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Near-Ideal Xylene Selectivity in Adaptive Molecular Pillar[n]arene Crystals

Jie, Kecheng, Liu, Ming, Zhou, Yujuan, Little, Marc A., Pulido, Angeles, Chong, Samantha Y., Stephenson, Andrew, Hughes, Ashlea R., Sakakibara, Fumiyasu, Ogoshi, Tomoki, Blanc, Frédéric, Day, Graeme M., Huang, Feihe, Cooper, Andrew I.
Journal of the American Chemical Society 2018 v.140 no.22 pp. 6921-6930
X-ray diffraction, adsorption, carbon, constitutional isomers, coordination polymers, crystal structure, crystals, energy efficiency, nuclear magnetic resonance spectroscopy, solids, sorbents, stable isotopes, xylene, zeolites
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.