Enantioseparation with liquid membranes

Chiral resolution of racemic products is a challenging and important task in the pharmaceutical, agrochemical, flavor, polymer and fragrances industries. One of the options for these challenging separations is to use liquid membranes. Although liquid membranes have been known for almost four decades and have been used for optical resolutions, no comprehensive review has been published about the use of this technology for enantioseparations. In this review, the various liquid membrane-related technologies are described and compared, including bulk liquid membranes, emulsion liquid membranes, micelle-extraction and micellar enhanced ultrafiltration, as well as supported liquid membranes. Next to technological advances, an overview of recent developments in chiral recognition chemistry in liquid-liquid equilibria is presented. The following extractant classes have recently been reported in conjunction with chiral separation: cyclodextrines, BINOL's, calixarenes, crown ethers, BINAP's, tartaric acids and ionic liquids. The use of two supported (non-liquid) membranes with an inner loop of extract phase appears to be the most stable liquid membrane configuration, allowing for a large degree of freedom in operational conditions such as solvent to feed ratio. The library of solvents still needs broadening to make the technology more versatile and based on the variety of successes with catalytically active organometallic complexes, development of new chiral selector systems based on asymmetric catalysis literature is suggested for future selector screening studies.