CAU-63, an Ultramicroporous Al-MOF with a Honeycomb-Shaped 2D IBU

The hydrothermal synthesis of the new aluminum metal-organic framework (Al-MOF) CAU-63 [Al₇(OH)₁₂O₃(2,4-HPydc)₃] and two new Al coordination polymers (CPs) Al-Pydc-CP1 [Al₂(OH)₅(2,4-HPydc)] and Al-Pydc-CP2 [Al(OH)(H₂O)(2,4-Pydc)] linked by anions of lutidinic acid (pyridine-2,4-dicarboxylic acid, 2,4-H₂Pydc) is reported. High-throughput investigations of the Al³⁺/2,4-H₂Pydc/NaOH/H₂O system were carried out to determine the fields of formation. An increase of the molar ratio of metal to linker was found to be the key parameter for the formation of higher condensed inorganic building units (IBU), changing from dimeric to one- and two-dimensional structures. The crystal structures were determined by 3D electron diffraction with subsequent Rietveld refinement against powder X-ray diffraction data. The pyridine nitrogen atoms of the linker molecules coordinate to aluminum ions in all three compounds, resulting in crystal structures deviating from the typically observed MIL-53 and CAU-10 type frameworks. The coordination polymers Al-Pydc-CP1 and Al-Pydc-CP2 contain edge-sharing Al-O/N polyhedra leading to dimeric and helical IBUs, while in CAU-63, tetrameric [Al₄O₁₄N₂] units are bridged by Al³⁺ ions, leading to a honeycomb Al-O-N network with organic moieties interconnecting the layers. This linkage results in channel-like ultramicropores, which are accessible to H₂O and NH₃ molecules but too small to adsorb N₂ and even CO₂.