Supercritical hydrogenation and acid-catalysed reactions "without gases"

Article abstract of DOI:10.1039/b404181j

The high temperature catalytic decomposition of HCO₂H and HCO₂Et are used to generate the high pressure H₂ and the supercritical fluids needed for micro-scale hydrogenation of organic compounds; our approach overcomes the problems and limitations of handling high pressure gases on a small-scale and opens the way to the widespread use of continuous supercritical reactions in the laboratory.

Full text of DOI:10.1039/b404181j

Supercritical hydrogenation and acid-catalysed reactions “without gases”
Jason R. Hyde and Martyn Poliakoff
School of Chemistry, University of Nottingham, Nottingham, UK NG7 2RD.
E-mail: martyn.poliakoff@nottingham.ac.uk; Fax: +44 115 9513058; Tel: +44 115 9513386
Received (in Cambridge, UK) 18th March 2004, Accepted 21st April 2004
First published as an Advance Article on the web 27th May 2004
The high temperature catalytic decomposition of HCO2H and
HCO2Et are used to generate the high pressure H2 and the
supercritical fluids needed for micro-scale hydrogenation of
organic compounds; our approach overcomes the problems and
limitations of handling high pressure gases on a small-scale and
opens the way to the widespread use of continuous supercritical
reactions in the laboratory.
This high pressure mixture of H2 and CO2 can be used for
catalytic hydrogenation with a noble metal catalyst in reactor 2
(Fig. 1). The hydrogenation products can be collected directly from
the back-pressure regulator, where phase separation between CO2
and products occur. Table 1-1 shows that cyclohexene can be
hydrogenated quantitatively in this way at 80 °C and 8 MPa,
conditions similar to other scCO2 hydrogenation methodolo-
gies.2,11 Oct-1-ene, Table 1-2, can also be hydrogenated quantita-
tively to octane under similar conditions. However, as expected,
oct-1-yne, Table 1-3, is fully hydrogenated to octane under these
conditions.
Thus, the decomposition of HCO2H is a practical means of
performing supercritical hydrogenation as shown in Table 1.
However the ratio of CO2 : H2 is fixed at 1 : 1. This is particularly
unfortunate because we have previously shown that the control of
H2 concentration is the key to the success of SCF hydro-
The value of micro-scale reactors is becoming increasingly
apparent.1 The use of micro-reactors with supercritical fluids
(SCFs) would be particularly useful on grounds of safety and
environmental acceptability. Unfortunately, the marriage of SCFs
and micro-reactors has been thwarted by difficulties in handling
gases under high-pressure conditions on a very small scale. Here we
describe a new approach in which the SCF and other gases are
generated from liquid precursors, thus overcoming the problems of
gas handling because delivering small quantities of liquids under
high pressure is simple.
Supercritical hydrogenation, particularly in supercritical carbon
dioxide, scCO2, has several advantages over traditional solvent-
based hydrogenation.2–5 Mass transport limitations are minimised,
and access to the catalytic centres can be enhanced. There is
improved heat transfer, avoiding hot-spots, and SCFs are thought to
increase catalyst lifetime by extracting residues from the surface of
the catalyst.6 These factors alone make SCF hydrogenation
attractive, but the real advantage is the solvent-power of the SCF,
which can be tuned by changes in both temperature and pressure.5,7
Changing SCF conditions can alter the selectivity of reactions. The
disadvantage, however, is the need for specialised equipment and
bulky gas cylinders.
Miniaturisation of SCF equipment has the additional advantage
of a faster response to changes in system conditions, with faster
reaction optimisation. Furthermore, if gas cylinders could be
eliminated, SCF chemistry would be transformed, becoming much
more accessible to non-specialists.
Our solution to this problem is to generate both the CO2 and H2
needed for scCO2 hydrogenation in situ by the catalytic decomposi-
tion of formic acid HCO2H.8,9 This is achieved by passing cold
HCO2H over a 5% Pt catalyst preheated to 450 °C, as shown
schematically in Fig. 1.† Under these conditions the alternative
decomposition products, CO and H2O are only generated in
relatively small quantities.10
Fig. 1 Schematic experimental set-up. HCO2H and organic substrate were
delivered by HPLC pumps. System pressure was maintained by a back
pressure regulator. For reaction conditions see Table 1.
Table 1 Results obtained from the “without gases” approach to SCF chemistry
Flow rate/
mL min21
(HCO2H)
Flow rate/
mL min21
(HCO2Et)
Tempera-
ture/°C
Pressure/
MPa
Entry
Starting material
Product
%Yieldd
1
2
3
4
5
6
7
Cyclohexenea
Cylohexane
Octane
Octane
Hydrocinamaldehyde
Tetrahydrofuran
1,3,5-Trimethylisopropylbenzene
3-Methyl-4-isopropylphenol
100
100
100
91
100
45
0.4
0.4
0.4
0.4
0
0
0
0
0
0.4
0.4
0.4
80
80
80
200
180
200
200
8
8
8
20
20
10
10
Oct-1-enea
Oct-1-yneb
trans-Cinnamaldehydeb
Butane-1,4-diolc
1,3,5-Trimethylbenzenec
m-Cresolc
0
0
30
a Deloxan AP/II Pt (5%). b JM Supported Pd (2%). c Amberlyst-15. d All yields determined by GLC. All reactions were performed over a minimum of an
8 hour period without change in conversion or selectivity.
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C h e m . C o m m u n . , 2 0 0 4 , 1 4 8 2 – 1 4 8 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

genations.2,5 We have overcome this limitation by exploiting the
similar critical temperatures of CO2 (31.1 °C) and C2H6 (32.6 °C)
and generating mixtures of scCO2 and scC2H6 by the catalytic
decomposition of HCO2Et, which leads to formation of CO2 and
C2H6.
The control of H2 concentration is achieved by the parallel
decomposition of HCO2Et and HCO2H over a single catalyst bed,
to produced C2H6 and CO2 and H2 (Fig. 2). By controlling the
relative flow rates of HCO2H and HCO2Et, a mixture of H2 + CO2
+ C2H6 can be produced with the desired partial pressure of H2.
presence of a C2H6–CO2 mixture appear to be slightly higher than
in pure scCO2, Table 1-5. Entries 6 & 7 show that the same
approach works with acid-catalysed Friedel-Craft alkylations using
propan-2-ol as the alkylating agent. Again the yields are compara-
ble to previously reported alkylations in “pure” scCO2.15 These
examples demonstrate that the “without gases” approach to SCF
chemistry is versatile and provides a convenient methodology to
anyone who wishes to experiment with SCFs.
There is one complication. The high temperature reverse water
gas shift reaction is unavoidable. If uncontrolled, the concentration
of CO in the gases generated will rise. However, the concentration
of CO can be greatly reduced by using 90% HCO2H in H2O. The
residual H2O and CO do not appear to hinder unduly the fluid
generated from being used as a hydrogenation medium, nor did they
appear to deactivate/poison the catalyst significantly during the
course of our experiments. Currently we are investigating how the
presence of CO in the H2/CO2 mixture can be exploited for
supercritical hydroformylation “without gases”.
Fig. 2 The key principle of the “without gases” approach to SCF
hydrogenation. Changing the flow ratio of HCO2H and HCO2Et changes the
concentration of H2 in the SCF.
We thank EPSRC (GR/R41644), Thomas Swan & Co. Ltd and
HEL Ltd for support. We are grateful to M. Guyler, P. Fields and R.
Wilson, R. W. K. Allen, P. Styring, J. Singh, S. K. Ross, B. Walsh
and P. Licence for their help and advice.
Thus by using two high pressure liquid pumps, we can achieve
control of not only both the overall flow rate of the SCF solvent (a
mixture of scCO2 and scC2H6) but also the concentration of H2
within that mixture. We demonstrate this control of the fluid by
“titrating” H2 into the fluid during the hydrogenation of oct-1-yne,
Fig. 3.
Fig. 3 shows the effect of controlling H2 concentration by this
methodology. Oct-1-yne was hydrogenated whilst the flow rate of
HCO2H was increased to deliver the desired H2 concentrations. It is
important to stress that the two precursor liquids are decomposed
over the same catalyst (either Pt or Pd). Therefore, tuning the
concentration of H2 does not increase the complexity of the
apparatus unduly. We and other authors12–14 have previously
demonstrated that hydrogenation is possible in scC2H6 and,
although the solvent properties of scCO2 and scC2H6 will clearly be
different, we believe that these differences are not so large as to
mask the overall trends in the reactions.
Notes and references
†
All equipment was constructed from 318-SS (SwageLok®) with HPLC
pumps (Gilson 802) and a back pressure regulator (Jasco 880). HCO2H
(90%, Aldrich) and HCO2Et (99.9%, Aldrich) and organic substrates were
used as supplied. Pt 5% (7–10 g, reactor 1) was heated to 450 °C. The flow
of HCO2H and HCO2Et was then started until system pressure was reached.
Organic substrates were introduced after a period of ca. 30 min. after the
system pressure stabilised. Samples were collected directly from the back
pressure regulator and analysed by GLC.
1 W. Ehrfeld, V. Hessel and H. Loewe, Microreactors: New Technology
for Modern Chemistry, Wiley-VCH, Wienheim, 2000.
2 M. G. Hitzler and M. Poliakoff, Chem. Commun., 1997, 1667.
3 A. Baiker, Chem. Rev., 1999, 99, 453.
4 J. W. King, R. L. Holliday, G. R. List and J. M. Snyder, J. Am. Oil Chem.
Soc., 2001, 78, 107.
The decomposition of HCO2Et in the absence of HCO2H opens
up the possibility of SCF reactions other than hydrogenation. This
is demonstrated in Table 1 entries 5–7. The dehydration of
1,4-butandiol to form tetrahydrofuran (THF) has previously been
reported,12 and interestingly the conversions obtained in the
5 M. G. Hitzler, F. R. Smail, S. K. Ross and M. Poliakoff, Org. Proc. Res.
Dev., 1998, 2, 137.
6 B. Subramaniam, V. Arunajatesan and C. Lyon, Coking of solid acid
catalysts and strategies for enhancing their activity, in Catalyst
Deactivation, B. Delmon and G. F. Froment (eds.), Studies in Surface
Science and Catalysis, Vol. 126, Elsevier, Amsterdam, 1999,p. 63.
7 J. R. Hyde, P. Licence, D. N. Carter and M. Poliakoff, Appl. Catal., A.,
2001, 222, 119.
8 Y. Gao, J. K. Kuncheria, H. A. Jenkins, R. J. Puddephatt and G. P. A.
Yap, J. Chem. Soc., Dalton Trans., 2000, 3212.
9 E. Shustorovich and A. Bell, Surf. Sci., 1989, 222, 371.
10 J. S. Francisco, J. Chem. Phys., 1992, 96, 1167.
11 V. Arunajatesan, B. Subramaniam, K. W. Hutchenson and F. E. Herkes,
Chem. Eng. Sci., 2001, 56, 1363.
12 W. K. Gray, F. R. Smail, M. G. Hitzler, S. K. Ross and M. Poliakoff, J.
Am. Chem. Soc., 1999, 121, 10711.
13 S. van den Hark and M. Härröd, Appl. Cat., A, 2001, 210, 207.
14 S. van den Hark, M. Härröd and P. Moller, J. Am. Oil Chem. Soc., 1999,
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Fig. 3 Effect of increasing the relative concentration of H2 on the
hydrogenation of oct-1yne to octane.
15 M. G. Hitzler, F. R. Smail, S. K. Ross and M. Poliakoff, Chem.
Commun., 1998, 359.
C h e m . C o m m u n . , 2 0 0 4 , 1 4 8 2 – 1 4 8 3
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