Chemistry Reference
In-Depth Information
Fig. 21.1
Schematic phase diagram of
pure carbon dioxide.
medium for chemical synthesis, with a strong
emphasis on homogeneous and heterogeneous
metal catalysis. The material is organised according
to the impact that CO
2
has on a synthetic process in
the context of 'green chemistry'. It ranges from the
replacement of organic solvents to the simultaneous
use of CO
2
as a reagent and solvent, including a dis-
cussion of some unique effects based on the specific
properties of the supercritical state. In this approach,
we found it appropriate to select examples that we
find most illustrative for the individual effects.
Arguably, such an organisation and selection must
remain arbitrary and therefore we refer the inter-
ested reader to some recent reviews with other
emphasis for further reading [1,7,8].
and pressures above the critical values to obtain a
homogeneous mixture in the presence of other com-
ponents. Although a large body of data on the phase
behaviour of CO
2
with various other components
is available, much less is known about systems that
are related directly to synthetic procedures. Model
calculations based on various equations of state can
help to give first estimates about the phase behav-
iour but become rather challenging for multicom-
ponent systems, with varying concentrations as
encountered in typical reaction mixtures.
The phase behaviour of the system CO
2
/H
2
/cyclo-
hexane was studied in the context of heterogeneous
hydrogenation of organic compounds in a continu-
ous flow reactor (see Section 5.1) [9]. As expected,
the miscibility of the system decreases with increas-
ing partial pressures of H
2
under otherwise identical
conditions. At higher temperatures, relatively
small amounts of CO
2
were found to be sufficient
to ensure high miscibility. The phase behaviour of
H
2
/CO
2
/amine systems was investigated as part of
a study devoted to the homogeneously catalysed
hydrogenation of scCO
2
to formic acid and its deriv-
atives (see Section 7) [10]. In the case of triethy-
lamine, a large amount of amine is totally miscible
with scCO
2
in the absence of hydrogen, but the
amount of amine soluble in a single phase is reduced
by a factor of five when H
2
is present at partial pres-
sures typically used in the reaction. If the secondary
2 Phase Behaviour and Solubility in
Supercritical Reaction Mixtures
The phase behaviour of a reaction mixture is an
important parameter for chemistry in scCO
2
. In order
to use the properties of a supercritical phase during
a reaction, one generally will try to find conditions
where a single homogeneous phase is present. On
the other hand, it could be interesting to establish a
two-phase system to achieve a separation after the
reaction. The phase behaviour of mixtures is much
more complex than that shown in Fig. 21.1 for pure
CO
2
and it is not sufficient to work at temperatures
