Enantioselective adsorption characteristics of aluminum-substituted MFI zeolites

Using computer simulations, we have studied the adsorption of racemic and scalemic mixtures of 4-ethyl-4-methyloctane molecules in aluminum-substituted MFI zeolites containing positively charged ions. Our results show that aluminum distribution affects the shape of the enantiopure adsorption isotherms, which is contrary to previously established results on linear alkanes. In addition, we examined the enantiospecific adsorption at maximum loading conditions. For enantiopure and racemic mixtures, the fraction of S- and R-enantiomers in the adsorbed phase is identical to that of the gas phase. However, when the external gas contains a scalemic mixture, the dominant enantiomeric fraction might either increase or decrease upon adsorption, depending on the aluminum distribution and on the type of cations. We provide a theoretical explanation for both types of adsorption characteristics. Our findings open perspectives for new approaches of enantioseparation that do not require a chiral adsorbent.