1 research outputs found
Conformation-Controlled Sorption Properties and Breathing of the Aliphatic Al-MOF [Al(OH)(CDC)]
The Al-MOF CAU-13 ([AlÂ(OH)Â(<i>trans</i>-CDC)]; <i>trans</i>-H<sub>2</sub>CDC = <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 N<sub>2</sub>, H<sub>2</sub>, CH<sub>4</sub>, CO, CO<sub>2</sub>, and water were recorded, and the adsorption
enthalpies for the gases were determined by microcalorimetry. The
structural changes upon adsorption of CO<sub>2</sub> 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 CO<sub>2</sub> 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-13Â(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<sup>2–</sup> 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”