(248c) Installing 3-Dimensional Linkers to Fine Tune the Pore Size of Metal–Organic Frameworks for Efficient Gas Separations

Metal–organic frameworks (MOFs) are highly tunable materials with strong potential for use as porous media in non-thermal adsorption- or membrane-based separation processes. However, many prominent industrial gas separations target molecules with sub-angstrom differences in size, requiring precise control over the pore size of MOFs. Specifically, separating isomers of hexane is particularly challenging due the similar kinetic diameters and boiling points of the molecules. Our group has demonstrated that substituting 3-dimensional linkers (3DLs) for 2-dimensional, planar linkers in flexible MOFs is an effective strategy to tune pore size for selective separations. 3DLs have additional sites for host-guest interactions within the framework and offer steric bulk that results in increased structural rigidity. In this study, we substituted bicyclo[1.1.1]pentane-1,3-dicarboxylic acid (BPDCA) for benzene-1,4-dicarboxylate (BDC) in MIL-53(Al). MIL-53 is known to exhibit structural breathing triggered by temperature or guest molecules adsorption. Using variable temperature X-ray diffraction studies, we show that installing the 3DL limits structural flexibility to a ~2% change in unit cell volume, while MIL-53 exhibits a ~40% change. Further, we use single crystal X-ray diffraction of host-guest interactions and single component adsorption isotherms to demonstrate the efficacy of this material for separating hexane isomers based on their different sizes and shapes.