Conformation-Controlled Sorption Properties and Breathing of the Aliphatic Al-MOF [Al(OH)(CDC)]

Abstract

The Al-MOF CAU-13 ([Al­(OH)­(<i>trans</i>-CDC)]; <i>trans</i>-H₂CDC = <i>trans</i>-1,4-cyclohexanedicarboxylic acid) is structurally related to the MIL-53 compounds that are well-known for their “breathing” behavior, i.e., the framework flexibility upon external stimuli such as the presence of adsorbate molecules. The adsorption properties of CAU-13 were investigated in detail. The sorption isotherms of N₂, H₂, CH₄, CO, CO₂, and water were recorded, and the adsorption enthalpies for the gases were determined by microcalorimetry. The structural changes upon adsorption of CO₂ were followed with in situ synchrotron powder X-ray diffraction (PXRD). The patterns were analyzed by parametric unit cell refinement, and the preferential arrangement of the CO₂ molecules was modeled by density functional theory calculations. The adsorption and separation of mixtures of <i>o</i>-, <i>m</i>-, and <i>p</i>-xylene from mesitylene showed a preferred adsorption of <i>o</i>-xylene. The structures of <i>o</i>/<i>m</i>/<i>p</i>-xylene-loaded CAU-13 were determined from PXRD data. The adsorption of xylene isomers induces a larger pore opening than that in the thermal activation of CAU-13. In the crystal structure of the activated sample CAU-13­(empty pore), half of the linkers adopt the a,a confirmation and the other half the e,e conformation, and the presence of a,a-CDC^2– ions hampers the structural flexibility of CAU-13. However, after the adsorption of xylene, all linkers are present in the e,e conformation, allowing for a wider pore opening by this new type of “breathing”