Ultrahigh Size Exclusion Selectivity for Carbon Dioxide from Nitrogen/Methane in an Ultramicroporous Metal-Organic Framework

Separations based on molecular size (molecular sieving) are asolution for environmental remediation. We have synthesized andcharacterized two new metal-organic frameworks (MOFs) (Zn2M;M=Zn, Cd) with ultramicropores (<0.7 nm) suitable for molecular sieving. Weexplore the synthesis of these MOFs and the role that the DMSO/H2O/DMF solvent mixture has on the crystallization process. We further explorethe crystallographic data for the DMSO and methanol solvated structures at273 and 100 K; this not only results in high-quality structural data but alsoallows us to better understand the structural features at temperatures aroundthe gas adsorption experiments. Structurally, the main difference between thetwo MOFs is that the central metal in the trimetallic node can be changedfrom Zn to Cd and that results in a sub-A 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 CO2while N2and/or CH4is excluded from entering the pore. Furthermore, due to the size exclusion behavior, the MOF has an adsorptionselectivity of 4800:1 CO2/N2and 5x1028:1 CO2/CH4. When M = Cd, the pore aperture of the MOF increases slightly, allowing N2and CH4to 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 thatis not able to function as a molecular sieve for these gases. The data delineate how subtle sub-A changes to the pore aperture of aframework can drastically affect both the adsorption selectivity and separation selectivity