Structures, Sorption Characteristics, and Nonlinear Optical Properties of a New Series of Highly Stable Aluminum MOFs

Employing high-throughput methods, the synthesis conditions for a series of six new MOFs based on aluminum ions and the V-shaped linker molecule 1,3-benzene dicarboxylic acid, denoted as CAU-10-X (CAU = Christian-Albrechts-University) with the sum formula [Al(OH)(C8H3O4X)]·solvent, were established (X = functional group in 5-position of the aromatic ring; X = H (1), CH3 (2), OCH3 (3) NO2 (4), NH2 (5), or OH (6)). Because of the absence of macroscopic crystals, the obtained compounds were structurally characterized employing XRPD-methods. The crystal structures of 1, 2, and 3 were refined using Rietveld methods. Although the described MOFs are isoreticular, they crystallize in several, sometimes noncentrosymmetric space groups (1, 4, 6), because of slight structural changes induced by the functionalization. These space groups were confirmed with second-harmonic generation measurements. All compounds are highly stable as confirmed by temperature-dependent XRPD and IR experiments and decompose at temperatures above 350 °C. The stabilities of all compounds in aqueous solutions of varying pH were confirmed by XRPD measurements and their sorption properties toward nitrogen, hydrogen, carbon dioxide, and water vapor at low pressures are reported. A drastic influence of the functional group on affinity, capacity, and accessibility of the pores for these gases is observed. These properties depend on the polarity and size of the functional group as well as on subtle structural differences between the CAU-10-X compounds.