It has been observed in a previous study that small amounts of the chiral substance hydroquinine-4-methyl-2-quinolylether (HMQ) increased the yield of the preferential crystallization processes of (+)-(S)-mandelic acid without contaminating the solid product. In this work, systematic investigations were performed to verify the influence of HMQ on the nucleation and crystal growth of mandelic acid. It has been found that HMQ inhibits both primary nucleation of the enantiomer and the racemate. Besides, it leads to a retarded crystal growth rate of the enantiomer with an increased amount of HMQ. Further, a series of preferential crystallization runs with varied HMQ amounts were conducted, and the enhancement of the yield was quantitatively evaluated. Different from common cases, the additive suppresses primary nucleation of both enantiomers and the racemate.

The resolution of chiral compound-forming systems using hybrid processes was discussed recently. The concept is of large relevance as these systems form the majority of chiral substances. In this study, a novel hybrid process is presented, which combines pertraction and subsequent preferential crystallization and is applicable for the resolution of such systems. A supported liquid membrane applied in a pertraction process provides enantiomeric enrichment. This membrane contains a solution of a chiral compound acting as a selective carrier for one of the enantiomers. Screening of a large number of liquid membranes and potential carriers using the conductor-like screening model for realistic solvation method led to the identification of several promising carriers, which were tested experimentally in several pertraction runs aiming to yield enriched (+)-(S)-mandelic acid (MA) solutions from racemic feed solutions. The most promising system consisted of tetrahydronaphthalene as liquid membrane and hydroquinine-4-methyl-2-quinolylether (HMQ) as chiral carrier achieving enantiomeric excesses of 15% in average. The successful production of (+)-(S)-MA with a purity above 96% from enriched solutions by subsequent preferential crystallization proved the applicability of the hybrid process.

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