Novel improved ruthenium catalysts for the generation of hydrogen from alcohols

Article abstract of DOI:10.1039/b613785g

The dehydrogenation reaction of alcohols to generate hydrogen at ambient conditions has been studied, it is shown for the first time that ruthenium in the presence of easily available amine ligands constitutes an active catalysts for this reaction: high turnover frequencies (TOF) up to 519 h^-1 (after 2 h) and catalyst stability (>250 h) were achieved for the dehydrogenation of isopropanol with a combination of [RuCl₂(p-cymene) ]₂/TMEDA. The Royal Society of Chemistry.

Full text of DOI:10.1039/b613785g

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Novel improved ruthenium catalysts for the generation of hydrogen
from alcohols
Henrik Junge, Bjo¨rn Loges and Matthias Beller*
Received (in Cambridge, UK) 21st September 2006, Accepted 16th October 2006
First published as an Advance Article on the web 31st October 2006
DOI: 10.1039/b613785g
presence of RuCl3?xH2O and adamantylphosphines or biarylpho-
sphines.8 The best results were observed applying 2-di-tert-butyl-
phosphinyl-1-phenyl-1H-pyrrole as the ligand.9
The dehydrogenation reaction of alcohols to generate hydrogen
at ambient conditions has been studied, it is shown for the first
time that ruthenium in the presence of easily available amine
ligands constitutes an active catalysts for this reaction: high
turnover frequencies (TOF) up to 519 h21 (after 2 h) and
catalyst stability (.250 h) were achieved for the dehydrogena-
tion of isopropanol with a combination of [RuCl2(p-cymene)]2/
TMEDA.
From a catalyst development point of view it is also interesting
to note that recently various research groups, e.g. Milstein,10
Williams,11 Park,12 Hulshof13 and Hartwig14 investigated the
related oxidations of alcohols in the presence of ruthenium
phosphine catalysts. Here, hydrogen is produced as a side-product.
However, the catalyst efficiency with respect to hydrogen
production is in general low.
The use of alternative feedstocks for energy generation is one of
the central challenges of this century. Due to the progressive
depletion of fossil fuel reserves and the continuously increasing
energy demands, hydrogen is becoming an increasingly attractive
feedstock for energy generation.1 An obvious advantage of a
hydrogen economy is the significant reduction of the emission of
green house gases.
Apart from the ultimate solution, which is water cleavage,2
renewable resources such as biomass or its fermentation products,
e.g. alcohols are a promising basis for the on-time production of
hydrogen for stationary devices. However, efficient hydrogen
production from renewable resources remains difficult and
improved chemical and engineering technologies for generating
hydrogen at higher reaction rates and under milder conditions are
required. In this respect the development of more efficient catalysts
and their understanding will be a key issue. With regard to
catalysts, improvements have been made in the past mainly in the
field of heterogeneous catalysis.3 Consequently, the use of soluble
transition metal complexes for hydrogen generation has been
largely neglected.4
Clearly, for future applications in this area it is crucial to
improve the catalysts with respect to economy, stability, and easy
availability. Here, we report an important step towards this
direction. For the first time convenient catalysts consisting of
ruthenium and easily available amines are shown to be active in
the generation of hydrogen under mild conditions (isopropanol:
TOF .500 h21; 90 uC).
As a starting point and model system the dehydrogenation of
isopropanol to hydrogen and acetone in a 1 : 1 molar ratio was
chosen (Fig. 1) due to easy product analysis, availability and
handling. Hydrogen production was measured both by volumetric
methods and at the same time by a gas sensor. In addition, gas-
chromatographic measurements of the produced gas have been
made. In all cases only hydrogen and argon (from the inert gas
atmosphere) were detected as gaseous products.15 In the liquid
phase, hydrogen and acetone, but also minor amounts (1–15%) of
the aldol condensation, partly followed by hydrogenation
(Guerbet reaction16,17), products could be detected. Hence, it is
important to note that a small part of the evolved hydrogen is
consumed by this side-reaction.
Typically, catalyst tests were performed at 90 uC in the presence
of sodium isopropylate (0.8 mol L21; 6.7–6.9 mol%). In our
previous work we have demonstrated that without base, hydrogen
generation is very slow.9
ð1Þ
To date, only few molecularly defined catalysts have been
developed towards the production of hydrogen.5–7 Among these
reports the work of Cole-Hamilton et al. who used catalysts of the
type [Ru(H2)(X2)(PPh3)3] (X2 = N2, H2) is especially noteworthy.6
Here, turnover frequencies (TOF) of 150 h21 for ethanol, 330 h21
for isopropanol and up to .500 h21 for glycol and n-butanol were
achieved at 150 uC in the presence of NaOH. More recently, we
have demonstrated that the dehydrogenation of isopropanol also
proceeds at a comparably lower temperature (90 uC) in the
In order to develop more economical and less sensitive
ruthenium catalysts we turned our attention to nitrogen ligands
instead of phosphines (Fig. 1). Comparison of different ruthenium
precursors, e.g. RuCl3?xH2O and [RuCl2(p-cymene)]2 without any
ligand present revealed that the latter system already displayed
significant activity (TOF after 2 and 6 h: 192 and 120 h21
,
respectively) (Fig. 2, entry 1). Therefore a broad screening of
various mono-, bi-, and tridentate nitrogen ligands has been done
in the presence of 16 ppm of this ruthenium source (Ru : N = 1 : 2
for bidentate, 1 : 3 for tridentate ligands) (Fig. 2). Almost all of the
tested amines showed some dehydrogenation activity in the model
reaction. However, the addition of trialkylamines gave much better
results compared to secondary and primary amines (Fig. 1,
Leibniz-Institut fu¨r Katalyse e. V. an der Universita¨t Rostock, Albert-
Einstein-Strabe 29a, D-18059, Rostock, Germany.
E-mail: matthias.beller@catalysis.de; Fax: +49 381 12815000;
Tel: +49 381 128113
522 | Chem. Commun., 2007, 522–524
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2-aminomethylpyridine showed a significant inhibition of the
reaction (TOF after 2 h: ,60 h21). On the other hand, simple
aliphatic amines, e.g. triethylamine and n-butyldimethylamine
increased the catalyst activity (TOF after 2 h: .250 h21). This can
not be explained by a simple base effect because N,N-dimethylani-
line gave even better results and the amount of the respective
ligand is small in solution (64 ppm).
Best results were observed in the presence of TMEDA, the
N,N,N9,N9-tetramethyl-1,3-diamine 1, the tridentate TMEDA
derivative 5,2-dimethylaminoethanol and N,N-dimethylaniline
(TOF after 2 h: .300 h21). At this point it is also interesting to
compare the described activity with the previous results obtained
by Cole-Hamilton’s catalyst [Ru(H4)(PPh3)3],6 which still belongs
to the most active catalysts published for the dehydrogenation of
alcohols in the absence of a hydrogen acceptor. Previously, a
catalyst turnover frequency of about 50 h21 was obtained at 90 uC
after 2 h.9
On the basis of the promising results, we studied the ruthenium–
TMEDA and ruthenium–9 in situ catalysts in more detail (Table 1).
Decreasing the [RuCl2(p-cymene)]2 concentration from 80 ppm to
only 4 ppm increased the catalyst activity, which might be
explained by the higher dilution of deactivating aldol condensation
products (mesityloxide and water) (Table 1, entries 1–3). At this
low ruthenium concentration it is beneficial to increase the amount
of the nitrogen ligand. Applying a Ru : N ratio of 1 : 20 led to a
turnover frequency of 519 h21 (Table 1, entry 5). To the best of
our knowledge this is the highest catalyst activity reported below
100 uC for alcohol dehydrogenations. In general, the TOF values
decreased slightly for all catalytic systems after 6 and 24 h,
respectively. This deactivation seems to correlate with the
formation of aldol by-products and water, which are both
generated by self-condensation of acetone. In agreement with this
finding, the addition of mesityloxide and water decreased the rate
of hydrogen formation. Nevertheless, the catalyst system is stable
for several days. Exemplarily, it has been shown that the
combination of [RuCl2(p-cymene)]2 and 10 equiv. TMEDA is still
active after 11 days! Within this time 71 mmol hydrogen have been
produced, which corresponds to a total turnover number (TON)
of 17215 and a potential electric energy of 4.8 Wh.18
Fig. 1 Selected nitrogen ligands applied in the catalyst screening.
Finally, the dehydrogenation of 1-phenylethanol and ethanol
were studied under modified reaction conditions (Table 2).
In the presence of [RuCl2(p-cymene)]2–9 both alcohols generate
hydrogen below 100 uC. However, the reactivity is almost one
order of magnitude lower compared to isopropanol. Besides
hydrogen acetophenone or acetaldehyde are formed. The latter is
mainly converted into higher alcohols by the Guerbet reaction, C4,
C6 and C8-alcohols could be detected by GC/MS. In the case of
Fig. 2 Screening of several nitrogen containing ligands.{
compare reactions with tetramethylethylenediamine (TMEDA),
with dimethylethylenediamine 6 and ethylenediamine 7). To our
surprise, often applied coordinating ligands such as terpyridine and
Table 1 Variation of catalyst to substrate ratio
a
a
a
Catalyst
amount/ppm
Ligand : Ru
Ratio
VH2 /
mL2 h
TOFb/
212 h
VH2 /
mL6 h
TOFb/
216 h
VH2 /
mL24 h
TOFb/
h
2124 h
Ru-Precursor
Ligand
h
h
1c
2c
3d
4d
5d,e
6d
a
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
80
16
4.0
4.0
4.0
4.0
TMEDA
TMEDA
TMEDA
TMEDA
TMEDA
9
1
1
1
3
10
10
166
64
90
104
107
64
161
309
439
505
519
313
c
270
116
177
194
195
144
87
190
288
315
317
233
—
—
—
—
466
337
d
—
—
—
—
189
137
b
Measured by gas burette. Calculated concerning to values measured by gas burette. 10 mL i-PrOH, 1.0 mL ISTD. 40 mL i-PrOH,
4.0 mL ISTD. after 268 h: VH2 1756 mL, TOF 64 h21
.
e
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Chem. Commun., 2007, 522–524 | 523

View Online
selectivity compared with heterogeneous catalysts. See for example:
Table 2 Dehydrogenation of various alcohols (5.0 mL) with
20.5 mmol [RuCl2(p-cymene)]2–9 at 90 uC
J. S. M. Samec, J.-E. Ba¨ckvall, P. G. Andersson and P. Brandt, Chem.
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1507.
TOFb/
212 h
VH2 /
mL6 h
TOFb/
h
216 h
a
a
VH2
mL2 h
/
Entry
1
Substrate
h
134
66
208
34
2
3
15
7.6
3.0
25
11
4.1
1.8
6.0
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a
b
Gas burette. Calculated concerning to values measured by gas
burette.
ethanol also traces of methane as a second gaseous product could
be detected.
8 For other catalytic applications of these ligands see: R. Jackstell,
S. Harkal, H. Jiao, A. Spannenberg, C. Borgmann, D. Ro¨ttger,
F. Nierlich, M. Elliot, S. Niven, K. Cavell, O. Navarro, M. S. Viciu,
S. P. Nolan and M. Beller, Chem.–Eur. J., 2004, 10, 3891; A. Moballigh,
A. Seayad, R. Jackstell and M. Beller, J. Am. Chem. Soc., 2003, 125,
10311; A. Seayad, M. Ahmed, H. Klein, R. Jackstell, T. Gross and
M. Beller, Science, 2002, 297, 1676; K. Selvakumar, A. Zapf and
M. Beller, Org. Lett., 2002, 4, 3031; R. Jackstell, H. Klein, M. Beller,
K.-D. Wiese and D. Ro¨ttger, Eur. J. Org. Chem., 2001, 3871.
9 H. Junge and M. Beller, Tetrahedron Lett., 2005, 46, 1031.
10 J. Zhang, G. Leitus, Y. Ben-David and D. Milstein, J. Am. Chem. Soc.,
2005, 127, 10840; J. Zhang, M. Gandelmann, L. Shimon, H. Rozenberg
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In conclusion, we have shown for the first time, that it is possible
to generate significant amount of hydrogen from alcohols in the
presence of in situ generated ruthenium–amine complexes.
Compared to previously known catalysts the presented complexes
are more easily available, more stable, and show improved
turnover frequencies for the dehydrogenation of isopropanol
below 100 uC. Interestingly, the catalyst systems are active for
more than 11 days, which has so far never achieved for such
reactions.
In addition, our catalyst systems might be also of interest for
transfer-hydrogenation reactions and the dynamic kinetic resolu-
tion processes of chiral alcohols.19 Work towards the further
improvement of the catalyst is in progress in our laboratories.
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 Mrs A.
Lehmann, and Mrs S. Buchholz (all LIKAT) for their excellent
analytical support.
11 G. R. A. Adair and J. M. J. Williams, Tetrahedron Lett., 2005, 46, 8233.
12 J. H. Choi, N. Kim, Y. J. Shin, J. H. Park and J. Park, Tetrahedron
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14 J. Zhao and J. F. Hartwig, Organometallics, 2005, 24, 2441.
15 All experiments were carried out under an inert gas atmosphere (argon)
with exclusion of air. For the standard reaction procedure sodium is
dissolved in 4.0/8.0/38.0 mL of isopropanol at 90 uC in a double walled
thermostated reaction vessel. This vessel is vigorously purged with argon
to remove any hydrogen before starting the catalytic reaction.
Hexadecane, 0.5/1.0/4.0 mL, has been added as internal standard. The
solution containing the catalyst and the ligand in 1.0/2.0 mL isopropanol
is prepared in a Schlenk tube and added to the reaction vessel at 90 uC
via septa and a small teflon tube after refluxing became stationary.
Starting with the addition of the catalyst the progress of the reaction is
followed by several analytical methods. The amount of generated
hydrogen is measured by gas burette. In addition a hydrogen sensor of
the Fa. Hach Ultra Analytics GmbH is used for analysis of hydrogen
and a GC for analyzing gases is applied (gas chromatograph HP 5890,
permanent gases: Carboxen 1000, TCD, external calibration; alkanols,
aldehydes/ketones: HP Plot Q, 30 m, FID). Aldol condensation
products and hydrogenation of the aldol condensation products are
analyzed by gas chromatography (HP 1, 50 m, FID, internal standard).
Typically, the reproducibility of the volumetric determined hydrogen is
between 5–15%.
16 C. Carlini, M. D. Girolamo, A. Macinai, M. Marchionna, M. Noviello,
A. M. Raspolli Galletti and G. Sbrana, J. Mol. Catal., 2003, 204, 721.
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18 The energy content of hydrogen is calculated for the conversion H2 +
0.5 O2 A H2O in a fuel cell assuming 100% efficiency (lower heating
value: 33.33 kWh kg21).
19 W. Barratta, E. Herdtweck, K. Siega, M. Toniutti and P. Rigo,
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M. Hoff and H. Trauthwein, Tetrahedron Lett., 2005, 46, 3403;
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Inorg. Chem., 2005, 4341; A. S. Y. Yim and M. Wills, Tetrahedron,
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Lett., 2004, 45, 6799.
Notes and references
{ For ligand 6 values are not reproducible; in 5 experiments turnover
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524 | Chem. Commun., 2007, 522–524
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