Page 161 - Chiral Separation Techniques

5.3 Membrane-Assisted Chiral Separations 139

5.3.1 Liquid–Liquid Extraction

In principle, the same selectors can be used for chiral separation in conventional liq-
uid–liquid extraction as are applied in supported liquid membranes. Generally, enan-
tioselectivity is obtained by addition of a chiral selector to just one of the two phases.
Although relatively high separation factors can be obtained in these types of sys-
tems, a multistage configuration is required to achieve high optical purities. In order
to reach a large number of stages, good countercurrent flow is a prerequisite. The
countercurrent extraction described by Takeuchi et al. [56] meets these specifica-
tions, though scale-up of the proposed apparatus is expected to be difficult. The per-
formance of conventional extraction is often limited by back-mixing and flooding.
This can be avoided by immobilizing the liquid–liquid interface in the pores of a
membrane (Fig. 5-10), so-called “hollow-fiber membrane extraction”. Advantages
of hollow-fiber membrane extraction as compared to conventional extraction are the
high surface area per unit volume, as well as the fact that flow ratios can be chosen
independently [57–61].

Fig. 5-10. Schematic representation of
hollow-fiber membrane extraction.

For the separation of D,L-leucine, Ding et al. [62] used poly(vinyl alcohol) gel-
coated microporous polypropylene hollow fibers (Fig. 5-11). An octanol phase con-
taining the chiral selector (N-n-dodecyl-L-hydroxyproline) is flowing countercur-
rently with an aqueous phase. The gel in the pores of the membrane permits diffu-
sion of the leucine molecules, but prevents convection of the aqueous and octanol
phase. At a proper selection of the flow ratios it is possible to achieve almost com-
plete resolution of the D,L-leucine (Fig. 5-12).
Important numbers in the design of (membrane-based) liquid– liquid extractions
are the extraction factors for both enantiomers, E and E , defined as
L D
E = m L ⋅ F e (2)
L
F
r