Direction to practical production of hydrogen by formic acid dehydrogenation with Cp?Ir complexes bearing imidazoline ligands

Article abstract of DOI:10.1039/c5cy01865j

A Cp?Ir complex with a bidentate pyridyl-imidazoline ligand achieved the evolution of 1.02 m³ of H₂/CO₂ gases by formic acid dehydrogenation without any additives or adjustments in the solution system. The pyridyl-imidazoline moieties provided the optimum pH to be 1.7, resulting in high activity and stability even at very acidic conditions.

Full text of DOI:10.1039/c5cy01865j

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Direction to practical production of hydrogen by
formic acid dehydrogenation with Cp*Ir
complexes bearing imidazoline ligands†
Cite this: DOI: 10.1039/c5cy01865j
Received 3rd November 2015,
Accepted 4th November 2015
Naoya Onishi,a Mehmed Z. Ertem,b Shaoan Xu,a Akihiro Tsurusaki,ac
b
b
acd
Yuichi Manaka,cd James T. Muckerman, Etsuko Fujita and Yuichiro Himeda
*
DOI: 10.1039/c5cy01865j
www.rsc.org/catalysis
A
Cp*Ir complex with
a
bidentate pyridyl-imidazoline ligand
since the reaction rate decreased with the change in FA to
amine ratio because of FA consumption. Furthermore, due to
the loss of volatile amines during the hydrogen release pro-
cess, the turnover numbers (TONs) in the amine systems
were limited.13 Catalysts that function in aqueous solutions
without any base can avoid these problems, although
their catalytic activities and durabilities are generally
unsatisfactory.8,18
achieved the evolution of 1.02 m3 of H2/CO2 gases by formic acid
dehydrogenation without any additives or adjustments in the solu-
tion system. The pyridyl-imidazoline moieties provided the opti-
mum pH to be 1.7, resulting in high activity and stability even at
very acidic conditions.
Hydrogen (H2) is considered as a clean energy source because
it produces only water when it undergoes combustion.1
However, due to the difficulties associated with the storage
and transport of H2 gas, simple and safe methods to handle
it easily are desired. One such approach is the storage of H2
gas by incorporating it into a liquid organic compound.
Formic acid (FA) is viewed as one of the promising hydrogen
storage materials because it can be interconverted with H2/
CO2.2–6 Based on this idea, much recent attention has been
focused on transition metal-catalysed interconversion
between H2/CO2 and FA.
In 2008, two distinct studies on FA dehydrogenation under
mild reaction conditions without CO contamination were
reported independently by Beller and Laurenczy.7,8 After
these discoveries, other catalysts for the dehydrogenation of
FA have been studied extensively. Fe,9–11 Ru,12,13 and Ir14–17
complexes were shown to be effective catalysts in the pres-
ence of amines and/or organic solvents. However, for the
amine system, control of FA concentration with periodic
additions of FA to maintain the reaction rate was needed,
Pentamethylcyclopentadienyl (Cp*) iridium complexes
with aromatic N,N bidentate ligands (e.g., consisting of pyri-
dine, pyrimidine, or azole moieties) have been found to show
high catalytic activity for the dehydrogenation of FA in aque-
ous solutions.19–23 Furthermore, previous studies have
revealed that both hydrogenation of CO2 and dehydrogena-
tion of FA are catalytically activated by an increase in electron
donating ability from the N,N bidentate ligand to the Ir
centre. Recently, we have reported that electron-rich azole
moieties such as imidazole are also very effective ligands for
FA dehydrogenation.21 Similarly, Xiao et al. have reported
FA dehydrogenation by a Cp*Ir complex with a phenyl-
imidazoline derivative, which was the C–C saturated analogue
of azole, in a FA–amine (5 : 2) azeotrope.14 Although the cata-
lyst showed excellent activity for the dehydrogenation reac-
tion in 10 s, with a turnover frequency (TOF) corresponding
to 145 000 h−1, the repeated addition of FA at short intervals
was required for sustained FA dehydrogenation. We have also
reported that an imidazoline derivative, dihydroxypyrimidyl-
imidazoline, displayed high activity due to the presence of a
pendant OH group as a proton donor site.24 Very recently, Li
reported that Cp*Ir complexes with imidazoline and tetra-
hydropyrimidine moieties were highly efficient in small-scale
experiments under reaction conditions similar to ours.25
Based on these reports, imidazoline moieties could be con-
cluded to be promising ligands for FA dehydrogenation as
well as CO2 hydrogenation.26
a National Institute of Advanced Industrial Science and Technology, Tsukuba
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan.
E-mail: himeda.y@aist.go.jp
b Chemistry Department, Brookhaven National Laboratory, Upton, New York
11973, USA
c CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan
d Renewable Energy Research Centre, National Institute of Advanced Industrial
Science and Technology, 2-2-9 Machiikedai, Koriyama, Fukushima, 963-0298,
Japan
A significant challenge for FA dehydrogenation catalysts is
to satisfy both high catalytic efficiency and durability require-
ments.10,12,13 In our previous report, 50 L of H2 (total volume:
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy01865j
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Table 1 Dehydrogenation of FA in watera
Run Cat./μM Time/h TOFc/h−1 TON
100 L, TON of 2 050 000) was evolved from 200 ml of 6 M FA
solution, although the reaction rate decreased gradually.24
Further efforts in the development of efficient and robust cat-
alysts for the supply of large amounts of H2 are required to
enable the use of FA dehydrogenation on a practical scale.
Herein, we report effective catalysts bearing imidazoline
derivatives for the dehydrogenation of FA that can produce a
large volume of H2 and can sustain their activities for
extended periods of time in water without any organic addi-
tives. Particularly effective and stable H2 production at highly
acidic conditions (e.g., pH 1.7) was achieved by the incorpora-
tion of a bidentate ligand that combines an imidazoline and
a pyridine moiety.
Conv Ref.
1
2
3
4
5
6
7
C1/500
C2/100
C3/100
C4/50
4
5
3
2.5
2.5
4
800
4000
5500
13 300
15 000
54 700
81 900
2000
98
22
10 000 100
10 000 100
20 000 100
20 000 100
100 000 100
22
24
This work
This work
This work
This work
C5/50
C6/10
C6/10b
4
784 000
98
a
b
The reaction was carried out in 1 M aqueous FA solution at 60 °C.
The reaction was carried out in 8 M aqueous FA solution. Average
c
TOF over the initial 10 min.
We have previously reported on FA dehydrogenation using
imidazole complexes C1, C2,22 and imidazoline complex C324
(Scheme 1) in acidic aqueous solutions (Table 1, runs 1, 2 vs. 3).
The results indicated that the non-aromatic imidazoline
moiety could be more efficient than the aromatic imidazole
ones. Based on this hypothesis, we recently prepared
imidazoline complexes C4–C6, and demonstrated their high
catalytic activity for CO2 hydrogenation under basic condi-
tions.27 Firstly, the dehydrogenation of FA for H2 evolution
using C4–C6 is investigated at standard conditions (1 M
aqueous FA solution at 60 °C). In comparing the TOF value
of C1 with that of C4, the imidazoline moiety was concluded
to be more effective than the imidazole moiety for FA dehy-
drogenation (Run 1 vs. 4). This result showed a similar trend
as in the hydrogenation of CO2.27 Additionally, FA dehydroge-
nation by the N-methylated complex C5 (Run 5) was almost
the same as that of C4. From this result, it can be concluded
that the proton in the N–H functionality has little involve-
ment in the dehydrogenation reaction.14 This conclusion is
further supported by density functional theory (DFT) calcula-
tions of the free-energy profiles of the catalytic reactions (see
the ESI†). A high TOF of the bisimidazoline complex C6 was
observed, which is similar to the results of Li.25 Previously,
we have shown that it is necessary to increase the electron
donation ability from the N atom of the ligand to the Ir cen-
tre in order to improve the catalytic activity for FA dehydroge-
nation.20 Based on our recent papers,22,27 we suggested that
an imidazoline moiety would show the highest activity com-
pared to imidazole and pyridine. In fact, C6 with two
imidazoline moieties was confirmed by activity studies at
standard conditions to exhibit the highest TOF among all the
employed catalysts (Table 1).
Next, we examined the pH-dependence of dehydrogena-
tion with C4, adjusted by changing the ratio of FA and
sodium formate (SF) to a total FA + SF concentration fixed at
1 M or by adding H2SO4 (Fig. 1). In our previous report, C3
showed maximum catalytic activity at pH 3.0, influenced by a
pendant OH group on the pyrimidine moiety (Fig. 1, black
solid line).24 However, this result meant that a pH adjust-
ment would be needed for maximum performance of C3. For
practical use in a hydrogen storage system, complicated oper-
ations such as adding compounds for pH adjustment should
be avoided. On the other hand, the reaction rate and conver-
sion by C4 between pH 0.8 and 7.0 were found to be maxi-
mized at pH 1.7 (Fig. 1, blue solid line, Fig. S2(a),† and Table
S1†). The maximum in the activity vs. pH profile of C4 is
shifted to a lower pH than that of C6 (pH 2.2, Fig. 1, red solid
line). Based on these results, we investigated the pH-
dependent rate-determining steps (RDSs) of C4 and C6 via
kinetic isotope effect (KIE) studies (Tables S2 and S3†) and
DFT calculations.21,24,25 FA dehydrogenation using a homoge-
neous catalyst generally proceeds through three steps: (i) for-
mation of a formato complex; (ii) release of CO2 by β-hydride
elimination to generate a metal–hydride complex, [M]–H; and
Fig. 1 pH-dependence of FA dehydrogenation at 60 °C in 1 M FA/SF
solution (pH 1.7–7.0) or 1 M FA containing H2SO4 (pH 0.8–1.6). The pH
of the solution is adjusted by changing the ratio of FA and SF while
keeping their total concentration constant. Black solid line, C3 (100
μM); blue solid line, C4 (50 μM); and red solid line, C6 (10 μM).
Scheme 1 Catalytic dehydrogenation of FA by catalysts C1 through
C6 bearing bidentate ligands 1 through 6, respectively.
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(iii) production of H2 from the reaction of [M]–H and a pro-
ton.21 This plausible mechanism for FA dehydrogenation
using imidazoline-type catalysts was supported by NMR stud-
ies in which C4-C6 reacted with SF to generate the inferred
Ir–H complex (Fig. S3–S5†). From the KIE studies, the forma-
tion of the [Ir]–H complex was found to be the RDS at low pH
where activity is rising, switching to the formation of H2 at
higher pH for both C4 and C6 (Fig. 1 and S2†), where activity
is decreasing. This switchover is further supported by the
free-energy profiles generated by DFT calculations at the M06
level of theory28 (Schemes S1–S14 and Table S3†). In our pre-
vious reports, we have demonstrated that the presence of pro-
ton responsive sites in the ortho-pyridinol moiety of a
bidentate ligand and their associated pKa values play a signif-
icant role in the switching of the RDS from β-hydride elimi-
nation to H2 formation.24 However, in the case of C4 and C6,
there are no proton responsive sites in the associated pH
range, and the change in the RDS occurs at more acidic pH
values (1.7 for C4 and 2.2 for C6) compared to our previously
reported catalysts (e.g., the switchover point for C3 is at pH ≅
3.0). As a consequence of this, C4 and C6 display their opti-
mum activity at more acidic conditions rendering them more
efficient catalysts for H2 generation via FA dehydrogenation.
We also examined the temperature dependence of the
dehydrogenation reaction by C4 (Fig. S6(a) and Table S4†). By
analysing the Arrhenius plot (Fig. S6(b)†), the activation
energy (Ea) was calculated to be 72.0 kJ mol−1, which is
almost the same as the value obtained using the
bisimidazoline analogue C6.25
reaction. Furthermore, the percentage of the components in
the released gases measured by GC was about 1 : 1 of H2 : CO2
during the reaction (Fig. S7†). On the other hand, although
C6 showed excellent activity in the 1 M FA solution, 2% of
the FA remained in the 8 M FA solution after 4 h (Table 1,
run 6 vs. 7). In fact, C6 was clearly degraded in large-scale
experiments, while the use of C4 led to complete conversion
under the same conditions (Fig. S8†). Since high stability and
activity of catalysts are prerequisites for practical use in
hydrogen storage, C4 would be more useful than C6. In addi-
tion, a significant observation is that C4 showed the highest
TOF at pH 1.7 in the presence of only FA in the reaction solu-
tion, which indicates that adjustment of the solution pH is
not required to achieve optimum activity for C4. This indi-
cates that although a pyridyl moiety does not further enhance
catalytic activity, it gives much higher tolerance and provides
more stability than an imidazoline moiety.
For the production of a larger volume of H2, the dehydro-
genation reaction catalysed by C4 was carried out in 1 L of 10
M FA solution at 50 °C (Fig. 3). After the reaction started, it
maintained a very high average TOF value of 7340 h−1. The
total volume of the released gases reached about 0.4 m3 after
114 h. After the reaction rate became slow due to the
decrease in substrate concentration, 10 mol of FA in 100 mL
of H2O was added to the reaction solution (Fig. 3). This sec-
ond stage of reaction maintained high catalytic activity for a
total of 363 h, and the average TOF value was 6250 h−1. This
means that the catalyst was still active in the second stage of
the reaction. Overall, 1.02 m3 of gases (H2 : CO2 ratio of 1 : 1)
was released from 20 mol of FA. HPLC analysis of the reac-
tion solution showed that FA was completely converted to H2
and CO2 gases, and the final TON was 2 000 000. Although
this epoch-making value was obtained only on the laboratory
scale, C4 could be a promising catalyst for a future chemical
hydrogen storage system. Furthermore, these reaction condi-
tions (i.e., aqueous FA solution without any additives) can be
assumed to be optimal because gas evolution was sustained
as long as the FA concentration remained in a suitable range.
For example, the addition of SF to the FA solution led to a
The dependence on the concentration of FA in the range
from 1 M to 20 M was investigated using C4 at 60 °C. The
TOFs and the volumes of released gases are shown in Fig. 2a
(detailed data in Table S5†). The time courses of the volumes
of released gases are shown in Fig. 2b. In 8 M and 12 M FA
solution, the gases were released at a constant rate by the
end of the reaction. Interestingly, although the reaction rate
was slow at the beginning of the reaction in the 20 M solu-
tion, it increased gradually due to the decrease in FA concen-
tration and the solution becomes closer to the optimal pH. In
all cases, FA was completely decomposed by the end of the
Fig. 2 (a) The plots of the TOF versus concentration of FA (solid line)
and the volumes of released gases (dashed line) for the
dehydrogenation of FA by C4 (50 μM) at 60 °C. (b) The time courses of
the volumes of the released gases in various FA concentrations at 60
°C by C4 in 8 M (red), 12 M (blue), and 20 M (black).
Fig. 3 The time courses of the volume and rate of released gases by
FA dehydrogenation in 1 L of 10 M FA solution with C4 (10 μM at the
beginning of the reaction) at 50 °C (red: rate, black: volume of
released gases).
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frequent additions of substrates or additives, or monitoring
the solution system were not required. Furthermore, the gases
could be released under high pressure conditions without any
significant decrease in the reaction rate. An important feature
of the C4 complex is that it displays optimum activity and high
stability in acidic (pH 1.7) conditions. From the viewpoint of
practical use in a hydrogen storage system, these results will
contribute to the development of novel catalyst designs and the
realization of a viable hydrogen storage system.
Acknowledgements
A. T., Y. M. and Y. H. thank the Japan Science and Technol-
ogy Agency (JST), CREST for financial support. The work at
BNL was carried out under contract DE-SC00112704 with the
U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences.
Fig. 4 The time courses of the volume and rate of released gases, and
the pressure of the vessel for FA dehydrogenation in 8 M FA solution
(80 mL) at 60 °C by C4 (25 μM). (red: rate of released gases, blue:
pressure, black: volume of released gases, conditions: pressure gauge
of the back pressure valve was set at 1 MPa).
Notes and references
significant decrease in the rate owing to the increase in pH
caused by consumption of FA (Fig. S1†). Practical production
of H2 requires ease of handling (e.g., no need for any addi-
tives) and control of the reaction system, both of which were
achieved in FA dehydrogenation by C4.
We also investigated high pressure H2 gas production in a
closed vessel. The reaction was carried out using C4 at 60 °C
in a glass autoclave with a back pressure valve set at 1 MPa
(Fig. 4). After 47 min, the pressure reached 1 MPa and the
gases started to be released through the back pressure valve.
Even under high pressure conditions, no significant decrease
in reaction rate was observed, and the average TOF was
17 700 h−1, which was almost the same as that at atmospheric
pressure (19 700 h−1, Table S5†). Compared to the results in
Fig. 3, the reaction rate appeared to decrease earlier because
a relatively small amount of FA (0.64 mol) was loaded.
1 N. Armaroli and V. Balzani, ChemSusChem, 2011, 4, 21.
2 B. Loges, A. Boddien, F. Gartner, H. Junge and M. Beller,
Top. Catal., 2010, 53, 902.
3 Q.-L. Zhu and Q. Xu, Energy Environ. Sci., 2015, 8, 478.
4 T. C. Johnson, D. J. Morris and M. Wills, Chem. Soc. Rev.,
2010, 39, 81.
5 S. Enthaler, J. von Langermann and T. Schmidt, Energy
Environ. Sci., 2010, 3, 1207.
6 W.-H. Wang, Y. Himeda, J. T. Muckerman, G. F. Manbeck
and E. Fujita, Chem. Rev., 2015, DOI: 10.1021/acs.
chemrev.5b00197.
7 B. Loges, A. Boddien, H. Junge and M. Beller, Angew. Chem.,
Int. Ed., 2008, 47, 3962.
8 C. Fellay, P. J. Dyson and G. Laurenczy, Angew. Chem., Int.
Ed., 2008, 47, 3966.
After the gas evolution stopped, the residual FA concentra-
tion was only 0.4 mM (conversion yield, 99.995%). It is clear
that the equilibrium between CO2/H2 and FA even under high
pressure conditions favours gas evolution. It was confirmed
by GC that the released gases were composed of only H2 and
CO2 without CO (<1 ppm, Fig. S9†). This result could be very
important for the supply of H2 gas to devices such as fuel
cells, which are easily degraded by a small amount of CO.
9 A. Boddien, D. Mellmann, F. Gartner, R. Jackstell, H. Junge,
P. J. Dyson, G. Laurenczy, R. Ludwig and M. Beller, Science,
2011, 333, 1733.
10 E. A. Bielinski, P. O. Lagaditis, Y. Zhang, B. Q. Mercado, C.
Wuertele, W. H. Bernskoetter, N. Hazari and S. Schneider,
J. Am. Chem. Soc., 2014, 136, 10234.
11 T. Zell, B. Butschke, Y. Ben-David and D. Milstein, Chem. –
Eur. J., 2013, 19, 8068.
12 G. A. Filonenko, R. van Putten, E. N. Schulpen, E. J. M.
Hensen and E. A. Pidko, ChemCatChem, 2014, 6, 1526.
13 P. Sponholz, D. Mellmann, H. Junge and M. Beller,
ChemSusChem, 2013, 6, 1172.
14 J. H. Barnard, C. Wang, N. G. Berry and J. Xiao, Chem. Sci.,
2013, 4, 1234–1244.
15 S. Fukuzumi, T. Kobayashi and T. Suenobu, J. Am. Chem.
Soc., 2010, 132, 1496.
16 R. Tanaka, M. Yamashita, L. W. Chung, K. Morokuma and
K. Nozaki, Organometallics, 2011, 30, 6742.
Conclusions
Effective catalysts for FA dehydrogenation for H2 evolution
were synthesized by the use of Cp*Ir catalysts with
imidazoline derivatives. The C4 catalyst featuring a bidentate
ligand with imidazoline and pyridine moieties displayed an
improved stability and activity, especially in low pH solu-
tions. C4 completed FA dehydrogenation with a fixed reaction
rate even under higher FA concentration conditions, and as a
consequence, 1.02 m3 of gas was produced and a TON of
2 000 000 was achieved using 20 mol of FA and 10 μmol of C4
in 363 h. In this experiment, complicated operations such as
17 S. Oldenhof, B. de Bruin, M. Lutz, M. A. Siegler, F. W.
Patureau, J. I. van der Vlugt and J. N. Reek, Chem. – Eur. J.,
2013, 19, 11507.
Catal. Sci. Technol.
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18 Y. Maenaka, T. Suenobu and S. Fukuzumi, Energy Environ.
Sci., 2012, 5, 7360.
19 Y. Himeda, N. Onozawa-Komatsuzaki, H. Sugihara, H.
Arakawa and K. Kasuga, J. Mol. Catal. A: Chem., 2003, 195, 95.
20 Y. Himeda, Green Chem., 2009, 11, 2018.
24 W.-H. Wang, M. Z. Ertem, S. Xu, N. Onishi, Y. Manaka, Y.
Suna, H. Kambayashi, J. T. Muckerman, E. Fujita and Y.
Himeda, ACS Catal., 2015, 5, 5496.
25 Z. Wang, S.-M. Lu, J. Li, J. Wang and C. Li, Chem. – Eur. J.,
2015, 21, 12592.
21 W.-H. Wang, S. Xu, Y. Manaka, Y. Suna, H. Kambayashi, J. T.
Muckerman, E. Fujita and Y. Himeda, ChemSusChem,
2014, 7, 1976.
26 N. Onishi, S. A. Xu, Y. Manaka, Y. Suna, W. H. Wang, J. T.
Muckerman, E. Fujita and Y. Himeda, Inorg. Chem.,
2015, 54, 5114.
22 Y. Manaka, W.-H. Wang, Y. Suna, H. Kambayashi, J. T. Muckerman,
E. Fujita and Y. Himeda, Catal. Sci. Technol., 2014, 4, 34.
23 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.
27 S. Xu, N. Onishi, A. Tsurusaki, Y. Manaka, W.-H. Wang, J. T.
Muckerman, E. Fujita and Y. Himeda, Eur. J. Inorg. Chem.,
DOI: 10.1002/ejic.201501030.
28 Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120,
215.
This journal is © The Royal Society of Chemistry 2015
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