Water-Soluble Iridium-NHC-Phosphine Complexes as Catalysts for Chemical Hydrogen Batteries Based on Formate

Article abstract of DOI:10.1002/cssc.201500808

Molecular hydrogen, obtained by water electrolysis or photocatalytic water splitting, can be used to store energy obtained from intermittent sources such as wind and solar power. The storage and safe transportation of H₂, however, is an open and central question in such a hydrogen economy. Easy-to-synthesize, water-soluble iridium-N-heterocyclic carbene-phosphine (Ir^I-NHC-phosphine) catalysts show unprecedented high catalytic activity in dehydrogenation of aqueous sodium formate. Fast reversible generation and storage of hydrogen can be achieved with these catalysts by a simple decrease or increase in the hydrogen pressure, respectively.

Full text of DOI:10.1002/cssc.201500808

DOI: 10.1002/cssc.201500808
Communications
Water-Soluble Iridium-NHC-Phosphine Complexes as
Catalysts for Chemical Hydrogen Batteries Based on
Formate
[a]
[a]
[b]
[b]
[a, b]
Henrietta Horvµth,* Gµbor Papp,* Roland Szabolcsi, gnes Kathó, and Ferenc Joó*
Molecular hydrogen, obtained by water electrolysis or photo-
catalytic water splitting, can be used to store energy obtained
from intermittent sources such as wind and solar power. The
genation of CO is feasible only in the presence of inorganic or
2
[
4]
organic bases, for example various amines. In other cases,
[
5c]
I
the activity of the often exceedingly active pH-responsive Ir -
based catalysts has to be switched between hydrogenation of
storage and safe transportation of H , however, is an open and
2
À
À
central question in such a hydrogen economy. Easy-to-synthe-
HCO3 and dehydrogenation of HCO2 (in fact: HCO H) by set-
2
size, water-soluble iridium-N-heterocyclic carbene-phosphine
ting the optimum pH for the given reaction with external acid
I
[5]
(
Ir -NHC-phosphine) catalysts show unprecedented high cata-
and base in each cycle.
lytic activity in dehydrogenation of aqueous sodium formate.
Fast reversible generation and storage of hydrogen can be
achieved with these catalysts by a simple decrease or increase
in the hydrogen pressure, respectively.
As a possible answer to these problems, we developed the
[
6]
first purely inorganic reaction system in which the formate–
bicarbonate charge–discharge cycles (Scheme 1) are catalyzed
The boundless accumulation of carbon dioxide in the atmos-
phere is caused in a large part by burning fossil fuels. Fossil
fuels can be replaced by renewable energy, however, new
methodologies have to be developed to make the latter suita-
ble for use in everyday life, and one possibility is its storage in
the form of easy-to-handle chemicals. Hydrogen has long been
suggested as such a chemical because its reaction with oxygen
in fuel cells provides electric energy for universal use (i.e., the
hydrogen economy). Owing to natural fluctuations in the
supply of solar, wind, and other forms of renewable energy, H2
2
Scheme 1. Storage and release of H in the bicarbonate/formate equilibrium.
II
by the same Ru -complex, [{RuCl (mtppms-Na) } ] (mtppms-
2
2 2
Na=monosulfonated triphenylphosphine sodium salt). Dehy-
drogenation of formate in this aqueous reaction mixture yield-
ed CO-free H together with bicarbonate. As a result, storage
2
and delivery of H could be alternated simply by changing the
2
pressure of H with no need of any additives. Very recently
2
a similar system was described, with inorganic formate salts as
II
(
obtained by water electrolysis or photocatalytic water split-
storage material and Ru -PNP pincer complexes as catalysts in
[
7]
ting) can be useful as an energy storage material provided
there are suitable physical or chemical systems in which the
hydrogen itself can be stored and from which it can be liberat-
ed at will.
water–THF or water–dioxane mixtures.
During our research into the synthesis and catalytic applica-
tion of water-soluble iridium(I)-N-heterocyclic carbene-phos-
phine complexes in hydrogenation and in redox isomerization
[
8a]
Formic acid is one of the most promising compounds for hy-
in aqueous solutions, we noticed that the same complexes
[1]
drogen storage. Several efficient homogeneous and hetero-
also catalyzed the decomposition of aqueous sodium formate
[
2]
[8b]
geneous catalysts have been developed for decomposition of
to yield CO-free gaseous H2.
catalysts, that is,
Na [Ir(cod)(emim)(mtppts)] (2) (cod=1,5-octadiene, emim=1-
Therefore, the most active
HCO H; the reaction supplies hydrogen that can be used in
[Ir(cod)(emim)(mtppms)] (1) and
2
fuel cells. However, decomposition of formic acid also yields
CO , in addition to H . Although direct hydrogenation of CO
2
ethyl-3-methylimidazol-2-ylidene,
mtppts-Na =trisulfonated
3
2
2
2
to formic acid (required to construct a closed hydrogen stor-
triphenylphosphine sodium salt; Scheme 2), were studied in
detail both in formate dehydrogenation and in bicarbonate hy-
[3]
age/delivery cycle) has been demonstrated, practical hydro-
drogenation.
These
catalysts,
as
well
as
Na [Ir(bmim)(cod)(mtppts)] (3) (bmim=1-butyl-3-methylimida-
2
[
a] Dr. H. Horvµth, Dr. G. Papp, Prof. F. Joó
MTA-DE Homogeneous Catalysis and Reaction Mechanisms Research Group
P.O. Box 7, 4010 Debrecen (Hungary)
E-mail: henrietta.horvath@science.unideb.hu
joo.ferenc@science.unideb.hu
[
b] R. Szabolcsi, Dr. . Kathó, Prof. F. Joó
Department of Physical Chemistry
University of Debrecen
P.O. Box 7, 4010 Debrecen (Hungary)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500808.
Scheme 2. Water-soluble iridium-N-heterocyclic carbene-phosphine catalysts.
ChemSusChem 2015, 8, 3036 – 3038
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications
Figure 2. Changes in the H
2
pressure (above atmospheric) upon catalytic for-
Figure 1. Time course of hydrogen evolution from aqueous HCO
2
Na solu-
mate dehydrogenation in a closed reactor. [1]=2 mm; [mtppms]=4 mm;
HCO Na]=2400 mm; V(H O)=10 mL; T=808C. At t=100 min and 220 min
P(H ) was set to atmospheric.
tions catalyzed by Na
2
[Ir(cod)(emim)(mtppts)] (2)+2 mtppts at various
[
2
2
À
[
HCO
2
]/[Ir] ratios: * 3000; * 4200; * 5000; * 7500; * 10000. [2]=0.5 mm;
O)=10 mL; T=808C, P(total)=1 bar.
2
mtppts=1.0 mm; V(H
2
zol-2-ylidene) can also be obtained in situ in the reaction of
[
IrCl(cod)(NHC)] and mtppms-Na or mtppts-Na (NHC=emim
3
[
8a]
or bmim).
Figure 1 shows the rapid evolution of H2 from aqueous
HCO Na solutions on catalysis by 2+2 mtppts (the highest ini-
2
À1
tial TOF of H evolution was 11 240 h ; Supporting Informa-
2
tion, Table S1). Note that the catalyst tolerates very high sub-
strate/catalyst ratios (ꢀ10000)—this is important for construct-
ing hydrogen batteries of high capacity. In general, 1 is more
active than 2 (Table S1), however, it has lower solubility and
does not dissolve completely in ꢀ2.1m HCO Na solutions.
2
Figure 3. Reversible hydrogen storage in a formate–bicarbonate system.
However, in the presence of 2 equiv mtppts the solutions of
[
[
Ir]=10 mm, [mtppts]=20 mm; [Ru]=10 mm, [mtppms]=42.5 mm;
H CO Na]=257 mm; V(H O)=0.5 mL; T=808C.
2 2
1
remained homogeneous even at 5m HCO Na concentration
13
2
and yielded H with an unprecedented high turnover frequen-
2
À1
cy (TOF) of 15110 h at 808C.
Dehydrogenation of formate in a closed system results in in-
new cycles on Figure S3 (Supporting Information). One cycle of
[
6]
creased H pressure. Hydrogen evolution stops when thermo-
hydrogen storage from our prior work with [{RuCl (mtppms-
2
2
dynamic conditions of the bicarbonate–formate equilibrium
Na) } ] catalyst is also shown in Figure 3.
2
2
(
Scheme 1) are reached. Under our conditions, in the first cycle
The activities of various catalysts applied so far for hydrogen
storage in aqueous formate systems are shown in Table 1. It is
seen that the activity of [Ir(cod)(emim)(mtppms)]+mtppts is
superior to that of all other catalysts. Because of this high ac-
tivity, one discharge–charge cycle is completed in ꢁ60 min
the equilibrium pressure was 5.9 bar, corresponding to 0.4 L of
H from 10 mL aqueous solution. When releasing the pressure,
2
hydrogen generation resumed and reached a new equilibrium
at a lower pressure (owing to decreased formate concentra-
tion) (Figure 2).
II
(Figure 3) while a similar discharge–charge cycle with the Ru -
[
7]
Discharge–charge cycles of a hydrogen battery were mod-
pincer complexes required 4 h in each direction. Note that
13
elled in a 5 mm high-pressure NMR tube using H CO Na.
other Ir-based catalysts are not listed in Table 1; in fact, the
2
Figure 3 shows that in the dis-
À
charge step HCO /H O was de-
2
2
À
Table 1. Activities of various catalysts of formate dehydrogenation.
composed to H2 and HCO3 in
about 30 min. Increasing the H2
Catalyst
Solvent
M in
HCO M
T
[8C]
iTOF
[h ]
Ref.
À1 [a,b]
pressure to 100 bar resulted in
2
À
a fast hydrogenation of HCO
3
Pd/C nanocat.
Pd nanocat.
Pd/rGO
H
H O
H
DMF/H
DMF/H
2
dioxane/H O
2
O
Na
NH4
K
30
20
80
60
60
84
80
80
80
80
24.4
1800
11 299
2923
2592
820
2600
19
196
[9]
À
to HCO2
(charging; TOFꢀ
[10]
[11]
[12]
[12]
[7]
2
À1
[c]
3
10 h ; see also Supporting In-
2
O
[
d]
[
[
{RuCl
2
2
(benzene)}
(benzene)}
2
]+dppm
]+dppm
2
O
O
Li
formation, Table S2). Upon re-
lease of the pressure, dehydro-
genation of formate took place
again. The catalyst retained its
full activity even after storing
the reaction mixture under
{RuCl
II
2
2
Na
Na
Na
Na
Cs
Na
Ru -PNP pincers
[{RuCl (mtppms-Na) } ]+mtppms
2
2
2
2
2
H O
2
[6]
[{RuCl
(mtppts-Na
(mtppts-Na
3
)
)
2
}
}
2
]+mtppts
]+mtppts
H
H
H
2
O
2
O
2
O
[13]
[13]
this work
[
[
{RuCl
3
2
2
Ir(cod)(emim)(mtppms)]+mtppts
15110
[
ꢁ
a] Initial TOF. [b] iTOF values taken from the literature or calculated from published data for reaction times
1 h. [c] rGO=reduced graphite oxide. [d] dppm=bis(diphenylphosphino)methane.
1
00 bar H2 for 71 days at room
temperature—as shown by four
ChemSusChem 2015, 8, 3036 – 3038
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications
À1
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[14]
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2
805–808.
I
In conclusion, we demonstrate that the water-soluble Ir-
NHC-phosphine type complexes have exceedingly high activity
in dehydrogenation of aqueous sodium formate. This is the
first time such complexes are used to catalyze these two reac-
tions. The same complexes are also remarkably active in the
hydrogenation of bicarbonate to formate. These features allow
construction of additive-free, purely aqueous reversible hydro-
gen storage systems in which the fast charge–discharge pro-
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[
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Keywords: carbenes · homogeneous catalysis · hydrogen ·
iridium · phosphorus
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Received: June 15, 2015
4
Revised: July 14, 2015
Published online on August 19, 2015
ChemSusChem 2015, 8, 3036 – 3038
www.chemsuschem.org
3038
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim