Page 149 - Chiral Separation Techniques

Chiral Separation Techniques: A Practical Approach, Second, completely revised and updated edition
Edited by G. Subramanian
Copyright © 2001 Wiley-VCH Verlag GmbH
ISBNs: 3-527-29875-4 (Hardcover); 3-527-60036-1 (Electronic)
5 Membranes in Chiral Separations

Maartje F. Kemmere and Jos T. F. Keurentjes

5.1 Introduction

At present moment, no generally feasible method exists for the large-scale produc-
tion of optically pure products. Although for the separation of virtually every
racemic mixture an analytical method is available (gas chromatography, liquid chro-
matography or capillary electrophoresis), this is not the case for the separation of
racemic mixtures on an industrial scale. The most widely applied method for the
separation of racemic mixtures is diastereomeric salt crystallization [1]. However,
this usually requires many steps, making the process complicated and inducing con-
siderable losses of valuable product. In order to avoid the problems associated with
diastereomeric salt crystallization, membrane-based processes may be considered as
a viable alternative.
During the past decades, the range of conventional separation techniques has been
extended by a wide range of membrane separation processes. The first applications
of membranes are found in biomedical applications such as hemodialysis and
plasmaphoresis [2]. The first industrial membrane application has been the desali-
nation of water streams, mainly for potable water production [3]. A significant effort
in membrane materials development has led to many industrial applications of a
variety of processes, including gas separations, pervaporation, pertraction, electro-
dialysis and various filtration processes (reverse osmosis, RO; nano filtration, NF;
ultrafiltration, UF; micro filtration, MF) [4]. Membrane separations often provide
opportunities as a cost-efficient alternative to separations that are troublesome or
even impossible, using classical methods. Additionally, since most membrane pro-
cesses are performed at ambient temperature, they can offer clear advantages com-
pared to other separation processes, e.g. reducing the formation of by-products.
For the separation of racemic mixtures, two basic types of membrane processes
can be distinguished: a direct separation using an enantioselective membrane, or sep-
aration in which a nonselective membrane assists an enantioselective process [5].
The most direct method is to apply enantioselective membranes, thus allowing selec-
tive transport of one of the enantiomers of a racemic mixture. These membranes can
either be a dense polymer or a liquid. In the latter case, the membrane liquid can be
chiral, or may contain a chiral additive (carrier). Nonselective membranes can also