Influence of Thermal and Mechanical Stimuli on the Behavior of Al-CAU-13 Metal-Organic Framework.

The response of the metal-organic framework aluminum-1,4-cyclohexanedicarboxylate or Al-CAU-13 (CAU: Christian Albrecht University) to the application of thermal and mechanical stimuli was investigated using synchrotron powder X-ray diffraction (SPXRD). Variable temperature in situ SPXRD data, over the range 80-500 K, revealed a complex evolution of the structure of the water guest containing Al-CAU-13H2O, the dehydration process from ca. 310 to 370 K, and also the evolution of the guest free Al-CAU-13 structure between ca. 370 and 500 K. Rietveld refinement allowed this complexity to be rationalized in the different regions of heating. The Berman thermal Equation of State was determined for the two structures (Al-CAU-13H2O and Al-CAU-13). Diamond anvil cell studies at elevated pressure (from ambient to up to ca. 11 GPa) revealed similarities in the structural responses on application of pressure and temperature. The ability of the pressure medium to penetrate the framework was also found to be important: non-penetrating silicone oil caused pressure induced amorphization, whereas penetrating helium showed no plastic deformation of the structure. Third-order Vinet equations of state were calculated and show Al-CAU-13H2O is a hard compound for a metal-organic framework material. The mechanical response of Al-CAU-13, with tetramethylpyrazine guests replacing water, was also investigated. Although the connectivity of the structure is the same, all the linkers have a linear e,e-conformation and the structure adopts a more open, wine-rack-like arrangement, which demonstrates negative linear compressibility (NLC) similar to Al-MIL-53 and a significantly softer mechanical response. The origin of this variation in behavior is attributed to the different linker conformation, demonstrating the influence of the S-shaped a,a-conformation on the response of the framework to external stimuli.

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-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".

Aluminum-1,4-cyclohexanedicarboxylates: high-throughput and temperature-dependent in situ EDXRD studies.

The system AlCl3·6H2O/cis-H2CDC/trans-H2CDC/solvent was systematically investigated with high-throughput methods to study the influence of the two 1,4-cyclohexanedicarboxylate isomers (cis- and trans-H2CDC) as flexible aliphatic linker molecules on the formation of new crystalline compounds. Using the cis-isomer, the layered inorganic-organic hybrid compound [Al(OH)(cis-CDC)] (1) is formed. The use of trans-H2CDC leads to the microporous MOF [Al(OH)(trans-CDC)]·H2O (2) denoted CAU-13. Its framework is related to the well-known MIL-53, which was previously described for trivalent cations and rigid terephthalate linker molecules. The crystal structures of 1 and 2 were derived from powder X-ray diffraction data. Temperature-dependent in situ energy dispersive X-ray diffraction (EDXRD) experiments for the synthesis of 2 were carried out at HASYLAB, DESY, Hamburg. The kinetic analysis, applying the Gualtieri model to the experimental data, revealed Arrhenius activation energies of 76 kJ/mol for both the nucleation and the growth process. These values do not differ much from the activation energies reported for MOFs with aromatic rigid linker molecules.