Near-Ideal Xylene Selectivity in Adaptive Molecular Pillar[<i>n</i>]arene Crystals

The energy-efficient separation of alkylaromatic compounds is a major industrial sustainability challenge. The use of selectively porous extended frameworks, such as zeolites or metal–organic frameworks, is one solution to this problem. Here, we studied a flexible molecular material, perethylated pillar­[<i>n</i>]­arene crystals (<i>n</i> = 5, 6), which can be used to separate C8 alkylaromatic compounds. Pillar[6]­arene is shown to separate <i>para</i>-xylene from its structural isomers, <i>meta</i>-xylene and <i>ortho</i>-xylene, with 90% specificity in the solid state. Selectivity is an intrinsic property of the pillar[6]­arene host, with the flexible pillar[6]­arene cavities adapting during adsorption thus enabling preferential adsorption of <i>para</i>-xylene in the solid state. The flexibility of pillar[6]­arene as a solid sorbent is rationalized using molecular conformer searches and crystal structure prediction (CSP) combined with comprehensive characterization by X-ray diffraction and ¹³C solid-state NMR spectroscopy. The CSP study, which takes into account the structural variability of pillar[6]­arene, breaks new ground in its own right and showcases the feasibility of applying CSP methods to understand and ultimately to predict the behavior of soft, adaptive molecular crystals