Ligand Effect on the Stability of Water-Soluble Iridium Catalysts for High-Pressure Hydrogen Gas Production by Dehydrogenation of Formic Acid

Article abstract of DOI:10.1002/cphc.201900137

Aiming to develop a highly effective and durable catalyst for high-pressure H₂ production from dehydrogenation of formic acid (DFA), the ligand effect on the catalytic activity and stability of Cp*Ir (Cp*:pentamethylcyclopentadienyl anion) complexes were investigated using 5 different kinds of N,N’-bidentate ligands (bipyridine, biimidazoline, pyridyl-imidazoline, pyridyl-pyrazole and picolinamide). The Ir complex with biimidazoline ligand exhibited the highest catalytic activity, but deactivation occurred readily at high pressure. The pyridine moiety in the ligand can enhance the stability of Ir complex catalysts for the high-pressure reaction. The Ir complex catalyst containing pyridyl-imidazoline ligand showed the high activity and best stability under the high-pressure conditions.

Full text of DOI:10.1002/cphc.201900137

Accepted Article
Title: Ligand Effect on the Stability of Water-soluble Iridium
Catalysts for The High-pressure Hydrogen Gas Production by
Dehydrogenation of Formic Acid
Authors: Masayuki Iguchi, Naoya Onishi, Yuichiro Himeda, and Hajime
Kawanami
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10.1002/cphc.201900137
ChemPhysChem
COMMUNICATION
Ligand Effect on the Stability of Water-soluble Iridium Catalysts
for The High-pressure Hydrogen Gas Production by
Dehydrogenation of Formic Acid
Masayuki Iguchi,[a] Naoya Onishi,[b] Yuichiro Himeda,[b] and Hajime Kawanami*[a]
Dedication ((optional))
Abstract: Aiming to develop a highly effective and durable catalyst
for the high-pressure H2 production from dehydrogenation of formic
acid (DFA), the ligand effect on the catalytic activity and stability of
Cp*Ir (Cp*:pentamethylcyclopentadienyl anion) complexes were
thermodynamically favorable occurs as side reaction, hampers
applications of DFA in a polymer electrolyte membrane fuel cell.[4]
HCO2H(l) → H2(g) + CO2(g)
HCO2H(l) → H2O(l) + CO(g)
DG0 = −32.9 kJ/mol
DG0 = −12.4 kJ/mol
(1)
(2)
investigated using
5 different kinds of N,N’-bidentate ligands
(bipyridine, biimidazoline, pyridyl-imidazoline, pyridyl-pyrazole and
picolinamide). The Ir complex with biimidazoline ligand exhibited the
highest catalytic activity, but deactivation occurred readily at high
pressure. The pyridine moiety in the ligand can enhance the stability
of Ir complex catalysts for the high-pressure reaction. The Ir complex
catalyst containing pyridyl-imidazoline ligand showed the high activity
and best stability under the high-pressure conditions.
Various homogeneous and heterogeneous catalysts were
developed for the selective DFA at atmospheric pressure and mild
temperature of less than 100 °C.[5] However, there are few reports
on the production of H2 by DFA at high-pressure above 10 MPa.[6]
In this context, water is a promising solvent for DFA because FA
is highly miscible in water. Hence the reaction has been
intensively reported using iridium (Ir) complexes with the Cp*
ligand (Cp*: pentamethylcyclopentadienyl anion).[7] Recently, our
group demonstrated DFA to generate high-pressure H2 gas using
water-soluble Cp*Ir catalysts at mild temperatures without adding
any base.[8] As a hydrogen carrier, FA provides the distinct
advantage of producing the high-pressure H2 at mild
temperatures,[9] which avoids the requirement of large amount of
energy for compression. The high-pressure reaction conditions
also provide an efficient separation of CO2 and H2 after the
reaction, without necessitating any absorbents or membrane as
separator or low-temperature distillation process. Generated
gases can be compressed through the chemical reaction, which
results high volumetric energy density of H2, and easy handling of
CO2 for its delivery and storage as well as further reaction with H2
to produce FA.
Growing concern over rapidly increasing energy issues with the
depletion of fossil fuels and increasing levels of CO2 in the
atmosphere, the use of renewable energy becomes one of the
most urgent and critical targets in the 21st century. In this context,
molecular hydrogen (H2) is considered as one of the most
promising clean and renewable energy carriers for bridging gaps
between production and consumption of energies.[1] However,
very low density of H2 cause difficulties in its storage and
transportation. Thus, H2 is preferred to be stored physically or
chemically in a material (hydrogen carrier), which can be released
readily on demand.[2] Formic acid (FA, HCO2H) has been
attracted as one of the liquid organic hydrogen carriers (LOHCs)
because it is stable under ambient conditions and
a
biodegradable compound.[3] Compare to other organic acids, FA
has the lowest chemical oxygen demand (COD) and
comparatively good hydrogen content per weight (4.3 wt%) as
well as per volume (53 kg/m3) compared with other LOHCs such
as methylcyclohexane (6.2 wt%, 47 kg/m3). The dehydrogenation
of FA to H2 and CO2 (DFA)is a thermodynamically favorable
reaction (Eq. 1), thus, it can occur under mild temperature and
high-pressure conditions because of the low reaction enthalpy
and Gibbs energy. However, FA dehydration (Eq. 2) is also
According to our previous report, Cp*Ir complex containing
2-(2-pyridyl)imidazoline ligand (1 in Figure 1) catalyzed DFA to
generate the pressure as high as 153 MPa at 80 °C.[8c] The
complex 1 was highly active and selective for DFA, and
maintained its stability even under the continuous addition of neat
FA at high pressure condition. Furthermore, the gas flow was
controlled through the addition of neat FA to the reaction solution.
Although the complex
1 is a stable catalyst for DFA at
atmospheric pressure (turnover number, TON, value of 2×106), it
was deactivated at high pressures due to the ligand elimination.[8c]
To adopt FA as a hydrogen storage material for practical
applications, efficient and robust catalysts are required to tolerate
severe reaction conditions, which is a big challenge.
[a]
Dr. Masayuki Iguchi, Dr. Hajime Kawanami
Research Institute for Chemical Process Technology
Department of Materials and Chemistry
National Institute of Advanced Industrial Science and Technology
Sendai, Miyagi, 983-8551 Japan
Systematic studies of the ligand effect on DFA at
atmospheric pressure revealed that the nature of the N,N’-
bidentate ligand in the Cp*Ir complex largely affected the catalytic
performance.[10] The Ir complex having 2,2-bipyridine without any
substituents (6) was slightly active for DFA,[10a] however, the
catalytic activity enhanced by changing the ligand moiety from
pyridine to 2-imidazoline (1).[10c] Although, the presence of 2,2’-
biimidazoline (2) in Ir complex exhibits high activity, less stability
than 1.[10c] Incorporation of N-phenyl-2-pyridinecarboxamide (3)
E-mail: h-kawanami@aist.go.jp
[b]
Dr. Naoya Onishi, Dr. Yuichiro Himeda
Research Institute of Energy Frontier
Department of Energy and Environment
National Institute of Advanced Industrial Science and Technology
Tsukuba, Ibaraki, 305-8565
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cphc.201900137
ChemPhysChem
COMMUNICATION
ligand in the Ir complex also shows better activity than 1,[10d]
possibly due to the electro-donating ability of amide ligand. The
catalytic activity of Ir complex having 1-(2-pyridyl)pyrazole without
any functional groups (7) was as low as the complex having 2,2-
bipyridine (6). Introduction of the electro-donating group such as
hydroxyl, on the pyrazole moiety (4) enhanced the activity,[10e]
which is comparable with Ir complex having hydroxyl groups on
2,2’-biprydine (5) ligand.[10a] The catalyst was stable under
refluxing conditions of aqueous solution of FA under atmospheric
pressure.[10e], but shows low activity compared to 1.
In this work, we aimed to develop a highly efficient and
durable catalyst for the high-pressure H2 production from DFA.
Using 5 different kinds of N,N’-bidentate ligands anchoring Cp*Ir
complex we investigated the ligand effect on the activity and
stability of the complexes for high-pressure DFA.
50
40
30
20
10
0
2+
0
2
4
6
8
N
2-
SO4
Ir
H2
O
N
Time /h
HCO2
H
H2
CO2
OH
+
Figure 2. Time course of the generated pressure by FA decomposition using
different Ir complexes: 1 (orange), 2 (green), 3 (red), 4 (blue). Reaction
condition: 60 ºC, aqueous solution of FA (16 mol/L, 40 mL), complex (0.4
mmol/L). Time is defined as the time after heating the reaction solution.
HN
N
N
N
N
N
N
N
N
N
N
N
N
N
=
N
N
NH
N
O
NH
N
N
OH
HO
1
2
3
4
5
6
7
TOF / h-1
13,300
54,700
20,100
6,400
2,500
=
30
20
After reaching the pressure of 40 MPa, while using the initial
concentration of 16 mol/L (complexes 1, 2, 4) and 8 mol/L
(complex 3), the high-pressure gas was exhausted from the back-
pressure regulator (Table 1). The initial turnover frequency (TOF)
calculated from the gas flow rate was the highest on 2 (14,400 h-
1) followed by 3, 1 and 4, at 60 °C (entry 2 in Table 1), which
agrees well with the results of atmospheric pressure (Figure 1).
The flow rate of generated gas was constant in the beginning of
the reaction and then decreased with time for 1,[8c] whereas in the
case of 2 and 4, the gas flow rate gradually increased with time
during the reaction (Figures S4 and S5 in the Supporting
Information). This time dependence of the reaction rate might be
correlated with the FA concentration.[10d, 10e] The effect of gas
pressure on the reaction rate was estimated from the ratio of TOF
value at 40 MPa to that at atmospheric pressure. The reaction
rate at the high pressure decreased about 1/4 from that at
atmospheric pressure, which agrees with the other complexes as
described in the literature.[8c] These results indicate that the gas
pressure generated by DFA barely affect the reaction mechanism
under the applied conditions. The residual concentrations of FA,
after DFA, were same for 1 and 4 (entries 1 and 3) but much
higher for complex 2. Thus, the complexes 1 and 4 represents the
higher catalytic stability and longer life time for DFA than 2 under
the high-pressure conditions, indicating the pyridine moiety in the
ligand plays an important role on the catalytic stability.
Figure 1. Catalytic activity of Cp*Ir complexes containing different N,N’-
bidentate ligands in terms of turnover frequency (TOF) on DFA at 60°C and
atmospheric pressure.
The high-pressure reactions were carried out with 40 mL of
FA aqueous solution (16 mol/L) using 16 µmol of complexes 1-4
at 60 °C in an autoclave equipped with a back-pressure regulator
(Figure 2). The gas pressure generated by DFA reached to 10
MPa following the order of 2, 3, 1 and 4, which reached to 40 MPa
after 1 h, 3 h and 6 h using 2, 1 and 4, respectively. On the other
hand, the gas pressure generation stopped at 20 MPa after 10 h
for 3 however, reducing the FA concentration from 16 mol/L to 8
mol/L, 40 MPa gas was generated within 2 h using 3 (Figure S1
in the Supporting Information). Although the complex 3 was highly
active, the deactivation occurred quickly under the high-pressure
and high-FA concentration conditions due to the formation of Ir
trihydride complex ([(Cp*Ir)2H3]+) as confirmed from ESI-MS
analysis of reaction solution (Figure S2 in the Supporting
Information).[2a, 11] Since the trihydride complex was slightly active
for DFA,[12] the complex 3 might be deactivated by forming the
trihydride complex via the elimination of N,N’-bidentate ligand
under the high-pressure H2 (Figure S3 in the Supporting
Information).
The high-pressure reaction at 80 °C showed the higher TOF
compared to that at 60 °C using 1 and 4 (entries 1 vs. 5-7 and 4
vs. 8-9 in Table 1). In addition, changing the initial concentration
of FA from 8.2 mol/L to 15.6 mol/L lowers the TOF value of 1 and
enhanced the residual concentrations (entries 5 vs. 6). However,
after increasing the complex concentration, the TOF value of 1
and residual FA concentration were comparable independent to
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10.1002/cphc.201900137
ChemPhysChem
COMMUNICATION
the initial FA concentrations (15.4 mol/L 8.2 mol/L) (entries 5 vs.
7). On the other hand, initial FA concentrations of 15.5 mol/L and
7.7 mol/L realizes same TOF values over complex 4, but the
residual FA concentration was higher for 15.5 mol/L (entries 8 vs.
9). Under the reaction conditions of 40 MPa and 80 °C, the
equilibrium FA concentration was estimated as 0.38±0.03 mol/L
in the solution (entries 5, 7 and 8). Only the complex 1 was
capable for completing the high-pressure reaction in even using
the high initial concentration of 15.5 mol/L at 80 °C and 40 MPa
(entries 7, 9 and 10), hence, possesses high stability, which
followed by 5 and 4 according to the residual FA concentration
after the reaction.
Table 1. Catalytic dehydrogenation of FA at high-pressures (40 MPa) using Ir complexes[a]
Entry
Complex
Temperature
/ °C
Complex conc.
/ mmol/L
Initial FA conc.
/ mol/L
TOF value[b]
/ h-1
Reaction Time
/ h
Residual
conc. / mol/L
FA
1[c]
2
1
2
3
4
1
1
1
4
4
5
60
60
60
60
80
80
80
80
80
80
0.4
0.4
0.4
0.4
0.2
0.2
0.4
0.4
0.4
0.4
15.6
15.6
8.2
3,000
14,400
3,800
1,400
10,700
6,500
9,000
5,700
5,700
1,900
6.0
0.61
3.52
0.74
0.62
0.37
9.99
0.40
0.37
0.95
0.55
1.9
3
2.1
4
15.6
8.2
11.2
1.9
5
6
15.7
15.4
7.7
18.2
2.4
7
8
1.2
9
15.5
15.7
5.0
10
14.0
[a] Reaction conditions: 40 MPa, aqueous solution (40mL). [b] Average value over initial 30 min. [c] Data taken from previous work.[8c]
In practical applications, H2 is preferred to be produced by
DFA on demand and its production can be controlled by
measurement, neat FA was continuously added to the solution
for 10 h and waited for subsiding the gas generation, while
keeping the pressure constant. This cycle was repeated for
several times until the gas flow rate was decreased less than half
of initial value. For the complex 1, the flow rate of high-pressure
gas was maintained its constancy until 3 times of FA addition, as
the complex retained its stability over 30 h under the applied
conditions.[8c] The TON value that is decreased by 80% of initial
flow rate (TON80%) was calculated as 45,800 for 1 (entry 1 in
Table 2 and Figure S6 in the Supporting Information). Using the
complexes 3 and 4, gas generation remained constant until two
times of FA addition, and the TON80% values were obtained as
32,000 and 28,700 respectively (entries 2 and 3, and Figures S7
and S8 in the Supporting Information). The results in Table 2
revealed that 1 shows the highest catalytic stability, followed by
3 and 4. The deactivation of 4 might be attributed to the
generation of CO (entry 3), which is consistent with the Ir-based
catalysts.[6d] The IR spectrum of solution after the reaction also
confirmed carbonyl complex formation from 4 (Figure S9 in the
Supporting Information). The precise mechanism of catalyst
deactivation is still under investigations.
Table 2. Continuous decomposition of FA at high pressures (20MPa) using
Ir complexes.[a]
Entry
Complex
Stability[b]
/ h
CO
/ vol. ppm
TON80%
1
2
3
1
3
4
30
20
20
n.d.
n.d.
9±5
45,800
32,000
28,700
[a] Reaction conditions: 50 °C, complex (16 µmol), initial FA solution (5
mol/L, 40 mL). High-pressure FA was added at 0.6 mL/h over 10 h and then
stopped for several hours, which was repeated for several times. [b] Time
lapsed from the beginning until a decrease in the gas generation rate. [c]
Calculated value when the gas flow rate decreased by 80 % of initial rate.
In conclusion, we evaluated the effect of N,N‘-bidentate
ligands including bipyridine (5), biimidazoline (2), pyridyl-
imidazoline (1), pyridyl-pyrazole (4) and picolinamide ligands (3)
adjusting the FA addition. We evaluated the catalytic stability of
1, 3 and 4 under high-pressure conditions via continuous FA
decomposition at 20 MPa and 50 °C (Table 2). During the
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10.1002/cphc.201900137
ChemPhysChem
COMMUNICATION
anchoring Ir complex on the catalytic activity and stability for DFA.
The Ir complex having biimidazoline ligand (2) shows the highest
activity followed by 3, 1, 4 and 5, whereas, catalytic stability
exhibits different trend of pyridyl-imidazoline ligand (1)> 5> 4> 2>
3. Among the complex studied 1 and 5 emerged as the two mo
most stable catalysts and also shows excellent efficieny even at
high pressure of 100 MPa or above,[8a, 8c] indicating the catalytic
stability is an important criterion for the high-pressure DFA. This
work suggests that the pyridine moiety in the ligand plays an
important role on the catalytic stability, which can help to develop
a highly effective and durable catalyst for the high-pressure H2
production from DFA.
supported by the Japan Science and Technology Agency (JST),
CREST (No. JPMJCR1342), and the International Joint
Research Program for Innovative Energy Technology of the
Ministry of Economy, Trade, and Industry (METI) of Japan for
M.I., Y. H. and H.K.
Keywords: formic acid • hydrogen • homogeneous catalysis •
high-pressure chemistry • iridium
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Experimental Section
The
water-soluble
Ir
complexes,
[Cp*IrIII(2-(2-
pyridyl)imidazoline)(H2O)]SO4 (1),[13] [Cp*IrIII(biimidazoline)(H2O)]SO4
(2),[13] [Cp*IrIII(N-phenyl-2-pyridinecarboxamide)(H2O)]HSO4 (3),[10d]
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literature methods. Formic acid (FA, >99 %) was used as received from
FUJIFILM Wako Pure Chemical Industries, Ltd. Deionized water was
prepared through a distillation system (Toyo Roshi Kaisha, Ltd., GS-590)
and a filtration system (EMD Millipore Corp., ZFSQ240P4). All reagents
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The volume of generated gas was measured by a wet gas meter
(Shinagawa Co., Ltd., W-NK-0.5A). The measured volume was converted
to the value at standard ambient temperature and pressure by using the
ideal gas law. A GC-TCD system (Agilent Technologies, 3000A Micro
GC) was used to determine the concentrations of H2, CO2, and CO in the
generated gas. The FA concentration in the reaction solution was
measured by a HPLC-UV system on an ion exclusion column (Showa
Denko K. K., KC-811) with 20 mmol/L phosphoric acid aqueous solution.
The reaction solution was analysed by an electrospray ionization mass
spectrometer (ESI-MS) in a positive ion mode (Agilent Technologies,
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aqueous solution of FA (40 mL) were loaded into the reactor at room
temperature. The reactor was purged by argon gas several times and was
heated to the desired temperature under stirring. The pressure of gas
generated was controlled by a back-pressure regulator (JASCO Corp.,
BP-2080) within 0.1 MPa. The gas exhausted from the regulator was
analysed by the gas meter and GC system. For the continuous flow
measurements, neat FA was added to the solution at a constant rate
using a syringe pump (JASCO Corp., PU-980). After stopping the gas
generation, the reactor was cooled and then depressurized to
atmospheric pressure. Since the generated gas was composed of an
equimolar mixture of H2 and CO2 containing less than 100 vol ppm CO
for all experiments, the turnover frequency (TOF) and turnover number
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the gas flow rate decreased by 80 % of initial rate.
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Acknowledgements
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10.1002/cphc.201900137
ChemPhysChem
COMMUNICATION
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601.
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10.1002/cphc.201900137
ChemPhysChem
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
The N,N’-bidentate ligand in the Cp*Ir
complex affected the catalytic activity
and stability for the high-pressure
dehydrogenation of formic acid. The
pyridine moiety in the ligand can
enhance the catalytic stability for the
high-pressure reaction. The complex
having pyridyl-imidazoline ligand is the
high active and best stable catalyst
between 5 kinds of catalysts.
Masayuki Iguchi, Naoya Onishi, Yuichiro
Himeda, Hajime Kawanami*
2+
N
2-
SO4
Ir
Page No. – Page No.
H2O
N
H2 + CO2
HCO2H
Ligand Effect on the Stability of
Water-soluble Iridium Catalyst for The
High-pressure Hydrogen Gas
Production by Dehydrogenation of
Formic Acid
Stability
at 40 MPa
ꢀ
ꢃ
ꢂ
ꢁ
ꢀ
OH
OH
HN
N
N
N
>
N
N
N
N
<
TOF
at 0.1MPa
>
>
N
NH
N
O
N
N
NH
HO
This article is protected by copyright. All rights reserved.