Membrane-assisted enantiomer

A large variety of applications using either vapor permeation or pervaporation has been reported. These include the use of pervaporation for the removal of toxic organics from water (Schnabel et al., 1998) and wastewater streams (Moulin et al., 2002), sometimes using hybrid approaches with adsorptive techniques the use of pervaporation membranes in direct methanol fuel cells (Pivovar et al., 1999) and, more recently, the resolution of isomeric mixtures (Kusumocahyo etal., 2004) and membrane-assisted enantiomer enrichment (Paris et al., 2004), in both cases using membranes containing specific complexation agents such as cyclodextrins. 

Another possibility of constructing a chiral membrane system is to prepare a solution of the chiral selector which is retained between two porous membranes, acting as an enantioselective liquid carrier for the transport of one of the enantiomers from the feed solution of the racemate to the receiving side (Fig. 1-5). This system is often referred to as membrane-assisted separation. The selector should not be soluble in the solvent used for the elution of the enantiomers, whose transport is driven by a gradient in concentration or pH between the feed and receiving phases. As a drawback common to all these systems, it should be mentioned that the transport of one enantiomer usually decreases when the enantiomer ratio in the permeate diminishes. Nevertheless, this can be overcome by designing a system where two opposite selectors are used to transport the two enantiomers of a racemic solution simultaneously, as it was already applied in W-tube experiments [171]. 

Most of the chiral membrane-assisted applications can be considered as a modality of liquid-liquid extraction, and will be discussed in the next section. However, it is worth mentioning here a device developed by Keurentjes et al., in which two miscible chiral liquids with opposing enantiomers of the chiral selector flow counter-currently through a column, separated by a nonmiscible liquid membrane [179]. In this case the selector molecules are located out of the liquid membrane and both enantiomers are needed. The system allows recovery of the two enantiomers of the racemic mixture to be separated. Thus, using dihexyltartrate and poly(lactic acid), the authors described the resolution of different drugs, such as norephedrine, salbu-tamol, terbutaline, ibuprofen or propranolol. 

For the separation of racemic mixtures, two basic types of membrane processes can be distinguished a direct separation using an enantioselective membrane, or separation in which a nonselective membrane assists an enantioselective process [5]. The most direct method is to apply enantioselective membranes, thus allowing selective 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... 

A particular example of a membrane-assisted process applied on a large scale is the acylase-catalyzed resolution of N-acetyl-D,L-amino acid, as developed by De-gussa AG [128, 129]. Annually, the industrial plant produces several hundreds of tonnes of enantiomerically pure L-amino acid. D,L-amino acid is acetylated in a Schotten-Baumann reaction to N-acetyl-D,L-amino acid. Subsequently, the L-amino acid enantiomer is obtained via an acylase reaction. Figure 5.15 shows the reaction scheme. 

---