Zeolites and related crystalline microporous oxides—tetrahedrally coordinated atoms covalently linked into a porous framework—are of interest for applications ranging from catalysis to adsorption and ion-exchange1. In some of these materials (such as zeolite rho) adsorbates2, ion-exchange, and dehydration and cation relocation3,4 can induce strong framework deformations. Similar framework flexibility has to date not been seen in mixed octahedral/tetrahedral microporous framework materials, a newer and rapidly expanding class of molecular sieves5,6,7,8,9,10,11,12,13,14,15,16. Here we show that the framework of the titanium silicate ETS-4, the first member of this class of materials8, can be systematically contracted through dehydration at elevated temperatures to ‘tune’ the effective size of the pores giving access to the interior of the crystal. We show that this so-called ‘molecular gate’ effect can be used to tailor the adsorption properties of the materials to give size-selective adsorbents17 suitable for commercially important separations of gas mixtures of molecules with similar size in the 4.0 to 3.0 Å range, such as that of N2/CH4, Ar/O2 and N2/O2.
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We thank J. Curran for discussions and assistance in the preparation of the manuscript, and VTI Corp. for the data of Fig. 5. This work was supported by ATP/NIST, the David and Lucile Packard Foundation and NSF-CTS.
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Rietveld refinement of Sr-ETS-4 thermally treated at 150°C showing observed, calculated and difference plots from neutron diffraction data. Tick marks indicate possible Cmmm reflections.
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Rietveld refinement of Sr-ETS-4 thermally treated at 200°C showing observed, calculated and difference plots from neutron diffraction data. Tick marks indicate possible Cmmm reflections.
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Rietveld refinement of Sr-ETS-4 thermally treated at 250°C showing observed, calculated and difference plots from neutron diffraction data. Tick marks indicate possible Cmmm reflections.
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Rietveld refinement of Sr-ETS-4 thermally treated at 300°C showing observed, calculated and difference plots from neutron diffraction data. Tick marks indicate possible Cmmm reflections.
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Powder X-ray diffraction data showing the stability of framework contraction. (a) Room-temperature data before thermal treatment. (b) Two overlaying traces are shown for samples after thermal treatment at 330°C. One is immediately after thermal treatment and the overlay is after 12 days of exposure to humid room-temperature air. After 12 days only a small shift to smaller angles (larger d-spacing) is observed.
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Kuznicki, S., Bell, V., Nair, S. et al. A titanosilicate molecular sieve with adjustable pores for size-selective adsorption of molecules. Nature 412, 720–724 (2001). https://doi.org/10.1038/35089052
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