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Separation of rare gases and chiral molecules by selective binding in porous organic cages

Abstract

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.

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Figure 1: The porous organic cage molecule and its extended crystal packing.
Figure 2: Molecular simulations and adsorption measurements demonstrate dynamic gas permeability and high selectivity for xenon and radon.
Figure 3: Gas-loaded organic cage structures have parallels with gas hydrates.
Figure 4: Separation of valuable or harmful rare gases at low concentrations using organic cages.
Figure 5: Chiral separation using CC3.
Figure 6: Simulated molecular configurations of (S)-1-phenylethanol in the pores of CC3-R.

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Acknowledgements

We thank EPSRC (EP/H000925/1; EP/K018396/1), the European Research Council under FP7 (ERC grant agreement no. 321156) and Region PACA (Provence-Alpes-Cote-d’Azur) for funding. A.I.C. is a Royal Society Wolfson Merit Award holder. K.E.J. is a Royal Society University Research Fellow. We thank Diamond Light Source for access to beamlines I11 (EE7040) and I19 (MT8728) that contributed to the results presented here. We also thank the beamline staff for their assistance during the I11 experiments. We thank D. Dubbeldam for providing the RASPA simulation package. We thank the US Department of Energy (DOE), Office of Nuclear Energy, and in particular, J. Bresee, for their support. T. Todd (Idaho National Laboratory) and B. Jubin (Oak Ridge National Laboratory) provided programmatic support and guidance. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the US Department of Energy by Battelle Memorial Institute under Contract DE-AC05-76RL01830.

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Contributions

L.C., D.H. and K.E.J. performed the molecular simulations. P.S.R., J.L., D.M.S. and P.K.T. conceived and carried out the gas breakthrough measurements. S.Y.C. and M.A.L. carried out X-ray structure analyses. T.H., K.M.T., J.A.A. and J. Bell performed gas sorption analyses. R.N. and J. Busto carried out the radon adsorption measurements. A.K., M.E.B. and A.S. determined chiral selectivities. A.K., A.S., M.A.L., M.E.B., P.S.R., S.Y.C., and T.H. prepared the cage molecules. A.I.C. conceived the project. All coauthors contributed to the writing of the paper.

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Correspondence to Andrew I. Cooper.

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Chen, L., Reiss, P., Chong, S. et al. Separation of rare gases and chiral molecules by selective binding in porous organic cages. Nature Mater 13, 954–960 (2014). https://doi.org/10.1038/nmat4035

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