)
on the dehydrogenation of formic acid by introducing four
hydroxyl groups to the dinuclear Ir complex.2f
Formic acid is a potentially valuable hydrogen carrier in the
context of renewable fuels for the solution of today's energy
problems. It is a non-toxic liquid at room temperature and
contains H2 in 4.3 wt%, thereby offering an easy-handling H2
storage material compared with direct H2 storage as a com-
pressed gas. Moreover, the free energy for conversion of
formic acid into H2 and CO2 in aqueous solution is quite low
(Scheme 1).1
With a view toward developing a more efficient catalyst,
we explored a new design for increasing the electron donat-
ing ability of the ligand through the use of five-membered
aromatic N-heterocycles (azoles) as a ligand in an iridium or
rhodium complex. Pyridine, which was chosen in our previ-
ous studies,4,5 is well known as an electron deficient aromatic
heterocyclic ligand. In contrast, azoles, such as imidazole
and pyrazole, have an electron rich character because of
having six π-electrons delocalized on each five-membered
ring. The electron donating ability of imidazole to a coordi-
nated metal atom is stronger than that of pyridine, as has
been demonstrated on the basis of measured Mossbauer
spectra,6 absorption spectra,7,8 and cyclic voltammetry.7 This
prompted us to investigate the azole complexes as potentially
good catalysts in the dehydrogenation of formic acid. In
addition, some groups have already attempted to use azole-
containing complexes in catalytic reactions, e.g., hydro-
amination,9 hydrogenation of CO2,10 and dehydrogenation of
formic acid.11 In particular, the dehydrogenation of formic
acid using a phenylazole complex yielded a TOF of 536 h−1 in
formic acid–triethylamine (5/2) for the first 3 min at 25 °C.11
Herein, we report the highly efficient dehydrogenation of
formic acid in water with simple azole-containing complexes.
Scheme 1 Dehydrogenation of formic acid.
The generation of formic acid by hydrogenation of CO2
has been widely investigated during the last four decades.2 In
combination with the hydrogenation of CO2 for H2 storage,
the dehydrogenation of formic acid for H2 supply may provide
an energy-efficient H2 storage/supply system. Therefore, H2
generation through dehydrogenation of formic acid has been
studied intensively in the past few years (Scheme 1).3
Recently, we have investigated the catalytic dehydrogenation
of formic acid using iridium catalysts with 2,2′-bipyridine (bpy)
derivative ligands in water without additional organic base or
a National Institute of Advanced Industrial Science and Technology, Tsukuba
Central 5-2, 1-1-1 Higashi, Ibaraki, Tsukuba, 305-8565 Japan.
E-mail: himeda.y@aist.go.jp, ou-wanhui@aist.go.jp
b JST, ACT-C, 4-1-8 Honcho, Saitama, Kawaguchi, 332-0012 Japan
c Chemistry Department, Brookhaven National Laboratory, Upton,
NY 11973-5000, USA
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy00830d
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This difference in activity is attributed to the nitrogen position
on the azole. The two adjacent nitrogen atoms in the pyrazole
withdraw electrons to the aromatic ring, causing the electron
donating ability of the coordinating nitrogen atom of pyrazole
to be less than that of imidazole. When using pyridinyl
benzimidazole complex 5 (entry 5), the TOF value was slightly
lower than that of 3 owing to the electron withdrawing effect
of the benzene ring. The TOF value of biazole complex 6 was
3980 h−1 (entry 6), higher than that of pyridinylazole complexes
and 130 times higher than that of 1. As expected, the exchange
of pyridine with azole enhanced the catalytic activity remark-
ably. It is noteworthy that the TOF value was even higher than
that of complex 2, in which electron donating ability was
enhanced by hydroxyl substituents on the bpy ligand (entry 2).
The iridium tetramethyl biimidazole complex 7 (entry 7) shows
a high catalytic activity (TOF = 7020 h−1). This high value is
attributed to the electron donating ability of the ligand in 7
that is enhanced by the four methyl substituents. Thus, we
choose 7 as a catalyst in the following experiments. We tried
using rhodium catalyst 8 (entry 8), but its activity was much
lower than that of iridium complex 7. The same tendency was
observed when using bpy derivatives as a ligand.5
The temperature dependence of the TOF values was also
investigated. Fig. 2a shows the initial part of the time course of
gas evolution using 7 in aqueous formic acid at 40–80 °C. The
initial TOF value of 7 was observed to increase with increasing
reaction temperature. Fig. 2b shows the Arrhenius plots of
the TOF values of 7. The apparent activation energy of 7 is
78 kJ mol−1. This value is almost the same as that of complex 2
(76 kJ mol−1).4 The highest TOF value for 7 (34 000 h−1) was
obtained at 80 °C (Table 1, entry 9).
Fig.
1 Complexes for dehydrogenation of formic acid and
hydrogenation of carbon dioxide.
Fig.
1 shows the iridium and rhodium complexes
employed in this work. Imidazole and pyrazole were selected
as basic azole structures. These complexes are divided into
three categories according to the structure of the chelated
ligand. The first one is composed of bi-six membered nitrogen
heterocycles containing complexes 1 and 2. Complex 2 has
two hydroxyl groups for enhancing the electron donating
ability of the bpy ligand. The second series consists of
ligands with six and five membered nitrogen heterocycles
(pyridinylazole) containing complexes 3, 4 and 5. The last
series contains biazole complexes 6, 7 and 8. Complexes 7
and 8 have four electron donating methyl groups on the
biimidazole ligand. These complexes were prepared
according to literature procedures (see the ESI†).12
All the azole complexes were investigated for dehydrogena-
tion of formic acid in 1 M aqueous formic acid solution at
60 °C, and CO contamination was not detected by GC (Table 1,
entries 1–8). Initial TOF values of pyridinylazole complexes 3,
4 and 5 were 810, 400, and 570 h−1, respectively (entries 3, 4,
and 5). In comparison with the TOF value of non-substituted
complex 1 (entry 1), these TOF values are higher by more than
a factor of 13. The TOF value of 3 was twice as high as that of 4.
Dehydrogenation of formic acid using 7 under various
catalyst concentrations and formic acid concentrations was
investigated (Table S2†). Catalytic dehydrogenation of formic
acid also proceeded smoothly in a closed system under pres-
surized conditions (Fig. 3a). The pressure increased up
to 1.8 MPa after 1.5 h in this system. Formic acid was
dehydrogenated by
7 almost completely (>98%). These
results show that the pressure in the system did not
inhibit the reaction. It indicates that high pressure H2
gas can be supplied using 7. Moreover, no CO contaminant
Table 1 Results of dehydrogenation of formic acid
Entry
Catalyst
Amount (mM)
TOFa (h−1
)
Time (h)
Minimum TON
Conversion (%)
1b
2c
3
4
5
1
2
3
4
5
6
7
8
7
7
2.0
0.2
0.5
0.5
0.5
0.1
0.1
0.5
0.1
0.1
30
2400
810
400
570
3980
7020
250
34 000
9600
—
5
4
7
6
5
2
8
0.5
2.5
—
—
100
98
>99
>99
100
100
26
100
80
5000
1970
2000
2000
10 000
10 000
1020
10 000
8000
6
7
8c
9d
10e
a
Reaction conditions: the reaction was carried out at 60 °C in 10 mL of 1 M aqueous formic acid (pH 1.75). The average value during the first
b
c
d
e
10 min. The average value during the first 2 h. Used 2 M aqueous formic acid. Carried out at 80 °C. Used 1 M aqueous sodium formate–
formic acid (1/4) pH 2.8.
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Fig.
4 pH dependence of initial TOF values at 60 °C in 1 M
Fig. 2 (a) Time course of gas evolution using 0.1 mM of 7 in 10 mL of
1 M aqueous formic acid under 0.1 MPa (closed circles: 40 °C, open
circles: 50 °C, closed squares: 60 °C, open squares: 70 °C, closed
triangles: 80 °C). Experimental details are available in the ESI.† The
induction periods observed at low temperatures are due to the
detection limit of the gas meter used. (b) Arrhenius plot of initial TOF
values for dehydrogenation of formic acid using 7.
HCO2H/HCO2Na using 7.
values for the dehydrogenation of formic acid were lower
than that at pH 1.75 except for rhodium complex 8.
The hydrogenation reaction of CO2 to formate using azole
complexes was also carried out under 1 MPa of H2–CO2 (1/1)
in aqueous solution (Table S3†). The results of hydrogenation
using azole complexes were higher compared to 1. Therefore,
these azole complexes exhibit the possibility of catalysing an
interconversion cycle between HCO2H and CO2 using only
one catalyst.
In summary, the newly designed azole-containing com-
plexes exhibit higher catalytic activities than the unsubstituted
bpy analogues in the dehydrogenation of formic acid in aque-
ous solutions. Especially, the iridium tetramethyl biimidazole
complex 7 achieved a TOF value as high as 34000 h−1 (80 °C)
which is higher than the TOF values of any other mononuclear
complexes yet reported. The extraordinary activity of complex 7
is only inferior to the most active dinuclear complex that we
previously reported. Nevertheless, complex 7 is more robust in
concentrated aqueous solutions of formic acid. The remarkable
activity and stability are strongly affected by the imidazole
structure and electron donating methyl groups. A study of the
detailed activation mechanism with azole ligands is under way.
Fig. 3 (a) Time course of pressure value during dehydrogenation
processes using 7 (0.1 mM) at 80 °C in a 1 M formic acid aqueous
solution (7 mL) in a glass autoclave. (b) Gas chromatogram of the
evolved gas and CO containing gas using a FID with methaniser. 1000 μL
of 10 ppm of CO gas and 500 μL of evolved gas were injected.
was observed despite the high-pressure reaction conditions
(Fig. 3b).
The pH dependence of TOF values using catalyst 7 was
also investigated. The TOF values from a 1 M aqueous
sodium formate–formic acid solution were measured at
various pH values (Fig. 4). The highest TOF value of 9600 h−1
was obtained at pH 2.8 at 60 °C (Table 1, entry 10). When
the pH was higher than 7, no gas generation was observed.
The UV/Vis spectral changes of 7 were measured at pH 1.5 to
12.5 (see Fig. S1, ESI†). The average pKa1 of 7 for single
deprotonation of the imidazole and pKa2 for deprotonation of
the aqua ligand were respectively 8.8 and 11.4 as determined
from these measurements. These values are similar to
the reported pKa values of 9.0 and 11.3 of its ruthenium
analogue.8 Since the protonation status of 7 did not change
below pH 7, the pH dependence of dehydrogenation by 7 was
not due to the deprotonation status of the imidazole ligand.
It is inferred that the pKa of formic acid (3.75) affected
the pH dependence as previously reported.2f,13 The TOF value
of other azole complexes at pH 3.5 (close to the pKa of
formic acid, 1 M aqueous sodium formate–formic acid (1/1)
solution) was also measured (Table S1†). At pH 3.5, TOF
Acknowledgements
Y. H. and W.-H. W. acknowledge JST, ACT-C for financial
support. Work at Brookhaven National Laboratory was
carried out under contract DE-AC02-98CH10886 with the
U.S. Department of Energy and supported by its Division
of Chemical Sciences, Geosciences, and Biosciences, Office
of Basic Energy Sciences.
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