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

Abstract

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.

Supramolecular‐Macrocycle‐Based Crystalline Organic Materials

“…'' This is related to seemingly nonporous materials possessing lattice voids or cavities available to guest molecules in spite of non‐atomic‐scale channels in the crystal structure. Another intriguing feature for certain macrocycle‐based COMs is that they are nonporous but are structurally adaptive, which are termed as nonporous adaptive crystals (NACs) . There are no voids or cavities in the original state of NACs as a result of efficient molecular packing, but certain void space is induced upon specific guest uptake on account of the structural adaptivity of soft macrocycles.…”

Highly Selective, High‐Capacity Separation of o‐Xylene from C8 Aromatics by a Switching Adsorbent Layered Material

“…Recently,asupermolecular cage with intrinsic porosity reported by Coopersg roup exhibits exceptionally high selectivity but it also suffers from low adsporption capacity. [6] Nassimbeni and co-workers have also reported inclusion compounds with high selectivity but low working capacity. [7] Indeed, there is no physisorbent material which combines the desirable criteria (selectivity > 5and capacity > 50 wt %) that would enable aparadigm shift towards physisorptive separation of C 8 aromatics.…”

Separation of Aromatics/Cyclic Aliphatics by Nonporous Adaptive Pillararene Crystals

“…[7a-c] Moreover,M OFs,b uilt from reversible metal-organic bonds,a re not stable enough for practical recycled use.T hus,t he discovery of stable and recyclable adsorbents for the separation of aromatics and cyclic aliphatics is still demanding. [10] Thenonporous character with "intrinsic porosity" to accommodate specific vapors and structural adaptivity Scheme 1. [8] With rich host-guest chemistry,they have been applied in various areas, including supra-amphiphiles,a rtificial channels,a nd supramolecular polymers.…”

“…[8,9] Recently,o ur group have pioneered the research of pillar[n]arene-based nonporous adaptive crystals. [10] Thenonporous character with "intrinsic porosity" to accommodate specific vapors and structural adaptivity Scheme 1. Chemical structures:a )benzene (Ben), toluene (Tol), cyclohexane (CH), and methylcyclohexane (MCH);b )perethylated pillar-[5]arene (EtP5) and pillar [6]arene (EtP6).…”