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

Abstract

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.