Controlled generation of hydrogen from formic acid amine adducts at room temperature and application in H2/O2 fuel cells

Article abstract of DOI:10.1002/anie.200705972

(Figure Presented) Forever blowing bubbles: Hydrogen is generated from formic acid amine adducts at room temperature used directly in fuel cells (see picture for apparatus). Ruthenium phosphine systems act as catalysts in this transformation.

Full text of DOI:10.1002/anie.200705972

Communications
DOI: 10.1002/anie.200705972
Hydrogen Storage
Controlled Generation of Hydrogen from Formic Acid Amine Adducts
at Room Temperature and Application in H /O Fuel Cells**
2
2
Björn Loges, Albert Boddien, Henrik Junge, and Matthias Beller*
Dedicated to Professor Boy Cornils on the occasion of his 70th birthday
One of the central challenges of this century is the sufficient
and sustainable supply of energy. In this respect, advance-
ments in hydrogen technology, such as the generation of
hydrogen from suitable starting materials, its storage and
conversion into electrical energy, are of particular interest.
Besides methane and methanol, renewable resources,
such as (bio)ethanol and glycerol, are considered as promising
[
1]
sources for hydrogen production. Nevertheless, their use
remains difficult, as the applied reforming processes run at
high temperature (> 2008C). Thus, improved technologies for
generating hydrogen at higher reaction rates and under
milder conditions are required. At present, hydrogen can be
only produced at ambient temperatures by the reaction of
Figure 1. A CO -neutral cycle for the storage of hydrogen in formic
acid base adducts.
2
metals or metal compounds, for example, NaBH , with water.
4
However, these compounds have obvious disadvantages, such
as toxicity, price, and safety.
formic acid base adducts, which is a favorable enthalpy-driven
reaction.
[
5]
To the best of our knowledge there is no reaction system
known at present which is able to generate hydrogen from
organic products in a controlled manner at room temper-
In contrast to hydrogen generation from formic acid, the
homogeneous hydrogenation of CO in the presence of base is
2
well established, and high catalyst activities and selectivities
have been achieved. The reaction was first reported in 1976 by
[
2]
ature. Herein we demonstrate the possibility of generating
hydrogen on demand from mixtures of formic acid and
amines at room temperature. Notably, formic acid as a
hydrogen source is non-toxic and can be handled and stored
[
6]
Inoue et al. Later, seminal work was carried out by Noyori,
[
5,7]
Jessop, Leitner, Jóo, and Himeda.
However, the decom-
position of formic acid was almost disregarded, although it is
the prerequisite for catalytic transfer hydrogenations with
formic acid as the hydrogen donor. Few examples include the
[3]
easily.
Our previous work on the development of low-temper-
ature hydrogen generating systems used alcohols as feed-
[
8]
[9]
[10]
reports of Coffey and the groups of Otsuka, Strauss, and
[2e–g]
[11]
stock.
More recently, we had the idea to apply carbon
Trogler et al., who between 1967 and 1982 described the
dioxide as storage media for hydrogen. Based on the catalytic
processes of formation and decomposition of formic acid, a
power supply system should be possible. Figure 1 depicts a
CO -neutral hydrogen storage cycle. Although CO is avail-
decomposition of formic acid with platinum, ruthenium,
iridium, or rhodium complexes. Catalyst activities are low
with two exceptions: A turnover frequency (TOF) of 100 h
À1
after 15 min at room temperature was observed applying a
2
2
[
9]
À1
able in huge amounts on the earth, the use of carbon dioxide
for hydrogen storage has been largely neglected until now and
platinum phosphine catalyst, and a TOF of 1187 h at 100–
[
8]
1178C with an iridium phosphine complex. Alternatively,
[4]
should be paid more attention in future.
Pd/C has been used for the decomposition of formic acid at
[
4b]
Hydrogenation of carbon dioxide is thermodynamically
an uphill process, and therefore a base is needed to give
room temperature,
1408C.
and of formate salts at 70 to
[
4d,e,12,13]
From the mid-1990s, the decomposition of formates or
formic acid base adducts has been studied as the undesired
[
*] Dipl.-Chem. B. Loges, A. Boddien, Dr. H. Junge, Prof. Dr. M. Beller
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-5000
E-mail: matthias.beller@catalysis.de
Homepage: http://www.catalysis.de
back-reaction in catalytic CO hydrogenation. With rhodium
2
[4a]
[14]
phosphine catalysts, Leitner et al. and Jóo et al. reached
turnover frequencies of approximately 30 h at room tem-
À1
perature and 708C, respectively. Himeda et al. obtained TOFs
À1
[15]
of 238 h with rhodium bipyridine catalysts at 408C.
[
**] This work has been supported by the State of Mecklenburg-
Vorpommern, the BMBF, the DFG (Leibniz prize), and the Fonds
der Chemischen Industrie (FCI). We thank Dr. C. Fischer, S.
Bucholz, and C. Mewes (LIKAT) for their excellent analytical and
technical support and Carbo-Tex GmbH Paderborn for a free sample
of CarboTex.
Puddephattꢀs group used the dinuclear complex [Ru (m-
2
CO)(CO) (m-dppm) ]for the decomposition of formic acid
4
2
À1
without base, and reported TOFs up to 500 h after 20 min in
NMR spectroscopic studies. Furthermore, this reaction has
been investigated with nitrite-promoted rhodium
[16]
[17]
and
3
962
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Angewandte
Chemie
[
18]
molybdenum phosphine catalysts, and efforts have been
Table 2: Decomposition of formic acid amine adducts using [{RuCl (p-
2
cymene)}2].
[19]
made in the field of heterogeneous catalysis.
Initially, we investigated the decomposition of formic acid
in the presence of different homogeneous catalysts at 408C.
As a starting point, 5HCO H/2NEt was chosen as model
Entry Amine
Vgas (2 h)
[mL]
TON
(2 h)
Vgas (3 h)
[mL]
TON
(3 h)
[
a]
[a]
2
3
1
2
NEt3
NH3
41
35
25
50
14
12
8.6
17
20
18
12
4.7
20
11
14
15
61
50
38
76
88
82
56
20
88
50
62
67
21
17
13
26
30
28
19
6.8
30
17
21
23
system, as it is known to act as a hydrogen source in transfer
[
20]
hydrogenation.
3
EtNMe2
Without catalyst, no gas evolution is detected.
4
BuNMe
2
5
6
HexNMe2 59
OctNMe2 53
RhCl ·xH O showed slow formation of hydrogen, whereas
3
2
RuCl ·xH O is not active (Table 1, entries 1 and 2). However,
3
2
7
Bu NMe 35
2
[
{RuCl (p-cymene)} ]is an appropriate catalyst precursor, as
2 2
8
PhNMe2 14
it showed significant activity at catalyst concentrations from
20 to 10000 ppm (Table 1, entries 3–5). [RuCl (p-cymene)-
9
DMAE
59
33
41
44
3
10
11
TMEDA
2
[
b]
(
(
tmeda)]is also active, but needed a longer induction period
Table 1, entry 6). For comparison, the common heteroge-
NEt
3
NEt3[
c]
12
neous transfer hydrogenation catalyst Pd/C was investigated
Table 1, entry 7). However, it is only active within the first
two hours, and thereafter the system is deactivated. Also
other metal precursors, for example, iron compounds
Reaction conditions: 5.0 mL preformed formic acid/amine mixture,
(
29.75 mmol [{RuCl (p-cymene)} ], 408C, 3 h, ratio HCO H/N=5:2.
2
2
2
[a] Measured by gas burette (H
c] 5.0 mL ethanol added.
:CO
=1:1). [b] 5.0 mL water added;
2
2
[
Table 1: Study of various catalysts for the generation of H from5HCO H/2NEt adduct.
2
2
3
gas is generated, which corresponds
to a conversion of 93%. After an
induction period, the hydrogen gen-
eration remained linear for approx-
Entry Vsubst
Catalyst
nmetal
Vgas (2 h)
[mL]
TON
(2 h)
Vgas (6 h)
[mL]
TON
(6 h)
[
a]
[a]
[
mL]
[mmol]
1
2
3
4
5
6
7
8
9
1
5
5
5
5
10
5
5
5
5
5
RhCl ·xH O
19.1
19.1
19.1
59.5
1191
1.9
2.0
-
18
0.1
36
123
666
99
21
0.9
–
19
–
3
2
[
b]
[b]
RuCl ·xH O
0.6
12
41
215
23
3
2
imately 24 hours before a rate
[{RuCl (p-cymene)} ]
13
14
3.7
38
42
11
2
2
acceleration occurred. This could
be explained by the increase in the
ratio of amine to formic acid. In
agreement with these results, addi-
tional experiments with varying
ratios of NEt to HCO H showed
[{RuCl (p-cymene)} ]
2
2
[
c]
[c]
[c]
[c]
[{RuCl (p-cymene)} ]
2
2
[RuCl (tmeda)(p-cymene)] 59.5
7.7
20
–
–
–
34
22
–
–
–
2
Pd/C
19.1
19.1
59.5
59.5
19
[
[
[
b]
b]
d]
[b]
Fe O /silica
0.4
0.6
3.6
2
3
nano-Fe O3
2
3
2
[d]
0
[CpFe(CO)2I]
4.1
that the catalyst activity is increased
Reaction conditions: substrate (subst): preformed formic acid/amine mixture, 408C, 6 h. [a] Measured by a higher amine content (TON
by gas burette (H :CO =1:1). [b] No hydrogen detected by GC. [c] Reaction performed at 26.58C. after 2 h: 0.9 for NEt /HCO H =
2
2
3
2
[d] Only CO and no hydrogen detected by GC.
1
:10 and TON after 2 h: 38 for
NEt /HCO H = 3:4). Notably, after
3
2
full conversion and addition of new
(
Table 1, entries 8–10) were included in the screening, but
HCO H, the catalyst is active again.
2
none of them showed any significant activity.
After demonstrating the possibility to generate hydrogen
The influence of different amines was investigated using
at low temperature from formic acid amine adducts with
1
000 ppm [{RuCl (p-cymene)} ]as the standard catalyst
[{RuCl (p-cymene)} ], we turned our attention to phosphine-
2
2
2
2
precursor, and a formic acid to nitrogen atom ratio of 5:2.
In comparison to the standard system (Table 2, entry 1),
formic acid in the presence of ammonia (Table 2, entry 2) is
less easily decomposed. The investigation of a series of
homologous aliphatic dimethyl amines (Table 2, entries 3–6)
revealed an increase of activity with increasing carbon chain
length.
containing ruthenium complexes. As a starting point we chose
the commercially available complex [RuCl (PPh ) ]. To our
2
3 3
The maximum volume of hydrogen is obtained in the
presence of N,N-dimethylhexylamine and N,N-dimethylami-
noethanol (DMAE) (Table 2, entries 5 and 9), and the lowest
amount of gas generated is observed using N,N-dimethylani-
line (Table 2, entry 8). Interestingly, the addition of water or
ethanol showed no influence on the activity towards formic
acid decomposition demonstrating the robustness of the
system (Table 2, entries 11–12).
In a long-term experiment, the catalytic system was active
for more than 42 h (Figure 2). During this time, 2728 mL of
Figure 2. Gas evolution in a long-term experiment with 5.0 mL
5HCO H/2NEt and 29.75 mmol [{RuCl (p-cymene)} ] at 408C.
2
3
2
2
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3963

Communications
delight this catalyst precursor gave much higher turnover
frequencies: up to 417 and 302 h after 2 and 3 hours,
the advantages of liquid fuels and established H /O fuel
2 2
À1
[21]
cells.
This might be interesting for new applications in
respectively, and a formic acid conversion of 90% after
portable electric devices.
3
hours (Table 3, entry 1). [RuCl (PPh ) ]is only slightly less
2 3 3
active at 26.58C compared to 408C (Table 3, entries 2 and 3).
Owing to the slow dissolution of [RuCl (PPh ) ], we applied Experimental Section
2
3 3
All catalytic experiments were carried out under an inert gas
atmosphere (argon) with exclusion of air. The substrates were dried
by standard laboratory methods prior to use. The formic acid to amine
Table 3: Decomposition of 5HCO
lyst systems.
2
H/2NEt
3
3
adduct with Ru/PPh cata-
1
ratio was determined by H NMR spectroscopy on a Bruker Avance
3
00 spectrometer. The catalyst precursors were purchased from
[
a]
[a]
Entry nRu
Solvent T
[8C] (2 h)
mL]
Vgas
TON Vgas
(2 h) (3 h)
[mL]
TON Conv.
(3 h) (3 h)
[%]
commercial suppliers: [RuCl (PPh ) ](ABCR), Pd/C (10 wt.%)
2
3 3
[mmol]
(Fluka), RuCl ·xH O, RhCl ·xH O, [{RuCl (p-cymene)} ], [CpFe-
3 2 3 2 2 2
[
(CO) I], nano-Fe O (all Aldrich). We prepared [RuCl (p-cymene)-
2
2
3
2
[
[
[
[
[
[
b]
b]
b]
b]
b]
f]
1
2
3
4
5
6
59.5
5.95
5.95
5.95 DMF
5.95 DMF
5.95 DMF
–
–
–
40
40
2436
139
834 2641
905 90
(tmeda)]and Fe O /silica (5 wt.%).
2 3
477
362
882
891
691
143
119
258
261
204
490
408
882
893
700
4.9
4.1
8.8
8.9
7.0
The amount of gas liberated was measured by a gas burette. In
addition, a GC for analyzing gases was applied (Agilent 6890N,
permanent gases: Carboxen 1000, TCD, external calibration; amines:
HP Plot Q, 30 m, FID), and a hydrogen sensor (Hach Ultra Analytics
GmbH) was used for analysis of hydrogen.
26.5 106
[c]
[d]
[d]
40
40
40
258
260
202
[e]
Fuel cell experiments were performed with a fuel cell from AMT
Conditions: 5.0 mL 5HCO H/2NEt , 3 h, catalyst. [a] Measured by gas
2
3
[22]
Analysenmesstechnik GmbH
0 wt.%, catalyst loading 1.5 mgcm , electrode area 6.25 cm ) and
applying Pt–Ru/C as anode (20/
burette (H :CO =1:1). [b] Catalyst: [RuCl (PPh ) ]. [c] 1.0 mL dimethyl-
2
2
2
3
3
À2
2
1
formamide (DMF) added to substrate. [d] Catalyst pretreated in 1.0 mL
À2
Pt/C as cathode catalyst (20 wt.%, catalyst loading 1.5 mgcm
,
À1
DMF (2 h, 808C). [e] TOF 2688 h
RuCl ·xH O+3PPh .
after 20 min. [f] Catalyst:
2
electrode area 6.25 cm ). Gas flow on the anode side was set by the
3
2
3
gas generation reaction; as oxidant, compressed air was used at a flow
À1
rate of 36.66 mLmin
.
For the fuel-cell experiments, the generated gas was cleaned by
leading it through a column (internal diameter 10 mm, length
170 mm) filled with 7.36 g CarboTex (BET 1200 m g -MP/I 1200,
diameter 0.4 mm). CarboTex was purchased from CarboTex Produk-
tions- und Veredelungsbetriebe GmbH Paderborn.
dimethylformamide as solvent for a pretreatment of the
catalyst precursor, which increased the activity further (TOF:
up to 445 h after 2 h; Table 3, entries 4 and 5) and improved
2
À1
À1
the reproducibility. It is noteworthy that the molecularly
Typical procedure for the decomposition of formic acid/amine
adducts: The premixed solution of 5HCO H/2NEt (5.0 mL,
À1
defined pre-catalyst gave a TOF after 20 min of 2688 h ,
2
3
which is the highest activity for formic acid decomposition
known to date (Table 3, entry 5). For comparison, a catalyst
formed in situ from RuCl ·xH O and 3 equivalents of tri-
0
.0595 mol HCO H) was warmed to 26.5 or 40.08C in a double-
2
walled thermostatic reaction vessel. The vessel was purged with argon
to remove any other gas before the reaction was started by addition of
the catalyst [{RuCl (p-cymene)} ](18.23 mg, 29.75 mmol).
3
2
phenylphosphine (Table 3, entry 6) was about 23% less active
than [RuCl (PPh ) ].
2
2
Typically, the standard deviation for the volumetrically deter-
mined hydrogen volumes and calculated activities is between 1–20%.
2
3 3
In the gas phase of all the experiments, only hydrogen and
carbon dioxide are detected alongside the inert atmosphere,
argon, and traces of the evaporated substrates. Thus, and
importantly for fuel cell applications, formic acid is selectively
converted into hydrogen and CO2.
We demonstrated that it is indeed possible to generate
hydrogen from organic compounds under mild conditions and
to directly use it for power generation. Thus, we combined the
Received: December 28, 2007
Revised: February 15, 2008
Keywords: catalysis · formic acid · fuel cells · hydrogen ·
.
ruthenium
hydrogen generation unit with a H /O PEM fuel cell. An easy
2
2
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2
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[
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2
2
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presented can be run at low temperatures without the need
of high-temperature reforming processes. The hydrogen
produced can be directly used in fuel cells, thus combining
[
3
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Angewandte
Chemie
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[
and stability to reach the same power densities as H /O2 fuel
2
[
cells.
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www.angewandte.org
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