Chemistry Reference
In-Depth Information
2.3 Applications of supercritical fluids
Due to their unique properties, SCFs have found wide applications in
several fields, including extraction, as a medium for catalytic reactions
and for materials synthesis and production. Supercritical fluids can be
utilised in chromatography and large-scale industrial extractions. The
high diffusion coecients and tuneable solvating power make SCFs ideal
in well established separation technologies. Uses in chromatography
include the detection of hydrocarbons, polymer additives and toxic
compounds in food.
12,13
Examples of successful large-scale industrial
processes are the extraction of medicinal compounds from plants,
decaffeination, and the extraction of seed oils.
14-17
Reactions have been carried out using SCF solvents for both
heterogeneous and homogeneous catalytic reactions. SCFs have been
shown to have applications as both solvents and reactants. In the field of
heterogeneous catalysis a range of reactions have been performed in a
range of SCFs, such as alkylation,
18
esterification,
19
Fischer-Tropsch
synthesis,
20
hydrogenations,
21,22
isomerisation
23
and oxidation re-
actions.
24
Benefits achieved using an SCF as solvent include the lack of a
gas/ liquid interface, enhanced mass transfer at the catalyst interface and
the ability to separate reactants and products by tuning conditions. The
ability to tune reaction conditions to selectivly favour different products
has been aptly demonstrated by Stevens et al. for the hydrogenation of
furfural in SC-CO
2
.
21
A comprehensive dedicated review by Baiker
provides a detailed discussion on the use of supercritical fluids as reaction
media for heterogeneous catalysis.
6
Homogeneous catalytic processes have
also been widely investigated, with many reactions including isomeriza-
tion, hydrogenation and cyclization reactions investigated, and these are
discussed in detail in Noyori's review on the topic.
25
The ability to separate
catalyst and products relatively easily by use of an SCF provides a flexibility
for application of homogeneous catalysts. Simple manipulation of the
pressure and temperature can easily achieve the separation of valuable
transition metal catalytic complexes and products.
In the context of preparation of materials, SCFs can be utilised in
several ways; as a reactant, a solvent or alternatively as an anti-solvent. A
vast array of materials for various applications have been prepared,
utilising the properties of SCFs. Some excellent reviews are available: Darr
and Poliakoff have published a comprehensive discussion on SCF tech-
niques and the subsequent inorganic materials that can be produced.
5
Yeo and Kiran have focussed on SCFs for the preparation of polymer
materials
26
and a review by Subramaniam et al. have concentrated on
SCFs for pharmaceutical processing.
27
The following section gives some
brief details on the SCF methods that can be used for materials prepar-
ation, but will not specifically give detail on the materials prepared.
3 Supercritical fluid methods for materials preparation
One of the first techniques to prepare materials using SCFs as a solvent
was the rapid expansion of supercritical solutions (RESS) technique.
28,29
RESS involves dissolving a solid in a supercritical solvent, followed by the
