Chemistry Reference
In-Depth Information
Chapter 21: Supercritical Carbon Dioxide as
an Environmentally Benign Reaction
Medium for Chemical Synthesis
NATHALIE TANCHOUX AND WALTER LEITNER
syntheses, however, has emerged only during the
last 10-15 years [1].
The critical point of a pure compound marks the
end point of the evaporation line in the phase
diagram. This point is characterised for CO
2
by the
critical temperature
T
c
= 31.0°C, the critical pressure
P
c
= 73.8 bar and the corresponding critical density
d
c
= 0.466 g ml
-1
(Fig. 21.1). Beyond this point, no
distinct liquid or vapour phase can exist and the new
supercritical phase exhibits properties that are rem-
iniscent of both states.
The physicochemical properties of scCO
2
as a
solvent for chemical synthesis are described in detail
elsewhere and will be summarised here only briefly
[1,4,6-8]. Like gases, scCO
2
exhibits a very low
surface tension, low viscosity and high diffusion
rates. It is completely miscible with many reaction
gases over a wide range of composition. This pro-
vides an ideal situation for reactions involving
gaseous reactants, because the availability of the gas
in the supercritical fluid is higher than in a liquid
solvent at comparable partial pressures. Without
losing these gas-like properties, the density of scCO
2
can be tuned to liquid-like values with relatively
small variations of temperature and/or pressure. At
densities around and above the critical density, many
liquid or even solid materials become reasonably
soluble in scCO
2
, whereby the solubility of individ-
ual materials depends strongly on the exact value of
the bulk density of the medium. This last property
forms the basis for the selective purification pro-
cesses in natural product extraction and may apply
equally to isolate products from reaction mixtures
and/or to recover catalytic systems. The density vari-
ations also affect other properties of the supercritical
fluid and sometimes can directly influence the
course of chemical reactions.
In the following parts of this chapter we will use
recent examples to highlight the potential of scCO
2
as an attractive and environmentally benign reaction
1 Introduction
Increasing efforts are being made to develop chemi-
cal processes with minimised ecological impact,
leading, for example, to reduced production of waste
or avoiding the use of hazardous or toxic organic
chemicals. One important aspect in this area is the
quest for replacement of organic solvents by alter-
native reaction media. The majority of chemical
processes in fine chemical synthesis are carried out
in solution and organic solvents usually are the first
choice. These solvents often bear considerable po-
tential risk owing to their toxicity, flammability or
environmental hazards. Furthermore, they have to
be separated from the products completely and recy-
cled or disposed of at the end of the process, adding
to the overall costs of production. In the present
paper we will discuss supercritical carbon dioxide
(scCO
2
) as an alternative solvent providing an envi-
ronmentally benign reaction medium with unique
properties for solution-phase chemistry [1].
Supercritical fluids have been known since the
discovery of the critical temperature more than
175 years ago [2]. Their chemical reactivity has
raised interest since the very beginning, even though
they have been studied mainly from a physical point
of view for a long time [3]. The technology to utilise
scCO
2
as a solvent for selective extraction processes
is well established nowadays in the food industry
and commercial applications include the production
of decaffeinated coffee and hops aroma [4]. These
applications have originated from the pioneering
work of Kurt Zosel at the Max-Planck-Institut für
Kohlenforschung in the middle of the twentieth
century [5]. The commercial success of these pro-
cesses has stimulated a number of efforts to use
scCO
2
in areas as diverse as dry cleaning, dyeing of
fabrics and polymers, metal degreasing and materials
processing. A broad and general interest in the use
of supercritical fluids as reaction media for organic
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