Hydrogen Production from a Methanol-Water Solution Catalyzed by an Anionic Iridium Complex Bearing a Functional Bipyridonate Ligand under Weakly Basic Conditions

Article abstract of DOI:10.1002/anie.201502194

An efficient catalytic system for the production of hydrogen from a methanol-water solution has been developed using a new anionic iridium complex bearing a functional bipyridonate ligand as a catalyst. This system can be operated under mild conditions [weakly basic solution (0.046 mol L^-1 NaOH) below 100 C] without the use of an additional organic solvent. Long-term continuous hydrogen production from a methanol-water solution catalyzed by the anionic iridium complex was also achieved.

Full text of DOI:10.1002/anie.201502194

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201502194
German Edition: DOI: 10.1002/ange.201502194
Hydrogen Generation
Hydrogen Production from a Methanol–Water Solution Catalyzed by
an Anionic Iridium Complex Bearing a Functional Bipyridonate
Ligand under Weakly Basic Conditions**
Ken-ichi Fujita,* Ryoko Kawahara, Takuya Aikawa, and Ryohei Yamaguchi*
Abstract: An efficient catalytic system for the production of
hydrogen from a methanol–water solution has been developed
using a new anionic iridium complex bearing a functional
bipyridonate ligand as a catalyst. This system can be operated
À1
under mild conditions [weakly basic solution (0.046 molL
NaOH) below 1008C] without the use of an additional organic
solvent. Long-term continuous hydrogen production from
a methanol–water solution catalyzed by the anionic iridium
complex was also achieved.
Scheme 1. Homogeneous catalysts for hydrogen production from
methanol–water mixtures. dme=dimethoxyethane.
H
ydrogen is attracting considerable attention as an alter-
[
1]
native energy source to fossil fuels. The advantages of
hydrogen as an energy source include 1) highest mass energy
density of any fuel, 2) facile conversion to electrical energy as
well as mechanical energy, and 3) generation of only water
upon conversion to energy. To work toward a “hydrogen
society” it is highly important to develop an efficient and
practical method for the production of hydrogen from
renewable and easy-to-handle resources. In this context,
methanol is an ideal candidate substrate, because it is
Scheme 2. Pathway for homogeneously catalyzed hydrogen production
from methanol–water mixtures.
À1
[7]
conditions (8.0 molL KOH) and/or an additional organic
[
2]
[4,8,9]
reproduced from carbon dioxide, is liquid under ambient
conditions, and possesses high hydrogen content.
solvent such as THF, toluene, and triglyme are required.
For practical applications, methanol–water mixtures without
an organic solvent and lower concentrations of base are highly
Although heterogeneously catalyzed reforming of meth-
anol producing hydrogen and carbon dioxide has been
[
5a]
desirable.
We previously reported the dehydrogenative oxidation of
[
3]
continuously studied and developed, this process usually
requires temperatures over 2008C. On the other hand, a few
reports on hydrogen production from methanol–water mix-
tures at temperatures below 1008C have recently been
reported. Most of these examples employ homogeneous
[10]
[10c]
[10d]
alcohols
catalyzed by iridium complexes 4
and 5
(structures shown in Scheme 3) bearing functional bipyridine
and bipyridonate ligands, respectively. Primary and secondary
alcohols were efficiently converted to carbonyl products with
accompanying evolution of hydrogen. Following the success-
ful dehydrogenative oxidation of a variety of alcohols
catalyzed by 4 and 5, dehydrogenation of methanol must be
an attractive target for hydrogen production. In this study, we
report the synthesis of a new water-soluble anionic iridium
[
4–6]
ruthenium catalysts such as 1–3 (Scheme 1).
These cata-
lytic reactions proceed through the following three elemen-
tary steps (Scheme 2): 1) dehydrogenation of methanol to
formaldehyde, 2) hydration of formaldehyde followed by
dehydrogenation generating formic acid, and 3) dehydrogen-
ation of formic acid to carbon dioxide. Hydrogen production
under relatively mild conditions below 1008C has been
achieved by these catalytic systems; however, strongly basic
[
*] Prof. Dr. K. Fujita, Dr. R. Kawahara, T. Aikawa, Prof. Dr. R. Yamaguchi
Graduate School of Human and Environmental Studies
Kyoto University
Sakyo-ku Kyoto 606-8501 (Japan)
E-mail: fujita.kenichi.6a@kyoto-u.ac.jp
yamaguchi.ryohei.75s@st.kyoto-u.ac.jp
[
**] This work was partially supported by KAKENHI (no. 25105732,
6620153, and 26288047) and by the Asahi Glass Foundation. We
2
thank Yuhi Tanaka and Kazuki Toyomura for their valuable technical
assistance.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502194.
Scheme 3. Hydrogen production from a methanol–water solution cata-
lyzed by iridium complexes 4–7.
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complex and its high catalytic activity for hydrogen produc-
tion from a methanol–water solution.
Table 1: Optimization of the reaction conditions for hydrogen produc-
tion from a methanol–water solution catalyzed by 8.
[a]
Firstly, hydrogen production from a methanol–water
solution was examined using 4 and 5 (Scheme 3). When the
methanol–water solution (molar ratio of methanol/water=
[11]
1
:4) was refluxed
for 20 h in the presence of 4 or 5
Entry CH OH/H O
NaOH pH of solution Evolved
Yield of
3
2
(
0.50 mol%), the reaction did not proceed at all. However,
À1
[
mmolmmol
]
[mol%] start/end
H [mmol] H [%]
2
2
addition of a catalytic amount of base (NaOH) greatly
improved the activity, giving a mixed gas of hydrogen and
carbon dioxide (H /CO = 3:1) in 61% (with 4) and 60%
1
2
3
4
5
6
7
8
20/20
20/80
20/140
20/80
20/80
20/80
20/80
20/80
0
0
0
0.30
0.50
1.0
5.0
9.5/7.4
9.0/7.0
8.4/6.9
10.2/7.6
11.1/8.2
11.6/8.4
>13.0/10.3
>13.0/>13.0
4.9
6.2
2.8
34.9
50.2
48.0
37.2
0.9
8
10
5
58
84
80
62
2
2
2
[
12]
(
with 5) yields. These results strongly suggest that a new
catalytically active species must be generated from 4 and 5
under basic conditions. Conversely, complex 6, with a bipyr-
idine ligand without a hydroxy moiety, and complex 7, with
a 4,4’-dihydroxy-2,2’-bipyridine ligand, did not exhibit any
catalytic activity, indicating that a 6,6’-difunctionalized ligand
is critical to the effective catalyst for hydrogen production
from a methanol–water solution.
50
[a] Reaction was performed with methanol (20 mmol), water (20–
40 mmol), 8 (0.50 mol%), and NaOH (0–50 mol%) under reflux
1
conditions for 20 h.
Because basic conditions gave rise to effective hydrogen
production, we attempted to isolate the new complex
generated by the reaction of 5 with a base. When the reaction
of 5 with NaOH (1.5 equiv) was conducted in water, anionic
hydroxo complex 8 was obtained in 72% yield [Eq. (1)]. The
(0.50 mol%) without the addition of base for 20 h, a mixed
gas of hydrogen and carbon dioxide (H /CO = 3:1) was
2
2
evolved in 8% yield (entry 1). The optimum ratio of methanol
and water was determined to be 1:4, which afforded a 10%
yield of hydrogen (entries 1–3). In the reaction in entry 2, the
pH value of the solution decreased from 9.0 to 7.0 during the
reaction, which could be ascribed to the generation of carbon
dioxide. Considering the pH-dependent reversible intercon-
version of the complexes (Scheme 4), catalytically active 8
must be converted to inactive 5 at the end of the reaction.
Hence, addition of base (NaOH) was examined to keep the
solution basic (entries 4–8). The highest yield of hydrogen
(84%) was obtained by addition of 0.50 mol% NaOH
structure of 8 was elucidated by spectroscopic analysis and
single-crystal X-ray analysis (see the Supporting Information,
SI). Complex 8 was highly soluble in water and stable in air.
Interestingly, complexes 4, 5, and 8 were reversibly
interconverted in water upon changing the pH value
[
16]
(entry 5). In this case, the pH of the solution was kept at
above 8.2 during the reaction, thus maintaining the catalyti-
cally active structure of 8. However, addition of too much
[17]
NaOH suppressed the reaction (entry 8).
Under the
[
13]
(
Scheme 4). When NaOHaq was added to the solution of
optimum conditions shown in entry 5, the concentration of
À1
base in the system was as low as ca. 0.046 molL . Thus,
efficient catalytic hydrogen production from a methanol–
water solution under desirable and mild conditions (weakly
basic solution without an additional organic solvent below
1
008C) has been achieved using 8 as the catalyst.
Hydrogen production from a methanol–water solution
catalyzed by 8 would proceed via formaldehyde and formic
acid, as shown in Scheme 2. To obtain information about the
reaction pathway, we conducted the dehydrogenation of
[18]
a formaldehyde–water solution (Table 2). Although 4 and 5
showed very low catalytic activities (entries 1 and 2), 8
exhibited higher activity, affording hydrogen in 57% yield
Scheme 4. pH-Dependent interconversion of iridium complexes 4, 5,
and 8.
(
entry 3). Furthermore, addition of NaOH (0.50 mol%)
[14]
4
in water, precipitation of 5 was observed at pH 6.8. Upon
greatly improved the yield to 89% (entry 4). Dehydrogen-
further addition of NaOH , a clear solution of 8 was obtained
ation of a sodium formate solution in water was also
aq
[
15]
[19,20]
at pH 12.0. In the reverse process, addition of HOTfaq to the
solution of 8 resulted in precipitation of 5 at pH 6.5 and a clear
solution of 4 at pH 2.7.
conducted [Eq. (2)].
The reaction of a sodium formate
Using anionic complex 8, we investigated the optimum
conditions for catalytic hydrogen production from a metha-
nol–water solution (Table 1). When an equimolar mixture of
methanol and water was refluxed in the presence of 8
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Table 2: Dehydrogenation of a formaldehyde–water solution catalyzed
by 4, 5, and 8.
ation of formaldehyde through a formate ion to carbon
[
a]
dioxide would occur by a similar catalytic mechanism (see SI).
Based on these findings, we have designed a new system
for long-term continuous hydrogen production from a meth-
anol–water solution catalyzed by 8. To construct the new
system, the following two points were considered: 1) the
substrates (methanol and water) should be continuously
added to the system at the same rate as they are consumed,
and 2) a small amount of base should be continuously added
to keep the solution slightly basic and maintain the structure
of 8. Thus, we performed the reaction as follows (Figure 1).
Entry Catalyst NaOH [mol%] Evolved H [mmol] Yield of H [%]
2
2
1
2
3
4
4
5
8
8
0
0
0
0.50
4.9
5.1
22.7
35.5
12
13
57
89
[
a] Reaction was performed with 37% formaldehyde solution (20 mmol),
water (80 mmol), iridium complex (0.50 mol%), and NaOH (0 or
.50 mol%) under reflux conditions for 20 h.
0
solution in water in the presence of 8 (0.050 mol%) and
NaOH (0.50 mol%) at 908C for 20 h resulted in the evolution
of hydrogen in 99% yield. These results, shown in Table 2 and
Equation (2), clearly suggest that hydrogen production from
a methanol–water solution catalyzed by 8 proceeds through
the pathway shown in Scheme 2.
A possible catalytic cycle for the dehydrogenation of
a methanol–water solution is presented in Scheme 5. Firstly,
Figure 1. Long-term continuous hydrogen production from a metha-
nol–water solution catalyzed by 8.
Methanol (20 mmol), water (80 mmol), 8 (0.020 mmol), and
NaOH (0.10 mmol) were placed in a flask and heated at
reflux. During the reaction, a solution composed of methanol,
water, and NaOH was continuously added through a syringe
À1
pump. The optimized rates of addition were 0.60 mmolh for
À1
methanol and water, and 0.001 mmolh for NaOH. Accord-
ing to this procedure, continuous hydrogen production was
accomplished for 150 h giving 210 mmol (64% based on
methanol) of hydrogen, with the turnover number of 8
approaching 10510.
In summary, we have developed a new and efficient
catalytic system for the production of hydrogen from
a methanol–water solution using new anionic iridium complex
Scheme 5. Possible mechanism for the hydrogen production from
a methanol–water solution catalyzed by 8.
8
bearing a functional bipyridonate ligand as a catalyst. This
system can be operated under mild conditions [weakly basic
À1
solution (0.046 molL NaOH) below 1008C] without using
anionic methoxo species A would be formed by the reaction
of 8 with methanol. b-Hydrogen elimination of Awould occur
to release formaldehyde and give anionic hydrido species
an additional organic solvent. Long-term continuous hydro-
gen production from a methanol–water solution catalyzed by
8 was also achieved.
[21]
B. Protonation of the bipyridonate ligand by methanol or
[22]
water would give neutral hydrido species C. Protonolysis of
the hydride on iridium with the hydroxy proton on the
functional ligand would then occur to give neutral unsatu-
rated species D accompanied by evolution of hydrogen.
Finally, 8 or Awould be regenerated by the reaction of D with
Keywords: dehydrogenation · homogeneous catalysis ·
hydrogen · iridium · methanol
How to cite: Angew. Chem. Int. Ed. 2015, 54, 9057–9060
Angew. Chem. 2015, 127, 9185–9188
[23]
a hydroxide or methoxide anion. Successive dehydrogen-
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[
12] Volume of the evolved gas was measured using a gas burette.
The ratio of H and CO in the evolved gas was confirmed by GC
[
[
1] a) S. Dunn, Int. J. Hydrogen Energy 2002, 27, 235; b) L. Barreto,
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2
2
analysis. See SI for details.
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7
995. See also reference [5d].
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ascertained by H NMR analysis in D O.
2
[
[
[16] Organic bases were also effective. For example, the reaction of
3
a methanol–water (1:4) solution in the presence of 8
Appl. Catal. B 2010, 99, 43.
(0.50 mol%) and 2,6-lutidine (0.50 mol%) resulted in 63%
yield. We thank the referee for the valuable suggestion.
[17] Suppression of the reaction by too much NaOH is rationalized
by considering the catalytic mechanism (Scheme 5). See
Ref. [22].
[18] Recently, catalytic systems for hydrogen production from
a formaldehyde–water solution have been reported: a) L. E.
Heim, N. E. Schlçrer, J.-H. Choi, M. H. G. Prechtl, Nat.
Commun. 2014, 5, 3621; b) T. Suenobu, Y. Isaka, S. Shibata, S.
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Rodríguez-Lugo, M. Trincado, M. Vogt, F. Tewes, G. Santiso-
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649.
[
[
[
[
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6] Recently, Crabtree et al. reported the dehydrogenation of
methanol catalyzed by iridium N-heterocyclic carbene com-
plexes: J. Campos, L. S. Sharninghausen, M. G. Manas, R. H.
Crabtree, Inorg. Chem. 2015, 54, 5079.
[19] There has been a large number of publications on the catalytic
decomposition of formic acid to H and CO . For examples of the
2
2
iridium-complex-catalyzed reactions, see: a) Y. Himeda, Green
Chem. 2009, 11, 2018; b) S. Fukuzumi, T. Kobayashi, T. Suenobu,
J. Am. Chem. Soc. 2010, 132, 1496; c) R. Tanaka, M. Yamashita,
L. W. Chung, K. Morokuma, K. Nozaki, Organometallics 2011,
30, 6742.
7] Although the reaction catalyzed by 1 proceeds even under
À1
weakly basic conditions such as 0.5 molL NaOH, strongly
À1
basic conditions (8.0 molL KOH) are necessary for the
[20] When a formic acid–water solution was heated at 808C for 1 h in
the presence of 8, the reaction proceeded quantitatively.
Because 8 must be immediately converted to 4 under acidic
conditions, 4 would be highly active for dehydrogenation of
a formic acid–water solution. Actually, the similar reaction
catalyzed by 4 also proceeded almost quantitatively. See SI for
details.
efficient hydrogen production. See Ref. [4a].
8] For a PNP-pincer iron complex-catalyzed hydrogen production
from a methanol – water mixture under strongly basic conditions
À1
(
8.0 molL KOH), see: E. Alberico, P. Sponholz, C. Cordes, M.
Nielsen, H.-J. Drexler, W. Baumann, H. Junge, M. Beller,
Angew. Chem. Int. Ed. 2013, 52, 14162; Angew. Chem. 2013, 125,
1
4412.
[21] We observed spectroscopic evidence of the formation of B from
1
[
9] For a bicatalytic system for base-free hydrogen production from
a methanol – water mixture using triglyme as an organic solvent,
see: A. Monney, E. Barsch, P. Sponholz, H. Junge, R. Ludwig, M.
Beller, Chem. Commun. 2014, 50, 707.
8 by H NMR analysis. We also performed the ESI-MS analysis
of catalytically important iridium species in hydrogen produc-
tion from a methanol–water solution catalyzed by 8, and
observed a peak probably corresponding to B. See SI for details.
[22] It is highly probable that protonation of B leading to C could be
suppressed under too strongly basic conditions. Therefore, the
reaction shown in entry 8 of Table 1 gave poor result.
[
10] a) K. Fujita, N. Tanino, R. Yamaguchi, Org. Lett. 2007, 9, 109;
b) K. Fujita, T. Yoshida, Y. Imori, R. Yamaguchi, Org. Lett. 2011,
1
3, 2278; c) R. Kawahara, K. Fujita, R. Yamaguchi, J. Am.
Chem. Soc. 2012, 134, 3643; d) R. Kawahara, K. Fujita, R.
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Sano, R. Yamaguchi, ACS Catal. 2014, 4, 1010.
[23] We spectroscopically observed the conversion of B to 8 that
could be formed via C and D. See SI for details.
[
11] Reflux temperature of the reaction mixture (methanol/water=
Received: March 15, 2015
1
:4) was 888C.
Published online: June 17, 2015
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