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Activation-controlled Structure Deformation of Pillared-bilayer Metal-organic Framework Membranes for Gas Separations
M.-Y. Kan, J. H. Shin, C.-T. Yang, C.-K. Chang, L.-W. Lee, B.-H. Chen, K.-L. Lu, J. S. Lee*, L.-C. Lin*, and D.-Y. Kang*
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 CO2 and CH4, 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 CO2 and no breathing effect under CH4. These results imply the possible ligand rotation occurring during the adsorption of CO2 and CH4 in Zn-AIP-AZPY. The Zn-AIP-AZPY membranes were further prepared using the seeded growth method, and these membranes showed an exceptionally high H2 permeability (over 105 barrer) with a good H2/CO2 selectivity (ideal selectivity beyond 8). A reverse CO2/CH4 selectivity, i.e., CH4 permeates faster than CO2, was surprisingly found. Monte Carlo simulations were conducted for probing the adsorption properties of CO2 and CH4, as well as their adsorption energy landscape in Zn-AIP-AZPY. It was found that this MOF energetically favored the adsorption of CO2 over CH4 by nearly 10 kJ/mol. Such a strong adsorption is anticipated to create high energy barriers for CO2 hopping between neighboring adsorption sites.