Ultra-High Size Exclusion Selectivity for Carbon Dioxide from Nitro-gen/Methane in an Ultramicroporous Metal-Organic Framework

Berdichevsky, Ellan K. and Downing, Victoria and Hooper, Riley W. and Butt, Nathan W. and McGrath, Devon T. and Donnelly, Laurie J, and Michaelis, Vladimir K. and Katz, Michael J. (2022) Ultra-High Size Exclusion Selectivity for Carbon Dioxide from Nitro-gen/Methane in an Ultramicroporous Metal-Organic Framework. Inorganic Chemistry, 61 (20). pp. 7970-7979. ISSN 1520-510X

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Abstract

Separations based on molecular size (molecular sieving) are a solution for environmental remediation. We have synthesized and characterized two new metal-organic frameworks (Zn2M; M = Zn, Cd) with ultramicropores (<0.7 nm) suitable for molecular sieving. We explore the synthesis of these MOFs and the role that the DMSO/H2O/DMF solvent mix-ture has on the crystallization process. We further explore the crystallographic data for the DMSO and methanol solvated structures at 273 and 100K; this not only results in high quality structural data, but also allows us to better understand the structural features at temperatures around the gas adsorption experiments. Structurally, the main difference between the two MOFs is that the central metal in the trimetallic node can be changed from Zn to Cd and that results in a sub-Å change in the size of the pore aperture, but a stark change in the gas adsorption properties. The separation selectivity of the MOF when M = Zn is infinite given the pore aperture of the MOF can accommodate CO2 while N2 and/or CH4 is excluded from en-tering the pore. Furthermore, due to the size exclusion behaviour, the MOF has an adsorption selectivity of 4800:1 CO2:N2 and 5×1028:1 CO2:CH4. When M = Cd, the pore aperture of the MOF increases slightly, allowing N2 and CH4 to enter the pore, resulting in a 27.5:1 and a 10.5:1 adsorption selectivity, respectively; this is akin to UiO-66, a MOF that is not able to function as a molecular sieve for these gases. The data delineates how subtle sub-Å changes to the pore aperture of a framework can drastically affect both the adsorption selectivity and separation selectivity.

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