Influence of flexibility on the separation of chiral isomers in the STW-type zeolite

Abstract

Molecular simulation, through the computation of adsorption isotherms, is a useful predictive tool for the selective capacity of nanoporous materials. Generally, adsorbents are modelled as rigid frameworks, as opposed to allowing for vibrations of the lattice, and this approximation is assumed to have negligible impact on adsorption. In this work, we test this approach in an especially challenging system by computing the adsorption of the chiral molecules 2-pentanol, 2-methylbutanol and 3-methyl-2-butanol in the all-silica and germanosilicate chiral zeolites STW, and study their lattice vibrations upon adsorption. The analysis of single and multicomponent adsorption isotherms shows the suitability of the STW-type zeolites as molecular sieves for chiral separation processes, which pose a challenging task in the chemical and pharmaceutical industries. We also provide new experimental adsorption data that validate the force field employed. Our results reveal that the lattice vibrations of the all-silica framework are sorbate-independent while those of germanosilicate STW on the other hand display host-guest coupling modulated by uptake and sorbate type that disrupt the chiral recognition sites. This study points out that the effects of intrinsic flexibility on the selective capacity of nanoporous materials may range from low to high impact, some of which could not have been foreseen even after the examination of the structural dynamics of an empty framework.