Development of an Iridium-Based Catalyst for High-Pressure Evolution of Hydrogen from Formic Acid

Article abstract of DOI:10.1002/cssc.201600697

A highly efficient and recyclable Ir catalyst bearing a 4,7-dihydroxy-1,10-phenanthroline ligand was developed for the evolution of high-pressure H₂ gas (>100 MPa), and a large amount of atmospheric pressure H₂ gas (>120 L), over a l

Full text of DOI:10.1002/cssc.201600697

DOI: 10.1002/cssc.201600697
Communications
Development of an Iridium-Based Catalyst for High-
Pressure Evolution of Hydrogen from Formic Acid
[a, d]
[b, d]
[c, d]
[a, d]
Masayuki Iguchi,
Yuichiro Himeda,
Yuichi Manaka,
and Hajime Kawanami*
A highly efficient and recyclable Ir catalyst bearing a 4,7-dihy-
droxy-1,10-phenanthroline ligand was developed for the evolu-
storage capacity and easy handling and transportation with
[2a,3]
the existing infrastructures used for gasoline and diesel.
It
tion of high-pressure H gas (>100 MPa), and a large amount
is necessary that both the process of releasing storage of H2
should occur at mild temperature for the reduction of material
2
of atmospheric pressure H gas (>120 L), over a long term
2
[2c,3]
(3.5 months). The reaction proceeds through the dehydrogena-
losses and energy consumption during the reactions.
Re-
tion of highly concentrated aqueous formic acid (FA, 40 vol%,
leasing compressed H from a storage material for mobile ap-
2
À1
10 molL ) at 808C using 1 mmol of catalyst, and a turnover
plications, such as transportation, is a challenging issue be-
cause of space limitations, as typically a large volume system is
required. H -fueled vehicles currently use H in the gaseous
number (TON) of 5000000 was calculated. The Ir catalyst pre-
cipitated after the reaction owing to its pH-dependent solubili-
ty in water, and 94 mol% was recovered by filtration. Thus, it
can be treated and recycled like a heterogeneous catalyst. The
catalyst was successfully recycled over 10 times for high-
pressure FA dehydrogenation at 22 MPa without any treatment
or purification.
2
2
[
1a,c,2b]
form from high-pressure H gas tanks that they carry.
The
2
generation of high-pressure H consumes a large amount of
2
energy during compression, which corresponds to approxi-
[1a,2b]
mately 10–15% of the H energy content.
The energy con-
2
sumed during the compression of H2 could be reduced by
generating high-pressure H through a chemical reaction.
2
Recently, formic acid (FA) has attracted considerable atten-
tion as a liquid hydrogen storage material because it is stable,
moderately flammable, and readily biodegradable under ambi-
Growing concerns over the depletion of fossil fuels and anthro-
pogenic global warming has led to the search for alternative
[4]
renewable energy resources. Molecular H is considered as one
ent conditions. FA contains a relatively high content of H2,
2
of the perfect choices to act as a future energy source because
of its high energy density and environmentally benign proper-
and the low reaction enthalpy permits the release of H at
2
[2c,4e]
mild temperature
through dehydrogenation, which is
[
1]
[4e]
ties. Although H is a promising energy source, the gaseous
a thermodynamically favorable process. CO , a co-product of
2
2
nature of H makes it difficult to store, transport, and use in
FA dehydrogenation, can also be converted to FA by photo- or
2
[
1]
[4a–d]
mobile applications. H can be stored by physical adsorption
electro-chemical reduction in the presence of catalysts.
FA
2
[
5]
on some specific materials, by chemical bonding, or in a com-
has been recognized as a H2 storage material since 1978.
[1a,c,2]
plexed form that is incorporated into small molecules.
However, the development of a suitable process for the gener-
Chemical storage of H in liquid materials gives several advan-
ation of H gas from FA has progressed slowly because of the
2
2
tages over other hydrogen storage materials, such as a high H2
requirement of severe reaction conditions, low product selec-
[2c]
tivity, and regeneration of the catalyst. In addition, the oc-
currence of CO as a by-product deactivates the catalyst and
[
a] Dr. M. Iguchi, Prof. Dr. H. Kawanami
Research Institute for Chemical Process Technology
Department of Material and Chemistry
National Institute of Advanced Industrial Science and Technology
Sendai, Miyagi 983-8551 (Japan)
[6]
hampers the application of the generated H in fuel cells.
2
In the presence of homogeneous catalysts, FA can be de-
composed selectively at mild temperature. Various catalysts
have been developed for the selective decomposition of FA to
E-mail: h-kawanami@aist.go.jp
produce H2 with a high rate at temperatures of less than
[b] Dr. Y. Himeda
[7]
1
008C. Many researchers investigated the catalytic dehydro-
Research Institute of Energy Frontier
Department of Energy and Environment
National Institute of Advanced Industrial Science and Technology
Tsukuba, Ibaraki 305-8565 (Japan)
genation of FA under atmospheric pressure conditions, which
releases high-pressure H2 gas, and although the process is
energy-efficient, it faces the problem of H separation. To date,
2
[
c] Dr. Y. Manaka
Renewable Energy Research Center
Department of Energy and Environment
National Institute of Advanced Industrial Science and Technology
Koriyama, Fukushima 963-0298 (Japan)
there have been very few reports of the dehydrogenation of
[8]
FA under high-pressure conditions, above 10 MPa, because
the catalyst must be capable of withstanding severe reaction
conditions, especially high pressures and high concentrations
of FA over long time periods. In practical applications of FA as
[
d] Dr. M. Iguchi, Dr. Y. Himeda, Dr. Y. Manaka, Prof. Dr. H. Kawanami
Core Research for Evolutional Science and Technology
Japan Science and Technology Agency
a H carrier, a high concentration of FA is generally used for
2
the fast and efficient production of large amounts of high-
pressure H2.
Tokyo, 102-0076 (Japan)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cssc.201600697.
Recently, we developed water-soluble Ir catalysts for fast
[9]
and selective FA decomposition under mild temperatures.
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The introduction of hydroxyl groups into a bipyridine ligand
activated pentamethylcyclopentadienyl Ir (Cp*Ir) complexes
[
9a,e,f]
toward FA dehydrogenation.
Cp*Ir complex containing 4,4’-dihydroxy-2,2’-bipyridine
4DHBP, catalyst 1 in Figure 1) catalyzed selective FA decompo-
We also reported that the
(
Figure 1. Cp*Ir complexes for the development of a FA recycling system.
[
10]
sition at the high pressure of 123 MPa. However, the main
drawback is the separation and consequent recycling of the
catalyst. Even though catalyst 1 has a long lifetime of over
Figure 2. The time courses of the volume of evolved gases/rate of evolution
of gases from FA decomposition in 10 molL of FA aqueous solution
3
3 h with turnover number (TON) of 100000 and turnover fre-
À1
À1
quency (TOF) of 3100 h at 608C under atmospheric pres-
sure,
(500 mL) at 608C, catalyzed by catalyst 2 (1 mmol) in a glass autoclave [black
À1
[
9a]
line: volume of evolved gas (L), red cross: gas evolution rate (Lh )].
both the TON and TOF decreased to 38100 and
À1
2510 h , respectively, under the high-pressure condi-
tions of 30 MPa and 808C owing to the deactivation
of the catalyst (Figure S1 in the Supporting Informa-
tion). As the gas pressure increases owing to the de-
composition of FA, the catalyst undergoes partial hy-
drogenolysis as a result of the presence of high-pres-
[
a]
Table 1. Dehydrogenation of FA under atmospheric pressure conditions.
Entry Cat. Catal. Conc. FA Conc. Bath temp. React. time TOF
TON
À1 [b]
[
mM]
[M]
[8C]
[h]
[h ]
1
2
3
4
5
6
7
8
9
1
2
2
2
2
2
2
2
2
100
100
100
100
100
100
25
1
1
3
60
60
60
60
60
60
79.8
89.8
98.8
7
2020
3010
3640
3220
2480 129000
910 203000
17000 160000
33800 160000
62900 320000
10000
sure H in the system, resulting in a change to an in-
2
5
12
20
45
178
12
7
10000
30000
60000
soluble compound, which then precipitates after the
[
11]
reaction (Scheme S1). We predicted that the bipyri-
dine ligand might be changing from its chelating
conformation in 1 to another conformation under
the high-pressure H2 conditions (Figure S2 in sup-
porting information). Thus, after precipitation, the
catalyst loses its activity towards decomposition of
FA.
6
[
[
d]
e]
12.9
20.3
4
4
4
[c]
c]
[f]
[f]
[f]
[
25
12.5
[
c]
6.5
[
a] The reaction was carried out in degassed aqueous FA solution (20 mL) until gas
evolution ceased; the volume of the evolved gas was temperature-corrected. [b] The
TOF was determined 30 min after beginning the reaction. [c] The reaction was carried
out in a degassed aqueous 4m FA solution (40 mL). [d] 50 wt% FA. [e] 80 wt% FA.
In this work, we have developed an effective cata-
lyst for the dehydrogenation of FA under high-pres-
sure conditions, which has a long lifetime and that
can be recycled several times. Here, we introduce an
Ir catalyst containing 1,10-phenanthroline-4,7-diol
[
f] Temperature of the reaction solution was measured by a thermocouple probe.
À1
(catalyst 2) as a chelating ligand, which prevents cis/trans iso-
obtained TOF value of catalyst 2 was 3010 h
(Table 1,
merization of the pyridine skeleton by bridging, and investi-
gate its potential in terms of catalytic activity, durability, and
reusability.
entry 2), which is superior to that of catalyst 1 (Table 1,
entry 1), and CO was below the detection limit (<5 ppm, Fig-
ure S3). With increasing concentration of FA, the TOF reaches
its maximum between 3 and 6m (Table 1, entries 2-6). Interest-
ingly, the TOF decreased when 80% of highly concentrated FA
was used, but the catalyst maintained its activity during the re-
action. Moreover, the TON increased to 203000 (Table 1,
entry 6). With increasing reaction temperature, a high TOF of
The Ir catalysts 1 and 2 were synthesized as reported in the
[
12]
literature.
using catalyst 2. When we tested the catalyst durability with
mmol of catalyst under atmospheric pressure using 10m of
FA, catalyst 2 continued the dehydrogenation of FA for 2600 h
about 3.5 months), and almost 100% of FA was transformed
to H and CO (Figure 2). The evolved gas volumes (H and
First, we investigated the FA dehydrogenation
1
À1
(
62900 h and high TON of 320000 were obtained at 98.88C.
Hence, catalyst 2 shows comparable activity with catalyst 1 for
the dehydrogenation of FA under atmospheric pressure, as
well as good durability, even from a highly-concentrated FA so-
lution.
2
2
2
CO ) increased linearly and the evolution rate was in the
2
À1
region of 0.11–0.12 mLh for the first 1000 h. The calculated
TON value of catalyst 2 was 5000000. Table 1 compares the re-
sults of the dehydrogenation of FA with catalysts 2 and 1. The
&
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We further evaluated the catalyst durability under high-
pressure conditions. Previously, we reported that catalyst
1
could generate high-pressure gas over 100 MPa by FA dehy-
drogenation, but the catalytic activity gradually decreased as
the reaction progressed. Catalyst 2 can also produce high-pres-
sure gas at 110 MPa due to the dehydrogenation of FA from
a 16m FA solution (Figure 3). It was observed that the rate of
Figure 5. Images of the reactant (catalyst 2: 8 mmol, water: 3 mL, FA (100%):
1
mL) during the reaction at different stages: (a) before the reaction at RT
(
208C), pH 6.8; (b) during reaction after addition of FA at 508C under high-
pressure condition (22 MPa), pH 0.9; and (c) after the reaction and precipita-
tion of catalyst after cooling down to RT (208C), pH 1.9.
À1
(
<0.1 MPah ). Interestingly, after the reaction, catalyst 1 was
completely dissolved in aqueous solution, whereas catalyst 2
had precipitated (Figure 5c). The precipitate could be easily
separated and recovered by filtration. The structural stability of
the precipitated catalyst 2 was confirmed by NMR (Figure S5).
After cooling down the system to 48C, catalyst 2 was filtered
and the Ir complex remaining in the filtrate was less than
6 mol% of the initial catalyst loading [31 ppm by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) analy-
sis]. Catalyst 2 in each of its conformations has a pH-depen-
Figure 3. Time course of high-pressure evolution of gases from FA decom-
position in the presence of catalyst 1 (black cross) and 2 (red circle). Reac-
tion conditions: autoclave: 24 mL internal volume, FA aqueous solution:
À1
À1
16 molL , 13 mL, catalyst: 2.0 mmolL , reaction temperature: 808C.
[12a]
dent solubility in aqueous medium.
We observed that
À1
increase of pressure was comparatively faster than that of cata-
lyst 1 (Figure S4).
To compare the durability of catalysts 1 and 2, we recycled
both of the catalysts under 22 MPa gas evolution conditions
before the reaction, the pH of the 6.5 molL FA solution was
0.9, which changed to a pH of 1.9 after the decomposition of
À1
FA at 22 MPa (0.65 molL ). Thus, the catalyst precipitated as
the reaction progresses owing to the change in pH of the
system (Figure 4 and S6). As a result, catalyst 2 spontaneously
precipitated and 94 mol% was recovered from the reactant
without further pH adjustment.
(Figure 4) by simply removing the aqueous FA under reduced
pressure after the reaction. When catalyst 2 was used, the gas
st
evolution rate remained practically unaltered in the 1 and
nd
th
2
runs. However, by the 4 run, it had decreased to 1/3 of
Recycling of catalyst 2 was conducted for high-pressure de-
hydrogenation of FA at a pressure above 22 MPa (Figure 6).
Catalyst 2 can be successfully recycled over 10 times while
st
that of the 1 run. The rate in the case of catalyst 2 was much
faster (initial rate of 1.3 MPah ) than that of catalyst 1
À1
Figure 4. Time course of high-pressure evolution of gas from FA in the presence of catalyst (a) 1 and (b) 2. The reaction was carried out at 608C in an auto-
À1
À1
st
clave (internal volume is 7 mL) with FA aqueous solution (7 molL , 4 mL) and catalyst (0.1 mmolL ). The catalyst was recycled 4 times (red: 1 run, black:
nd
th
2
run, blue: 4 run) without any purification.
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1
were analyzed by H NMR in [D ]DMSO (Bruker Corp., AVANCE III
6
4
00). The catalyst concentration in solution after the reaction was
monitored by inductively coupled plasma atomic emission spec-
troscopy (ICP-AES, SPS3100, SII Nano Technology Inc.).
A reactor (7–24 mL) was equipped with a pressure transducer
(Kyowa Electronic Instruments Co., Ltd., PGM-500 KE or PG-2TH)
and a stop valve. In a typical experiment, the catalyst aqueous so-
lution and FA were loaded into the reactor at room temperature.
After purging air in the reactor with He, the reactor was pressur-
ized to the desired pressure with He and then heated to the de-
sired temperature. When the reaction was completed, the reactor
was cooled and then depressurized to atmospheric pressure. The
released gas was collected during depressurization.
Figure 6. Recycling experiments of high-pressure gas evolution from FA in
the presence of catalyst 2. The catalyst was recycled after the former reac-
tion without any purification except filtration. Reaction conditions: 508C,
Acknowledgements
À1
À1
The authors thank Dr. David C. Grills of Brookhaven National
Laboratory, and Dr. Maya Chatterjee of AIST for their help with
manuscript preparation. The authors would like to acknowledge
the financial support of Japan Science and Technology Agency
2
8
MPa He, FA aqueous solution (7 molL , 4 mL), catalyst (2 mmolL
mmol). FA was added after depressurization (1 mL). The upper horizontal
,
axis represents times of high-pressure gas release.
(
JST), CREST, and the International Joint Research Program for In-
maintaining its activity, and it has a durability of over 200 h of
total reaction time under 22 MPa. In each experiment, all the
evolved gases were confirmed as H and CO without any de-
novative Energy Technology of the Ministry of Economy, Trade,
and Industry (METI) of Japan.
2
2
tectable amount of CO (<6 ppm, Figure S7). Catalyst 2 is ho-
mogeneous but can be recycled like a heterogeneous catalyst
many times under high-pressure conditions.
Keywords: catalysis · formic acid · high pressure · hydrogen ·
iridium
In conclusion, we have developed an Ir catalyst (catalyst 2)
bearing the 1,10-phenanthroline-4,7-diol ligand, which can be
used for the generation of high-pressure hydrogen from FA
with an obtained highest TON value of 5000000. It can be suc-
cessfully recycled over 10 times without losing any catalytic ac-
tivity. Catalyst 1 with a 4,4’-dihydroxy bipyridine ligand has the
ability to generate high-pressure gas from FA aqueous solution
effectively, but it is not suitable for the development of a high-
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Experimental Section
FA (>99.0%) was used as received from Wako Pure Chemical In-
dustries, Ltd. Deionized water was prepared through a filtration
system (EMD Millipore Corp., ZFSQ240P4) and distillation system
[
[
(
Toyo Roshi Kaisha, Ltd., GS-590). All liquid reagents were degassed
to remove air by bubbling helium gas (He, >99.995%) before use.
The Ir catalysts, [Cp*Ir(4DHBP)(H O)][SO ] (1) and [Cp*Ir(DHP-
2
4
[12]
T)(H O)][SO ] (2) were synthesized according to the literature.
A
2
4
GC-mTCD system (Agilent Technologies, 3000A Micro GC) was used
to determine the concentrations of H , CO , and CO in the released
gas. The concentration of FA in the reaction solution was measured
by a HPLC-UV system on an ion exclusion column (Showa Denko
K. K., KC-811) with phosphoric acid aqueous solution. The pH value
of the reaction solution was determined with a glass electrode
2
2
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983; f) Y. Suna, M. Z. Ertem, W.-H. Wang, H. Kambayashi, Y. Manaka, J. T.
Muckerman, E. Fujita, Y. Himeda, Organometallics 2014, 33, 6519–6530;
Received: May 25, 2016
1
Revised: July 9, 2016
Published online on && &&, 0000
ChemSusChem 2016, 9, 1 – 6
www.chemsuschem.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

COMMUNICATIONS
M. Iguchi, Y. Himeda, Y. Manaka,
H. Kawanami*
Robust recycle: A highly durable and
recyclable Ir catalyst containing a 4,7-di-
hydroxy-1,10-phenanthroline ligand was
developed for high-pressure gas evolu-
tion under 100 MPa by dehydrogena-
tion of formic acid. Simple filtration can
recover 94% of the catalyst after the re-
action and it can be successfully recy-
cled over 10 times without deactivation.
&& – &&
Development of an Iridium-Based
Catalyst for High-Pressure Evolution
of Hydrogen from Formic Acid
&
ChemSusChem 2016, 9, 1 – 6
www.chemsuschem.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!