Comparison of the structure-directing effect of ephedrine and pseudoephedrine during crystallization of nanoporous aluminophosphates

Abstract

We present here a systematic study about the behavior of two diastereomeric chiral organic molecules, (1R,2S)-ephedrine and (1S,2S)-pseudoephedrine, for directing the crystallization of crystalline nanoporous aluminophosphates. We apply a combination of extensive synthetic experiments, characterization and molecular simulations in order to relate the different structure-directing effects of the two isomers experimentally observed with their particular molecular structure. Our results show a higher efficiency of pseudoephedrine to direct the crystallization of AFI materials: in contrast, ephedrine tends to give a layered-like phase with a strong supramolecular aggregation together with AFI materials. In addition, aggregation of ephedrine within the AFI channels is stronger than that of pseudoephedrine. Molecular simulations show a different conformational behavior for the two diastereoisomers, which is manifested in a different supramolecular behavior, and as a consequence a different structure-directing behavior. This multi-level study shows the intricacy between the absolute configuration of the structure-directing agents, and hence their molecular structure which results in the development of distinct intramolecular H-bonds, the molecular conformational space, the supramolecular chemistry associated, and the structure-directing mode of action during the crystallization of AFI-aluminophosphate materials, providing a molecular-level understanding of the structure-directing phenomena of these molecules.