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

The Al-MOF CAU-13 ([Al­(OH)­(trans-CDC)]; trans-H2CDC = trans-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 o-, m-, and p-xylene from mesitylene showed a preferred adsorption of o-xylene. The structures of o/m/p-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-CDC2– 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”.