Page 150 - Chiral Separation Techniques

128 5 Membranes in Chiral Separations

provide essential nonchiral separation characteristics in combination with chirality
outside the membrane. The required enantioselectivity can stem either from a selec-
tive physical interaction, or from a selective (bio)conversion.
In this chapter we will provide an overview of the application of membrane sep-
arations for chiral resolutions. As we will focus on physical separations, the use of
membranes in kinetic (bio)resolutions will not be discussed. This chapter is intended
to provide an impression, though not exhaustive, of the status of the development of
membrane processes for chiral separations. The different options will be discussed
on the basis of their applicability on a large scale.

5.2 Chiral Membranes

5.2.1 Liquid Membranes

In general, a liquid membrane for chiral separation contains an enantiospecific car-
rier which selectively forms a complex with one of the enantiomers of a racemic
mixture at the feed side, and transports it across the membrane, where it is released
into the receptor phase (Fig. 5-1).

Fig. 5-1. Schematic representation of a liquid membrane for chiral separation.

The carrier should not dissolve in the feed liquid or receptor phase in order to
avoid leakage from the liquid membrane. In order to achieve sufficient selectivity,
minimization of nonselective transport through the bulk of the membrane liquid is
required. Liquid membranes can be divided into three basic types [6]: emulsion; sup-
ported; and bulk liquid membranes, respectively (Fig. 5-2).