Enantioselective separations using chiral supported liquid crystalline membranes

Abstract

Porous and nonporous supported liquid crystalline membranes were produced by impregnating porous cellulose nitrate supports with cholesteric liquid crystal (LC) materials consisting of 4-cyano-4'-pentylbiphenyl (5CB) mixed with a cholesterol-based dopant (cholesteryl oleyl carbonate [COC], cholesteryl nonanoate [CN], or cholesteryl chloride [CC]). The membranes exhibit selectivity for R-phenylglycine and R-1-phenylethanol because of increased interactions between the S enantiomers and the left-handed cholesteric phase. The selectivity of both phenylglycine and 1-phenylethanol in 5CB/CN membranes decreases with effective pore diameter while the permeabilities increase, as expected. Phenylglycine, which is insoluble in the LC phase, exhibits no transport in the nonporous (completely filled) membranes; however, 1-phenylethanol, which is soluble in the LC phase, exhibits transport but negligible enantioselectivity. The enantioselectivity for 1-phenylethanol was higher (1.20 in 5CB/COC and 5CB/CN membranes) and the permeability was lower in the cholesteric phase than in the isotropic phase. Enantioselectivity was also higher in the 5CB/COC cholesteric phase than in the nematic phase of undoped 5CB (1.03). Enantioselectivity in the cholesteric phase of 5CB doped with CC (1.1), a dopant lacking hydrogen bonding groups, was lower than in the 5CB/COC phases. Finally, enantioselectivity increases with the dopant concentration up to a plateau value at approximately 17 mol%.

Temperature-Dependent Gas Transport Behavior in Cross-Linked Liquid Crystalline Polyacrylate Membranes

“…The activation energy for permeation is higher in the nematic phase than the isotropic phase for all gases studied. This is expected, as the ordering in the nematic phase acts as a barrier to transport, and is in agreement with activation energies for transport in small molecule LC materials [16]. The relative activation energies for different gases do not correlate directly with the size of the gas molecules, indicating that interactions between the gases and polymer are probably more important.…”

“…Investigations of the selective pervaporation of volatile organic compounds and the enantioselective properties of cholesteric liquid crystalline membranes have been reported [13,14,15,16]. In addition, there have been several studies of side-chain LCPs focusing on the relationship between polymer microstructure and transport properties [17,18,19,20,21,22].…”

Separation of Enantiomers by Continuous Preferential Crystallization: Experimental Realization Using a Coupled Crystallizer Configuration

“…This allows the separation of many racemates. − A version of chiral chromatography successfully applied in industrial scale is continuous countercurrent simulated moving-bed (SMB) chromatography enabling an increase in productivity and realizing the goal of achieving optically pure enantiomers . Nevertheless, highly expensive chiral stationary phases and a dilute separation product have initiated the search for and development of alternative technologies which, among others, include (1) diastereomeric (or classical) resolution, − (2) chiral membranes, − (3) dynamic resolution, − (4) liquid–liquid extraction, − and (5) preferential crystallization. − Preferential crystallization, in particular, has attracted great attention, the reason being that the majority of chiral pharmaceutical, agrochemical, food, and cosmetic products, and their intermediates are produced as highly pure solids.…”

Principles, Concepts and Strategies of Stereoselective Synthesis

“…149 Adsorption-type enantioselective membranes and membrane-assisted resolution systems with non-enantioselective solid membranes have attracted much attention recently. [150][151][152][153] If the enantioselective membrane is possible to recycle, the enantiomer membrane extraction technology will afford an efficient, practical way for the chiral resolution. As a result of the separation process, diastereomeric mixtures can be resolved, and enantioselectivities up to 95% ee for some racemates can be then achieved.…”