Transcript Hydrogen generation from alcohols by

Hydrogen Generation from Alcohols by Homogeneous Catalysts
Tarn C. Johnson, David J. Morris and Martin Wills
Department of Chemistry, The University of Warwick, Coventry, CV4 7AL, UK
HDelivery
Introduction
Mechanism of Alcohol Oxidation
As fossil fuel reserves are progressively depleted, the development of a clean
energy supply that is sustainable and can meet the rising global demand for
energy is rapidly becoming one of the greatest challenges of the 21st century. A
promising solution to this problem is the use of hydrogen as an energy vector.
This has the advantage of a significant reduction in greenhouse gas emissions
as hydrogen can be either combusted or converted directly into electricity via
fuel cells, liberating water as the sole by-product. For this reason the catalytic
oxidation of primary and secondary alcohols to yield molecular hydrogen is
desirable (Scheme 1).
Figure 4. Mechanism of alcohol
oxidation
In considering the mechanism of
hydrogen gas production from
alcohols, two discrete steps can be
envisaged; in the first step the
active catalyst removes two
hydrogen atoms from the alcoholic
substrate, resulting in a metal
hydride complex and a carbonyl
compound, the second step being
the release of hydrogen gas to
regenerate the active catalyst
(Figure 4).
Scheme 1. The oxidation of
alcohols with release of hydrogen
gas.
The ruthenium-catalysed oxidation of alcohols with subsequent elimination of
hydrogen gas has been investigated.[1] As an alternative, the use of a hydrogen
acceptor has an accelerating effect on the reaction and allows the generation of
low molecular weight alcohols such as methanol, ethanol and iso-propanol
which could have applications in fuel cells. The selection of an efficient
hydrogen-transfer catalyst (Figure 1) allows this process to be carried out in
high yields.
Phenylethanol Oxidation
by Acetone
Conversion (%)
100
80
60
1
2
3
4
40
20
1
2
0
0
5
10
15
Time (h)
3
4
A
B
C
D
Scheme 3. Conditions; 3 mmol A, 3 mmol B, 0.5 mol % [RuCl2(pcymene)]2, 8 mol % PPh3, 15 mol % LiOH.H2O, toluene, 110 °C, 15 h.
Alcohol oxidation reactions with
Identifying the Ratesubsequent release of hydrogen gas
Determining Step
occur slowly and require raised
100
temperatures. By performing such
80
reactions in the presence of a
60
hydrogen acceptor the hydrogen
A -> C
40
transfer behaviour of the catalyst was
B -> D
probed.
20
The reaction shown in Scheme 3
0
was carried out with monitoring by
0
5
10
15
NMR spectroscopy after 1, 2, 3, 6, 9
Time (h)
and 15 h and the conversions are
plotted in the graph shown in Figure Figure 5. The conversion of A to C
and B to D.
5. The conversions are high after the
first hour which indicates a fast equilibration process between species A, B, C
and D. This shows that the first mechanistic step, the removal of hydrogen, is
fast. The slow rise of the blue line and decline of the red line as the reaction
progresses show a decrease in the quantities of alcohols A and D as hydrogen
is gradually lost from the system. This shows that the second mechanistic
step, the elimination of hydrogen gas, is slow and therefore, the ratedetermining step.
Cyclone Catalysts
Conversion (%)
Catalyst Selection
Figure 1. The oxidation of phenylethanol by ruthenium catalysts in the presence
of acetone.
The application of acetaldehyde or formaldehyde (via paraformaldehyde) as a
hydrogen acceptor in conjunction with catalyst 3
results in essentially
quantitative conversion of phenylethanol to acetophenone and the formation of
ethanol or methanol respectively.
Δ
Substituent Effects in the Substrate
Scheme 2. The simultaneous oxidation of three alcohols with release
of hydrogen gas.
Competition Experiment
Conversion (%)
100
80
60
R=
-H
-OMe
-Me
40
20
0
0
5
10
15
Time (h)
Figure 2. The simultaneous oxidation
of three alcohols with the release of
hydrogen gas.
Oxidations in the Absence of
PPh3
100
Conversion (%)
The oxidation of phenylethanol and
two derivatives bearing increasingly
electron-donating
substituents
indicates a trend whereby more
electron-rich alcohols are oxidised
more
readily.
A
competition
experiment in which three alcohols
are oxidised simultaneously in one
reaction vessel is illustrated in
Scheme 2 and the results in Figure
2. This demonstrates the importance
of substituent effects in the catalysis.
80
60
R=
-H
-OMe
-Me
40
20
Figure 6. The Shvo catalyst and the mechanism by which it oxidises alcohols.
Catalysts bearing a cyclopentadienone ligand, so-called ‘cyclone catalysts’ are
very effective for hydrogen transfer reactions. The Shvo catalyst (Figure 6)
splits into two catalytically active species which take part in hydrogen transfer
through a bifunctional mechanism utilising a pendant oxygen atom.[2] An
analogous iron complex has also been shown by Casey et al.[3] to be an
efficient catalyst for ketone reductions. Current work focuses on the synthesis
of novel iron cyclone catalysts (Figure 7) with higher activities that operate at
lower temperatures.
0
0
5
10
15
Time (h)
Figure 3. The oxidation of three
alcohols with the release of hydrogen
gas with a less efficient catalyst.
This trend is emphasized by the use of
a less efficient hydrogen-transfer
catalyst. By removing the additive PPh3
from the catalytic system used in the
competition experiment, a less efficient
active catalyst
is generated. This
results in lower conversions with the
notable exception that the most
electon-rich
alcohol,
pmethoxyphenylethanol, is still oxidised
in a moderate yield (Figure 3).
Figure 7. Complexes
under investigation
Conclusions
The oxidation of alcohols by the method presented is very substrate sensitive,
with more electon-rich alcohols being oxidised more readily. The ratedetermining step in this process is the release of hydrogen gas from the
catalyst. For this reason high temperatures and long reaction times are
necessary to achieve high conversions. The use of a hydrogen acceptor
results in a dramatic rate increase and leaves scope for the transfer of
hydrogen from complex alcohols to form more simple ones for use in fuel cell
applications.
Acknowledgements. We thank the EPSRC (via SUPERGEN IV) for funding of
this project.
References. [1] Tarn C. Johnson, David J. Morris and Martin Wills, Chem. Soc. Rev. 2010, 39,
81-88. [2] R. Karvembu, R. Prabhakaran and K. Natarajan, Coord. Chem. Rev., 2005, 249, 911918. [3] C.P. Casey and H. Guan, J. Am. Chem. Soc., 2007, 129, 5816-5817.