Engineering Metal–Organic Frameworks for Selective Separation of Hexane Isomers Using 3-Dimensional Linkers

Abstract

Metal−organic frameworks (MOFs) are highly tunable materials with potential for use as porous media in non-thermal adsorption or membrane-based separations. However, many separations target molecules with sub-angstrom differences in size, requiring precise control over the pore size. Herein, we demonstrate that this precise control can be achieved by installing a three-dimensional linker in an MOF with onedimensional channels. Specifically, we synthesized single crystals and bulk powder of NU-2002, an isostructural framework to MIL-53 with bicyclo[1.1.1]pentane-1,3-dicarboxylic acid as the organic linker component. Using variable-temperature X-ray diffraction studies, we show that increasing linker dimensionality limits structural breathing relative to MIL-53. Furthermore, single-component adsorption isotherms demonstrate the efficacy of this material for separating hexane isomers based on the different sizes and shapes of these isomers.

Expanding Linker Dimensionality in Metal‐organic Frameworks for sub‐Ångstrom Pore Control for Separation Applications

“…In their first study, a 3D‐linker analogue of MIL‐53(Al) was synthesized through substituting pdc for bdc as the organic linker component in MIL‐53(Al) giving [Al(OH)(pdc)] (NU‐2002 ( 5 )) (Figure 7a). [58] Installation of the 3D‐linker resulted in a reduction of the structural flexibility to ≈2 % between 105 K and 398 K using X‐ray diffraction studies (Figure 8a). In contrast, MIL‐53(Al) shows a ≈40 % increase in unit cell volume between 100 K and 450 K [59] .…”

“…In their first study, a 3D-linker analogue of MIL-53(Al) was synthesized through substituting pdc for bdc as the organic linker component in MIL-53(Al) giving [Al(OH)(pdc)] (NU-2002 ( 5)) (Figure 7a). [58] Installation of the 3D-linker resulted in a reduction of the structural flexibility to � 2 % between 105 K and 398 K using X-ray diffraction studies (Figure 8a). In contrast, MIL-53(Al) shows a � 40 % increase in unit cell volume between 100 K and 450 K. [59] Furthermore, SCXRD studies of host-guest interactions, showed that adsorption of hexane isomers did not result in significant changes in the unit cell parameters, which lends to the rigidity of the structure.…”

Crystallographic and Compositional Dependence of Thermodynamic Stability of [Co(II), Cu(II), and Zn(II)] in 2-Methylimidazole-Containing Zeolitic Imidazolate Frameworks

“…15 By employing different metal−ligand combinations, 17,18 a wide range of frameworks have been prepared with tunable material properties, 19−21 from sorption selectivity to material porosity. 8,10,22 Moreover, ZIFs and related composites also offer exceptional thermal and hydrothermal stability, including under acidic or basic conditions, 23,24 which is considered a prerequisite for any widespread application. 25,26 Furthermore, various ZIF polymorphs are accessible through variations of the synthesis conditions.…”

“…By employing different metal–ligand combinations, , a wide range of frameworks have been prepared with tunable material properties, − from sorption selectivity to material porosity. ,, Moreover, ZIFs and related composites also offer exceptional thermal and hydrothermal stability, including under acidic or basic conditions, , which is considered a prerequisite for any widespread application. , Furthermore, various ZIF polymorphs are accessible through variations of the synthesis conditions. ,, For example, real-time studies of the mechanochemical ball-milling preparation of ZIF-8, a popular ZIF material based on divalent zinc nodes and 2-methylimidazolate linkers (MeIm – ), have revealed the sequential formation of the highly porous SOD-topology material ZIF-8, followed by the appearance of katsenite (kat) and finally diamondoid ( dia )-topology polymorphs. ,, …”

“…Metal–organic frameworks (MOFs) have emerged over the last two decades as a promising class of advanced materials based on metal-containing nodes bridged through organic linkers, capable of forming two- (2D) and three-dimensional (3D) frameworks . The linker-node assembly offers a high-degree of tunability, − providing access to potentially porous materials with a wide range of applications, from catalysis and gas sorption to rocket propellants. − A subclass of these materials are zeolitic imidazolate frameworks (ZIFs), ,,− based on tetrahedrally coordinated metal nodes bridged through azolate linkers, ,, resulting in materials with analogous topologies as those found in natural zeolites …”