(255a) Supported Liquid Crystal Membrane for Chiral Separation

There is an increasing demand for optically pure enantiomers in chemical industries such as pharmaceuticals, foods, agrochemicals as one enantiomer is effective while the other enantiomer is ineffective even toxic. Conventional chiral separation techniques, chromatography separation, crystallization, chiral synthesis, and capillary electrophoresis1,2 are energy intensive, batch and complex processes. On the other hand, a membrane based separation is a promising technique due to its simplicity, scalability, energy efficiency, and ability to operate continuously.3 Here we present a novel chiral separation membrane using thermotropic liquid crystals (LCs), 4-pentyl-4'-cyanobiphenyl (5CB). A mixture of 5CB and chiral dopant showed a helically oriented structure known as cholesteric LC (ChLC) phase which was verified using a polarized optical microscopy. The chiral separation membrane was fabricated by impregnation of LC chloroform solution in cellulose nitrate membrane (0.45µm) support under a vacuum. The membrane performance was evaluated for 1-phenylethanol and phenylglycine enantiomers using a nonchiral based HPLC (High Performance Liquid Chromatography) coupled with circular dichroism detection. Theoretical pore size of the membrane after LC impregnation, which was calculated with an assumption of cylindrical pore structure, was increased with an increase of LC chloroform concentration until a critical concentration, 25w/v%. Higher selectivity and lower permeability was observed for a membrane with a smaller theoretical pore size. Selectivity was increased with an increase of chiral dopant composition until a 17 mol% of chiral dopant. The LC membranes showed enantiomer selectivity with an excess of righthanded enantiomer due to more diffusion barrier as a result of more interactions between lefthanded enantiomer and lefthanded ChLC phase.

References 1 Aoki, T.; Tomizawa, S.; Oikawa, E. J. Mem. Sci. 1995, 99, 117-125 2 Rmaile, H. H.; Schlenoff, J. B. J. Am. Chem. Soc. 2003, 125, 6602-6603 3 Xie, R.; Chu, L.; Deng, J. Chem. So. Rev. 2008, 37, 1243-1263