Page 39 - Chiral Separation Techniques

14 1 Techniques in preparative chiral separations

Fig. 1-5. Enantioselective transport processes.

of the compound to be transported [154]. The membrane is then simply a technical
tool which permits a type of liquid–liquid extraction to be performed. In all cases the
membrane should contain the chiral selector to carry out the separation of enan-
tiomers.
The nature of enantioselective solid membranes can be very diverse. Chiral syn-
thetic and semisynthetic polymers have been applied directly for this purpose, but
other chiral molecules have also proved to be useful after immobilization on a
nonchiral porous membrane. Polysaccharide derivatives, especially cellulose carba-
mates [156–159], acrylic polymers, poly(α-amino acids) [160–162] and polyacety-
lene-derived polymers are some of the polymeric selectors that have been successful
in the resolution of racemic mixtures by this method. The high loadability of these
compounds, already demonstrated in HPLC and other classical applications, makes
them very attractive in continuous processes. Moreover, the filmogenic properties of
some of them, such as the polysaccharide derivatives, are interesting characteristics
when the formation of a membrane is envisaged. More recently, the introduction of
molecular imprinted polymers (MIPs) to membrane technologies has been described
as a promising alternative [163–166]. Among the chiral molecules immobilized on a
nonchiral rigid support membrane to perform an enantioselective separation are
amino acids and proteins, such as BSA [167–169]. The main limitation in the case
of solid membranes is the silting that occurs when all recognition sites have been
occupied and there is no real transport through the membrane. An ingenious system
has been described [159] to take advantage of this phenomenon for the separation of
enantiomers.
Liquid membranes can be constituted by liquid chiral selectors used directly [170]
or by solutions of the chiral molecules in polar or apolar solvents. This later possi-
bility can also be an advantage since it allows the modulation of the separation con-