ARTICLES PUBLISHED ONLINE: 20 JULY 2014 | DOI: 10.1038/NMAT4035 Separation of rare gases and chiral molecules by selective binding in porous organic cages Linjiang Chen 1 , Paul S. Reiss 1 , Samantha Y. Chong 1 , Daniel Holden 1 , Kim E. Jelfs 1 , Tom Hasell 1 , Marc A. Little 1 , Adam Kewley 1 , Michael E. Briggs 1 , Andrew Stephenson 1 , K. Mark Thomas 2 , Jayne A. Armstrong 2 , Jon Bell 2 , Jose Busto 3 , Raymond Noel 3 , Jian Liu 4 , Denis M. Strachan 4 , Praveen K. Thallapally 4 and Andrew I. Cooper 1 * The separation of molecules with similar size and shape is an important technological challenge. For example, rare gases can pose either an economic opportunity or an environmental hazard and there is a need to separate these spherical molecules selectively at low concentrations in air. Likewise, chiral molecules are important building blocks for pharmaceuticals, but chiral enantiomers, by definition, have identical size and shape, and their separation can be challenging. Here we show that a porous organic cage molecule has unprecedented performance in the solid state for the separation of rare gases, such as krypton and xenon. The selectivity arises from a precise size match between the rare gas and the organic cage cavity, as predicted by molecular simulations. Breakthrough experiments demonstrate real practical potential for the separation of krypton, xenon and radon from air at concentrations of only a few parts per million. We also demonstrate selective binding of chiral organic molecules such as 1-phenylethanol, suggesting applications in enantioselective separation. W ith the exception of argon, which makes up almost 1% of air, the rare or ‘noble’ gases are all commonly encountered in low concentrations: xenon (Xe) occurs naturally in the atmosphere at 0.087 parts per million by volume (ppmv); krypton (Kr) at 1.14 ppmv (ref. 1). Cryogenic methods are used to extract commercially valuable rare gases such as xenon from air, but this is costly because of the low concentrations involved. Rare gases are therefore valuable: high-purity xenon, for example, has uses including commercial lighting, medical imaging, anaesthesia and neuroprotection, and it sells for more than $5,000 kg -1 . Other rare gas isotopes can be harmful. Radon gas, which occurs naturally in a radioactive form ( 222 Rn), can accumulate in buildings, and is a leading cause of lung cancer 2 , accounting for around 21,000 deaths per year in the USA alone. Likewise, unstable, hazardous radioisotopes of krypton and xenon, such as 85 Kr and 133 Xe, are produced in nuclear fission and can enter the atmosphere during the reprocessing of spent nuclear fuel 3 or via nuclear accidents, such as the Fukushima Daiichi nuclear power plant catastrophe in Japan 4 . Cryogenic processes have been suggested for the removal of radioactive rare gases from off-gas streams in future nuclear reprocessing plants, but again this is energy intensive and expensive because of the low rare gas concentrations. Alternative separation technologies therefore could save energy, protect the environment, and produce valuable resources: for example, the reduction of 85 Kr concentrations to permissible levels in xenon-rich nuclear reprocessing streams would create an entirely new source of xenon for industrial use. In principle, gas mixtures can be separated with greater energy efficiency by using porous solids that bind specific components in the mixture, as suggested by early experiments on the adsorption of ‘radium emanations’ (radon) on charcoal by Rutherford 5 . A wide range of task-specific porous materials now exists, such as activated carbons 6,7 , zeolites 8 , metal–organic frameworks (MOFs; refs 9,10), porous molecular crystals 11 , and polymers 12 . It remains a major challenge, however, to efficiently separate gas molecules that are present in low concentrations (<500 ppmv) from the principal components in the gas mixture. For rare gases, this is exacerbated by their lack of chemical reactivity and the small size difference between the higher-mass rare gases, such as Kr (diameter = 3.69 Å; ref. 13), Xe (4.10 Å) and Rn (4.17 Å), and the common constituents of air. The spherical nature of the rare gases precludes strategies based on shape selectivity 14 ; hence precise tuning of the dimensions of the pores is required to achieve selective separations. Ideally, an adsorbent should exhibit both high adsorption selectivity and high adsorption capacity for the component of interest. The provision of a large physical surface area may not give good separation selectivity, but adequate adsorption capacity is nonetheless required to create economically viable separation methods. Porous MOFs show promise for Xe/Kr separations 15–17 and computational screening studies suggest that better materials remain to be discovered 13,18 . Few materials, however, provide effective separations of rare gases at low concentrations of just a few parts per million in air. The leading material is the nickel-based MOF, Ni/DOBDC, which was shown to separate 400ppm Xe from 40 ppm Kr in air containing O 2 ,N 2 and CO 2 with a Xe/Kr selectivity of 7.3 (ref. 19). We reported previously an organic cage molecule, CC3 (ref. 20), which we show here to have an internal cavity that is precisely the right size to accommodate a single xenon or radon atom. The largest inclusion sphere 21 in this cavity (d = 4.4 Å) is very close to the diameters of xenon (4.10 Å; Fig. 1c) and radon (4.17 Å). The cage packs in the crystalline state to give a robust 3D pore structure 1 Department of Chemistry and Centre for Materials Discovery, University of Liverpool, Crown Street, Liverpool L69 7ZD, UK, 2 Wolfson Northern Carbon Reduction Laboratories, Drummond Building, Newcastle University, Newcastle upon Tyne NE1 7RU, UK, 3 CPPM, Aix-Marseille Université, CNRS/IN2P3, 163 avenue de Luminy, case 902, 13009 Marseille, France, 4 Pacific Northwest National Laboratory, Richland, Washington 99352, USA. *e-mail: aicooper@liv.ac.uk NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials 1 © 2014 Macmillan Publishers Limited. All rights reserved.