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[n]­arene crystals
(n = 5, 6), which can be used to separate C8 alkylaromatic
compounds. Pillar[6]­arene is shown to separate para-xylene from its structural isomers, meta-xylene
and ortho-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 para-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.