Separation of Racemic Mixture by Ultrafiltration of Enantioselective Micelles. 1. Effect of pH on Separation and Regeneration

Abstract

Many enantiomer separation systems are studied to meet the increasing demand for enantiopure compounds. One way to obtain pure enantiomers is to apply enantioselective micelles in ultrafiltration systems. We have studied the separation of phenylalanine (Phe) enantiomers by the ultrafiltration of nonselective nonionic micelles containing selector molecules, cholesteryl-l-glutamate:CuII (CLG:CuII). Because the net charges of enantiomer and CLG are pH-dependent, it is foreseen that pH will be an important factor in the design of a cascaded separation process that yields enantiopure products. Experiments at pH 7, 9, and 11 showed that the complexation can be described by multicomponent Langmuir isotherms. The CLG enantioselectivity for d-Phe increases with decreasing pH, being 1.4, 1.7, and 1.9 for pH 11, 9, and 7, respectively. Accordingly, the saturation concentration and the affinity constants decrease with decreasing pH, finally resulting in no complexation at pH 6. To design an economically attractive separation process, the regeneration of d-Phe-saturated micelles leaving the multistage system is inevitable. Regeneration, i.e., recovery of enantioselective micelles for reuse, is possible at pH ≤ 4. To keep salt production to a minimum, the shift in pH between the separation and regeneration processes must be minimized. Therefore, a separation process at pH 7 seems most attractive.