Chemistry Reference
In-Depth Information
high costs are associated with purification of the
products and disposal of nickel residues. The better
mass transfer properties of scCO
2
result in continous
processing with higher space-time yields along with
less formation of undesirable by-products. Further-
more, improved catalyst lifetimes were noticed in
scCO
2
compared with conventional organic solvents,
and the products could be obtained in solvent-free
form.
These promising results have stimulated the inves-
tigation of hydrogenation processes using heteroge-
neous catalysts in various fields of fine chemicals
synthesis. Additional impetus is provided by the
consideration that reaction parameters in a super-
critical flow reactor can be controlled more or less
independently and that pressure effects may be
utilised in supercritical fluids at much lower pres-
sures than those required for traditional liquid-phase
'high pressure' reactions. The hydrogenation of a,b-
unsaturated aldehydes using a Pt/Al
2
O
3
catalyst in
scCO
2
occurred with very high rates in a batch
reactor, with high selectivity towards C=O bond
reduction producing almost exclusively the unsatu-
rated alcohols [59]. The combination of high rate
and selectivity is very hard to achieve in this process
with conventional solvents.
An exploratory study of continuous-flow hydro-
genations was performed with very small reactors of
5- or 10-ml volume using fixed-bed noble metal
catalysts on aminosiloxane supports (Deloxan) [9].
The hydrogenation of a very wide range of sub-
strates, including alkenes, alkynes, ketones, aldehy-
des and epoxides, occurred efficiently in scCO
2
. The
hydrogenation of nitro compounds was performed
in supercritical propane to avoid the formation of
insoluble carbamic salts from the amines and CO
2
.
Enhanced reaction rates were found in almost all
cases and the chemoselectivity during hydrogenation
of multifunctional substrates could be changed dras-
tically by small variations in the reaction parameters
(see Scheme 21.14).
The hydrogenation of isophorone, which is of com-
mercial interest in the fine chemical industry, was
found to exhibit high conversion and high selectivity
towards the desired product dihydroisophorone in
scCO
2
(see Scheme 21.15). The product could be
isolated in sufficient purity and in solvent-free form
with a remarkably high throughput of material in the
small flow-reactor system: 7.5 kg of isophorone could
be hydrogenated continuously using only 2 g of 5%
Pd on Deloxan in a 10-ml reactor with a substrate
flow rate of 7 ml min
-1
. The results indicate also that
Scheme 21.14
Hydrogenation of benzaldehyde in scCO
2
.
Scheme 21.15
Hydrogenation of isophorone in a continuous-flow reactor in scCO
2
.
