Activation-Controlled Structure Deformation of Pillared-Bilayer Metal–Organic Framework Membranes for Gas Separations

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We investigated gas adsorption and diffusion in an emerging pillared-bilayer metal–organic framework (MOF), Zn-AIP-AZPY (aip, 5-aminoisophthalic acid; azpy, 4,4′-azobipyridine). This work demonstrated that the crystal structure of this flexible MOF could be controlled by two different activation methods: the thermal activation and the chemical activation. The pore limiting diameter of the thermally activated compound was ∼3.7 Å, whereas that of the fully activated one via methanol extraction was 2.92 Å. Although the aperture size of the fully activated structure was smaller than the kinetic diameter of CO 2 and CH 4, Zn-AIP-AZPY still presented fair adsorption quantity to these two gases. In situ XRD experiments under various gases at different pressures also suggest a minor breathing effect when Zn-AIP-AZPY was exposed to CO 2 and no breathing effect under CH 4 . These results imply the possible ligand rotation occurring during the adsorption of CO 2 and CH 4 in Zn-AIP-AZPY. The Zn-AIP-AZPY membranes were further prepared using the seeded growth method, and these membranes showed an exceptionally high H 2 permeability (over 10 5 barrer) with a good H 2 /CO 2 selectivity (ideal selectivity beyond 8). A reverse CO 2 /CH 4 selectivity, i.e., CH 4 permeates faster than CO 2, was surprisingly found. Monte Carlo simulations were conducted for probing the adsorption properties of CO 2 and CH 4, as well as their adsorption energy landscape in Zn-AIP-AZPY. It was found that this MOF energetically favored the adsorption of CO 2 over CH 4 by nearly 10 kJ/mol. Such a strong adsorption is anticipated to create high energy barriers for CO 2 hopping between neighboring adsorption sites.